EXTRACHROMOSOMAL INHERITANCE Dr. S. Ramgopal Rao Associate Professor Biotechnology SNIST
INTRODUCTION • 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 PLANTS • 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 Mixed chloroplasts White/green Mutant chloroplast White non-photosynthetic Normal chloroplast Green photosynthetic
They may be intercrossed in a variety of different combinations by transferring pollen from one flower to another
Two features are surprising • There is difference between reciprocal crosses • 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): Normal Mutant Mixed Assumption: Pollen (male) contributes no information
POKY NEUROSPORA • In 1952, Mary Mitchell isolated a mutant strain of Neurospora that she called poky. • Poky Neurospora is: • Slow growing • It shows maternal inheritance • 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 poky Wild 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. protoperitheca (sexual mating type) conidia (asexual mating type) Reciprocal crosses of poky and wild-type Neurospora.
But where in the cytoplasm is the mutation carried? • Slow growth suggest lack of ATP energy, which is produced by mitochondria • 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 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 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 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 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 MUTATION • Nuclear petites • Neutral or recessive petites • Suppressive petites
NUCLEAR 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
NEUTRAL 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
Further 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 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 CHLOROPLAST 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 strS and strR 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
SHELL 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
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