Cytoplasmic Inheritance and Genes | Botany Optional for UPSC PDF Download

Cytoplasmic Inheritance

• Cytoplasmic inheritance, also known as maternal inheritance or maternal effect, is a phenomenon related to the transmission of genetic traits through cellular structures found in the cytoplasm. These structures include autonomous organelles like mitochondria and plastids, which possess their own genetic material. Mitochondria are present in both animal and plant cells, while plastids are exclusive to plant cells. In the traditional understanding of inheritance, genes carried by DNA and RNA are responsible for passing on an individual's genetic traits from one generation to the next.
• However, the concept of cytoplasmic inheritance introduces an additional layer to the inheritance process. In cytoplasmic inheritance, both the genes located in the cytoplasm and the cytoplasmic particles themselves play a role in the inheritance of traits. Researchers have extensively studied cytoplasmic inheritance in various plants and animals, revealing that elements outside the cell nucleus, specifically cytoplasmic components, can influence the process of inheritance.
• In this form of inheritance, the female gamete, which contributes the cytoplasm, plays a crucial role. It carries both nuclear and cytoplasmic material, while male gametes do not carry cytoplasmic material. This characteristic is why cytoplasmic inheritance is often referred to as maternal effect, as the phenotype or observable characteristics of the offspring are determined primarily by the mother or female that provides the cytoplasmic material.
• Following cross-fertilization, a zygote is formed, and the cytoplasm within the zygote is predominantly derived from the female gamete. This highlights the significant role of the female in cytoplasmic inheritance. The concept of cytoplasmic inheritance encompasses the inheritance of traits mediated by genetic material located outside the cell nucleus.

• In summary, cytoplasmic inheritance involves the transmission of genetic traits through cytoplasmic elements, with the female gamete playing a central role, and it has been studied extensively in various organisms.

Definition of Cytoplasmic Inheritance

Cytoplasmic inheritance is a type of genetic inheritance in which genetic traits are transmitted to the next generation through the utilization of cytoplasmic elements in organisms, including animals and plants. The cytoplasm contains autonomous organelles such as mitochondria and plastids, which possess their own DNA.

Key Features of Cytoplasmic Inheritance

• Transmission through Plasmagenes: Cytoplasmic inheritance involves the passage of genetic information between different organisms through entities known as plasmagenes.
• Localization in the Cytoplasm: Plasmagenes are located within the cytoplasm or exist outside the cell nucleus.
• Self-Replication: Plasmagenes have the ability to self-replicate, similar to chromosomal genes (DNA or RNA).
• Maternal Inheritance: Offspring inherit cytoplasmic elements exclusively from the female gamete, not from the male. As a result, only the female gamete is responsible for transferring plasmagenes to the offspring. This characteristic gives rise to the term "maternal inheritance" for cytoplasmic inheritance.
• Mutation Potential: Plasmagenes can undergo mutations.
• Reciprocal Crossing Over: Cytoplasmic inheritance can be investigated through reciprocal crossing-over experiments.
• Non-Uniform Outcomes: Reciprocal crossing-over experiments do not yield identical results, indicating variability in the inheritance patterns associated with cytoplasmic elements.

Examples of Cytoplasmic Inheritance

Several examples can provide a more detailed understanding of the concept of cytoplasmic inheritance. Experiments involving factors such as kappa particles, carbon dioxide sensitivity, and shell coiling have demonstrated the existence of maternal or cytoplasmic inheritance.

Kappa Particles Inheritance in Paramecium

• In 1938, scientist Sonneborn conducted a study on Paramecium aurelia, focusing on kappa particle inheritance. Paramecium aurelia exhibited two distinct strains: the killer strain and the sensitive strain. The killer strain carried a combination of a dominant gene and kappa particles, which produced a toxin called paramycin. The sensitive strain, on the other hand, possessed either the dominant gene or kappa particles.
• To grasp the concept of kappa particle inheritance, consider the crossing of a pure killer strain with a pure sensitive strain. The pure killer strain contained both dominant genes (KK) and the Paramycin protein factor, while the pure sensitive strain only had the recessive gene.
• During the process of crossing over, conjugation occurred between the KK gene of the killer strain and the kk gene of the sensitive strain. If the duration of conjugation was less than 3 minutes, only nuclear exchange took place. Consequently, one dominant gene and one recessive gene exchanged between the killer and sensitive strains during crossing over. This resulted in the production of two offspring known as ex-conjugants, with one being a killer strain (Kk) and the other a sensitive strain (Kk). The ex-conjugant with the Kk genotype behaved as a killer strain due to the presence of the dominant gene 'K' and kappa particles.

• Now, consider the concept of nucleoplasmic inheritance in the same context. If the duration of conjugation between the KK and kk genes extended beyond 3 minutes, nuclear exchange occurred alongside cytoplasmic exchange. During crossing over, one dominant and one recessive gene exchanged along with the exchange of cytoplasmic material, leading to nucleoplasmic inheritance. This process resulted in the production of two offspring called ex-conjugants, both of which exhibited the killer strain phenotype.

• In summary, the study explored kappa particle inheritance in Paramecium aurelia and highlighted the outcomes of crossing over, including the potential for nucleoplasmic inheritance under specific conditions of conjugation duration.

