EXTRACHROMOSOMAL INHERITANCE IN Paramecium

June 4, 2017 | Autor: Ogar Offumobi | Categoria: Genetics, Cell Biology
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EXTRACHROMOSOMAL INHERITANCE IN Paramecium


BY
OGAR, GODWIN OFFUMOBI
139072003


A Written Term Paper Submitted in Partial Fulfilment of the Requirements for the Course (CBG 804) for the degree of Master of Science (Cell Biology and Genetics) in the
Universty of Lagos



AUGUST 2014

SUPERVISOR: DR. A. KHALID


OUTLINE
INTRODUCTION
EXTRACHROMOSOMAL INHERITANCE
ADVANTAGES
AGRICULTURE
CLINICAL
TYPES OF EXTRANUCLEAR INHERITANCE
KAPPA PARTICLES IN Paramecium
DIAGRAMMATIC MECHANISM
REFERENCES











INTRODUCTION
EXTRACHROMOSOMAL INHERITANCE
In normal conditions, the phenotype of the progeny depends on the genes present in the nucleus and is referred as Chromosomal Inheritance, where there is a simple connection between the genes located on the chromosomes and observed phenotypes in the progeny. The male gametes and female gametes equally contribute to the phenotype of progeny and there is no differences observed with phenotypes with reciprocal crosses. In some exceptional cases, the phenotype is not dependent on the nuclear genes and is controlled by the genetic content present in the cytoplasm. The control of phenotype of the progeny by the non nucleus component present in the cytoplasm is called extrachromosomal inheritance or organellar inheritance or cytoplasmic inheritance. Extrachromosomal inheritance is distinct from the Maternal effect, where the phenotype of progeny depends on the mother's genotype and on nuclear gene products (mRNA or protein) present in the cytoplasm of the egg. The genetic material present in the mitochondria or chloroplast (apart from nuclear chromosomal DNA) are responsible for the extrachromosomal inheritance and hence called organellar inheritance since these organs are responsible for phenotype of the progeny. In few cases apart from mitochondria or chloroplast, the phenotype of the progeny depends on the extra-chromosomal particles present in the cytoplasm, hence also referred as cytoplasmic inheritance. In extrachromosomal inheritance, the reciprocal crosses between male and females will give different results.

One of the classical examples of extrachromosomal inheritance is the variegated leaves found in the Mirabilis jalapa commonly known as four-o'clock plant. There are three kinds of phenotypes observed in leaves as white, green and variegated (patches of white and green on same leaf). The progeny derived from the seeds of the white and green plant gave rise to white and green respectively irrespective of the kind of pollens used from three phenotypes. The variegated plant gave rise to all three phenotype branches and leaves irrespective of the phenotype of the pollen plant. The phenotype of the seed primarily depends on the phenotype of the plant that produces the egg and the genes responsible for the phenotypes are present in chloroplast. The chloroplasts are transferred from the mother to offspring through cytoplasm of the female gamete and male gamete has no role in transmission of chloroplast DNA.

Carbon dioxide (CO2) sensitivity in Drosophila is also a good example for extrachromosomal inheritance. When certain strains of Drosophila exposed to CO2, become unconscious and are referred as CO2 sensitive strains. When crosses were made between CO2 sensitive strain and normal wild type strain, CO2 sensitive phenotype was always transferred from mother side to progeny. 100% of the progeny obtained from the CO2 sensitive mother are always CO2 sensitive irrespective of kind of father's phenotype. CO2 sensitivity phenotype of Drosophila was observed in files due to the presence of virus like particles called Sigma factors present in the cytoplasm of the sensitive Drosophila flies.

