principles of plant genetics and breeding (acquaah/principles of plant genetics and breeding) ||...

24

Polyploidy in plantbreeding

Purpose and expected outcomes

Hybridization, as previously discussed, is a means of reorganizing genes from the parents involved in the cross in anew genetic matrix. Whereas the contents of the chromosomes may change because of the phenomenon of geneticrecombination, normal hybridization does not alter the chromosome number of the species. However, certainnatural processes can result in altered chromosome numbers. Similarly, the breeder may develop new variabil-ity by altering the number of chromosomes in the species through various processes. Furthermore, a number ofthe major crop species contain altered chromosome numbers. After studying this chapter, the student should beable to:

1 Define the term polyploidy.2 Discuss the variations in chromosome number in plants.3 Discuss the effects of polyploidy on plants.4 Discuss the importance of autoploidy in crop production.5 Discuss the genetics of autoploidy.6 Discuss the implications of autoploidy in plant breeding.7 Discuss the occurrence of alloploidy in nature.8 Discuss the genetics and breeding of alloploidy in plant breeding.9 Discuss the applications of aneuploidy.

24.1 Terminology

Ploidy refers to the number of copies of the entirechromosome set in a cell of an individual. Thecomplete chromosome set is characteristic of, orbasic to, a species. A set of chromosomes (the

genome) is designated by “x”. Furthermore, thebasic set is called the monoploid set. The haploidnumber (n) is the number of chromosomes thatoccurs in gametes. This represents half the chromo-some number in somatic cells, which is designated2n. A diploid species such as corn, has n¼ 10 and

Principles of Plant Genetics and Breeding, Second Edition. George Acquaah.� 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.

2n¼ 20. Also a diploid species has 2n¼ 2x in itssomatic cells and n¼ x in its gametes. A specieswith a higher ploidy, for example autotetraploid(four basic sets of chromosomes) has somatic cellswith 2n¼ 4x and gametes with n¼ 2x. For corn,for example, 2n¼ 2x¼ 20, while for wheat, a hexa-ploid with 42 chromosomes and a basic set of 7,2n¼ 6x¼ 42. Sometimes species that have morethan two genomes comprise sets from different ori-gins. To distinguish the source, each genome isdesignated by a different letter. For example, wheathas chromosome sets from three different originsand, hence, has a genetic designation (genomicformula) of AABBDD (Figure 24.1). To indicatethe number of haploids derived from individuals of

different ploidy levels for a single genome, a prefixis added to the term “haploid”’ to denote thenumber of sets (x) of the basic genome present.For example a monohaploid (n¼ 1x) is derivedfrom a diploid, while a dihaploid (n¼ 2x) is froma tetraploid, and so on.

In some species of higher plants, a pattern of ploidyemerges whereby the gametic (haploid) and somatic(diploid) chromosome numbers increase in an arith-metic progression, as illustrated by oats and wheat(Table 24.1). The set of species displaying this patternconstitute a polyploid series.

24.2 Variations in chromosome number

In nature, there exist two types of variation in chro-mosome number. In one type, called euploidy,the individuals contain multiples of the completeset of chromosomes that is characteristic of thespecies (the basic number, x). In another, calledaneuploidy, individuals contain incomplete sets ofchromosomes that may be equivalent to the euploidnumber plus or minus one or more specific chromo-somes (Table 24.2). The state of having multiples ofthe basic set in the somatic cell in excess of the diploidnumber is called polyploidy, and the individuals withsuch cells, polyploids. Polyploids are euploids. Wheneuploids comprise multiples of the genome (i.e.,duplicates of the genome from the same species) theyare called autoploids and the condition autoploidy(or autopolyploidy). However, when a combinationof genomes from different species are involved, theterm alloploid or allopolyploid (and, similarly, allo-ploidy or allopolyploidy) is used. Alternatively, theterm amphiploid or amphidiploid (and, similarly,amphiploidy or amphidiploidy) is also used todescribe polyploids with different genomes. It shouldbe pointed out that autoploidy and alloploidy are

Triticum monococcum(2n = 2x = 14)(AA)

X

2x = 14

Unknown(Aegilops speltoides?)2n = 2x = 14(BB)

Chromosome doubling

T. turgidum2n = 4x = 28(AABB)

X

3x = 21(ABD)

T. tauschii2n = 2x = 14(DD)

Chromosome doubling

T. aestivum2n = 6x = 42(AABBDD)

Figure 24.1 The proposed origin of common wheatTriticum aestivum.

Table 24.1 Polyploid series of selected species.

Oat (Avena species) Wheat (Triticum species)

Diploid (2n¼2x¼ 14) A. brevis (short oat) T. monococcum (einkorn)A. strgoga (sand oat) T. tauschii (wild oat)

Tetraploid (2n¼ 4x¼28) A. barbata (slender oat) T. timopheevii (wild)A. abyssinica (Abyssinia oat) T. turgidum (emmer)

Hexaploid (2n¼ 6x¼42) A. sativa (common oat) T. aestivum (common bread wheat)A. byzantina (red oat)

POLYPLOIDY IN PLANT BREEDING 453

extreme forms of polyploidy. Intermediates occurbetween them on a continuum of genomic relation-ships. C.L. Stebbins called the intermediates segmen-tal alloploids. Polyploids are named such that theprefix to the standard suffix (ploid) refers to the basicchromosome set (Table 24.3). For example “triploid”refers to a cell with three genomes (3x) while“hexaploid” refers to a cell with six genomes (6x).

24.3 General effects of polyploidy of plants

In terms of general morphology, an autoploid wouldresemble the original parent whereas an alloploidwould tend to exhibit a phenotype that is intermedi-ate between its parental species. Autoploidy increasescell size, especially in meristematic tissues. Autoploidsusually have thicker, broader, and shorter leaves.Other plant organs may increase in size compared totheir corresponding parts in diploids, an effect calledgigas features. The gigas luxuriance contributes moreto moisture content of the plant parts than to bio-mass. The plants tend to be determinate in growth.

The growth rate of polyploids is less than that ofdiploids. This may be due to their lower auxin con-tent than their diploid counterparts, as found fortetraploids. Polyploids tend to flower later and overa longer period. In grasses, autoploidy tends toreduce branching or tillering.

Polyploidy also affects the chemical composition ofplant parts. For example, artificial polyploidyincreased the synthesis of artemisinin (an antimalarialsesquiterpene) produced by diploid Artemisia annua,L) twofold to five fold in induced tetraploids. Simi-larly, the vitamin C content of vegetables and fruitshas been known to increase following chromosomedoubling. The nicotine content of tetraploidtobacco is about 18–% higher than in diploid spe-cies. Autoploids, generally, have fertility problemsand have poor pollen production. In some cases,reduction in fertility as compared to their diploidcounterparts may be as high as 80–95%. This reduc-tion in fertility is attributed to genetic imbalancefollowing chromosome doubling that leads to dis-harmonies in development (e.g., abnormal pollensac, failure of fertilization). Some changes in eco-logical requirements such as photoperiod and heatrequirements have been reported in some speciesfollowing chromosome doubling.

