Cytoplasmic Inheritance

Cytoplasmic inheritance also refers to Maternal inheritance or Maternal effect. In the cytoplasm, autonomous organelles are present like mitochondria and plastids which are having their own genetic material. Mitochondria is present in both animal and plant cell whereas plastids only found in the plant cell.

The gamete which contributes the cytoplasm is the female gamete which carries both nuclear and cytoplasmic material whereas a male gamete does not carry cytoplasmic material. This is the reason, why cytoplasmic inheritance also refers to “Maternal effect” where the phenotype of the offspring is decided by the mother or a female which holds the cytoplasmic material.

After the cross-fertilization, a zygote forms and whatever cytoplasm is coming to the zygote is due to the only female gamete.

Diagrammatic representation of cytoplasmic inheritance

As we know, nucleic acid like DNA and RNA are responsible for the inheritance of the gene. DNA is the hereditary material which transfers or inherit a gene from one generation to the next generation by the individual to individual. Therefore, the genes play an important role in forwarding the genetic features from the parents to the offspring.

By further study on inheritance, the idea of cytoplasmic inheritance has been originated. In cytoplasmic inheritance, the cytoplasmic genes and the cytoplasmic particles participate in the process of inheritance. The study of cytoplasmic inheritance has been studied in many plants and animals. Several studies have revealed that the extranuclear material i.e. cytoplasmic elements can carry out the biological process refer as “Inheritance”.

Content: Cytoplasmic Inheritance

  1. Definition
  2. Characteristics
  3. Examples

Definition of Cytoplasmic Inheritance

Cytoplasmic inheritance can define as the process, which takes part in the process of inheritance that transfers the genetic characters by the means of cytoplasmic elements in organisms like animals and plants. The cytoplasm consists of autonomous organelles like mitochondria and plastids which are having their own genes.

Characteristics of Cytoplasmic Inheritance

The cytoplasmic inheritance includes the following characteristics:

  • The cytoplasm inherits information to one another by the means of “Plasmagenes”.
  • The plasmagenes are found on the cytoplasm that is located outside the chromosome.
  • Plasmagenes also have the capability of “Self-replicating” like the chromosomal genes i.e. DNA or RNA.
  • The cytoplasm is received by the offspring only from the female gamete, not from the male. Therefore, only the female gamete is responsible to transfer plasmagenes to the offspring. Hence, cytoplasmic inheritance refers as “Maternal inheritance”.
  • Plasmagenes are also capable of mutation.
  • Cytoplasmic inheritance can be studied by reciprocal crossing over.
  • The results of the reciprocal crossing are not the same.

Examples of Cytoplasmic Inheritance

To understand the theory of cytoplasmic inheritance there are certain examples which will give us the whole idea of the cytoplasmic inheritance. There are several experiments like the presence of kappa particles, carbon dioxide sensitivity, shell coiling etc which shows the maternal or cytoplasmic inheritance.

Kappa Particles Inheritance in Paramecium

It was studied in the strain of Paramecium aurelia by the scientist Sonneborn in the year 1938. Paramecium aurelia gives two types of strains namely Killer strain and Sensitive strain. The killer strain should have a dominant gene plus kappa particles. The sensitive strain contains anyone from the two elements i.e. either dominant gene or kappa particles. The dominant gene will produce kappa particles which will produce a toxin “Paramycin”.

To understand the concept of kappa particle inheritance, let us cross a pure killer and sensitive strain. The pure killer strain will have both dominant genes i.e. KK plus a protein factor Paramycin. The pure sensitive strain will only have the recessive gene. During crossing over, there will be conjugation between KK gene with kk gene. If the duration of conjugation is less than 3 minutes, then there will be an only nuclear exchange.

Therefore at the time of crossing over, one dominant and one recessive gene will exchange between the killer and sensitive strains. After crossing over, two offspring will produce refer as “Ex-conjugants”. One killer strain (Kk) and one sensitive strain (Kk) will produce as a result of this type of crossing over. The ex-conjugant Kk will behave like a killer strain because it is having both dominant gene ‘K’ and kappa particles as there is only nuclear exchange.

nuclear inheritance in paramecium

Now, let us understand the concept of nucleoplasmic inheritance by the same crossing over. As there will be conjugation between KK gene with kk gene. If the duration of conjugation is more than 3 minutes, then there will be nuclear exchange along with Cytoplasmic exchange. Therefore at the time of crossing over, one dominant and one recessive gene will exchange along with cytoplasmic exchange. Therefore, this type of inheritance will refer to as “Nucleoplasmic inheritance”. After crossing over, two offspring will produce refer as “Ex-conjugants”. Both killer strain will produce as a result of this type of crossing over.

nucleocytoplasmic inheritance in paramecium

Carbon Dioxide Sensitivity in Drosophila

Carbon dioxide acts as “Paralyzing or anaesthetic agent”, which paralyzes several strains of Drosophila. In 1958, Heritier and Teissier studied the carbon dioxide sensitivity in Drosophila melanogaster, where he found high sensitivity towards carbon dioxide.

