Investigating sex expression and modification in dioecious Coccinia grandis through an integrated transcriptomic and proteomic approach

Abstract

Majority of the angiosperm species are co-sexual. Only ~ 6% of angiosperm species are dioecious having separate male and female individuals. Coccinia grandis is one such dioecious species having heteromorphic sex chromosomes (Male: 22A+XY & Female: 22A+XX). C. grandis belongs to Cucurbitaceae, a family known for its diverse sexual systems. We have also identified a rare gynomonoecious form of C. grandis (22+XX) that bears pistillate (GyM-F) and hermaphrodite flowers (GyM-H). Interestingly, application of silver nitrate (AgNO3) on female plants induces stamen development leading to the formation of morphologically hermaphrodite flowers (Ag-H). A comprehensive morphological, histological and cytological characterization of flower buds has been carried out. Pollen viability assays and mating experiments showed that pollens from GyM-H and Ag-H flowers were sterile. We have also assembled a de novo transcriptome from RNA-Seq of flower buds of all sexes at early and middle stages of development followed by differential gene expression analyses to identify sex-biased genes. Simultaneously, our proteomic approach has resulted in detection of 3385 proteins and many of these were expressed in a sex-biased manner. Ethylene responses were suppressed in both male and Ag-H compared to female buds suggesting a probable role of ethylene in stamen suppression similar to monoecious cucurbits such as melon and cucumber. Also, pollen fertility associated GO-terms were depleted in middle-staged GyM-H buds compared to male buds indicating the necessity of Y-chromosome for pollen fertility. In absence of a genetic transformation tool, we have standardized an efficient ALSV-mediated virus induced gene silencing (VIGS) technique in C. grandis. Overall, this study has enabled the identification of new sex-biased genes for further investigation of stamen arrest, pollen fertility, and AgNO3-mediated sex modification in C. grandis. Additionally, the VIGS technique standardized in this study would be useful for the functional characterization of such sex-biased genes.