The origin and mode of function of the Female locus in cucumber

Gynoecy in cucumber is controlled by the Female (F) locus that can be modified by other sex determining genes, environmental conditions and plant hormones. As in most of the Cucurbitaceae, the gaseous plant hormone ethylene induces female flower development and thus simulates the activity of the F locus. Monoecious (ff) cucumbers posses a single CsACS1 gene, while gynoecious plants posses an additional female-specific copy, denoted CsACS1G that was mapped to the cucumber Female (F) locus. To study the origin and function of the F locus we compared their genomic structure and investigated their expression pattern and the effect of reducing CsACS1/G transcript level in gynoecious plants. The CsACS1G and CsACS1 genes are identical twin genes except for the distal promoter region that is unique to each gene. The proximal promoter and the distal promoter of CsACS1G contain auxin response regulatory elements (AuxRE) while GA (gibberellic acid) response and repression elements are present in the proximal and distal promoters of both genes. Expression analyses indicated CsACS1/G is expressed in stamen primordia of gynoecious floral buds and that its expression is induced by auxin and repressed by GA. Gynoecious plants with reduced CsACS1/G transcript level present a monoecious phenotype. Altogether our data indicates that the female-specific CsACS1G, and thereby gynoecy in cucumber arose by gene duplication and chromosomal recombination and rearrangement. Furthermore our findings confirm that CsACS1G is indeed the F locus and that one of its activities is to repress stamen development in gynoecious floral buds.Cucurbitaceae 2008. IX EUCARPIA Meeting. Avignon (France), May 21-24th, 200