L. Heritier and Teissier experimented the carbon dioxide sensitivity in the breeding strain of D. melanogaster, where they crossed the sensitive strain with the normal strain. After the experiment, L. Heritier and Teissier concluded that the sensitive mother will produce sensitive offspring according to the cytoplasmic inheritance.

Sometimes sensitive male produce sensitive offspring when crossed with normal female Drosophila. The sensitivity produced by the male Drosophila only lasts for the first generation. L. Heritier and Teissier then further study to know about the sensitivity factor where they found the virus-like particle in the cells of sensitive flies and named it as “Sigma particle”.

L. Heritier and Teissier conclude that the sensitivity in Drosophila is due to the presence of sigma factor. Sigma particle is present in different strains of Drosophila. Sigma has a particle size of 0.07-micron diameter and it is heritable due to the presence of DNA.

  • Examples of sensitive drosophila strains: D. melanogaster, D. Algonquin, D. pseudoobscura, D. robusta, D. affinis etc.

Shell Coiling in Limnaea

In 1920, Arthur Boycott was the first to study the maternal effect of shell coiling in the snail. Arthur Boycott studied the maternal effect in Limnaea peregra. There are two types of coiling found in the Limnaea peregra:

  • Dextral coiling: This type of coiling is towards right-handed.
  • Sinistral coiling: This type of coiling is towards left-handed.

For the dextral coiling, a dominant gene i.e. ‘D’ is responsible. If there is either “DD” or “Dd” is present in a snail, then there will be dextral coiling. For the Sinistral coiling, a recessive gene i.e. ‘d’ is responsible. If there is “dd” gene present in a snail, then there will be Sinistral coiling. The shell coiling in snail is only decided by the genotype of the mother.

Let us understand the concept of the maternal effect of shell coiling in Linnaea by taking a couple of crossing over.

Crossing over between DD female and dd male

By this crossing over of pure dextral female and a Sinistral male, “Dd” zygote will produce with dextral coiling in the F1 generation. Then by intercrossing DD, Dd, Dd and dd will produce in the F2 generation with dextral coiling. According to the study of Alfred Sturtevant, the dd offspring will have dextral coiling due to maternal effect due to the dominant ‘D’ gene. Then in the F3 generation, three dextral and one Sinistral offspring will produce having a phenotype of 3:1.
shell coiling in Limnaea

Crossing over between dd female and DD male

By this crossing over of pure Sinistral female (dd) and a dextral male (DD), “Dd” zygote will produce with dextral coiling in the F1 generation. Then by intercrossing, DD, Dd, Dd and dd will produce in the F2 generation with dextral coiling. The dd offspring will again have dextral coiling due to the maternal effect of the dominant gene ‘D’. Then in the F3 generation, three dextral and one Sinistral offspring will produce having a phenotype of 3:1.
shell coiling in snail

Plastid Inheritance in Mirabilis jalpa

Mirabilis jalpa is also known as 4- O’ Clock plant act as a model organism for the study of cytoplasmic inheritance. In 1900, the scientist Carl Correns first study the Plastid inheritance in Mirabilis jalpa. In his experiment, he found that there are three types of branches in Mirabilis jalpa.

The branches are of green colour, pale green colour and variegated colour. As the female carries the cytoplasmic material, therefore if a female branch is green in colour will produce green leaves only no matter what is the colour of the male branch. Similarly, if a female branch is green in colour then it will produce pale green leaves. The variegated branch of a different colour will produce different coloured leaves. The green colour is due to the presence of “Chloroplast”. The pale green colour is due to the presence of “Leucoplast”. The variegated colour is due to the presence of all three pigments namely chloroplast, leucoplast and chromoplast.

Let us understand this concept by taking an equation. In the first circle, there is a female individual of all three types. A second circle represents the male individual of all three types. In the third circle, there is a progeny (an outcome of female and male branch crossing over).

plastid inheritance in mirabilis jalpa

If the female branch is green then all the progenies will be green in colour. If the female branch is pale green then, all the progenies will be pale green and if the female branch is variegated all the progenies will be variegated.

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