Transcriptome Sequencing and Comparative Analysis of Ovary and Testis Identifies Potential Key Sex-Related Genes and Pathways in Scallop Patinopecten yessoensis

Abstract

Bivalve mollusks have fascinatingly diverse modes of reproduction. However, research investigating sex determination and reproductive regulation in this group of animals is still in its infancy. In this study, transcriptomes of three ovaries and three testes of Yesso scallop were sequenced and analyzed. Transcriptome comparison revealed that 4394 genes were significantly different between ovaries and testes, of which 1973 were ovary-biased (upregulated in the ovaries) and 2421 were testis-biased. Crucial sex-determining genes that were previously reported in vertebrates and putatively present in bivalves, namely FOXL2, DMRT, SOXH, and SOXE, were investigated. The genes all possessed conserved functional domains and were detected in the gonads. Except for PySOXE, the other three genes were significantly differentially expressed between the ovaries and testes. PyFOXL2 was ovary-biased, and PyDMRT and PySOXH were testis-biased, suggesting that these three genes are likely to be key candidates for scallop sex determination/differentiation. Furthermore, GO and KEGG enrichment analyses were conducted for both ovary- and testis-biased genes. Interestingly, both neurotransmitter transporters and GABAergic synapse genes were overrepresented in the ovary-biased genes, suggesting that neurotransmitters, such as GABA and glycine, are likely to participate in scallop ovary development. Our study will assist in better understanding of the molecular mechanisms underlying bivalve sex determination and reproductive regulation.

Gonad transcriptome of golden musselLimnoperna fortuneireveals potential sex differentiation genes

“…Of these, 2,661 (68.1%) are more expressed in males whereas 1,245 (31.9%) are more expressed in females. This prevalence of male-biased transcripts was similarly observed in other bivalve mollusks, such as the yesso scallop Patinopecten yessoensis (22) and the Manila clam Ruditapes philippinarum (12), but it differs from the Pacific oyster C. gigas (11) and other recent study with P. yessoensis (54). However, Shi and collaborators (2018) (55) observed this male-biased pattern in 19 out of 21 analyzed species from several other animal taxa, including mice (Mus musculus), zebra fish (Danio rerio), C. elegans and Drosophila species.…”

“…To identify genes potentially involved with sex differentiation in L. fortunei, we used the BLASTx and Diamond results to search for genes previously associated to these processes in bivalves and model organisms (11,(14)(15)(16)18,22,51,58) and investigated their expression profile in male and female gonads. Similar to Zhang and collaborators (2014), we assumed that if a gene is involved with sex-determining mechanisms in other organisms, especially bivalves, and it shows sex-biased expression according to its known function, then it may be related to these processes in L. fortunei.…”

“…In fact, currently only three species (Homo Sapiens, Mus Musculus, and Macaca fascicularis) have the Sox30 sequence curated/ (64). The Sox30/SoxH gene was found with male-biased expression in bivalves such as C. gigas (11), P. yessoensis (22,54), and Ruditapes philippinarum (12). The gene was also identified in the snail (Lottia gigantea) and the California mussel Mytilus californianus (data not available) (63).…”

“…Our group is the first to publish the gonad transcriptome of the freshwater bivalve L. fortunei, an organism that has a highly negative impact on aquatic systems by changing the ecological environment and by colonizing and clogging pipes, causing huge economic damage. Golden mussel seems to share key genes related to the sexual development of model organisms from vertebrates to insects and nematodes and that have been identified in other bivalve species such as C. gigas, P. margaritifera and P. yessoensis (11,75,14,22). Our next step is to analyze the expression levels of these genes across all developmental stages and experimentally validate their role in sex determination/differentiation processes during L. fortunei development.…”

“…Previous research on sex-determining pathway genes has focused on homologous sequences of model organisms. However, identification and expression of genes related to these processes have been studied in some bivalves, including the vertebrate female-determining gene forkhead box L2 (FoxL2) (19,20,14,21), the male-determining genes double-sex-and mab-3-related transcription factor (Dmrt) (18,20,21), and Sox (Sryrelated HMG box) family genes (11,12,15,16,22,23). FoxL2 and Dmrt are the key genes for sex determination and maintenance, respectively, and their antagonic role in the control of gonadal sex has been characterized in mammals and has been suggested to play similar antagonic roles in bivalve species (12,24).…”