CYTOPLASMIC INHERITANCE

 What is Cytoplasmic Inheritances .........? 

Extra nuclear inheritances or cytoplasmic inheritances 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 the chloroplast or from cellular parasites like viruses or bacteria. 

Inheritance of most of the characters in eukaryotic organisms :-

1. The contributions by both male and female parents are equal so that the results from reciprocal crosses are identical.

2. Segregation produces the characteristic 3:1 ratio in the F 2 generation of a monohybrid cross and a typical 9:3:3:1 in dihybrid crosses.

- These features of inheritance were first demonstrated by Mendel: consequently, such an inheritance pattern is referred to as Mendelian inheritance. It is universally accepted that genes showing Mendelian inheritance are located in the chromosomes of eukaryotic nuclei. 

- Therefore Mendelian inheritance pattern is regarded as a sufficient evidence for a gene to be located in the chromosomes, such genes are termed as nuclear genes or more commonly simply as genes.

Non Mendelian Inheritance:- 

 Some characters in several organisms do not show Mendelian inheritance or they show a non Mendelian inheritance pattern. In such cases, the following characteristic features are observed.

1. There is consistent difference between the results from reciprocal crosses; generally only the trait from female parent is transmitted.

2. In most cases, there is no segregation in the F2 and subsequent generations. Characters showing non Mendelian inheritance may be grouped under three broad categories:

(1) those related to cellular structures and patterns,

(2) those produced by intracellular parasites, symbionts and viruses

(3) those associated with DNA containing cell organelles viz., mitochondria and

chloroplasts.

Plastid inheritance:- 

The inheritance pattern of plastid characters due to plasma genes located in plastid is known as plastid inheritance.

-  Plastid inheritance was first case of cytoplasmic inheritance to be discovered independently by Correns and Baur in 1908.

-  Variegation refers to the presence of white or yellow spots of variable size on the green back ground of leaves. Variegation may be produced by some environmental factors, some nuclear genes and in some cases, plasma genes.

Inheritance of plastids in Mirabilis jalapa:-

The inheritance of plastids in Four ‘O’ clock plant Meiabilis jalapa was first described by Correns (1908). In M. Jalapa, some of the branches may have normal green leaves, while in the same plant, some other branches may have only pale green or white leaves and still others may have variegated leaves. 

Flowers on branches with normal green leaves produce seeds that grow into plants with normal green leaves irrespective of whether they are pollinated by pollen from branches with normal green variegated or pale green leaves.

Progeny of a variegated four ‘O’ clock plant:- 

It is clear that variegation is determined by agencies transmitted through the female and  that it is not influenced by the type of pollen used. These agencies are the chloroplast. 

They are capable of self-duplication and are transmitted from generation to generation through the cytoplasm of the egg. 

Seeds borne on a green branch have three gene only green plastids, seeds borne on a pale green branch have three gene only pale green plastids and seeds borne on a variegated branch have green or pale green or a mixture of the two types of plastids.

Variegation is thus a heredity character determined by stable, self-duplicating, extra nuclear particles called plastids. Neither the nucleus of the female gamete nor the male gamete is involved in the control of this type of heredity character.

Characteristics of cytoplasmic inheritance:- 

1. Reciprocal differences:- 

Reciprocal crosses show marked differences for the characters governed by plasmagenes.

 In most cases, plasmagenes from only one parent, generally the female parent are transmitted, this phenomenon is known as uniparental inheritance.

2. Lack of segregation:- 

 In general, F2 F3 and the subsequent generations do not show segregation for a cytoplasmically inherited trait. 

This is because the f1 individuals generally receive plasma genes from one parent only.

3. Irregular segregation in biparental inheritance:- 

In some cases, plasma genes from both the parents are transmitted to the progeny, this is known as biparental inheritance.

4. Somatic segregation:- 

Plasma genes generally show somatic segregation during mitosis, a feature of rare occurrence in the case of nuclear genes.

5. Association with organelle DNA:- 

Several plasma genes have been shown to be associated with cp-DNA or mt-DNA.

6. Nuclear transplantation:- 

If nuclear transplantation revealas a trait to be governed by the genotype of cytoplasm and not by that of nucleus, cytoplasmic inheritance of the trait is strongly indicated.  

In nuclear transplantation, nucleus of a cell is removed and replaced by a nucleus of another genotype from a different cell. Generally nuclei of somatic cells are transplanted into zygotes before the first mitotic division is initiated.

7. Transfer of nuclear genome through back crosses: - The nucleus of a variety or species may be transferred into the cytoplasm of another species or variety through repeated back crossing with the former, which is used as the recurrent male parent. 

Lines produced in this way are known as alloplasmic lines since they have nuclei and cytoplasms from two different species.

 A comparison of the various characters of alloplasmic lines with those of the corresponding euplasmic line (lines having nuclei and cytoplasms from the same species) demonstrates cytopalsmic effects, if any on these traits. This technique is time consuming. 

8. Mutagenesis:- 

Some mutagens eg: Ethidium bromide are highly specific mutagens for plasma genes while nuclear genes are not affected by them.Induction of mutation by such agenets in a gene indicates it to be a plasma gene.

9. Lack of chromosomal location:- 

In many organism, extensive linkage maps of nuclear genes are available. If a gene is shown to be located in one of these linkage groups, it cannot be a plasma gene. 

Failure to demonstrate the location of a gene in one of the linkage groups of an organism is indicative of its cytoplasmic location, but this is highly tentative.

10. Lack of association with a parasite, symbiont or virus:- 

In many cases, a cytoplasmically inherited character is associated with a parasite, symbiont or virus present in the cytoplasm of the organism. Such cases cannot be regarded as cases of cytoplamic inheritance. 

Only those cytoplasmically inherited characters which are not associated with parasites, symbionts or viruses can be regarded as governed by plasma genes.

The known cases of true cytoplasmic inheritance are concerned with either choloroplast or mitochondrial traits and are usually associated with their DNA. Such cases are therefore often referred to as organellar inheritance, plastid inheritance and mitochondrial inheritance.

Significance of Cytoplasmic Inheritance

1. Development of cytoplasmic male sterility several crop plants like maize. Pearl millet, sorghum, cotton etc.

2. Role of mitochondria in the manifestation of heterosis.

3. Mutation of chloroplast DNA and mitochondrial DNA leads to generation of new variation.