Sox9 gene, a crucial member of the Sox gene family, is present in various organisms and involved in many physiological processes, especially in sex determination and gonad development. In this study, we reported a sox9 gene (designated as Spsox9) from Scylla paramamosain through analyzing published gonad transcriptome data. Meanwhile, the accuracy was validated by PCR technology, and the 3' sequences were cloned with 3' RACE technology. The full-length cDNA of Spsox9 is 2843 bp, consisting of a 243 bp 5' UTR, an 1124 bp 3' UTR, and a 1476 bp ORF encoding 491 amino acids. Furthermore, to better understand its conservation among crustacean species, the sox9 gene ortholog was identified in several other crustaceans species with their published transcriptome data, respectively. All of the Sox9 proteins identified in the current study had the common feature of Sox proteins (HMG domain) and were highly conserved among analyzed crustacean species. In all examined tissues, the Spsox9 was mainly expressed in the gonad (testis and ovary), eyestalk, and cerebral ganglion. During embryo development, Spsox9 was highly expressed in 5 pairs of appendages, 7 pairs of appendages, and eye-pigment formation stage. During ovary development, the expression level of Spsox9 remained stable in the first 4 stages (O1-O4) and decreased in the tertiary vitellogenesis (O5) stage. During testis development, the expression level of Spsox9 was highest in the spermatid stage (T2) and was significantly different from that in the spermatocyte stage (T1) and mature sperm stage (T3) (p < 0.05). In addition, Spsox9 exhibited a sex-biased expression pattern in T1 and O1. These present results indicated that the Spsox9 gene might play crucial roles in the gonad and embryo development of mud crab.