Kappa particles in Paramecium are other classical example for extrachromosomal inheritance, where the phenotype not only depends on the cytoplasm but also on the genetic material of the nucleus. Paramecin is the toxic substance produced by certain strains of Paramecium which kills the sensitive strains. The strain of Paramecium that produces paramecin is called Killer strain and hosts particles called Kappa particles which are responsible for paramecin production. Kappa particles require dominant allele K for its multiplication and genotypes KK and Kk can support Kappa particles, while genotype kk can't support Kappa particles. Different results are observed when Killer strain is allowed to conjugate with sensitive strain depending on the duration of conjugation. In short conjugation, where only genetic material is exchanged without the cytoplasmic exchange, the exconjugates will have 1:1 ratio of killer to sensitive strain and the further asexual division results in 1:3 of killer to sensitive strains. In prolonged conjugation, which involves exchange of both genetic and cytoplasmic content between conjugates, the exconjugates will have all killer strains and further asexual division results in 1:1 ratio of killer to sensitive strains.
ADVANTAGES
AGRICULTURE
Extrachromosomal inheritance has some practical advantages in agriculture and can also be used in prediction of diseases in humans for counseling. In plants Cytoplasmic Male Sterility trait can be used in plant breeding.
CLINICAL
A recent study involving congenital heart disease parents showed that the risk of getting congenital heart disease in the progeny was higher if the mother is affected rather than father. The advanced research on extrachromosomal inheritance associated with human diseases will go a long way in genetic counseling.
(Chandra,2012)
TYPES OF EXTRANUCLEAR INHERITANCE
Three general types of extranuclear inheritance exist. These are vegetative segregation, uniparental inheritance and biparental inheritance.
Vegetative segregation results from random replication and partitioning of cytoplasmic organelles. It occurs with chloroplasts and mitochondria during mitotic cell divisions and results in daughter cells that contain a random sample of the parent cell's organelles. An example of vegetative segregation is with mitochondria of asexually replicating yeast cells (Birky et al., 1978).
Uniparental inheritance occurs in extranuclear genes when only one parent contributes organellar DNA to the offspring. A classic example of uniparental gene transmission is the maternal inheritance of human mitochondria. The mother's mitochondria are transmitted to the offspring at fertilization via the egg. The father's mitochondrial genes are not transmitted to the offspring via the sperm. Very rare cases which require further investigation have been reported of paternal mitochondrial inheritance in humans, in which the father's mitochondrial genome is found in offspring (Schwartz and Vissing, 2003). Chloroplast genes can also inherit uniparentally during sexual reproduction. They are historically thought to inherit maternally, but paternal inheritance in many species is increasingly being identified. The mechanisms of uniparental inheritance from species to species differ greatly and are quite complicated. For instance, chloroplasts have been found to exhibit maternal, paternal and biparental modes even within the same species (Hansen and Jansen, 2006).
Biparental inheritance occurs in extranuclear genes when both parents contribute organellar DNA to the offspring. It may be less common than uniparental extranuclear inheritance, and usually occurs in a permissible species only a fraction of the time. An example of biparental mitochondrial inheritance is in the yeast, Saccharomyces cerevisiae. When two haploid cells of opposite mating type fuse they can both contribute mitochondria to the resulting diploid offspring (Birky, 1994).


KAPPA PARTICLES IN PARAMECIUM

Cytoplasmic Inheritance Involving Dispensable Hereditary Units (nuclear genes may or may not be involved). Kappa particles are found in certain killer strains of Paramecium and are responsible for production of substance paramecin, which is toxic to strains not possessing kappa (sensitive strain). The production of kappa particles is dependent on a dominant allele K, so that killer strains are KK or Kk and sensitive strains are ordinarily kk. In absence of dominant allele K, kappa particles cannot multiply and in absence of kappa particles, dominant allele K cannot produce them de novo. Consequently sensitive strains with genotypes KK or kk can be obtained. These will not carry any kappa particles. However, killer strain with genotype kk cannot be obtained, because even if kappa particles are present, these would be lost in absence of dominant allele. If Paramecium clones with genotypes KK or Kk are allowed to multiply asexually at such a fast rate, that division of kappa particles cannot keep pace with division of cells, kappa particles will be eventually lost. Consequently sensitive strains with dominant genotype (KK, Kk)having no kappa particles would be obtained.

If the killer (KK)and sensitive (kk)strains are allowed to conjugate, all exconjugants (the cells separating after conjugation) will have same genotype Kk. Phenotypes of these exconjugants will, however, depend upon duration for which conjugation is allowed. If conjugation does not persist long enough for exchange of cytoplasm, heterozygote (Kk)exconjugants will only have parental phenotypes. It means that killers will remain killers and sensitive will remain sensitive even after conjugation (Figure 1). If conjugation persists, sensitive strain will receive kappa particles and will become killer, so that exconjugants will be killers having genotype Kk .



 


Fig. 1.Results of a cross between a killer (KK) and a sensitive (kk)strain of Paramecium, when no cytoplasmic exchange is allowed.









Fig. 2. Results of a cross between a killer (KK)and a sensitive (kk)strain of Paramecium, when cytoplasmic exchange is allowed.




References
Birky, C.W. (1994).Relaxed and Stringent Genomes: Why Cytoplasmic Genes Don't Obey Mendel's Laws. Journal of Heredity; 85, 355-366.
Birky, C and William, Jr. (1995). Uniparental inheritance of mitochondrial and chloroplast genes: mechanisms and evolution. Proceedings of the National Academy of Sciences; 92, 11331-11338.
Birky, C.W., Robert, L.S., Jean, L.F. and Philip, S.P.(1978). Vegetative segregation of mitochondria in yeast: Estimating parameters using a random model. Molecular and General Genetics; 158, 251-261.
Chandra, K. (2012). Extrachromosomal Inheritance - Cytoplasmic Influence on Phenotype. Genetics;15:19:35
Duff, C. (1996). HIV infection in women. Primary Care Update for OB/GYNS ;3, 45-49.
Hansen, K and Jansen, R.K. (2006). Paternal, maternal, and biparental inheritance of the chloroplast genome in Passiflora (Passifloraceae): implications for phylogenic studies. The American Journal of Botany; 94, 42-46.
Sapp 2004. The dynamics of symbiosis: an historical overview. Canadian Journal of Botany; 82, 1046–1056.
Schwartz, M. and Vissing, J. (2003). New patterns of inheritance in mitochondrial disease. Biochemical and Biophysical Research Communications; 310, 247-251.






















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