L'eterosi nelle piante: dall'ipotesi genetica di Jones all'era genomica. Parte II: Blocchi cromosomici e basi genetiche dell'eterosi.

Chromosome blocks are the genomic units of genetic transmission in sexually reproducing plants. Breeders work with chromosome blocks, not individual genes in their selection programs. Thus, chromosome blocks support heterosis and affect estimates of gene action and interaction. Chromosome blocks vary in size according to the intensity of linkage and so the frequency of recombination, the number of sexual generations (i.e., the approach to linkage equilibrium) and the position of crossing-over sites. Even in the transfer of simple traits by backcross strategies, the amount of undesirable genetic material associated to the gene of interest is usually not known, situation often referred as linkage drug. D.F. Jones clearly recognized the role of chromosome blocks in 1917 when he proposed the dominance of linked factors as a means of accounting for heterosis. The proposition is elegant because it acknowledges the cumulative effect of linked dominant genes as transmission units. In the following years there was much debate about gene action, and heterosis was sometimes interpreted as true overdominance, that is single loci at which the heterozygous phenotype exceeds that of both homozygotes. Maize researchers were careful to point out that estimates of dominance variance exceeding that for straight dominance could be due to either overdominance or linkage disequilibrium of linked loci with favourable alleles in repulsion phase (pseudo-overdominance). Degrees of dominance in F2 populations in linkage disequilibrium was compared with populations in F8 through F16 in linkage equilibrium. Estimates for degree of dominance decreased with the approach to linkage equilibrium indicating that the initial heterosis was more likely due to linked dominant factors in linkage disequilibrium than to true overdominance. In autotetraploid alfalfa, E.T. Bingham reached the same conclusion from results indicating linked dominant factors in chromosome blocks, and not multiple gene interactions, as the basis for progressive heterosis. On the whole, genetic data so far collected indicate that superior dominant alleles at different loci complement each other by masking deleterious recessive alleles at the respective loci. The cumulative action of genes in chromosome blocks not only explains the breeding behaviour of diploid crops, but also explains the fixation of transgressive traits in self-pollinated allopolyploids and the relatively high levels of heterosis maintained in cross-pollinated autopolyploids. Thus, chromosome blocks provide an efficient model to explain heterosis and a unifying concept for all categories of plants.