24.4 Origin of polyploids

The strategies employed in the breeding of polyploidsare determined primarily on their origin. J.R. Harlanand J.M. de Wet concluded from an extensive reviewof the literature that nearly all polyploids arise by thepath of unreduced gametes. They pointed out thatthe most common factor leading to polyploidy is thefusion of 2n and n gametes to form a triploid,followed by either backcrossing or selfing to produce

Table 24.2 Classification of polyploidy.

Ploidy Genome Description

Diploidy AA BB Contains two of a basic chromosome set

Euploidy(a) Autoploidy AAA BBBB Multiples of a basic set (n) of one specific genome.(b) Alloploidy AAB AABB Multiples of the basic number but of different genomes(c) Segmental

AlloploidyAA’B AB’B’ Multiples of the basic number but the genomes have similar

parts

Aneuploidy AA 2nþ/� 1,2, . . . . . . k

Table 24.3 Naming of polyploids.

Genomeformula General name

Specificname

n A haploid(monoploid)

2n AA diploid3n AAA triploid autotriploid

AAB triploid allotriploid4n AAAA tetraploid autotetraploid

AABB tetraploid allotetraploid6n AAAAAA hexaploid autohexaploid

AABBDD hexaploid allohexaploid

454 CHAPTER 24

a tetraploid. Further, they observed that the occur-rence of unreduced gametes is variable and pervasivein the plant kingdom.

The unreduced (2n) gametes arise by one of twomechanisms – first division restitution(FDR) orsecond division restitution(SDR) – during meio-sis (Figure 24.2). Each mechanism has a differentgenetic consequence. In the FDR, the 2n gametesresult from parallel spindle formation after the nor-mal first division of meiosis. The cleavage furrowsoccur across the plane of the parallel spindles, pro-ducing dyads and 2� 2n pollen. The genetic conse-quence of the mechanism is that most of the

heterozygosity of the diploid hybrid is conserved inthe 2n gametes. In the SDR mechanism, the firstmeiotic division if followed by cytokinesis, but thesecond division is absent. This results in a dyadwith 2� 2n gametes. However, in terms of geneticconsequence, SDR results in significantly reducedheterozygosity in the 2n gametes. Researchers suchas T. Bingham have proposed the fusion of twoFDR 2n gametes to harness the heterosis thatresults in breeding potato. This heterosis can befixed; the elite lines produced will then be clonallypropagated. Seed-propagated species (e.g., alfalfa)cannot benefit from this strategy.

Causal event: Parallel spindle(abnormal) orientation

2n gametic ratio 1 AaBB 2 AaBb 1 Aabb

A B

Aa b

a b

B

A B

A

2x

B

Non-sisterchromatids

Non-sisterchromatids

Sisterchromatids

Sisterchromatids

A b

Aa

(a)

B

a b

b

a b

a B

a b

a B

A B

A b

Causal event: Prematurecytokinesis before 2nd meiosis

2n gametic ratio 1 AABb 2 aaBb

A B

Aa b

a b

B

A B

A

a

(b)

B

a b

b

2x

2x

2x

Figure 24.2 The origin of polyploidy by (a) first division restitution (FDR) and (b) second division restitution(SDR). Dyads occur in telophase II. FDR is caused by the presence of parallel or fused spindles, while SDR is causedby the presence of a cell plate before anaphase II. 2n pollen tends to be bigger in size than 1n pollen.


24.5 Autoploidy

As previously defined, autoploids comprise duplicatesof the same genome. Autoploids are useful in makingalloploids and wide crosses.

24.5.1 Natural autoploids of commercialimportance

Autoploidy is not known to have profoundlyimpacted the evolution of species. Having increasedsets of chromosomes does not necessarily increaseperformance. Autoploids of commercial value includebanana, a triploid, which is seedless (diploid bananashave hard seeds and not desirable in production forfood). Other important autoploids are tetraploidcrops such as alfalfa, peanut, potato, and coffee. Spon-taneous autoploids are very important in the horticul-tural industry where the gigas feature has producedsuperior varieties of flowering ornamentals of narcis-sus, tulips, hyacinths, gladiolus, and dahlia among

others. Autoploid red clovers and ryegrasses withlusher and larger leaves, taking advantage of the gigasfeature of polyploidy, have been bred for commercialuse as palatable and digestible livestock forage. Itshould be mentioned that there is no overwhelmingevidence to suggest that autotetraploids are produc-tively superior to their diploid counterparts.

24.5.2 Cytology of autoploids

Autoploids contain more than two homologous chro-mosomes. Consequently, instead of forming bivalentsduring meiosis as in diploids, there are also multiva-lents (Figure 24.3). For example, autoploids havemostly trivalents but some bivalents and univalentsare also present. Tetraploids have quadrivalents orbivalents as well as some trivalents and univalents.These meiotic abnormalities are implicated in sterilityto some extent, more so in triploids. The microsporesand megaspores with x or 2x genomes are usuallyviable.

Trivalent

Trivalent

Non-functionalgametes

Possible functionalgametes

Bivalent

Bivalents

Quadrivalents

Univalent

Univalent

12

3

(a) Triploidy

(b) Autotetraploidy

Figure 24.3 Cytology of polyploids: (a) triploidy and (b) autotetraploidy. Bivalents and quadrivalents usuallyproduce functional gametes while univalents and trivalents produce sterile gametes.

456 CHAPTER 24

The amount and nature of chromosome pairingdirectly impacts the breeding behavior of autoploids.Autoploids are induced artificially by chromosomedoubling using colchicine. Doubling a hybridbetween two diploid cultivars would produce a tetra-ploid in which there may be a tendency for thedoubled set of chromosomes from one parent to pairindependently of the doubled set of chromosomes ofthe other parent. This propensity is called preferen-tial or selective pairing, a phenomenon with geneticconsequences. If preferential pairing is complete,there would be no new genetic recombination, andhence the progeny would look like the doubled F1.Furthermore, the bivalent pairing would contributeto sterility originating from meiotic disorders, whilepreserving heterosis indefinitely, should there be anyproduced by the original cross. The concept of prefer-ential pairing is applied in the modern breeding ofpolyploids whereby alloploids are stabilized and madereliable as diploids, a process called diploidization.

