inheritance of chromosome deficiency and duplication in avena sativa l
TRANSCRIPT
A.S. LARIK
Sind Agricultural University, Tandojam, Pakistan
Received April 13, I 9 76/Accepted January I, I9 78
The inheritance of chromosome deficiency and dupli- cation was studied in two mono-trisomic lines of oats, Auena sativa L. The mono-trisomics were produced by crossing two different monosomic lines with a line tetrasomic for an unidentified chromosome. The con- stitution of the progenies of the two mono-trisomic lines was determined by studying chromosome pairing at metaphase I. The inheritance of the deficiency and duplication in mono-trisomic lines was determined by the transmission of the deficiency in the respective monosomics and the duplication in the trisomic. The ability of aneuploid gametes to function was depen- dent on their chromosome constitution. Restoration of the haploid number of chromosomes did not im- prove the competitive ability of the pollen if the ha- ploid numbers involved a deficiency and a duplica- tion. Male gametes which were nullisomic and disomic for a particular chromosome were at a selective disad- vantage in competition with haploid gametes. Since the chromosomes involved in the two original mono- somic lines did not belong to the same homoeologous group as the chromosome in the tetrasomic line, there. was no indication of compensation at the gametic level. Nevertheless, it was possible to isolate the nulli-tri- somic lines from the progeny of both monotrisomic lines.
Nulli-tetrasomic genotypes of nulli I, II, IV, VI, VIII? IX and X with the unidentified tetrasome were composed, but revealed no morphological compensa- tion as they were very much weaker than 43- qnd 44- chromosome type plants. Non compensation in these genotypes indicates that the chromosomes involved in the parental monosomic and tetrasomic lines do not belong to the same homoeologous group.
Introduction
The cultivated oat A. sativa (2n = 6x = 42) can tolerate chromosome loss and addition on account of its cyto- genetic architecture with its many duplications. Aneu- ploid tolerance in hexaploid Avena might be of the same order as that observed in Triticum aestivum, since both species are allohexaploids. Gene duplica- tion in polyploids frequently results in a high toler- ance of aneuploidy, and this provides an alternative method of genetical study. There are various reports of aneuploid tolerance in Arena which suggest that the tolerance in Avena is at a similar level or even less than in Triticum (Hacker & Riley, 1965;McGinnis & Taylor, 1961; Lefever & Patterson, 1964; Sears, 1954; 1958).
In studies of various aneuploid lines involving dif- ferent monosomics of the oat variety Sun II (Hacker, 1965) it is apparent that there was considerable varia- tion in the frequencies with which these lines segre- gate nullisomics in their progenies. Chang & Sadanaga (1964) used nullisomic frequency as method for dis- tinguishing monosomics deficient for different chro- mosomes. However, comprehensive cytogenetic data comparable with the wheat group (Riley, 1955) are not available in oats. The present paper is an attempt to extend further the knowledge of the cytogenetic architecture of Avena sativa by providing new infor- mation on the inheritance of chromosome deficiency and duplication.
Material and methods
Two mono-trisomic lines Av 859 [Mono II (41) x Av 267/n(44)] and Av 868 [Mono X (41) x Av 267/n (44)]
Genetica 54, 251-260 (1981). 0016-6707/81/0543-0251 $ 2.00. 0 Dr. W. Junk B.V. Publishers, The Hague. Printed in The netherlands. 251
derivatives ofA. sativa L., variety Sun II (2n = 6x= 42) were used in these studies. Seeds for these mono- -trisomic lines were kindly supplied by Dr. H. Thomas, Welsh Plant Breeding Station, Aberystwyth, Wales, U.K. These lines were again crossed reciprocally with Sun II for assessing the functional ability of aneu- ploid gametes. A number of mono-tetrasomic lines involving Mono I, II, IV, VI, VIII, IX and X were also synthesized by crossing the mono-trisomics with the tetrasomic line to study the nuIli-tetrasomic compen- sation. The tetrasomic was also crossed with Sun II to obtain a trisomic line to study the transmission of the frequencies of functioning 2 1 -chromosome gametes.
