Reproduction-associated pathways in females of gibel carp (Carassius gibelio) shed light on the molecular mechanisms of the coexistence of asexual and sexual reproduction
Gibel carp (Carassius gibelio) is a cyprinid fish that originated in eastern Eurasia and is considered as invasive in European freshwater ecosystems. The populations of gibel carp in Europe are mostly composed of asexually reproducing triploid females (i.e., reproducing by gynogenesis) and sexually reproducing diploid females and males. Although some cases of coexisting sexual and asexual reproductive forms are known in vertebrates, the molecular mechanisms maintaining such coexistence are still in question. Both reproduction modes are supposed to exhibit evolutionary and ecological advantages and disadvantages. To better understand the coexistence of these two reproduction strategies, we performed transcriptome profile analysis of gonad tissues (ovaries), and studied the differentially expressed reproduction-associated genes in sexual and asexual females. We used high-throughput RNA sequencing to generate transcriptomic profiles of gonadal tissues of triploid asexual females and males, diploid sexual males and females of gibel carp, as well as diploid individuals from two closely-related species, C. auratus and Cyprinus carpio. Using SNP clustering, we showed the close similarity of C. gibelio and C. auratus with a basal position of C. carpio to both Carassius species. Using transcriptome profile analyses, we showed that many genes and pathways are involved in both gynogenetic and sexual reproduction in C. gibelio; however, we also found that 1500 genes, including 100 genes involved in cell cycle control, meiosis, oogenesis, embryogenesis, fertilization, steroid hormone signaling and biosynthesis were differently expressed in the ovaries of asexual and sexual females. We suggest that the overall downregulation of reproduction-associated pathways in asexual females, and their maintenance in sexual ones, allow for their stable coexistence, integrating the evolutionary and ecological advantages and disadvantages of the two reproductive forms. However, we showed that many sexual-reproduction-related genes are maintained and expressed in asexual females, suggesting that gynogenetic gibel carp retains the genetic toolkits for meiosis and sexual reproduction. These findings shed new light on the evolution of this asexual and sexual complex. Overall design: Here, the molecular mechanisms associated with reproduction in C. gibelio were analysed to study the coexistence of asexual and sexual forms. In particular, the expression of reproduction-related genes was expected to differ between asexual and sexual females, since meiosis-related genes are not important for asexually reproducing individuals. To test this hypothesis, transcriptome profile analyses of gonadal tissues (ovaries) from asexual females and sexual females of C. gibelio were performed. In addition, the transcriptomes of the closely-associated species C. carpio and C. auratus were also analysed, with a particular emphasis on the genes contributing to sexual reproduction.