Define inbreeding depression and hybrid vigour. How do these phenomena atfect the self. and cross-pollinated species? Explain with the help of suitable examples.

SouravSeptember 13, 2024

Answer

Definitions

• Inbreeding Depression: The reduction in biological fitness and vigor observed in offspring resulting from mating between closely related individuals. This decline in fitness often manifests as decreased fertility, lower survival rates, and overall reduced performance due to the increased expression of deleterious recessive alleles and reduced genetic diversity.
• Hybrid Vigor (Heterosis): The increased vigor, growth, or yield observed in hybrid offspring compared to their parent strains. This enhancement results from the combination of diverse genetic backgrounds, which can lead to improved overall performance, such as better disease resistance, higher yield, and greater adaptability.

Effects on Self-Pollinated and Cross-Pollinated Species

1. Self-Pollinated Species

• Inbreeding Depression: In self-pollinated species, inbreeding depression is more pronounced due to the continuous self-fertilization leading to homozygosity. Over generations, self-pollination can increase the likelihood of expressing harmful recessive traits, reducing the fitness of the plants. For instance, in self-pollinated crops like wheat and barley, persistent self-pollination can result in reduced grain size, lower yields, and increased susceptibility to diseases due to the accumulation of deleterious alleles.
• Hybrid Vigor: The application of hybrid vigor in self-pollinated species is more challenging. While hybrid varieties are developed, maintaining genetic diversity and producing hybrid seeds in large quantities can be difficult. However, some successful examples include hybrid rice, which has shown significant yield improvements compared to its inbred counterparts. The production of hybrid varieties in such crops involves creating hybrids through controlled cross-pollination before self-pollination resumes.

2. Cross-Pollinated Species

• Inbreeding Depression: Cross-pollinated species generally experience less inbreeding depression because they maintain genetic diversity through mating with unrelated individuals. However, if these species are subjected to inbreeding or self-pollination, such as in breeding programs or isolated populations, they can still suffer from reduced fitness. For example, inbreeding in maize (a cross-pollinated species) can lead to poorer growth and lower yields.
• Hybrid Vigor: Hybrid vigor is often prominently exploited in cross-pollinated species. Cross-pollination naturally introduces genetic diversity, and hybrid varieties of such species frequently exhibit enhanced growth, yield, and disease resistance. Maize is a classic example, where hybrid varieties have significantly outperformed inbred lines in terms of yield and adaptability. Similarly, hybrid varieties of sunflower and cotton show improved performance over their inbred counterparts.

Examples

• Self-Pollinated Species: In wheat, continuous self-pollination can result in inbreeding depression, which manifests as reduced seed size and lower yield. Hybrid varieties of rice, produced through controlled cross-pollination, demonstrate hybrid vigor with increased yield and resistance to stress.
• Cross-Pollinated Species: In maize, hybrid varieties show remarkable hybrid vigor, with increased growth and yield compared to inbred varieties. This phenomenon is utilized extensively in commercial maize production to maximize crop performance.