EXTRACHROMOSOMAL EXTRACHROMOSOMAL INHERITANCE INHERITANCE

Dr. S. Ramgopal RaoDr. S. Ramgopal RaoAssociate Professor

BiotechnologySNIST

INTRODUCTIONINTRODUCTION• Extra nuclear inheritance is defined as non mendelian

inheritance, usually involving DNA in replicating mitochondria and some other organelles of cell.

• The genes that have been called cytoplasmic genes, extrachromosomal genes, or extranuclear genes are located on a unique kind of chromosome inside cytoplasmic organelle.

• Commonly defined as transmission through the cytoplasm (or things in the cytoplasm, including organelles) rather than the nucleus

• Generally only one parent contributes

• Organelle heredity

Organelles that contain chromosomes• Chloroplasts and mitochondria

• Infectious heredity– Involves a symbiotic or parasitic association with a

microorganism

Criteria for extranuclear inheritance

VARIEGATION IN LEAVES OF HIGHER VARIEGATION IN LEAVES OF HIGHER PLANTSPLANTS

• In 1909, carl correns reported some surprising resuls from his study on four O clock plants ( Mirabilis jalapa).

• The blotchy leaves of these variegated plants showed patches of green and white tissues, but some branches carried only white and some carried only green leaves.

• Variegated-shoot phenotypes in four o’clocks

Normal chloroplastGreenphotosynthetic

Mutant chloroplastWhitenon-photosynthetic

Mixed chloroplastsWhite/green

• They may be intercrossed in a variety of different combinations by transferring pollen from one flower to another

• Two features are surprising

1. There is difference between reciprocal crosses2. Phenotype of maternal parent is solely responsible

for determining the phenotype of all progeny(This phenomenon is called maternal inheritance)

• How such curious results could be explained?

• The difference in leaf color was known to be due to presence of either green or colorless chloroplast

• The inheritance pattern might be explained if these cytoplasmic organelle are somehow genetically autonomous and further are never transmitted via the pollen parent

• For an organelle to be genetically autonomous, it must have its own genetic determinants.

• Thus this organelle has its own genome

• The process of segregation and recombination of organelle genotype is called “cytoplasmic segregation and recombination (CSAR)”

Chloroplasts are inherited via the seed cytoplasm

3 types of eggs (female):

NormalMutantMixed

Assumption:

Pollen (male) contributes no information

POKY NEUROSPORAPOKY NEUROSPORA

• In 1952, Mary Mitchell isolated a mutant strain of Neurospora that she called poky.

• Poky Neurospora is:1. Slow growing2. It shows maternal inheritance3. It has abnormal amount of cytochromes

It is possible to cross some fungi in such a way that one parent contributes the bulk of cytoplasm to the progeny and this cytoplasmic contributing parent is called female even though no true sex is involved

Maternal inheritance for the poky phenotype was established i the following crosses

Poky(female) x wild type (male) → all pokyWild type (female) X poky (male) → all wild type

• Mutant [poky] Neurospora possess altered mtDNA cytochrome complements that lead to slow growth.

• [poky] phenotype is inherited with the cytoplasm.

Reciprocal crosses of poky and wild-type Neurospora.

protoperitheca (sexual mating type)

conidia(asexual mating type)

• But where in the cytoplasm is the mutation carried?

1. Slow growth suggest lack of ATP energy, which is produced by mitochondria

2. There are abnormal amounts of cytochromes and cytochromes are known to be located in the mitochondria

These clues led to the conclusion that in this case mitochondria are involved and mutation is in the mtDNA.

THE KILLER TRAIT IN PARAMECIUMTHE KILLER TRAIT IN PARAMECIUM

• 1930s,----- sonneborn observed that when two stocks of P.aurelia are mixed together, some of them die.

• Certain strains of P. aurelia are called killer strains because they release paramecin, a substance toxic to sensitive strains

BASIS OF KILLING ACTIONBASIS OF KILLING ACTION

• The killing action was due to the possession by killer cells of a cytoplasmic particle that was named kappa

• A cell lacking kappa is sensitive to the effect of kappa

• Sensitive stocks are immune to kappa’s killing action during conjugation

• For a cell to be a killer, it must posses the kappa particles in cytoplasm

• However, to maintain kappa , the paramecium must posses a dominant allele (K) in nucleus.

