Sull'eterosi nelle piante: dall'ipotesi genetica di Jones all'era genomica.

Heterosis, or hybrid vigor, refers to the phenomenon that an F1 progeny obtained by crossing genetically divergent inbred or pure lines exhibit greater biomass, speed of development, and fertility than the two homozygous parents. This biological phenomenon has been exploited extensively for the constitution of crop varieties and it has also been a powerful force in the evolution of plant populations. Perhaps, heterosis is the greatest phenomenon in nature that can be exploited without understanding it. There have been two explanations of heterosis beginning with East and Shull in 1908: both believed that “different germplasms produce a developmental stimulus that increases with diversity of the uniting gametes”. This is now called the overdominance hypothesis for which the heterozygote has an advantage. It refers to the idea that allelic interactions occur in the hybrid so that the heterozygous class performs better than either homozygous class. Alternatively, heterosis can be produced by the masking of recessive alleles in each parental line by dominant, or nearly dominant alleles from the other parental line. This is the dominance hypothesis that was developed by Jones in 1917 formulating a model of linked dominant alleles on chromosome blocks. In this case heterosis results from the complementation in the hybrid of different deleterious alleles that were present in a parental genotype by superior alleles from the opposite parental genotype. The genetic basis of heterosis has been discussed for nearly a century, but little consensus has emerged. In maize, the species most studied, experimental evidence suggests that the genetic basis of heterosis is partial to complete dominance. Overdominance has long been discussed as the genetic basis of heterosis. However, many data supporting overdominance presumably resulted from pseudo-overdominance, arising from dominant alleles linked in repulsion phase. Epistasis, particularly between associated loci, may also be an explanation for heterosis. No data exclude the possibility of all three mechanisms contributing to heterosis, albeit in different proportions.Heterosis, or hybrid vigor, refers to the phenomenon that an F1 progeny obtained by crossing genetically divergent inbred or pure lines exhibit greater biomass, speed of development, and fertility than the two homozygous parents. This biological phenomenon has been exploited extensively for the constitution of crop varieties and it has also been a powerful force in the evolution of plant populations. Perhaps, heterosis is the greatest phenomenon in nature that can be exploited without understanding it. There have been two explanations of heterosis beginning with East and Shull in 1908: both believed that “different germplasms produce a developmental stimulus that increases with diversity of the uniting gametes”. This is now called the overdominance hypothesis for which the heterozygote has an advantage. It refers to the idea that allelic interactions occur in the hybrid so that the heterozygous class performs better than either homozygous class. Alternatively, heterosis can be produced by the masking of recessive alleles in each parental line by dominant, or nearly dominant alleles from the other parental line. This is the dominace hypothesis that was developed by Jones in 1917 formulating a model of linked dominant alleles on chromosomes blocks. In this case heterosis results from the complementation in the hybrid of different deleterious alleles that were present in a parental genotype by superior alleles from the opposite parental genotype. The genetic basis of heterosis has been discussed for nearly a century, but little consensus has emerged. In maize, the species most studied, experimental evidence suggests that the genetic basis of heterosis is partial to complete dominance. Overdominance has long been discussed as the genetic basis of heterosis. However, many data supporting overdominance presumably resulted from pseudo-overdominance, arising from dominant alleles linked in repulsion phase. Epistasis, particularly between associated loci, may also be an explanation for heterosis. No data exclude the possibility of all three mechanisms contributing to heterosis, albeit in different proportions. With the advent of the genomic era, the technical tools to establish the molecular basis of heterosis are at hand.