24.5.3 Genetics of autoploids

The ploidy level may also be defined as the number ofdifferent alleles that an individual can possess for a sin-gle locus on a chromosome. A diploid can have twoalleles per locus, whereas an autotetraploid can havefour different alleles. The genetics of autoploids iscomplicated by multi-allelism and multivalent associa-tion of chromosomes during meiosis. Consider thesegregation of alleles of a single locus (A, a). In a dip-loid species, there would be three possible genotypesAA, Aa, and aa. However, in an autotetraploid therewould be five genotypes ranging from multiplex(aaaa) to quadriplex (AAAA) (Table 24.4). The pro-portion of dominant (A) to recessive (a) genes is

different in two of the five genotypes (AAAA andAaaa) in autotetraploids from what obtains in dip-loids. The number of phenotypes observed dependson dominance relationship of A and a. If allele A iscompletely dominant to allele a, there would be onlytwo phenotypes. If dominance is incomplete or theeffect of allele A is cumulative, there could be up tofive phenotypes. Upon selfing, a dominant phenotypein a diploid (AA, Aa) would produce a progeny thatis all dominant, or segregate in 3 : 1 ratio. Selfing eachof the five categories would produce much differentoutcomes in autotetraploids, assuming random chro-mosome segregation (Table 24.5).

An autoploid individual can have up to four alleles(abcd) per locus. Five different genotype categoriesare similarly possible, except that there may be onlyfour multiplex genotypes (aaaa, bbbb, cccc, dddd),only one tetragenic genotype (abcd), but numerous

Table 24.4 Genetics of autoploids.

Diploid Polyploidy Name

CrossAa�Aa AAaa�AAaa

Products1/4 AA 1/36 AAAA Quadruplex2/4 Aa 8/36 AAAa Triplex1/4 aa 18/36 AAaa Duplex

8/36 Aaaa Simplex1/36 aaaa Nulliplex

Table 24.5 Genetic frequencies following chromosomesegregation of autotetraploid.

Genotype Gametic frequency

AA Aa aa

AAAA 1 0 0AAAa 1/2 1/2 0AAaa 1/6 4/6 1/6Aaaa 0 1/2 1/2Aaaa 0 0 1

Note Chromatid segregation occurs less frequently thanchromosomes segregation and produces alternative types ofsegregation. For example the simplex (Aaaa) can producegametes that are homozygous (AA) by the process calleddouble reduction.

Assuming complete dominance and chromosomesegregation the following phenotypic ratios are observed.Certain segregation ratios are sometimes indicative of thenature of autotetraploid inheritance.

Cross Progeny (dominant : recessive)

AAAA�AAAA 1 : 0AAAa�AAAa 1 : 0AAaa�AAaa 35 : 1AAaa�Aaaa 11 : 1AAaa� aaaa 5 : 1Aaaa�Aaaa 3 : 1Aaaa� aaaa 1 : 1Aaaa� aaaa 0 : 1


combinations for the intermediate (Table 24.6). Thepossible gametic array is shown for each genotype.Interallelic and intrallelic interactions may occur foras many as four alleles per locus in an autotetraploid.The degree to which intrallelic interaction occursdetermines the expression of heterosis and inbreedingdepression in an autotetraploid. Because four identi-cal alleles are required to achieve homozygosity in anautotetraploid compared to only two in a diploid,homozygosity is achieved at a less rapid rate in auto-tetraploids (Figure 24.4).

Another aspect of autoploid genetics with plantbreeding implication is the difficulty of distinguishingbetween a triplex and a quadriplex on the basis of aprogeny test (assuming random chromosome segre-gation). Both genotypes (AAAA and AAAa) willbreed true for the dominant allele. To identify atriplex plant, the breeder would have to advance theprogeny one more generation to identify the duplexplants of the S1. Achieving genetic purity in auto-tetraploid stocks is difficult, not only because it ischallenging to identify triplex plants, but also becausedeleterious genes may persist in an autotetraploid,manifesting themselves only rarely in the homozy-gous. The breeder would need an additional twogenerations in order to identify the homozygousdominant genotype unequivocally.

24.5.4 Induction of autoploids

Plant breeders were initially attracted to induce poly-ploidy primarily because of the gigas effects, whichincreased cell size (but also reduced fertility). Thispro and con of the gigas effects make the induction ofautoploids more suited to crops whose economic partis vegetative. The primary technique for inducingautoploids is the use of colchicine (C22H25O6), analkaloid from the autumn crocus (Colchicum autum-nale). This chemical compound works by disruptingthe spindle mechanism in mitosis, thereby preventingthe migration of duplicate chromosomes to oppositepoles at anaphase. Consequently, the nucleus isreconstituted with twice the normal number of chro-mosomes, without any nuclear or cell division.

Ryegrass is one of the species that has been suc-cessfully improved by the induction of autoploidy.Rye (Secale cereale) is perhaps the only grain pro-ducing crop for which synthetic autoploids havebeen developed. Meristematic tissue is most suscep-tible to colchicine treatment. Hence, a germinatingseed, a young seedling, or a developing bud are thecommonly used plant material for autoploid induc-tion. The chemical may be applied in aqueous solu-tion or through various media (e.g., agar lanolinpaste). Seeds may be soaked in aqueous colchicineat a concentration of 0.05–0.4% for 30 minutes tothree hours. Buds are treated differently, for exam-ple, by intermittently exposing the selectedplant material for 2–6 days at concentrations of0.2–0.5%. The breeder should determine the besttreatment condition by experimentation. The

Table 24.6 Multiple allelelism in autotetraploids.

Tetrasomic condition

a1a1a1a1 All alleles are identical; monoallelic; balanced.a1a1a1a2 Two different alleles; diallelic; unbalanced.a1a1a2a2 Two different alleles; diallelic; balanced.a1a1a2a3 Three different alleles; triallelic.a1a2a3a4 Four different alleles; tetraallelic.

The number of possible interactions are (a) first order (e.g.,a1a2, a1a3), (b) second order (e.g., a1a2a3, a1a3a4), and(c) third order interaction (a1a2a3a4). This depends on thetetrasomic condition.

Tetrasomic condition 1st 2nd 3rd Total

a1a2a3a4 6 4 1 11a1a1a2a3 3 1 0 4a1a1a2a2 1 0 0 1a1a1a1a2 1 0 0 1a1a1a1a1 0 0 0 0

F (c

oeffi

cien

t of

inbr

eedi

ng) Diploid

Autotetraploid

Generations of selfing0 4 8

10

5

0

Figure 24.4 The effect of ploidy on the inbreeding asdemonstrated by diploids and autotetraploid.

458 CHAPTER 24

material treated should be thoroughly washed afterapplication to remove excess chemicals.

24.6 Breeding autoploids

In developing and using autoploids in plant breeding,certain general guidelines may be observed.

� Generally, species tend to have an optimum chro-mosome number (optimum ploidy number) atwhich they perform best. Because chromosomedoubling instantly and drastically increases chromo-some number, selecting parents with low chromo-some number for autoploid breeding will reducethe risk of meiotic complications that are often asso-ciated with large chromosome numbers. This willincrease the chance of obtaining fertile autoploids.

� Autoploids tend to have gigas features and also ahigh rate of infertility. Consequently, autoploidy ismore useful for breeding species in which the eco-nomic product is not seed or grain (e.g., foragecrops, vegetables, ornamental flowers).

