Convergent Evolution of the Osmoregulation System in Decapod Shrimps.

In adaptating to different aquatic environments, seawater (SW) and freshwater (FW) shrimps have exploited different adaptation strategies, which should generate clusters of genes with different adaptive features. However, little is known about the genetic basis of these physiological adaptations. Thus, in this study, we performed comparative transcriptomics and adaptive evolution analyses on SW and FW shrimps and found that convergent evolution may have happened on osmoregulation system of shrimps. We identified 275 and 234 positively selected genes in SW and FW shrimps, respectively, which enriched in the functions of ion-binding and membrane-bounded organelles. Among them, five (CaCC, BEST2, GPDH, NKA, and Integrin) and four (RasGAP, RhoGDI, CNK3, and ODC) osmoregulation-related genes were detected in SW and FW shrimps, respectively. All five genes in SW shrimps have been reported to have positive effects on ion transportation, whereas RasGAP and RhoGDI in FW shrimps are associated with negative control of ion transportation, and CNK3 and ODC play central roles in cation homeostasis. Besides, the phylogenetic tree reconstructed from the positively selected sites separated the SW and FW shrimps into two groups. Distinct subsets of parallel substitutions also have been found in these osmoregulation-related genes in SW and FW shrimps. Therefore, our results suggest that distinct convergent evolution may have occurred in the osmoregulation systems of SW and FW shrimps. Furthermore, positive selection of osmoregulation-related genes may be beneficial for the regulation of water and salt balance in decapod shrimps.