Abstract
BackgroundCoccinia grandis (ivy gourd), is a dioecious member of Cucurbitaceae having heteromorphic sex chromosomes. Chromosome constitution of male and female plants of C. grandis is 22A + XY and 22A + XX respectively. Earlier we showed that a unique gynomonoecious form of C. grandis (22A + XX) also exists in nature bearing morphologically hermaphrodite flowers (GyM-H). Additionally, application of silver nitrate (AgNO3) on female plants induces stamen development leading to the formation of morphologically hermaphrodite flowers (Ag-H) despite the absence of Y-chromosome. Due to the unavailability of genome sequence and the slow pace at which sex-linked genes are identified, sex expression and modification in C. grandis are not well understood.ResultsWe have carried out a comprehensive RNA-Seq study from early-staged male, female, GyM-H, and Ag-H as well as middle-staged male and GyM-H flower buds. A de novo transcriptome was assembled using Trinity and annotated by BLAST2GO and Trinotate pipelines. The assembled transcriptome consisted of 467,233 ‘Trinity Transcripts’ clustering into 378,860 ‘Trinity Genes’. Female_Early_vs_Male_Early, Ag_Early_vs_Female_Early, and GyM-H_Middle_vs_Male_Middle comparisons exhibited 35,694, 3574, and 14,954 differentially expressed transcripts respectively. Further, qRT-PCR analysis of selected candidate genes validated digital gene expression profiling results. Interestingly, ethylene response-related genes were found to be upregulated in female buds compared to male buds. Also, we observed that AgNO3 treatment suppressed ethylene responses in Ag-H flowers by downregulation of ethylene-responsive transcription factors leading to stamen development. Further, GO terms related to stamen development were enriched in early-staged male, GyM-H, and Ag-H buds compared to female buds supporting the fact that stamen growth gets arrested in female flowers.ConclusionsSuppression of ethylene responses in both male and Ag-H compared to female buds suggests 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. Overall, this study would enable identification of new sex-biased genes for further investigation of stamen arrest, pollen fertility, and AgNO3-mediated sex modification.
Highlights
Coccinia grandis, is a dioecious member of Cucurbitaceae having heteromorphic sex chromosomes
In the context of anther development, we identified several GO terms (GO:0080110, GO:0010208, GO:0010584, GO:0009555, GO:0055046, GO:0048658, GO:0048653) differentially enriched in male buds compared to female buds at an early stage of floral development. qRT-PCR was done to validate the results of differential expression analysis for a few interesting Coccinia homologs of AMS (ABORTED MICROSPORES), CER3 (ECERIFERUM 3), DEX1 (DEFECTIVE IN EXINE FORMATION 1), DYT1 (DYSFUNCTIONAL TAPETUM 1), EIL1 (ETHYLENE INSENSITIVE 3-like 1), EXCESS MICROSPOROCYTES 1 (EMS1) (EXCESS MICROSPORO CYTES 1), FER (FERONIA), MMD1 (MALE MEIOCYTE DEATH 1), MS1 (MALE STERILITY 1), SHT (Spermidine hydroxycinnamoyl transferase), TPD1 (TAPETUM DETERMINANT 1) and ZAT3 (Zinc finger protein ZAT3)
De novo-assembled transcriptome developed from RNA-Seq of different sexual phenotypes has enabled identification of C. grandis homologs of many genes known to be involved in flower development in species such as Arabidopsis, melon, and cucumber
Summary
Coccinia grandis (ivy gourd), is a dioecious member of Cucurbitaceae having heteromorphic sex chromosomes. Application of silver nitrate (AgNO3) on female plants induces stamen development leading to the formation of morphologically hermaphrodite flowers (AgH) despite the absence of Y-chromosome. Coccinia grandis bears male and female unisexual flowers on separate plants. One of the ways is when both male and female sex organ primordia are initiated at early stages of flower development, but at later stages the opposite sex organs are aborted as in Silene latifolia [15]. Coccinia grandis shows sex modification upon application of AgNO3 leading to the development of stamens in female flower (such flower will be referred to as Ag-H) as described in our previous report [14]. Application of silver compounds such as silver nitrate (AgNO3) or silver thiosulphate (Ag2S2O3)
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