For mitotic counts seeds were germinated on filter paper in Petri-dishes at constant temperature of 21°C. Roots were collected when 8-10 mm long. The pro- cedure outlined by Melnyk & Unrau (1961) was adopted, detached root-tips were pretreated in cooled water (0-2°C) for 24 h, fixed in acetic alcohol (3:1), hydrolized in N HCL at 60°C for 10 minutes and stained with Feulgen reagent. Stained root-tips were squashed in aceto-carmine. Meiosis was studied by fixing the panicles in Carnoy’s fluid (6:3: 1) to which a few drops of ferric chloride were added as mordant. Anthers were stained in alcoholic-hydrochloric acid- carmine stain for 2 hours at 60°C (Snow, 1963).
Estimation of the frequencies of functioning 22- and 21-chromosome male and female gametes in the trisomic line were calculated directly from the pro- portion of 42, 43 and 44chromosome plants in the selfed progenies of trisomics, using the formula sug- gested by Hacker (1965) for the analysis of inheri- tance of aneuploidy in oats:
(N + 1 - U) +_ J(U - 1 - N)z - p ,E 4N 21 7.1
= 2
where N is the proportion of 42chromosome off- spring, U is the proportion of 44chromosome off- spring, Psi is the proportion of functional 21-chro- mosome pollen and Es, is the proportion of function- al 2 l-chromosome eggs. Using the values obtained from the formula for pollen and eggs and the values given by Hacker (1965) for EZO and PZO for Mono II and X, it is possible to calculate the expected frequency of different chromosome constitution progenies on self- ing the mono-trisomic.
Experimental results
Estimation of the frequencies of functioning 21-chromosome gametes
The chromosome number of a sample of 74 seeds from the progeny of the trisomic line, derived from the cross Mono II x Av 267/n(44) was determined as follows:
4ZChromosome 43Chromosome 44Chromosome 35 28 11
These data were used to determine the frequency with which the haploid (n) and disomic (ntl) func- tioned in fertilization as male and female gametes, using Hacker’s formula for the analysis of the inheri- tance of aneuploidy in oats.
According to that formula,
In the present observations
N= $ =0.47andU= $ =0.15
and it follows that
2E,, = 1.32 A J1.75 - 1.88
E 21
= 1.32 _+ Jib _ 1.88 2
This result incorporating a negative square root is impossible as in Hacker’s (1965) data for the mono- somic line VII. The square root term is therefore ap- proximated as zero giving the identical values of 0.66 for Es1 and Psr . Therefore the approximate fre- quency of 21 and 22-chromosome eggs and pollen would be 0.66 and 0.34 respectively. Such negative results may have been due to chance occurrence of an improbable segregation ratio, due to error in determi- nation of chromosome numbers or due to small sam- ple size. Using these values obtained from the equa- tion for pollen and eggs and the values given by Hacker (1965) for Es,, and PZO for monosomics II
252
and X, the expected frequency of different gametes the mono-trisomic are independent then the frequency produced by the mono-trisomic can be calculated. If of the four possible classes of male and female gametes the transmission of the deficiency and duplication in can be determined as follows:
In Av 859 (Mono-II)
Pollen
Trisome Monosome - +
0.32 0.68
- 201 211 0.66 0.21 0.45
oY34 191+ 0.11 111 201+ 0.23 111
In Av 868 (Mono-X)
Pollen
Monosome Trisome - +
0.04 0.96
- 201 211 0.66 0.03 0.63
0.+34 1% 0.01 + 111 201+ 0.33 111
5%
Trisome Monosome - +
0.88 0.12
- 201 211 0.66 0.58 0.08
0>4 191+ 0.30 111 201 0.04 l 111
Trisome Monosome
- 0.76 o.f24
- 201 211 0.66 0.50 0.16
0.+34 1% + 111 201+111
0.26 0.08
The calculated frequencies of the different gametes can be used to estimate the expected F2 progenies of the mono-trisomic. The calculated expected frequen- ties of plants of all the possible chromosome consti- tutions on selfing the two mono-trisomic lines are presented in Tables 1 and 2.
