Abstract
Understanding the evolution of sex determination in plants requires the cloning and the characterization of sex determination genes. Monoecy is characterized by the presence of both male and female flowers on the same plant. Andromonoecy is characterized by plants carrying both male and bisexual flowers. In watermelon, the transition between these two sexual forms is controlled by the identity of the alleles at the A locus. We previously showed, in two Cucumis species, melon and cucumber, that the transition from monoecy to andromonoecy results from mutations in 1-aminocyclopropane-1-carboxylic acid synthase (ACS) gene, ACS-7/ACS2. To test whether the ACS-7/ACS2 function is conserved in cucurbits, we cloned and characterized ClACS7 in watermelon. We demonstrated co-segregation of ClACS7, the homolog of CmACS-7/CsACS2, with the A locus. Sequence analysis of ClACS7 in watermelon accessions identified three ClACS7 isoforms, two in andromonoecious and one in monoecious lines. To determine whether the andromonoecious phenotype is due to a loss of ACS enzymatic activity, we expressed and assayed the activity of the three protein isoforms. Like in melon and cucumber, the isoforms from the andromonoecious lines showed reduced to no enzymatic activity and the isoform from the monoecious line was active. Consistent with this, the mutations leading andromonoecy were clustered in the active site of the enzyme. Based on this, we concluded that active ClACS7 enzyme leads to the development of female flowers in monoecious lines, whereas a reduction of enzymatic activity yields hermaphrodite flowers. ClACS7, like CmACS-7/CsACS2 in melon and cucumber, is highly expressed in carpel primordia of buds determined to develop carpels and not in male flowers. Based on this finding and previous investigations, we concluded that the monoecy gene, ACS7, likely predated the separation of the Cucumis and Citrullus genera.
Highlights
The majority of angiosperms are hermaphroditic producing exclusively bisexual flowers
To test whether the genetic determinant controlling monoecy to andromonoecy sexual transition is conserved between the Citrullus and Cucumis genus and whether the watermelon ortholog of CmACS-7/CsACS2 could be encoded by the watermelon A gene, we isolated all the acid synthase (ACS) genes from melon, cucumber and watermelon
We have demonstrated that the sexual transition from monoecy to andromonoecy, in melon and cucumber, is controlled by the common gene CmACS-7/CsACS2 [9,10]
Summary
The majority of angiosperms are hermaphroditic producing exclusively bisexual flowers. There has been concerted effort in the last few decades to pinpoint the master genes controlling sex determination and to understand the evolutionary forces that drive transitions between sexual morphs [2,3]. In these investigations, several plant families have emerged as model systems [4,5,6,7]. Throughout evolution of Cucurbitaceae, there have been numerous shifts between monoecy and other sexual morphs, which makes this plant family a practical model to investigate the molecular mechanisms controlling sex determination
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