The Female-Specific Cs-ACS1G Gene of Cucumber. A Case of Gene Duplication and Recombination between the Non-Sex-Specific 1-Aminocyclopropane-1-Carboxylate Synthase Gene and a Branched-Chain Amino Acid Transaminase Gene

Cucumber ( Cucumis sativus L.) is a monoecious plant in which female sex expression (gynoecy) is controlled by the Female ( F ) locus that can be modified by other sex-determining genes as well as by environmental and hormonal factors. As in many other cucurbits, ethylene is the major plant hormone regulating female sex expression. Previously we isolated the Cs-ACS1 (ACS, 1-aminocyclopropane-1-carboxylate synthase) gene that encodes the rate-limiting enzyme in the ethylene biosynthetic pathway. We proposed that Cs-ACS1 is present in a single copy in monoecious ( ffMM ) plants whereas gynoecious plants ( FFMM ) contain an additional copy Cs-ACS1G that was mapped to the F locus. To study the origin of Cs-ACS1G , we cloned and analyzed both the gynoecious-specific Cs-ACS1G gene and the non-sex-specific Cs-ACS1 gene. Our results indicate that Cs-ACS1G is the result of a relatively recent gene duplication and recombination, between Cs-ACS1 and a branched-chain amino acid transaminase ( BCAT ) gene. Taking into consideration that the Cs-ACS1G gene was mapped to the F locus, we propose that this duplication event gave rise to the F locus and to gynoecious cucumber plants. Computer analysis of the 1 kb region upstream of the transcription initiation site revealed several putative cis -acting regulatory elements that can potentially confer the responsiveness of Cs-ACS1G to developmental and hormonal factors and thereby control female sex determination in cucumber. These findings lead us to a model explaining the action of Cs-ACS1 and Cs-ACS1G in cucumber floral sex determination.