Table 1
The observed frequencies of the Fz populations from Av 8.59 and Av 868 are compared with the cal- culated theoretical expection in Table 3. The observ- ed frequencies in both the families deviated signifi- cantly from the theoretical expectations (Tab. 3). In both families no progenies which were nulli-tetraso-
Estimation of the frequencies of different types of F, plants expected in the Progenies of selfed F, mono-trisomic plants in Av 859 (Mono II)
Pollen constitution (frequency) , 201 211 1% + 111 201+ 111
Egg constitution (frequency) (0.21) (0.45) (0.11) (0.23)
201 (0.58) 2011 WI+ 11 WI + 1111 1% + ~III+ 11 0.12 0.26 0.06 0.13
211 (0.08) 2011 + 11 2111 1911 + 1111 + 11 2011 + 1111 0.02 0.04 0.01 0.02
19x+ 111 (0.30) 1911 + 1111 %I+ 1x11+ 11 1911 + 11~ WI+ ~IV + 11 0.06 0.14 0.03 0.07
201+111 (0.04) 1911 + 1111 + 11 2011 + 1111 1%1+ 11v + 11 2011+ 1IV 0.01 0.02 0.004 0.01
253
h)
Table
2
g Es
timat
ion
of t
he f
requ
encie
s of
diff
eren
t ty
pes
of F
, pl
ants
ex
pect
ed
in th
e pr
ogen
ies
of s
elfe
d F,
m
ono-
triso
mic
plan
ts i
n Av
868
(M
ono
X)
Polle
n co
nstit
utio
n (fr
eque
ncy)
20
1 21
1 1%
+
111
201+
111
Eg
g co
nstit
utio
n (fr
eque
ncy)
(0
.03)
(0
.63)
(0
.01)
(0
.33)
201
(0.5
0)
2011
W
I+
11
0.02
0.
32
211
(0.1
6)
2011
+11
2111
0.
01
0.10
1%
+
111
(0.2
6)
1911
+
1111
19
11
+ 11
11 +
11
0.
01
0.16
20
1+
111
(0.0
8)
1911
+
1111
+
II.
2011
+
1111
0.
002
0.05
1911
+
IIII
0.01
1%
1+
~III+
11
0.
002
1%1+
11
v 0.
003
1%1+
11
v +
11
0.00
1
1911
+
1111
+
11
0.17
20
11
+ 11
11
0.05
1%1+
1I
V +
11
0.08
W
I+
1IV
0.03
Table
3
Com
paris
on
of e
xpec
ted
and
obse
rved
fre
quen
cies
of c
hrom
osom
e nu
mbe
r of
pro
genie
s fro
m F
, hy
brid
s of
sel
fed
mon
o-tri
som
ics
Chro
mos
ome
Clas
s Fr
eque
ncy
of p
roge
ny
with
chr
omos
ome
cons
titut
ion
P co
nstit
utio
n 20
11
WI+
11
19
11
+ 11
11
1%1+
11
11+
11
2111
x1
11+
11
of p
aren
ts
l%r+
lrv
~%I+
~Iv+
~I
2011
+ 1
1~
Chi-
or
squa
re
2011
+
1 III
A~85
9 (M
ono
II)
obs.
16
10
15
11
3
1 0
0 0
1911
+
1111
+
11
exp.
6.
72
15.6
8 6.
72
16.2
4 2.
24
2.24
1.
68
3.92
0.
56
33.6
9 0.
001
Av 8
68(M
ono
X)
obs.
2
14
8 20
2
8 -
0 0
WI+
11
11 +
11
ex
p.
1.08
17
.82
1.08
17
.82
5.40
5.
40
- 4.
32
1.62
54
.24
0.00
1
x’(7)
=
24.3
2 x’(
8)
= 26
.12
mic, mono-tetrasomic and tetrasomic were recovered in the Fz . In Av 859 (Mono-II) there was an excess of both nullisomic and nulli-trisomic progenies and a deficiency of monosomic and mono-trisomics. In Av
Table 4
Proportion of functional gametes of different chromosome constitution in reciprocal crosses between mono-trisomic and euploids (actual numbers in brackets)
Cbromosome constitution Cross 211 201 19x+ 111 201+ 111
QAv 859 x Sun II 0 0.43 0.57 0 (12) (16)
Sun II x Av 8596 0.55 0.45 0 0 (6) (5)
QAv 868 x Sun II 0 0.28 0.72 0 (5) (13)
Sun II x Av 868d 0.86 0.14 0 0 (12) (2)
Table 5
868 (Mono X) the number of nulli-trisomics was higher than the expected frequency.
