THE RELATIONSHIP BETWEEN GENOME STRUCTURE AND HETEROSIS IN SUNFLOWER

Sunflower (Helianthus annuus) is an annual plant in the family Asteraceae, with a large inflorescence whose fruits are used mainly for oil and animal feed products. This plant is considered as one of the most important crops for human nutrition, so a high yield is an important goal never to be ignored. Heterosis, intended as the superior performance of hybrid progeny compared to their inbred parents, is a feature that has been widely exploited in plant breeding for many decades, but the molecular mechanisms underlying this phenomenon remain unclear. To date, most studies about molecular basis of heterosis have focused mainly on maize and other monocotyledons, suggesting that the superior performance of the hybrid over the mean of the inbred parents may result from the altered regulation of gene expression: differences between parental repetitive sequences should affect tissue specificity or temporal regulation of expression of genes, and such differences have been proposed to be the cause of heterotic complementation, whose effects are comparable to allelic interactions proposed by the overdominance theory. The aim of the present research was to study the molecular basis of heterosis in sunflower and verify if the model proposed for maize can be extended to this species too. To reach this aim we studied the repetitive component of the sunflower genome to identify sequences potentially variable among lines. Then, a number of genes were isolated and their allelic variation analysed to collect gene sequences and study the expression of single alleles in hybrids. Finally, the correlation between hybrid heterosis and genetic distance between parents (calculated on variations in intergenic repetitive sequences) was measured. The results of our work appear to confirm the hypothesis made for maize: retroelements are the main cause of variation between different sunflower inbreds, possibly affecting the transcription of neighboring genes and causing the heterotic complementation effect.