Extra nuclear inheritance

Extranuclear Inheritance

1. BANGALORE UNIVERSITY CENTRE FOR APPLIED GENETICS DEPARTMENT OF ZOOLOGY JNANA BHARATHI BENGALURU 560 056 EXTRA NUCLEAR INHERITANCE IN Chlamydomonas , Neurospora & Paramecium VIDYASHRI S.H. I st Year M. Sc., I st Semester Centre for Applied Genetics Department of Zoology Bangalore University GUIDE Dr. VENU.G Centre for Applied Genetics Department of Zoology Bangalore University

2. INTRODUCTION TYPES OF INHERITANCE EXTRA-NUCLEAR INHERITANCE Chlamydomonas reinhardtii Neurospora crassa Paramecium aurelia SUMMARY REFERENCES CONTENTS

3. Inheritance is the process by which genetic information is passed on from generation to generation. When organisms reproduce, all the information for growth, survival and reproduction for the next generation is found in the DNA passed down from the parent generation. INHERITANCE

4. Autosomal Dominant One mutated copy of the gene in each cell is sufficient for a person to be affected by an autosomal dominant disorder. Eg : Marfan’s syndrome Autosomal Recessive The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show any signs and symptoms of the condition. Eg : Sickle cell disease PATTERNS OF INHERITANCE

5. X- Linked dominant This is caused by mutations in genes on the X chromosome, one of the 2 sex chromosomes in each cell. Eg : Fragile X syndrome X linked recessive Mutation would have to occur in both copies of the gene to cause the disorder. Eg:Haemophilia

6. Y-linked inheritance A condition is considered Y linked if the mutated gene that causes the disorder is located on the Y chromosome. Eg : Swyer’s syndrome Non- Mendelian inheritance It is also known as Mitochondrial inheritance. Maternal inheritance applies to genes in Mitochondrial DNA. Eg : Leber hereditary optic neuropathy

7. Extranuclear inheritance  is the transmission of genes that occur outside the nucleus . It is found in most eukaryotes and is commonly known to occur in cytoplasmic organelles such as mitochondria and chloroplasts or from cellular parasites like viruses or bacteria . Most of the recorded cytoplasmically inherited characters would follow the maternal line i.e., Uniparental mode of transmission. It is also known as maternal inheritance or Trans-ovarian transmission . EXTRA NUCLEAR INHERITANCE

8. R.Sager and N.Gilham have reported some cases of extrachromosomal inheritance in green alga Chlamydomonas reinhardtii The alga reproduces asexual as well as sexual means. The sexual reproduction takes place by fusion between 2 morphologically similar but physiologically dissimilar haploid gametes. Chlamydomonas reinhardtii

9. A cross can occur only if the parents are of different mating types. The mating types are physically identical but physiologically different. Such species are called heterothallic (literally “different bodied ”). In Chlamydomonas , the mating type alleles are called mt +  and mt -. Chlamydomonas is a haploid unicellulate green alga. It has two mating types—’+’ and ‘-‘.The two mating types are governed by two alleles of a nuclear single gene. The alleles are named as mt +  and mt – .

10. The + mating type is considered as female, while the – mating type is regarded as male. During sexual reproduction one mt +  and one mt –  cell pair and fuse together to form a zygote where there is mixture of cytoplasm coming from both mt + and mt –  gametes.

11. R. Sager isolated two strains of Chlamydomonas : one strain was resistant and the other one is sensitive . The trait of streptomycin resistance is believed to be located in its cp-DNA (chloroplast DNA). Mating between mt + streptomycin resistant ( S r ) and mt sensitive (S s ) cells produce only resistant progeny but the nuclear genes for mating type segregate as expected. But the reciprocal cross between mt + susceptible and mt –  resistant shows again the expected segregation for mating type but all progenies are sensitive type. Therefore, it clearly provides an example for extra-nuclear inheritance.

12. There are many examples for mitochondrial enzyme deficiency which are cases of extra chromosomal inheritance—petite yeast came from the studies of Neurospora crassa , a member of ascomycetes group of fungi. In this fungus, there is a slow-growing mutant strain called poky. Such strain exhibits poorly differentiated mitochondria which are deficient in the membrane-bound cytochromes b, a 1  and a 3 , essential proteins of the respiratory electron transport chain. Poky strain in Neurospora

13. When poky as female parent is crossed with a normal strain as a male parent (mitochondrial parent), the progeny are found to be poky . In reciprocal cross, the progeny are normal. This non- Mendelian uniparental inheritance suggests that the cytoplasm of female parent is important because the only difference between reciprocal crosses is the contribution of cytoplasm.

14. In 1943, T.M.Sonneborn reported that some strains of P . aurelia contain kappa particles and are known as killer strain . Kappa particles are the symbiont bacteria called Caedobacter taeniospiralis . The strain of Paramecium in which the kappa particles are absent are called sensitive strain. The sensitive strains are killed by the killer strain. The destruction of sensitive strain occurs through secretion of a toxic substance called P aramecin . This toxic substance is believed to breakdown the food vacuole membrane of the sensitive strain. Kappa particles in Paramecium

15. When killer strain of Paramecium conjugates with sensitive strain under appropriate condition for brief period and no cytoplasm exchange occurs. T wo kinds of clones result- one from the original killer cell which contains allele K ( Kk ) and kappa particles and the other from the original sensitive cell which carries the allele K ( Kk ) and lacks kappa particles. Under some conditions of conjugation per­sists much longer; a long connection is established between conjugants (killer and sensitive). In this conjugation, cytoplasm as well as nuclear genes are exchanged. As a consequence both ex- conjugants will receive the genotype Kk and the cytoplasm with kappa particles.

16. Extranuclear inheritance or cytoplasmic  inheritance is the transmission of genes that occur outside the nucleus. Extra-chromosomal inheritance, extra-nuclear inheritance, and maternal inheritance are all synonyms. All these terms can be defined as the inheritance of characteristics of only one of the two parents, usually the female parent to the progeny. Evidence for cytoplasmic inheritance was first reported by Correns in Mirabilis jalapa and by Bar in Pelargonium zonule in 1908. Rhoades described cytoplasmic male sterility in maize in 1933. In 1943, Sonneborn discovered kappa particles in Paramecium and described its cytoplasmic inheritance. Presence of DNA in chloroplasts was first demonstrated by Ris in plant cell. SUMMARY

17. William.S.Klug&Michael.R.Cummings,1996.Essentials of genetics, 2 nd edition., Prentice Hall Internationals. Robert H.Tamarin,2002.Principles of Genetics,7 th Ed. mRata McGraw –Hill Ediyion , New Delhi , India Gardner.E.J.,Simmons.M.J.&Snustad.D.P.1991.Principles of Genetics , John Wiley& Sons Inc., N.Y. REFERENCES