Reciprocal crosses between mono-trisomic and euploid
An alternative method of assessing the functional ability of aneuploid gametes is possible by making reciprocal crosses between the mono-trisomic plants and the euploid Sun II. In the mono-trisomic there are four possible types of gametes formed viz., 201, 211, 19rtlrr and 2Ortlrr and the frequency with which they participate in fertilization on the female side can be calculated from the crosses mono-trisomic x euploid and on the male side by the reciprocal cross euploid x mono-trisomic (Tabs. 5 and 6). In the F, hybrid from these crosses the following genotypes are possible :
2011+11 2111 ~%I+~III+~I 2011+1III (monosomic) (euploid) (mono-trisomic) (trisomic)
Estimated frequencies of different aneuploids in selfed progenies of mono-trisomics based on the data from reciprocal crosses between mono-trisomic (Av 859) and euploid (Sun II)
PoUen constitution (frequency) Egg constitution (frequency) 21 I 1% + 111 201+ 111
(0.55) :tzl5, (0) (0)
211 (0) - - - -
201 (0.43) WI+ 11 2011 - -
0.24 0.19 1% + III (0.57) 1%1+ ~III+ 11 1%1+ 1111 -
0.31 0.26 201 + 111 (0) - - - -
Table 6
Estimated frequencies of different aneuploids in selfed progenies of mono-trisomics,based on the data from reciprocal crosses between mono-trisomic (Av 868) and euploid (Sun II)
Pollen constitution (frequency) Egg constitution (frequency) 211 201 1% + 111 201 + 111
10.86) (0.14) (0) (0)
211 (0) - - - -
201 (0.28) WI+ 11 2011 -
0.24 0.04 191+ 111 (0.72) 1911 + ~III+ 11 1%1+ 1111 - -
0.62 0.10 24+111 (0) - - - -
255
Hacker (1965) using the formula for calculating the proportion of functional gametes in monosomic lines found that the proportion of n and (n - 1) fe- male gametes was similar to the calculated propor- tion of the functional gametes. However, in the case of the male gametes the n gametes had a distinctive selective advantage over the n-l gametes and the proportion of functional n gametes was much higher than the actual proportion formed. This is also readily apparent in the present work (Tab. 4) since the most effective male gametes were the haploid pollen. This behaviour was more pronounced in the case of Av 868 (Mono X) than in Av 859 (Mono II). On the other hand, on the female side the n - 1 (2Or) and n - 1 +I (19rtlrr) were effective. However, it must be empha- sized that these are based on a low number of scores, and the inability to find progeny derived from the other aneuploid gametes could be due to sampling error.
Results obtained from this method were compared with those obtained from the equation used for the estimation of the frequencies of functioning 21- chromosome gametes.
Comparison of estimated and observed frequencies of chromosome numbers showed close similarities in Av 859 (Mono II) where the value of chi-square falls between 0.10-0.20 probability indicating that observed frequencies in this line do not deviate significantly from the theoretical expectations, whereas in Av 868 (Mono X) the observed frequencies deviated signifi- cantly (P = < 0.05) from the theoretical expectation (Tab. 7). In Av 868 there was an excess of nulli- -trisomics and low frequency of observed mono-triso- mics.
&genies obtained from a range of mono-tetrasomic plants
In order to investigate further the functional ability of aneuploid gametes the progeny of a series of mono- tetrasomic plants involving different nullisomic lines of Sun II and the tetrasomic line Av 267/n(44) were scored. In these genotypes there were two possible types of gametes, 19rtlrr and 201+111. The former class was nullisomic for one chromosome and disomic for another, while the latter was disomic for one chro- mosome. The ability of these gametes to function in competition was revealed by the frequency of mono- -tetrasomic and nulli-tetrasomic plants in the progeny of mono-tetrasomic plants. In competition with ha- ploid gametes both the 19rtltr and 2Ortlrr are at a selective disadvantage. From Figure I it is readily observed that with the exception of mono-1 the num- ber of nulli-tetrasomics was much lower than the number of 43 and 44-chromosome types. This indi- cates that the 2Ortlrr gametes function at a higher
1 n r it MONO IV MONO VI MONOVIII MONO IX MONO X
Fig. 1. Proportion of nulli-tetrasomics in the progenies of mono-tetrasomics. White columns: 2n = 43-44; hatched col- umns: nulli-tetrasomics, 1911 + 1Iv (2n = 42).
