The dosage may augment the effects of hybridity on gene expression and phenotypes. The effects of genome merger (hybridity) and doubling on growth vigor are testable in diploid hybrids (merger only) and allotetraploids (merger and doubling, “super” hybrids). The allotetraploids are genetically stable and tractable, which provides an excellent model system for mechanistic studies on polyploidy and hybrid vigor. We developed the Arabidopsis allotetraploids to study molecular mechanisms of hybrid vigor. A molecular model of heterosis suggests that expression alteration of regulatory genes modulate expression of housekeeping genes and output traits in hybrids and allopolyploids, leading to growth vigor. The inability to discern a molecular basis for hybrid vigor is partly because F1 hybrids segregate, making it difficult for molecular tests. The “dominance” model suggests complementation of slightly deleterious recessive alleles in the hybrids, whereas the “overdominance” model indicates that the genes have greater expression when they are heterozygous. Two models have been fascinatingly debated over a century. Although hybrid vigor has been commonly observed and widely utilized in plant and animal production, the mechanisms remain elusive. Hybrid vigor was also observed in embryo development and reproductive traits in mammals. Since the introduction of hybrids in 1920’s, the yield of maize has steadily increased sixfold. In 1910’s, heterosis was rediscovered in maize by George H. Heterosis is a widespread phenomenon that has intrigued many scientists including Charles Darwin (1876) who systematically characterized growth vigor in cross-pollinated and self-pollinated plants of many species. For example, hybrid plants like maize and polyploid plants like wheat grow larger and more vigorously than their parents. In general, heterosis refers to increase in stature, biomass, and fertility of a hybrid that is superior to one or both parents. Heterosis or commonly known as hybrid vigor is evolutionarily defined as that the heterozygotes have higher fitness in a population than the homozygotes.
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