Discuss the physiological bases of heterosis.

SouravSeptember 13, 2024

Answer

Physiological Bases of Heterosis

Heterosis, or hybrid vigor, refers to the phenomenon where hybrid offspring exhibit superior traits compared to their parent lines. The physiological bases of heterosis are complex and involve several mechanisms. Here’s a detailed discussion of the primary physiological bases:

1. Increased Cell Size and Number
   • Embryo and Seed Size: One of the early hypotheses suggested that heterosis resulted from larger embryo and seed sizes in hybrids compared to inbred parents. Larger embryos and seeds often lead to enhanced early growth and development.
   • Growth Rate: Hybrids tend to show faster growth rates during the seedling stage due to increased cell size and number. This increase in growth can be attributed to more robust and efficient cellular machinery.
2. Improved Metabolic Efficiency
   • Enzyme Systems: Whaley (1952) proposed that hybrid embryos have more efficient enzyme systems that mobilize stored food materials more effectively. This metabolic efficiency can result in faster growth and greater vigor during early development.
   • Growth Regulators: Hybrids often exhibit enhanced production of growth regulators, which can positively influence various physiological processes, including cell division and elongation.
3. Genetic Interactions
   • Reduced Amount of a Single Gene Product: In some hybrids, an intermediate level of a gene product may result in increased vigor. For example, in Neurospora crassa, the heterozygous pab+ pab produces intermediate amounts of p-aminobenzoic acid, which promotes faster growth compared to the homozygotes.
   • Separate Gene Products: Different alleles may produce proteins with distinct functions. The presence of both protein types in hybrids can enhance adaptability to environmental changes. For instance, in humans, the heterozygous state for sickle cell anemia provides resistance to malaria due to the combined effects of normal and defective hemoglobin.
4. Hybrid Enzyme Products
   • Combined Gene Products: Many enzymes are composed of multiple subunits from different alleles. Hybrids may produce hybrid enzyme molecules with unique properties compared to those produced by homozygotes. For example, some hybrid enzymes may be less sensitive to heat or other environmental stresses.
   • Electrophoretic Patterns: Hybrids often exhibit novel enzyme bands on electrophoretic gels, indicating the presence of new enzyme forms with potentially advantageous properties.
5. Effects in Different Tissues
   • Enzyme Activity in Multiple Tissues: Some hybrids show superior performance due to the expression of enzymes in multiple tissues at intermediate levels. For example, an enzyme that shows different levels of activity in pollen and other tissues may result in enhanced overall performance in hybrids.