Table I
Comparison of expected and observed frequencies of chromosome numbers of progenies from F, hybrids of selfed mono-trisomics
Parents Frequency of progeny with chromosome constitution Chi- P (chromosome constitution) 201 WI+ 11 1911 + 1111 + 11 1911 + 1111 square
Av 859 (Mono II) Exp. 9.88 12.48 16.12 13.52 6.06 0.10-0.20 (1911 + llIl+ 11) Obs. 16 10 11 1.5 Av 868 (Mono X) Exp. 1.75 10.56 27.28 4.4 9.51 0.0.5-0.02 (191I+ lIII+lI) Obs. 2 14 20 8
x2 (3) = 7.85 at 5% level of significance.
256
Fig. 2. The relationship between the proportion of nulli- somics on seffrng different monosomic lines and the propor- tion of nulli-tetrasomics derived from the corresponding mono-tetrasomics.
frequency than the 19I+lrr gametes. In Figure 2 and Table 8 the frequencies of the nulli-tetrasomics and mono-tetrasomics are presented with the correspond- ing frequency of nullisomics in the progeny of the corresponding monosomic lines. If the slope in Figure 2 was one of unity this would show that the 2Or+ltr gametes could compete as effectively with 19rtlrr as the 21r gamete does against 201 in monosomic lines. However, in the lines studied except for mono II and IV, the proportion of nulli-tetrasomics was greater than the corresponding proportion of nullisomics in
the progeny of monosomic lines. In these lines the 19rtlII gametes were able to compete more success- fully with 2Ortlrr than the nullisomic with haploid gametes in the monosomic lines. In the two lines Mono-II and IV the 19rtlrr gametes were not as suc- cessful as the corresponding nullisomic gametes. In all these lines, the 2Ortlrr gametes were identical since the tetrasomic line used was Av 267/n(44) and in- volved Sun II background and the differences observed between the lines must be a reflection of the compet- itive ability of the 19rtlrr pollen. This was enhanced in all the lines with the exception of mono II and IV.
Nulli-tetrasomic plants were isolated from the selfed progenies of mono-tetrasomics. Nulli-tetrasomic combinations involving different monosomic lines were compared morphologically with the 43 and 44- chromosome type plants to detect nulli-tetrasomic compensation. There was no evidence of any com- pensation in the nulli-tetrasomic plants as they were consistently weak, whereas 43- and 44chromosome type plants were vigorous.
Discussion
The results of the present investigation show that mono-trisomic lines involving mono-II and X had characteristic frequencies of nullisomics, monosomics, mono-trisomics, nulli-trisomics, trisomics and euploids in their progenies. It has been suggested that several factors affect the frequency of different chromosome numbers in the progenies derived from various aneu-
Table 8
Proportion of nullisomic plants in the progenies of selfed monosomics and nulli- tetrasomic plants in the progenies of selfed mono-tetrasomics
Av Monosomic numbers line
*Proportion of nullisomic
%
Proportion of nulli-tetrasomic
%*
Total plants scored
981 I 16 62.50 48 983 II 29 8.0 50 985 Iv 15 12.5 48 987 VI 17 41.66 48 991 VIII 1 19.0 48 992 IX 8 40.0 22 994 X 1 10.0 49
* Value taken from HACKER (1965)
257
ploids (Siddiqui, 1972a). These include the loss of univalents during gametogenesis, which will influence the frequency of the different chromosome numbers in the pollen and embryo sacs; the certation activity of the pollen tubes from different euploid and aneu- ploid pollen grains, and the viability of different zy- gotic combinations. The frequency of different chromosome numbers in the progenies of aneuploids is also influenced by the direction in which the crosses are made (Siddiqui, 197213). Differences in the func- tioning ability of pollen deficient for different chro- mosomes reflect the genetics of gametophyte devel- opment, just as the differences in vigour and fertility of monosomics or nullisomics deficient for different chromosomes reflect the genetics of sporophyte development. It was found earlier (Larik, 1975) that all monosomics obtained from mono-trisomic lines in the variety Sun II were vigorous and self-fertile, the related nullisomics varied considerably, some being vigorous and highly fertile, while others were dwarfed and of very low fertility.
