The transcription factor sox9 has been implicated in cartilage formation and testis determination in mammals. Here, two duplicates of sox9 were found in Japanese flounder (Paralichthys olivaceus) named Posox9a and Posox9b, respectively. Phylogenetic and gene structure analyses revealed that Posox9a and Posox9b were homologous to that of teleosts and tetrapods. Quantitative real-time polymerase chain reaction (qRT-PCR) showed that both Posox9a and Posox9b expressed higher in testis than in ovary of adult tissues. The in situ hybridization (ISH) analysis of gonads showed that Posox9a and Posox9b mRNA were both detected in oocytes, Sertoli cells and spermatocytes. During sex differentiation, the expression of Posox9a exhibited obvious sexual dimorphic expression from 60 days after hatch (dah) with higher expression in male preferred individuals than female preferred individuals and increased gradually from 30 to 100 dah. A similar pattern was detected in Posox9b expression. After injection of androgen (17α-methyltestosterone) of different concentrations, the expression level of Posox9b increased significantly, whereas Posox9a did not change obviously. These results indicated that the two sox9 genes of Japanese flounder had converse functions in sex differentiation, whereas their differences in 17α-methyltestosterone administration were obvious and worthwhile for exploring evolutionary and adaptive significance. This study provided a foundation for further exploration of the roles of Posox9 genes during the sex determination and differentiation, spermatogenesis and gonadal function maintenance of Japanese flounder.

Bivalve molluscs are filter-feeding organisms that can accumulate paralytic shellfish toxins (PST) through ingesting toxic marine dinoflagellates. While the effects of PST accumulation upon the physiology of bivalves have been documented, the underlying molecular mechanism remains poorly understood. In this study, transcriptomic analysis was performed in the gills of Zhikong scallop (Chlamys farreri) after 1, 3, 5, 10, and 15 day(s) exposure of PST-producing dinoflagellate Alexandrium minutum. Higher numbers of differentially expressed genes (DEGs) were detected at day 1 (1538) and day 15 (989) than that at day 3 (77), day 5 (82), and day 10 (80) after exposure, and most of the DEGs were only regulated at day 1 or day 15, highlighting different response mechanisms of scallop to PST-producing dinoflagellate at different stages of exposure. Functional enrichment results suggested that PST exposure induced the alterations of nervous system development processes and the activation of xenobiotic metabolism and substance transport processes at the acute and chronic stages of exposure, respectively, while the immune functions were inhibited by PST and might ultimately cause the activation of apoptosis. Furthermore, a weighted gene co-expression network was constructed, and ten responsive modules for toxic algae exposure were identified, among which the yellow module was found to be significantly correlated with PST content. Most of the hub genes in the yellow module were annotated as solute carriers (SLCs) with eight being OCTN1s, implying their dominant roles in regulating PST accumulation in scallop gills. Overall, our results reveal the gene set responding to and involved in PST accumulation in scallop gills, which will deepen our understanding of the molecular mechanism of bivalve resistance to PST.

As a member of the forkhead box L gene family, foxl2 plays a significant role in gonadal development and the regulation of reproduction. During the evolution of deuterostome, whole genome duplication (WGD)-enriched lineage diversifications and regulation mechanisms occurs. However, only limited research exists on foxl2 duplication in teleost or other vertebrate species. In this study, two foxl2 paralogs, foxl2 and foxl2l, were identified in the transcriptome of spotted knifejaw (Oplegnathus punctatus), which had varying expressions in the gonads. The foxl2 was expressed higher in the ovary, while foxl2l was expressed higher in the testis. Phylogenetic reconstruction, synteny analysis, and the molecular evolution test confirmed that foxl2 and foxl2l likely originated from the first two WGD. The expression patterns test using qRT-PCR and ISH as well as motif scan analysis revealed evidence of potentially functional divergence between the foxl2 and foxl2l paralogs in spotted knifejaw. Our results indicate that foxl2 and foxl2l may originate from the first two WGD, be active in transcription, and have undergone functional divergence. These results shed new light on the evolutionary trajectories of foxl2 and foxl2l and highlights the need for further detailed functional analysis of these two duplicated paralogs.