Identification and Characterization of Gene SpDMRT99B and Its Sex-Biased Expression Profile in the Mud Crab, Scylla paramamosain

The doublesex and mab-3 related transcription factor (DMRT) gene family is conserved from invertebrates to humans. The functions of DMRT are mainly involved in sex development and the formation of many tissues and organs. In this study, a DM (Doublesex/Mab-3)-domain gene was identified in the mud crab Scylla paramamosain, and was named SpDMRT99B because of its many similarities to arthropod DMRT99B and phylogenetically close relationship with arthropod DMRT99B. The cDNA of SpDMRT99B gene is 1249 bp in length, encoding 224 amino acids. From 254 bp to 928 bp there is a conserved DM domain. No transmembrane domain was identified. Through multiple amino acid alignment and phylogenetic tree analysis, the closest gene to SpDMRT99B is Eriocheir sinensis DMRT99B, followed by Macrobrachium rosenbergii and Drosophila melanogaster DMRT99B. The expressions of the gene were characterized in different tissues of female and male crabs during early development period of crab individuals, as well as in different development periods of gonads. The results showed that SpDMRT99B gene is significantly highly expressed in testis than in ovary and other tissues. The expression level of SpDMRT99B in testis at different stages is significantly higher than that in ovary, and it is particularly highly expressed in immature testis. In early developmental stages of larvae, the expressions of SpDMRT99B remain at a low level and reach a peak at zoea stage I when the body segments shape up. It is speculated that SpDMRT99B gene might be involved in the gonadal development process and somitogenesis of S. paramamosain.