In the present studies deficiency of chromosome II had little effect on the sporophyte generation, where- as deficiency of chromosome X had only moderate effects on pollen viability, but fairly extreme effects on the sporophyte, since nullisomic plants deficient for this chromosome were markedly dwarfed and had very low fertility. Tolerance to chromosome deficiency must imply either that the chromosome concerned carried no essential genes, or alternatively that such essential genes that it does carry are duplicated else- where in the genotypes (Siddiqui, 1972~).
Another marked effect of aneuploidy was the behaviour of gametes which deviate from the normal haploid number of chromosomes. Sears (1954) has clearly shown that male gametes whichwere n - 1 or n + 1 were at a selective disadvantage in competition with haploid gametes. The frequency of monosomics and nullisomics in the selfed progeny of monosomic lines was obviously dependent on the ability of the 20chromosome gametes to function and participate in fertilization. The frequency of 20 and 21-chromo- some gametes formed in monosomic lines can be calculated from scoring the number of micro-nuclei in the tetrads. Hacker (1965) proposed a formula to determine the frequency of 20chromosome gametes that function from the data on the occurrence of nullisomic and disomic progeny when a monosomic line is selfed. He compared the frequency of 20-
chromosome gametes and the proportion that was functional and concluded that variation between monosomic lines in the proportion of 40,41 and 42- chromosome progeny produced was dependent on the functioning ability of the pollen carrying the chromosome deficiency. The ability of the deficient pollen to function was dependent on the chromo- some that was missing. The loss of some chromosomes of the complement is more readily tolerated than that of others, in a similar manner to the differential effect that the absence of different chromosomes has on morphology and development. In A. sativa there is also variation in the effectiveness of gametes carry- ing a chromosome duplication to function in fertiliza- tion depending on the chromosomes involved (McGin- nis, 1966; Thomas and Bhatti, 1975).
In the present investigation an attempt has been made to study the combined effect of a deficiency and duplication on the competitive ability of gametes of mono-trisomic lines. Sears (1966) demonstrated that gametes nullisomic for one chromosome and disomic for a homoeologous chromosome functioned effectively since the extra dose of one chromosome compensated for the deficiency of the other. Similarly Gupta (1969 and 1971) used the functioning ability of rye substitution gametes in common wheat to classify the rye chromosomes into the 7 wheat homo- eologous groups. However, in the present study there . was no evidence of nulhsomic-tetrasomic compensa- tion in either of the two populations studied.
Reciprocal crosses between euploids and the two mono-trisomic lines Av 859 and Av 868 clearly showed that the functioning ability of aneuploid male gametes was reduced compared with the female gametes. When the mono-trisomic plants were used as the male parent the majority of the functioning gametes had the full haploid complement. In Sun II x Av 859 (Mono II) 55% of the gametes were haploid, whereas in Sun II x Av 868 (Mono X) the frequency of haploid gametes was considerably higher (86%). However, nullisomic gametes were functional in all combina- tions. Hacker (1965) has reported 33% nullisomic male gametes in Mono II and only 4% nullisomic male gametes in Mono X. In Sun II x Av 859 as well as in Sun II x Av 868 the nulli-disomic pollen (191 t lrr) was non-functional although the nullisomic (201) pol- len was functional. The only difference between these gametes is the disomic condition of one of the chromosomes in the nulli-disomic pollen and shows
258
that this duplication reduced the competitive ability of these gametes compared with the nullisomic pollen.
Using the calculated frequencies of the 20 and 21 gametes in the relevant monosomic lines, the 21 and 22 gametes in the trisomic lines, the expected fre- quencies of the functional male and female gametes in mono-trisomics have been computed (Tabs. 1 and 2). In the two mono-trisomic lines the n t I gametes were expected to form 23 and 33 percent of the functioning pollen respectively. However, in the crosses euploid x mono-trisomics these gametes did not function. The F i population samples were low and the discrepancies between the expected and realised frequencies could be partly due to sampling errors. Failure of the trivalent to form regularly in mono- trisomic lines would also affect the population of gametes formed with 201 t lrr chromosomes and dis- tort the frequencies. However, even taking these two factors into consideration it is reasonable to conclude that pollen with a chromosome duplication had lower competitive ability that the nullisomic pollen in the mono-trisomics.
