Enumerate the different causes of heterosis.

Genetic Basis of Heterosis

The genetic explanations for heterosis are centered around the interaction of alleles at multiple loci. Several theories attempt to explain this interaction:

1. Dominance Hypothesis

This is the oldest and most widely accepted theory. It proposes that heterosis arises from the masking of deleterious recessive alleles in the parents by dominant alleles in the F1 hybrid. The F1 hybrid, being heterozygous at many loci, expresses the favorable dominant alleles, leading to superior performance. This assumes that the parents carry different deleterious recessive alleles, and the hybrid benefits from having at least one dominant allele at each locus.

2. Overdominance Hypothesis

This hypothesis suggests that heterozygotes are superior to both homozygotes at certain loci. The interaction between different alleles at a single locus results in a phenotype that is better than either homozygous state. This is often observed in loci controlling crucial physiological processes. It implies that the heterozygote possesses a unique advantage not present in either parent.

3. Epistasis Hypothesis

Epistasis refers to the interaction between genes at different loci. Heterosis can arise when the interaction between alleles at different loci in the hybrid is more favorable than the interactions in either parent. This can involve masking effects, complementary gene action, or other complex interactions. The combined effect of these interactions leads to improved performance.

4. Cumulative Distribution Hypothesis

This theory suggests that heterosis is a result of the accumulation of favorable dominant alleles from both parents. The more favorable alleles the hybrid inherits, the greater the heterosis effect. This is particularly relevant when dealing with traits controlled by many genes (polygenic traits).

Physiological Basis of Heterosis

Beyond the genetic level, physiological factors contribute significantly to heterosis:

1. Increased Photosynthetic Efficiency

Hybrid plants often exhibit higher rates of photosynthesis due to improved leaf anatomy, chlorophyll content, and enzyme activity. This leads to increased biomass production and yield.

2. Enhanced Nutrient Uptake and Utilization

Hybrids may have more efficient root systems, allowing them to absorb more nutrients from the soil. They also exhibit improved nutrient translocation and utilization, leading to better growth and development.

3. Improved Water Use Efficiency

Hybrid plants can exhibit better water use efficiency, allowing them to thrive in water-limited environments. This is often linked to improved stomatal regulation and root architecture.

4. Altered Hormone Balance

Heterosis can be associated with changes in hormone levels, such as auxins, gibberellins, and cytokinins, which regulate growth and development. These hormonal changes can contribute to increased vigor and yield.

Biochemical Basis of Heterosis

Biochemical changes underpin the physiological improvements observed in hybrids:

1. Increased Enzyme Activity

Hybrids often exhibit higher levels of enzyme activity, particularly those involved in key metabolic pathways like photosynthesis and respiration. This leads to more efficient biochemical processes.

2. Altered Protein Profiles

The protein profiles of hybrid plants can differ from those of their parents, with increased levels of certain proteins that contribute to growth and development. This is a direct consequence of the altered gene expression patterns in the hybrid.

3. Enhanced Antioxidant Capacity

Hybrids may have a greater capacity to scavenge reactive oxygen species (ROS), protecting them from oxidative stress and improving their overall health and vigor.

Theory	Mechanism	Example
Dominance	Masking of deleterious recessive alleles	Maize hybrids with improved yield due to masking of recessive alleles affecting kernel size.
Overdominance	Heterozygote superior to both homozygotes	Certain loci controlling disease resistance in rice hybrids.
Epistasis	Favorable interaction between genes at different loci	Yield improvement in wheat hybrids due to epistatic interactions affecting grain number and weight.