� Producing autoploids from cross-fertilizing speciespromotes gene recombination among the poly-ploids for a better chance of obtaining a balancedgenotype.

D.R. Dewey summarized the properties of a speciessuited for induction of polyploidy as:

� The species has low chromosome number.� The economic part of the plant is the vegetativematerial (e.g., forage grasses).

� The plant is cross-pollinated (allogamous).� The plant is perennial in habit.� The plant has the ability to reproduce vegetatively.

24.6.1 Autotetraploids and autotriploids

Tetraploid rye (2n¼ 4x¼ 28) has about 2% more pro-tein and superior baking qualities than diploid cul-tures. However, it also has about 20% higherincidence of sterility per spike, resulting in lower grainyield than diploids. Autotriploids of commercialimportance include sugar beet (2n¼ 2x¼ 18;3x¼ 27, 2n¼ 4x¼ 36). Triploidy is associated withthe genetic consequences of sterility because of theodd chromosome number that results in irregularmeiosis. The sterility favors species that are grown forvegetative commercial parts (e.g., grasses) and orna-mentals and fruits (seedless). In sugar beet, triploid

cultivars of monogerm types have significantlyimpacted the sugar industry.

Triploid hybrids are produced by crossing diploidswith tetraploids. Breeders use three kinds of geno-types. The diploid is male sterile (female, cms) whilethe tetraploid is the pollinator. The third componentis a male sterility maintainer (a diploid, N). The tetra-ploid is derived from a diploid by colchicines treat-ment of the seed (soak in 0.2% for 15 hours at30 �C). Seedless watermelon (3x¼ 33) is also pro-duced by crossing diploid (2n¼ 2x¼ 22) with tetra-ploid (2n¼ 4x¼ 44).

24.7 Natural alloploids

A number of economically important crops are allo-ploids. These include food crops (e.g., wheat andoat), industrial crops (e.g., tobacco, cotton, and sug-arcane), and fruits crops (e.g., strawberry and blue-berry). These crops, by definition, contain acombination of different genomes. Researchers overthe years have attempted to elucidate the ancesteralorigin of some alloploids. One of the most widelyknown successes was the work of Nagaharu U, theJapanese scientist who described the genomic relation-ships among naturally occurring mustard (Brassica)species (Figure 24.5). Dubbed the triangle of U, itdescribes the origins of three Brassica species by allo-ploidy. The diploid species involved are turnip or rape(B. campestris, n¼ 10), cabbage or kale (B. oleracea,

B. oleracea(2n = 18)

(e.g., cabbage,cauliflower)

B. carinata(2n = 34)

(e.g., wild mustard)

B. juncea(2n = 36)

(e.g., brownmustard)

B. napus(2n = 38)

(e.g., rutabaga, rape)

B. nigra(2n = 16)

(black mustard)

B. campestris(2n = 20)

(e.g., Chinesecabbage, turnip)

n = 9 n = 9

n = 8 n = 10

n = 10n = 8

Figure 24.5 The triangle of U showing the origins ofvarious alloploids in Brassica.


n¼ 9), and black mustard (B. nigra, n¼ 8). For exam-ple, rutabaga (B. napus) has 2n¼ 38, being a naturalamphiploid of B. olercea and B. campestris.

In cereal crops, wheat is a widely studied alloploidthat comprises genomes from three species. Culti-vated common wheat (Triticum aestivum) is a hexa-ploid with 21 pairs of chromosomes and designatedAABBDD. The AA genome comes from Einkorn(T. monococcum). Tetraploid wheats have the geno-mic formula AABB. Emmer wheat (T. dicoccum)crossed naturally with Aegilops squarrosa (DD) toform the common wheat.

24.7.1 Genetics of alloploids

As previously indicated, alloploids arise from the com-bination and subsequent doubling of differentgenomes, a cytological event called alloploidy. Thegenomes that are combined differ in degrees ofhomology, some being close enough to pair witheach other, whereas others are too divergent to pair.Sometimes, only segments of the chromosomes ofthe component genomes are different, a conditionthat is called segmental alloploidy. Some of thechromosomes of one genome may share a function incommon with some chromosomes in a differentgenome. Chromosomes from two genomes are saidto be homeologous when they are similar but nothomologous (identical).

Most alloploids have evolved certain genetic sys-tems that ensure that pairing occurs between chromo-somes of the same genome. A classic example occursin wheat (2n¼ 6x¼ 42) in which a gene on chromo-some 5B, designated Ph enforces this diploid-like par-ing within genomes of the alloploid. When this geneis absent, pairing between homoeologous chromo-somes as well as corresponding chromosomes of thethree genomes occurs, resulting in the formation ofmultivalents at meiosis I.

Alloploids exhibit a variety of meiotic features.Sometimes chromosomes pair as bivalents, andthereby produce disomic ratios. Where the compo-nent genomes have genes in common, duplicate fac-tor ratios will emerge from meiosis, an event thatsometimes is an indication of alloploid origin of thespecies. Whereas significant duplications of geneticmaterial have been found in wheat, the genomes ofupland cotton have little duplication. Tetrasomicratios are expected for some loci where multivalentassociations are found in allotetraploids.

24.7.2 Breeding alloploids

Alloploids may be induced by crossing two specieswith different genomes, followed by chromosomedoubling of the hybrid. Compared to autoploids,inducing alloploids is not commonly done by plantbreeders. If successful, the newly induced amphiploidinstantly becomes a new species (unable to cross toeither parent). It also becomes reproductively isolatedfrom its parents. Success of induced alloploids isenhanced by the proper choice of parents. Usingparents with low ploidy levels increases the chance ofhigh fertility and seed set in the amphiploid especially.Commercially successful induced alloploids are few.The most noted success with induced alloploidy is thecommercially grown amphiploid, triticale (X Triticose-cale), derived from a cross between wheat (Triticum)and rye (Secale) (Figure 24.6). The objective of devel-oping triticale was to obtain a product that combinesthe qualities of wheat with the hardiness of rye. In lieuof doubling the F1 to produce the desired syntheticproduct, wheat x rye cross may be undertaken. The

Intercross

Embryo

(Secale cereale)

Method 1 Method 2

Bread wheat(Triticum aestivum)

Rye Durum wheat(Triticum turgidum)

2n = 42Genome = AABBDD

2n = 14Genome = RR

2n = 28Genome = AABB

x x

F1ABDRn = 28

Sterile haploid

F1ABR

n = 21Sterile haploid

culture

AABBDDRR2n = 56Fertile

AABBRR2n = 42Fertile

Double

Intercross

x

and select and select

AABBDDRR2n = 56

AABB (RR/DD)2n = 42

AABBRR2n = 42

Primary octoploid

Secondary hexaploid

Primary hexaploid

Figure 24.6 Steps in the development of triticale.