• A cell homozygous for recessive (kk) is sensitive

• A cell that carries the dominant (KK) may lack kappa is also sensitive

• Paramecium has diploid micronuclei• At the time of conjugation it contain two haploid

micronuclei that were formed following meiosis• It behaves as gametes• Micronuclei are exchanged between mating cells by

way of conjugation tube.• The micronuclei received from a mate unites with

the stationary one and restore the diploid state of micronucleus.

• Following a mating between killer (KK) and sensitive (kk), both F1 cells become Kk.

• After later divisions and self fertilization, homozygous KK and kk cells arise

• To become a killer a sensitive cell of genotype KK must gain kappa paticles through cytoplasmic exchange at the time of mating

• This comes by prolong mating and cytoplasmic bridge

THE PETITE MUTATION IN YEASTTHE PETITE MUTATION IN YEAST

• Cells carrying a petite mutation grow slowly and form tiny colonies on agar in contrast with the larger ones of the wild phenotype

• These petites posses enzyme defects and are deficient in aerobic respiration

LIFE CYCLE OF YEASTLIFE CYCLE OF YEAST

• It is a unicellular organism

• Haploid cells can be classified into either of two mating types, + or –

• The diploid zygote formed from fusion of a + and a – cell may grow by budding to produce a diploid colony

• The diploid cells can also be stimulated to undergo meiosis

• The cell then enlarge and forms four haploid nuclei, each of which becomes the nucleus of a spore

• The meiotic cells behave like an ascus or sac and thus four spore are called ascospores

• Two of the ascospores will be mating type + and two will be –

• This 2:2 indicates that the genetic determinants are nuclear

TYPES OF PETITE MUTATIONTYPES OF PETITE MUTATION

1. Nuclear petites2. Neutral or recessive petites3. Suppressive petites

NUCLEAR PETITESNUCLEAR PETITES

• These petites behave in the expected mendelian fashion

• A cross of petite with wild produce wild diploid cells

• The ascospore yield a 2:2 segregation of wild type to petite

Chapter 15 slide 27

NEUTRAL OR RECESSIVE PETITESNEUTRAL OR RECESSIVE PETITES

• A cross of neutral petite with a wild produce diploid cells that are normal in phenotype

• When sporulation is induced, the ascus yields spores that produce only wild type cells

• The segregation is thus 4:0• The petite phenotype is disappeared and not appear

in successive generations• Clearly the neutral petite is not behaving in

mendelian way

Chapter 15 slide 29

Further clarificationFurther clarification

• Yeast was treated with acriflavin• Almost whole population of normal cells can be

transformed into petite• No known mutagen can affect nuclear genes to such

an extent that all the population become mutated• This indicates that determination for petite mutation

reside in cytoplasm

SUPPRESSIVE PETITESUPPRESSIVE PETITE

• It also behave in non mendelian fashion

• When it is crossed with wild type , the result depends when the sporulation is induced

• If ascospore formation takes place very soon after the zygote forms, it is found that most of the asci will give a segregation of 0:4, that is all the spores will give rise to petites.

• The zygote if immediately plated out on agar after the mating, form diploid colonies that are also petite

• In contrast to neutral petite, it is as if the wild type were tending to disappear.

• However different results can be obtained from the same cross

• mtDNA of cytoplasmic petites has undergone some sort of alteration

CHLAMYDOMONAS CHLAMYDOMONAS CHLOROPLAST MUTATIONSCHLOROPLAST MUTATIONS

• Unicellular alga– Haploid• Mating gives diploid cell that immediately

undergoes meiosis to form haploid cells– Single chloroplast with 50 copies of cpDNA

• mt+ and mt- strains• strS and strR strains (streptomycin resistance)

Offspring of strOffspring of strSS and str and strRR Crosses Crosses

• always have str phenotype of mt+ parent– Only mt+ parent donates cytoplasm

• But 50% of offspring mt+ and 50% mt- – mt encoded by nuclear gene

strstrSS and str and strRR Crosses Crosses

SHELL COILING IN SNAILSHELL COILING IN SNAIL

• Hermaphroditic snails• Some shells have right-handed (DD or Dd) coiling

while others have left-handed (dd)coiling• Reciprocal crosses (reverse mail and female

genotypes) of true-breeding snails– Offspring phenotype depends upon maternal

genotype—not maternal phenotype

SHELL COILING IN SNAILSHELL COILING IN SNAIL

• This happens because the genotype of the mother’s body determines the initial cleavage pattern of the developing embryo

• These Segregation ratios would never appear in organelle genes

• The term maternal effect can be used for the cases like the shell coiling example in order to distinguish them from organelle based inheritance