Cytoplasmic Inheritance | Agriculture Optional Notes for UPSC PDF Download

Introduction

Inheritance refers to the transfer of traits or features from parents to their descendants. While genes usually play a central role in defining an organism's characteristics, there are instances where the passage of traits from one generation to the next is determined by cytoplasmic genes. This type of inheritance is referred to as cytoplasmic inheritance, maternal inheritance, or extranuclear inheritance. Examples of cytoplasmic inheritance can be observed in everyday plants such as maize, the Mirabilis jalapa plant, cotton plants, and more. This form of inheritance is prevalent among eukaryotic organisms and is typically associated with cytoplasmic components like mitochondria or single-celled organisms like bacteria.

Cytoplasmic Inheritance Overview

Cytoplasmic inheritance, or organelle inheritance, involves the transmission of genetic material from the parent's organelle DNA to the offspring. Unlike nuclear genetics, cytoplasmic inheritance does not adhere to the principles of Mendelian gene inheritance, where offspring inherit half of their genes from each parent. Instead, the inheritance of organelle DNA can vary depending on the species, with genetic material from organelles coming solely from one parent, either male or female, or sometimes in unequal proportions from both parents.
This type of inheritance is controlled by genes located within organelles like chloroplasts and mitochondria.

Cytoplasmic Inheritance Types

Cytoplasmic inheritance has two main categories:

1. Plastid Inheritance

• Chloroplasts, a type of plastid, are central to this form of inheritance.
• This inheritance pattern is exclusive to plants because plastids are found only in plant cells.

Examples of plastid inheritance include:

a. Mirabilis Jalapa: 

• In the Mirabilis Jalapa plant, leaves can be green, white, or variegated. 
• Chloroplasts control the inheritance pattern. 
• Green leaves contain normal chloroplasts, white leaves have mutant chloroplasts, and variegated leaves have a mixture of both normal and mutant chloroplasts.

b. Oenothera:

• Oenothera exhibits a unique inheritance pattern where the entire chromosome set from either the male or female parent is transferred to the gametes.
• Gametes inherit all chromosomes from one parent.
• This phenomenon results from a series of reciprocal translocations.

c. Iojap in Maize:

• Plastids determine the soap phenotype in Iojap maize.
• In this case, green leaves possess normal plastids, white leaves contain mutant plastids, and striped leaves have a combination of both normal and mutant plastids. 
• When a cross occurs between a green male and an Iojap female, three different categories of plastids can be observed: white, green, or a mixture of both.

2. Mitochondrial inheritance

Mitochondrial inheritance is prevalent in both plants and animals.
Some examples include:

a. Cytoplasmic Male Sterility in Plants:

• Male sterility can occur in crop plants through three types: cytoplasmic, genetic, and cytoplasmic.
• In maize, cytoplasmic sterility is determined by mitochondrial DNA.
• Cytoplasmic sterility has been identified in various other crop plants like cotton, sorghum, and pearl millet.

b. Pokiness in Neurospora:

• Neurospora, a type of bread mold, exhibits two strains: the wild strain and the poky strain.
• The wild strain of Neurospora displays normal growth, while the mutant or poky strain exhibits significantly slower growth.
• These differences suggest the presence of cytoplasmic inheritance, as evidenced by the variations in reciprocal crosses.
• In Neurospora, females contribute the majority of the cytoplasm.

c. Petite in Yeast:

• Petite yeast, also known as "little yeast," belongs to the Ascomycetes group.
• Petite yeast is characterized by its notably small size and represents a mutant form, distinct from the normal conditions.
• Cytoplasmic factors are responsible for controlling this mutation.

Characteristics

• Reciprocal Differences: Cytoplasmic inheritance is responsible for distinct characteristics observed in reciprocal crosses in F₁.
• Maternal Effects: The greater contribution of cytoplasm by the female parent to the zygote leads to distinct maternal effects.
• Mappability: Mapping chloroplast genes and mitochondrial genes, which were historically challenging, has become feasible. Examples include chloroplast genes in Chlamydomonas and maize and mitochondrial genes in humans and yeast.
• Non-Mendelian Segregation: Cytoplasmic inheritance deviates from the typical Mendelian segregation pattern.
• Somatic Segregation: Characteristics controlled by cytoplasmic genes display segregation in somatic tissues, as seen in phenomena like leaf variegation.
• Governed by Plasmagenes: Cytoplasmic inheritance is managed by genes present in chloroplast or mitochondrial DNA.

Significance

• Cytoplasmic inheritance contributes to understanding the role of cytoplasmic organelles in the transmission of traits across various organisms.
• It has facilitated the mapping of the chloroplast genome and mitochondrial genome in numerous species, including humans, yeasts, and maize.
• Cytoplasmic male sterility has been harnessed in crops like maize, cotton, and pearl millet, enabling the production of hybrid seeds.
• Mitochondria's significance is growing in the context of heterosis, which leads to robust plant growth and increased crop yields.
• Mutations in chloroplast and mitochondrial DNA generate new variations in characteristics, and these mutations are observed in cytoplasmic inheritance.

Important features of Cytoplasmic DNA and Nuclear DNA

• Cytoplasmic DNA exists in two forms: Mitochondrial DNA (mt-DNA) and Chloroplast DNA (cp-DNA).
• Cytoplasmic DNA is typically circular, although there is an exception with ciliate protozoa where it is linear. In contrast, nuclear DNA is linear in eukaryotes and circular in prokaryotes.
• While the synthesis of chloroplast DNA and mitochondrial DNA is continuous throughout the cell cycle, nuclear DNA synthesis primarily occurs during the interphase (S phase) of the cell cycle.
• Cytoplasmic DNA replicates in a semiconservative manner within chloroplasts and mitochondria, while nuclear DNA replicates similarly in the chromosomes.
• Mutations in cytoplasmic DNA result in alterations to plasma genes, whereas mutations in nuclear DNA lead to changes in nuclear genes.
• Cytoplasmic DNA is capable of encoding RNA and proteins, similar to nuclear DNA.
• In vitro synthesis of both cytoplasmic DNA and nuclear DNA is achievable within their respective organelles.
• The synthesis of cytoplasmic DNA and nuclear DNA can be impeded when exposed to substances like ethidium bromide, acriflavine, and actinomycin-D.

Conclusion

This inheritance can occur exclusively from either the maternal or paternal parent or a combination of both. We have examined several examples of cytoplasmic inheritance, including Iojap in maize, pokiness in Neurospora, and Oenothera. There are two primary types of cytoplasmic inheritance: plastid and mitochondrial inheritance. This phenomenon plays a significant role in understanding the transmission of traits through various cytoplasmic organelles and has practical applications, such as the production of hybrid seeds, among other uses.