460 CHAPTER 24

F1 plant possesses 28 chromosomes and exhibits inter-mediate traits that favor rye (hairy neck, spike length).All F1s are sterile because of the formation of univa-lents and irregular gametogenesis. F1s are backcrossedto wheat to produce progenies containing 42 chro-mosomes (seven from rye and the rest from wheat).The wheat chromosomes form bivalents at meiosis,while the rye chromosomes form univalents. Thebivalent wheat chromosomes are irregularly arranged.Fertilization of an ovule with 21þ 7 chromosomes bypollen with the same genomic constitution will con-tain the full complement of chromosomes for wheatand rye (56 chromosomes). This product would bethe synthetic alloploid called triticale. Hexaploid triti-cale (AABBRR, 2n.¼ .6x.¼ .42) is superior agro-nomically to octoploid triticale (AABBDDRR,2n.¼ .8x.¼ .56), but it requires embryo culturing toobtain F1s between durum wheat and rye.

All amphiploid breeding is a long-term projectbecause it takes several cycles of crossing and selectionto obtain a genotype with acceptable yield and prod-uct quality. Common undesirable features encoun-tered in triticale breeding include low fertility,shriveled seeds, and weak straw. Even though tetra-ploid (2n¼ 2x¼ 28), hexaploid (2n¼ 6x¼ 42) andoctoploid (2n¼ 8x¼ 56) forms of triticale have beendeveloped, the hexaploid forms have more desirableagronomic traits, and hence are preferred. Alloploidshave been used to study the genetic origins of species.Sometimes, amphiploidy is used by breeders as bridgecrosses in wide crosses.

24.8 Aneuploidy

Whereas polyploidy entails a change in ploidy num-ber, aneuploidy involves a gain or a loss of one or afew chromosomes that make up the ploidy of the

species (i.e., one of a few chromosomes less or morethan the complete euploid complement of chromo-somes). Just like polyploidy, aneulploidy has its ownnomenclature (Table 24.7).

24.8.1 Cytogenetics of autoploids

The diploid complement of chromosomes is desig-nated 2n. A nullisomic, for example, is an individualwith a missing pair of chromosomes (2n� 2), while atetrasomic has gained a pair of chromosomes(2nþ 2). Similarly, a monosomic has lost one chro-mosome from a homologous pair (2n� 1), while atrisomic has gained an extra chromosome (2nþ 1).

Aneulploidy commonly arises as a result of irregularmeiotic mechanisms, such as non-disjunction (failureof homologous chromosomes to separate), leading tounequal distribution of chromosomes to oppositepoles (Figure 24.7). Consequently, gametes resultingfrom such aberrant meiosis may have a loss or gain ofchromosomes. Furthermore, chromosome additionsoften cause chromosome imbalance and reducedplant vigor.

24.8.2 Applications of aneuploidy

Aneuploidy is used in various genetic analyses.

Chromosome additions

Chromosome addition lines are developed by back-crossing the synthetic alloploid (F1) as seed parent toa cultivated species as pollen parent. This strategy ispreferred because male gametogenesis is more readilyperturbed by chromosomal or genic disharmoniesthan is the case in the female gametophyte. For exam-ple, E.R. Sears transferred the resistance of leaf rust ofAegilops umbellulata to Triticum aestivum (bread

Table 24.7 Aneuploid nomenclature.

Chromosome number Term Nature of chromosomal change

2n diploid normal

Aneuploidy2n�1 monosomy 1 copy of a pair of chromosome missing2nþ1 trisomy 3 copies of one chromosome (i.e., an extra copy)2nþ2 tetrasomy 4 copies of one chromosome (i.e., two extra copies)2nþ3 pentasomy 5 copies of one chromosome (i.e., three extra copies)

The individual with the condition, e.g., trisomy, is called a trisomic.


wheat) via bridge crossing with T. dicocoides asfollows:

T. dicocoides (AABB) × A. umbellulata (UU )

(female) (male)

F1 × T. aestivum (AABBDD)

BC1F1

BC2F3

× T . aestivum

(One plant contained 21’’ wheat + 1’ Aegilops)

However, it had draws backs (sterile pollen, brittlespike axis, etc.). Subjecting these chromosome addi-tion lines to irradiation successfully translocated thesegment of the Aegilops’ chromosome with thedesired resistance genes to chromosome 6B of wheat,effectively removing the negative effects. The newgenotype has been used in breeding as a source ofresistance to leaf and stem rust.

Trisomics are important in genetic analysis. Thereare several types of trisomics. The term primarytrisomic is used to refer to a case in which theeuploid complement is increased by one complete

chromosome. A secondary trisomic is one in whichthe extra chromosome has identical arms (i.e., oneparticular arm occurs as a quadruplicate). Such achromosome is called an iso-chromosome. Some-times, the extra chromosome added is derived fromparts of different chromosomes. Such additives arisefrom chromosome breakage-fusion events.

Primary trisomics may be used to assign genes tochromosomes. Theoretically, there are as many possibletrisomics, as there are chromosome pairs. Scientists cangenerate trisomic stocks for a species. To assign a gene,the mutant (e.g., a) that is homozygous for the alleleof interest is crossed to all the trisomic tester stocks.Assuming that all stocks are homozygous for the wildtype and assuming normal meiotic segregation, two F1types will be produced. Those produced from union ofnormal gametes (n) will segregate with the normal dip-loid ratio 3A:1aa. However, where a trisomic plant(nþ 1 gamete) is involved, an aberrant ratio wouldresult. Because a trisomic stock is unique, the gene ofinterest would be located on the chromosome that thetrisomic stock represents. These results assume randomsegregation of the three chromosomes of the trisomicand equal viability of pollen regardless of genetic

1st meotic [1st meotic division]division (Abnormal

Disjunction)

2nd meoticdivision

(Normal Disjunction)

[2nd meotic division]

2n + 1(trisomic)

2n + 1(trisomic)

2n – 1(monosomic)

2n – 1(monosomic)

Zygotes 2nnormal

2nnormal

2n + 1monosomic

2n -1monosomic

(a) (b)

Figure 24.7 The origin of anueploidy. Abnormal disjunction may occur at the first meiotic division (a) or at thesecond meiotic division (b) producing gametes with a gain or loss in chromosomes.

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constitution. In reality there is a preponderance of ngametes and reduced function of nþ 1 gametes. Theconsequence of reduced functionality is that, sooner orlater, a trisomic will revert to a diploid, unless the sci-entist makes special efforts to maintain it. Trisomicshave been applied in creative ways in plant breeding,including their use in hybrid seed production in barley,using a balanced tertiary trisomic that carries a recessivemale sterility gene. The addition of chromosomes fromother species (called alien addition lines) has beenexplored in interspecies crosses such as wheat x rye.Chromosome addition lines may be unstable enoughto be developed as cultivars.

Chromosome deletions

Unlike chromosome addition, in which gene duplica-tion occurs (hence an implied duplication in func-tion), chromosome deletion leads to a loss offunction. The consequence of a deletion depends onthe functional roles of the genes of the chromosomethat is lost. Invariably, surviving plants have less vigorand more sterility problems. However, in polyploids,the presence of homeologous (in alloploids) orhomologous (in autoploids) chromosomes may makeup for the missing functions.

