L'eterosi nelle piante: dall'ipotesi genetica di Jones all'era genomica. Parte III: Analisi dell'eterosi a livello molecolare.

With the advent of the genomic era and biotechnology, the technical tools to establish the molecular basis of heterosis are at hand. Some important data are emerging from the fog. Studies at the genome, transcriptome and proteome levels in model species have led to unexpected results. Although allelic sequences can vary extensively in a given genome, it was generally assumed that each gene in one individual should have an allelic counterpart in another individual of the same species. Violation of genetic microcolinearity was reported in maize and potentially related both to the classification of heterotic groups and to the manifestation of hybrid vigor in this species. Moreover, allelic expression variation of fundamental genes has recently been uncovered in mouse and maize hybrids. Most of the investigated genes showed differences at the messenger level, ranging from unequal expression of the two alleles (biallelic) to expression of a single allele (monoallelic). Changes in mRNA expression levels and allele-specific transcript ratios were attributed mainly to differences in noncoding DNA sequences (i.e. cis- and trans-acting elements). Epigenetic regulation and parental imprinting had minimal effects. One of the most important finding was that genetically improved modern hybrids of maize express both alleles at each locus, whereas less improved old hybrids frequently show monoallelic expression. Furthermore, the two alleles in the hybrids respond differently in plant tissues and to environmental conditions. The allele-specific expression and variation in different tissues in responding to stress suggest an unequivalent function of the parental alleles in the hybrids. Additional studies on gene dosage effects in either diploids or polyploids revealed that the expression of many genes, in terms of transcripts and proteins, does not exhibit the midparent value expected in case of additive gene action. Overall results indicated that allele dosage effects can play important roles in determining phenotypic diversity and have also impacts on hybrid vigor. Since housekeeping genes that encode metabolic functions usually show a greater degree of dominant/recessive behaviour between allelic alternatives and are believed less influenced by dosage effects, it has been argued that dosage effects are a reflection of the dosage dependence of most regulatory genes. Following this train of thought, researchers are led to the idea that heterosis might be the result of different alleles being present at loci that contribute hierarchically to the regulatory networks controlling quantitative traits. Real-Time PCR and DNA microarray analyses will shed light in this fascinating area and might be able to provide some answers about the spectrum of genes that show regulated changes of expression patterns in hybrids. An eventual molecular explanation of heterosis will determine whether it can be manipulated in crop plants for the benefit of agriculture.