Sears (1966) and Gupta (1969,197l) have shown that there is little selection on the female side in the functioning ability of aneuploid gametes in wheat. Hacker (1965) has confirmed that this is also the case in monosomic lines of Arena. In the crosses mono- trisomic x euploid the calculated frequencies of the two gametes that did not function, i.e. 211 and 2Or + lrr, were relatively low in comparison with the ex- pected frequencies of the male gametes that did func- tion. In the case of the two types of gametes that did function in these crosses, viz. 2Or and 191 t lrr, the proportion of the nulli-disomic gametes (191 t lrr) in both crosses was higher than expected and the pro- portion of nullisomic eggs (201) was lower than ex- pectation. Again the size of the Fi sample scored was low and there could be some sampling error. Never- theless the data clearly show that on the female side the nulli-disomic gametes (191 t 1 rr) did readily func- tion in fertilization.
The expected frequencies of the different aneu- ploids in the selfed progeny of mono-trisomics were calculated (Tab. 3) but in both Av 859 and Av 868 the observed frequencies deviated significantly from expectation. In.Av 859 there was an excess of nulli- somic and nulli-trisomic plants and a deficiency of monosomic and mono-trisomic plants. In Av 868 there was an excess of nulli-trisomic. These calculated
expected frequencies are based on the transmission of the deficiency and duplications in monosomics and trisomics respectively and it is assumed that they behave independently. However, the crosses euploid x mono-trisomic have shown that the combined effect of a duplication and deficiency (191 t lrr) reduced the functioning ability of these gametes compared with the nullisomic gametes.
The proportion of the functional gametes deter- mined from the results of reciprocal crosses between the euploid and mono-trisomic plants were also used to calculate the estimated frequencies of different aneu- ploids in the F, of the mono-trisomic (Tabs. 5 and 6). When the frequencies of the four genotypes expected were compared with the observed, the x2 was signifi- cant in Av 868 and was barely significant in Av 859 (Tab. 7). The low number of plants sampled again reduced the validity of the comparison.
In the range of mono-tetrasomic lines studied there was no indication of nullisomic-tetrasomic compensation in any of the combinations and the proportion of nulli-tetrasomic plants in the selfed progeny of the mono-tetrasomics reflects the function- ing ability of 191 t lrr gametes. In Figure 2 the pro- portion of nulli-tetrasomic plants in the selfed progeny of mono-tetrasomics is compared with the proportion of nullisomics in the progeny of corresponding mono- somics. Nulli-disomic gametes (191 t 1 rr) have a higher functioning ability in the mono-tetrasomics than the nullisomic gametes in the corresponding monosomic lines, since the proportion of nulli-tetrasomics was higher in the former than nullisomics in the latter in all lines except Mono II and IV (Tab. 8). The ability of the 19r t lrr gametes to compete effectively with 2Or t lrr demonstrates again that chromosome dupli- cation and deficiency do not behave independently in these complex aneuploids. The present results demon- strate that the ability of aneuploid gametes to func- tion is basically dependent on their chromosome constitution and not on the chromosome number. The nulli-disomic pollen had the haploid number of chromosomes but was ineffective in competition with n and n t 1 gametes in the euploid x mono- trisomic crosses. This contradicts the conclusion of Smith (1963) that the restoration of the chromosome number in wheat-rye aneuploids was the important factor in determining the ability of substitution gametes to function. On the other hand Riley et al. (1966) showed that the chromosome constitution of
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the gametes was important in the transmission of an alien chromosome in wheat Aegilops aneuploids.
The author is greatly indebted to Dr. Hugh Thomas, Head, Department of Cytology, Welsh Plant Breeding Station for invaluable guidance and constant help and to Dr. K.A. Siddiqui, Head, Plant Genetics Division, AEARC, Tandojam for constructive criticism. The author is also grateful to the government of Pakistan and British Council authorities for financial help.
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