Monosomics (2n� 1) may be used just like triso-mics to assign genes to chromosomes in a polyploidspecies. This requires the development of monoso-mics for all the existing chromosome pairs in the spe-cies, as was done by E.R. Sears for the Chinese Springcultivar of wheat. Nullisomics (2n� 2) may also beused in this fashion, but with less success because ofsevere reduction in vigor and fertility.

Chromosome substitution

Whereas alien chromosome addition entails addingan alien chromosome to the genome of an existinggenotype, chromosome substitution entails replac-ing or substituting a chromosome of the recipientspecies with an alien chromosome. Intervarietal(between varieties of the same species) and inter-specific chromosome substitutions are moreimportant in plant breeding than addition of chro-mosomes. One of the well known substitutionsinvolves chromosome 1B of wheat and chromo-some 1R of rye. The resultant wheat cultivar pro-vided resistance to disease (leaf rust, stripe rust,powdery mildew). To use this technique, there

have to be monosomic lines for the species (linesare available for wheat, cotton, tobacco, oats).

The backcross breeding procedure may be used tosubstitute one chromosome for another in monoso-mics or nullisomics. Such chromosome substitutionmay be done within the species and involves other spe-cies (i.e., alien substitution). Researchers such as Searshave used the technique to assign numerous genes tochromosomes. However, the technique is challengingand requires a great amount of cytological analysis.

Supernumerary chromosomes

Also called accessory or B-chromosomes, supernu-merary chromosomes are natural additions of varyingnumbers of small chromosomes to the normalgenome. They have been found in all major taxonomicgroups of organisms. These chromosomes are oftenpredominantly heterochromatic and unstable in behav-ior. Although largely considered as genetically inert,studies in some species have indicated that the B-chro-mosomes increase the recombination frequency of A-chromosomes (the normal set of chromosomes) inspecies in which they occur. It is possible to use certainbreeding techniques to increase their number. In somespecies, such rye, fertility is reduced by the presence ofone or two supernumerary chromosomes. Howevercorn plants can accumulate at least 10 such materialsbefore an adverse effect on fertility is noticeable.

24.9 General importance of polyploidy inplant improvement

Polyploidy played a significant role in plant evolutionand domestication. Induced polyploids received sig-nificant attention when mitotic inhibitors (oryzalin,trifluralin, amiprophos-methyl, and N2O gas, colchi-cine) were discovered. Unfortunately, the inducedpolyploids seldom outperformed their diploid pro-genitors. In terms of generating genetic variability forbreeding, somatic doubling does not produce geneticmaterial but only produces additional copies of exist-ing chromosomes. Abnormalities noted by research-ers to be associated induced polyploidy include erraticfruit bearing, brittle wood, watery fruit, stunting ana-tomical imbalances (resulting from the gigas feature),somatic instability, and extreme genetic redundancy(resulting in chimeric tissues in high order ploidy).

Using polyploidy for cultivar development as theultimate goal may not be a worthy undertaking. On


the other hand, induced polyploids may be used inother ways in a breeding program, including:

� Enhancing heterozygosity. Induced polyploidsmay used as germplasm in a breeding program toincrease heterozygosity.

� Overcoming interploid blocks. Barriers to hybrid-ization resulting from differences in ploidy levelscommonly have their origin in endosperm imbalance.Instead of the normal maternal:paternal genomicmakeup of the endosperm (2 : 1), meiotic errors maycause a deviation from this ratio, resulting in under-development or abortion of the embryo. Genomicmanipulation such as chromosome doubling may beused to make the parents cross-compatible.

� Development of sterile cultivars. Irregularities inthe meiotic apparatus often end in the production ofsterile plants. In ornamental horticulture, sterileplants either do not bear flowers or bear sterile flow-ers that do not produce seed. These seedless plantsremain longer in the landscape and also have nopotential to become an invasive species. In fruit pro-duction, sterility results in seedlessness of fruits. Trip-loidy, as previously discussed, is a method of inducingseedless fruits. Triploids may also be developed fromthe nutritive tissue of the embryo in most angio-sperms. This tissue is triploid (produced from thefusion of three haploid nuclei). Autotetraploids canalso produce sterility from meiotic complications.

� Enhancing pest resistance. Some research hasshown that increasing the chromosome number (orgene dose) may increase the expression of certainsecondary metabolites and chemicals that promotepest resistance. Autotetraploid rye is more diseasetolerant than its diploid counterparts. Some evi-dence exists to suggest that polyploidy may enhancestress tolerance in some species.

� Restoring fertility. Polyploidy can also be used torestore fertility in sterile plants by doubling thechromosome number. Restoring fertility allows theplant to be used as germplasm in a breeding pro-gram, or even to be released as a cultivar.

� Producing large fruits. In species where large fruitsize is desirable, the gigas effect of polyploidy isadvantageous. Triploidy in apples is known to resultin increased fruit size without loss of quality andappearance. Unfortunately, tetraploid apples havelarger fruits than diploid apples, but have poor fruitquality.

� Increasing vigor. Polyploidy flowers are known tobe larger and last longer than their diploid counter-parts before withering.

24.10 Inducing polyploids

The most commonly used meiotic inhibitor in theinduction of polyploids is colchicine. It is applied togrowing regions of plants (meristems). Young seed-lings (or their epical meristems) may be soaked (orsprayed) in an appropriate concentration of the chem-ical for an appropriate amount of time (could be a fewhours or even days). Axillary or subaxillary buds mayalso be used. Verification of the success of the treat-ment may be conducted in several ways, the easiestbeing visual inspection. Such an inspection producesonly tentative results. Early physical evidence of poly-ploidy includes thicker and broader leaves, largerflowers and fruits, distorted growth, and shortenedinternodes. Putative polyploids need to be authenti-cated by more reliable, albeit more time consuming,methods, such as examination of the pollen (larger)and the chloroplast count (more chloroplasts perguard cell), flow cytometry (measures DNA content)or chromosome count (the ultimate and definitiveevidence of polyploidy). For chromosome count,root tips and anthers are popular materials to use. Itshould be noted that the induction of polyploidy isnot uniform throughout the treated material. Theconvention is to recognize three histogenic layers(L-1, L-2, L-3), which may be altered differently.Guard cells reflect the L-1 layer, while the corticallayer and reproductive tissues (e.g., anthers) reflectthe L-2 and L-3 layers, respectively.

24.11 Use of 2n gametes forintrogression breeding

When gametes have the somatic chromosome num-ber, 2n, they are called unreduced gametes. Thiscondition occurs widely among angiosperms andsome researchers believe this to be origin of poly-ploidy plant species. Whereas artificially inducedallopolyploids have fixed heterozygosity, sexualpolyploids lack this characteristic and, hence,recombination occurs between the alien parentalgenomes. Also, because of recombination, introgres-sion can be achieved. Successful introgression breed-ing has been achieved in species such as Alstroemeria,Lilium, Medicago, Solanum, and Primula.