Carbon Dioxide Sensitivity in Drosophila

• In 1958, researchers Heritier and Teissier conducted a study on carbon dioxide sensitivity in Drosophila, specifically Drosophila melanogaster. They observed that this species exhibited a high sensitivity to carbon dioxide, which could function as a paralyzing or anesthetic agent.
• Heritier and Teissier conducted experiments on Drosophila melanogaster using a breeding strain consisting of both sensitive and normal strains. In their experiments, they crossed the sensitive strain with the normal strain.
• Following their experiments, Heritier and Teissier reached the conclusion that offspring would inherit carbon dioxide sensitivity primarily from their sensitive mother, indicating a pattern of cytoplasmic inheritance. Interestingly, there were instances where sensitive males produced sensitive offspring when crossed with normal female Drosophila. However, it was noted that this sensitivity inherited from the male Drosophila only persisted for the first generation.
• Heritier and Teissier delved further into the sensitivity factor and identified virus-like particles present in the cells of sensitive flies. They named these particles "sigma particles." Based on their findings, Heritier and Teissier attributed the sensitivity observed in Drosophila to the presence of sigma particles. Sigma particles were found in various strains of Drosophila and were characterized by their small size, measuring 0.07 microns in diameter. Importantly, sigma particles were heritable due to the presence of DNA.
• Notable examples of Drosophila strains exhibiting carbon dioxide sensitivity include D. melanogaster, D. Algonquin, D. pseudoobscura, D. robusta, and D. affinis, among others. These findings shed light on the genetic basis and inheritance patterns of carbon dioxide sensitivity in Drosophila species.

Shell Coiling in Limnaea

In 1920, Arthur Boycott conducted pioneering research on the maternal effect of shell coiling in snails. He specifically focused on studying this phenomenon in Limnaea peregra, a species of snail that exhibits two distinct types of shell coiling:

• Dextral coiling: This type of coiling results in a right-handed orientation of the snail's shell.
• Sinistral coiling: This type of coiling leads to a left-handed orientation of the snail's shell.

The inheritance of shell coiling in Limnaea peregra is governed by specific genetic factors. For dextral coiling, a dominant gene labeled as 'D' is responsible. Snails carrying either two copies of the dominant gene (DD) or one dominant and one recessive gene (Dd) will exhibit dextral coiling. On the other hand, sinistral coiling is determined by a recessive gene denoted as 'd.' Snails with two copies of the recessive gene (dd) will display sinistral coiling.

Crucially, the orientation of shell coiling in snails is solely determined by the genetic makeup of the mother. This phenomenon underscores the concept of maternal effect, where the genotype of the mother directly influences the phenotype of the offspring in terms of shell coiling orientation.

To illustrate the concept further, one can examine the outcomes of crossing over involving these genetic factors.

Crossing over between DD female and dd male

Crossing a pure dextral female (DD) with a sinistral male (dd) in Limnaea peregra results in a zygote (Dd) in the F1 generation, leading to dextral coiling. Subsequent intercrossing of Dd individuals in the F2 generation yields a combination of DD, Dd, Dd, and dd genotypes, all of which display dextral coiling. This persistent dextral coiling in the presence of the recessive 'd' allele is attributed to the maternal effect, specifically the presence of the dominant 'D' gene. In the F3 generation, snails with the dd genotype also exhibit dextral coiling due to the maternal effect of the dominant 'D' gene, resulting in a phenotypic ratio of 3:1 with three dextral-coiled snails and one sinistral-coiled snail. 

Crossing over between dd female and DD male

When a pure sinistral female (dd) is crossed with a dextral male (DD) in Limnaea peregra, a zygote (Dd) is generated in the F1 generation, resulting in dextral coiling. Subsequent intercrossing of Dd individuals in the F2 generation leads to the presence of DD, Dd, Dd, and dd genotypes, all of which exhibit dextral coiling. The dd offspring in the F2 generation also display dextral coiling, attributed to the maternal effect of the dominant 'D' gene. Consequently, the F3 generation consists of three dextral-coiled snails and one sinistral-coiled snail, resulting in a phenotypic ratio of 3:1. 

Plastid Inheritance in Mirabilis jalpa

Plastid inheritance in Mirabilis jalpa, commonly known as the 4-O'Clock plant, serves as a valuable model organism for the study of cytoplasmic inheritance. In 1900, scientist Carl Correns conducted pioneering research on plastid inheritance within this species. His experiments revealed the existence of three distinct strains of Mirabilis jalpa, characterized by their leaf colors: green, pale green, and variegated.

It's essential to grasp the role of various types of plastids in these strains:

• Green leaves are the result of chloroplast presence.
• Pale green leaves contain leucoplasts.
• Variegated leaves possess a combination of chloroplasts, leucoplasts, and chromoplasts, resulting in their multi-colored appearance.

In Mirabilis jalpa, the inheritance of leaf color is primarily determined by the female branch. When a female branch is green, it consistently produces green leaves in its progeny, regardless of the male branch's color. Similarly, a female branch with pale green leaves will yield offspring with pale green leaves. In the case of a variegated female branch, its progeny will exhibit a variety of leaf colors, reflecting the presence of multiple plastid types.

This concept is illustrated through the analogy of three circles: the first circle (yellow-colored) represents a female individual with the potential for green (G), pale green (P), or variegated (V) plastid inheritance. The second circle (cream-colored) represents male individuals of all three types. Finally, the third circle (blue-colored) symbolizes the diverse offspring resulting from crosses between female and male branches. In summary, the leaf color inheritance in Mirabilis jalpa is predominantly determined by the maternal branch's plastid type, with specific plastids responsible for distinct leaf colors in the progeny.