2n gametes may be detected in plants throughapproaches such as pollen size examination, flowcytometry, and progeny analysis. Intergenomic

464 CHAPTER 24

recombination is critical for introgression. GISH(genomic in situ hybridization) is one of the mosteffective techniques used to detect chromosomalrecombination. In situ is used to locate the chromo-somal location of a specific DNA or RNA probe(labeled with a fluorescent probe). The 2n gameteintrogression technique has been used successfully inpotato, Brassica, Musa, Allium cepa, Lilium longiflo-rum and others.

24.12 Haploids in breeding

Haploids contain half the chromosome number ofsomatic cells. Anthers contain immature microsporesor pollen grains with the haploid (n) chromosomenumber. If successfully cultured (anther culture), theplantlets resulting will have a haploid genotype. Hap-loid plantlets may arise directly from embryos orindirectly via callus, as previously discussed. To havemaximum genetic variability in the plantlets, breedersusually use anthers from F1 or F2 plants. Usually, thehaploid plant is not the goal of anther culture. Rather,the plantlets are diplodized (to produce diploid plant-s) by using colchicine for chromosome doubling. Thisstrategy yields a highly inbred line that is homozygousat all loci after just one generation.

Methods used for breeding self-pollinated speciesgenerally aim to maintain their characteristic narrowgenetic base through repeated selfing over severalgenerations for homozygosity. The idea of using hap-loids to produce instant homozygotes by artificialdoubling has received attention.

Haploids may be produced by one of severalmethods:

� Anther culture to induce androgenesis.� Ovary culture to induce gynogenesis.� Embryo rescue from wide crosses.

24.12.1 Anther culture

Flower buds are picked from healthy plants. After sur-face sterilization, the anthers are excised from thebuds and cultured unto an appropriate tissue culturemedium. The pollen grains at this stage would be inthe uninucleate microspore stage. In rice the late uni-nucleate stage is preferred. Callus formation startswithin 2–6 weeks, depending on the species, geno-type, and physiological state of the parent source.

High nitrogen content of the donor plant and expo-sure to low temperature at meiosis reduces albinosand enhances the chance of green plant regeneration.Pre-treatment (e.g., storing buds at 4–10 �C for 2–10days) is needed in some species. This and other shocktreatments promote embryogenic development. Theculture medium is sometimes supplemented withplant extracts (e.g., coconut water, potato extract).To be useful for plant breeding, the haploid pollenplants are diplodized (by articifial doubling with 0.2%colchicines, or through somatic callus culture).

Applications

(a) Development of new cultivars. Through diplod-ization, haploids are used to generate instanthomozygous true breeding lines. It takes only twoseasons to obtain doubled haploid plants, versusabout seven crop seasons using conventional pro-cedures to attain near homozygous lines. Thegenetic effect of doubling is that doubled haploidlines exhibit variation due primarily to additivegene effects and additive x additive epistasis, ena-bling fixation to occur in only one cycle of selec-tion. Heritability is high because dominance iseliminated. Consequently, only a small number ofdoubled haploid plants in the F1 is needed, versusseveral thousands of F2 for selecting desirablegenotypes.

(b) Selection of mutants. Androgenic haploids havebeen used for selecting especially recessivemutants. In species such as tobacco, mutantsresistant to methionine analogue (methioninesulfoxide) of the toxin produced by Pseudomonastabaci have been selected.

(c) Development of supermales in asparagus. Hap-loids of Asparagus officinalis may be diplodized toproduce homozygous males or females.

Limitations

(a) The full range of genetic segregation of interest tothe plant breeder is observed because only a smallfraction of androgenic grains develop into fullsporophytes.

(b) High rates of albinos occur in cereal haploids (noagronomic value).

(c) Chromosomal aberrations often occur, resultingin plants with higher ploidy levels, requiring sev-eral cycles of screening to identify the haploids.


(d) Use of haploids for genetic studies is hampered bythe high incidence of nuclear instability of haploidcells in culture.

24.12.2 Ovule/ovary culture

Gynogenesis using ovules or ovaries has beenachieved in species such as barley, wheat, rice,maize, tobacco, sugar beet, and onion. The methodis less efficient than androgenesis because only oneembryo sac exists per ovary as compared to thou-sands of microspores in each anther. Ovaries rang-ing in developmental stages from uninucleate tomature embryo sac stages are used. However, it ispossible for callus and embryos to develop simulta-neously from gametophytic and sporophytic cells,making it a challenge to distinguish haploids fromthose of somatic origin. Generally, gynogenesis isselected when androgenesis is problematic (as insugar beet and onion).

24.12.3 Haploids from wide crosses

Certain specific crosses between cultivated and wildspecies are known to produce haploids. Well estab-lished systems include the interspecific crossesbetween Hordeum vulgare (2n¼ 2x¼ 14, VV) xHordeum bulbosum (2n¼ 2x¼ 14, BB), commonlycalled the bulbosum method, and also in wheat xmaize crosses. The F1 zygote has 2n¼ 2x¼ 14 (7Vþ 7B). However, during the tissue culture of theembryo, the bulbosum chromosomes are eliminated,leaving a haploid (2n¼ x¼ 7V). This is thendoubled by colchicines treatment to obtain2n¼ 2x¼ 14VV.

24.12.4 Haploid breeding versusconventional methods

The potential of haploid breeding to hasten cropimprovement is attractive. However, the method isyet to become a mainstream breeding approach. Hap-loids cannot be obtained in the high enough fre-quency that is needed for selection. Further, haploidswill express recessive deleterious traits. The methodmay not be cost effective, overall. Nonetheless, hap-loids have been used in breeding of many crops,including asparagus, barley, citrus, corn, grape,cucumber, pepper, peanut, wheat and potato.

Haploid breeding applied to species with polysomicinheritance (e.g., potato) is different from thatapplied to species like barley and rice. Polyhaploidsobtained from polyploids have polysomic inheritanceand may be homozygous of heterozygous. In an auto-tetraploid like potato, dihaploids may be AA, Aa, oraa, depending on the genotype of the tetraploid,which could be heterozygous (AAAa, AAaa, Aaaa).Consequently, doubling of haploids will not alwaysresult in homozygosity, unless dihaploids are usedagain for the production of monoploids.

24.13 Doubled haploids

Researchers exploit haploidy generally by doublingthe chromosome number to create a cell with thedouble dose of each allele (homozygous).

24.13.1 Key features

Inbred lines are homozygous genotypes produced byrepeated selfing with selection over several genera-tions. The technique of doubled haploids may beused to produce complete homozygous diploid linesin just one year (versus more than four years in con-ventional breeding) by doubling the chromosomecomplement of haploid cells. Such doubling may beaccomplished in vivo naturally, or through crossing ofappropriate parents, or in vitro through the use ofcolchicine. The success of doubled haploids as abreeding technique depends on the availability of areliable and efficient system for generating haploidsand doubling them in the species.

24.13.2 Applications

Doubled haploids have been successfully used inbreeding species in which efficient haploid generationand doubling systems have been developed. Theseinclude canola, barley, corn, and wheat. Additionally,doubled haploids are used to generate general geneticinformation that can be applied to facilitate the breed-ing process. Such information includes gene actionand interaction, estimating the number of genetic var-iances, calculating combining abilities, and detectionof gene linkages, pleiotropy, and chromosome loca-tions. Haploids are used in mutation studies (recessivemutants are observable instantly) and in selectingagainst undesirable recessive alleles.

466 CHAPTER 24

24.13.3 Procedure

The first step in using doubled haploids in breeding isidentifying the source of haploids.

Natural sources

Haploids originate in nature through the phenome-non of parthenogenesis (gamete formation withoutfertilization). The haploids may be maternal or pater-nal in origin. It is estimated that haploids occur incorn at the rate of 1 in 1000 diploids, 99% of whicharise from parthenogenesis of maternal origin. Spon-taneous doubling occurs in corn at the rate of 10% ofhaploids developed. The key is distinguishingbetween haploid and diploid plants. A marker systemfor this purpose was first developed by S.S. Chasebased on seedling color, purple plants being encodedby the dominant gene (P) while normal green plantsare recessive (p). A cross of F1(pp) x PP would yield999Pp (purple diploids) and 1pp (green haploid).Another marker used is the purple aleurone color.

To use this marker system, the breeder should crossa heterozygous female to a male with marker genes.The seed from those with dominant endospermmarker of the male is saved and planted, discardingseedlings with the dominant male marker. Next, cyto-logical evaluation of plants with the recessive femalemarker (by root tip squash) is conducted. The haploidplants are retained and grown in the greenhouse orfield, and self-pollinated to produce diploids.

Artificial sources

Haploid production through interspecific and inter-generic crosses is in use, one of the well known beingthe barley system (previously discussed). After dou-bling the chromosome, the diploid plants are grownto maturity. Seeds are harvested for planting plantrows. Because diploids produced by this method arenormally completely homozygous, there is no need

for growing segregating generations as obtains inconventional programs.

Advantages and disadvantages

The technique of doubled haploids has certain advan-tages and disadvantages, the key one including:

Advantages� Complete homozygosity is attainable in ashorter period.

� Duration of the breeding program can bereduced by several (2–3) generations.

� It is easier and more efficient to select amonghomogeneous progeny (versus heteroge-neous progeny in conventional breeding).

� The cultivar released is homogeneous.Disadvantages

� The procedure requires special skills andequipment in some cases.

� Additional technology for doubling mayincrease the cost of a breeding program.

� Frequency of haploids generated is notpredictable.

� There is a lack of opportunity to observe lineperformance in early generations prior tohomozygosity.

Genetic issues

Unlike the conventional methods of inbreeding, itis possible to achieve completely homozygous indi-viduals. Using an F1 hybrid or a segregating popu-lation as female parent in the production ofmaternally derived haploids increases genetic diver-sity in the doubled haploid line. It is advantageousif the female also has agronomically desirable traits.F1 hybrids are suitable because their female gameteswill be segregating.

Key references and suggested reading

Borojevic, S. (1990). Principles and methods of plant breed-ing. Elsevier, New York.

Bingham, E.T. (1980). Maximizing heterozygosity inautotetraploids, in Polyploidy: Biological relevance

(ed. Lewis W.H.). Plenum Press. New York, pp.471–489.

Chase, S.S. (1964). Analytic breeding of amphipolyploidplant varieties. Crop Science, 4:334–337.


Dewey, D.R. (1980). Some applications and misapplicationof induced polyploidy to plant breeding, in Polyploidy:Biological relevance (ed. W.H. Lewis). Plenum Press.New York, pp. 445–470.

Haynes, K.G. (1992). Some aspects of inbreeding inderived tetraploids of potatoes. Journal of Heredity,83:67–70.

Hermsen, J.G. Th. (1984). Nature, evolution, and breedingof polyploids. Iowa State Journal of Research, 58:411–420.

Hill, R.R. (1971). Selection in autotetraploids. Theoreticaland Applied Genetics, 41:181–186.

Poehlman, J.M., and Sleper D. A. (1995). Breeding fieldcrops, 5th edn. Iowa State University Press. Ames, IA.

Sterett, S.B., Henninger, M.R., Yencho, G.C., Lu, W., Vin-yard, B.T., and Haynes, K.G. (2003). Stability of internalheat necrosis and specific gravity in tetraploid x diploidpotatoes. Crop Science, 43:790–796.

Thomas, H. (1993). Chromosome manipulation and poly-ploidy, in Plant breeding: Principles and prospects (eds.Hayward M.D., Bosemark N.O. and Ramagosa I.).Chapman and Hall, London.

Sears, E.R. (1954). The aneuploids of common wheat.Missouri Agric. Exp. Stn. Res. Bull., 572:1–58.

Singh, A.K., Moss, J.P., and Smartt, J. (1990). Ploidymanipulations for interspecific gene transfer. Advances inAgronomy, 43:199–240.

Internet resources

http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Polyploidy.html – A good discussion on polyploidy(accessed March 15, 2012).

http://wheat.pw.usda.gov/ggpages/BarleyNewsletter/42/oral04.html – Application of doubled haploids in bar-ley breeding (accessed March 15, 2012).

Outcomes assessment

Part A

Please answer the following questions true or false.

1 A genomic formula of 2n� 1 refers to a trisomic.2 The regular set of chromosomes of a species is called A chromosomes.3 An individual in which the euploid complement of chromosomes is increased by one complete set of chromosomes is

called a secondary trisomic.4 The triangle of U describes genomic relationships among naturally occurring species of wheat.5 Triticale is an euploid.6 Colchicine is used for reducing the number of chromosomes in a cell.7 Aneuploids have a duplicate of the entire chromosome set.8 The genotype AAAA represents a duplex tetraploid.9 The genotype AADDEE represents an alloploid.

10 A hexaploid consists of six genomes.

Part B

Please answer the following questions.

1 Describe the triangle of U.2 Distinguish between homologous and homeologous chromosomes.3 Distinguish between a primary trisomic and a secondary trisomic.4 Discuss a common mechanism of aneuploidy.5 Distinguish between an aneuploid and euploid.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . is polyploidy with chromosomes from different genomes.7 Write the genetic formula for a triplex genotype.8 What is a segmental alloploid?

468 CHAPTER 24

Part C

Please write a short essay on each of the following topics.

1 Discuss the effect of polyploidy on plants.2 Discuss, with an example, a polyploidy series.3 Discuss the artificial induction of polyploids.4 Discuss the importance of doubled haploids to plant breeding.


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