Duplicated paralog of sulfide: quinone oxidoreductase contributes to the adaptation to hydrogen sulfide-rich environment in the hydrothermal vent crab, Xenograpsus testudinatus

Abstract

The hydrothermal crab, Xenograpsus testudinatus (xtcrab) inhabits shallow-water, hydrogen sulfide (H₂S)-rich hydrothermal vent regions. Until now, the adaptative strategy of xtcrab to this toxic environment was unknown. Herein, we investigated the sulfide tolerance and detoxification mechanisms of xtcrabs collected in their high-sulfide hydrothermal vent habitat. Experimental immersion of xtcrab in various sulfide concentrations in the field or in aquaria assessed its high sulfide tolerance. HPLC measurement of hemolymph sulfur compounds highlighted xtcrab detoxification capacity via catabolism of sulfide into much less toxic thiosulfate. We focused on a key enzyme for H₂S detoxification, sulfide: quinone oxidoreductase (SQR). Cloning and phylogenetic analysis revealed two SQR paralogs in xtcrab, that we named xtSQR1 and xtSQR2. As shown by qPCR, xtSQR2 and xtSQR1 were expressed in the digestive gland, suggesting the involvement of both paralogs in the detoxification of food-related H₂S. In contrast, xtSQR1 transcript was highly expressed in the gill, while xtSQR2 was not detectable, suggesting a specific role of SQR1 in gill detoxification of H₂S of environmental origin. Comparison between xtcrabs in their hydrogen sulfide-rich hydrothermal habitat, and xtcrabs maintained for one month in sulfide-free seawater aquarium, showed higher transcript levels of gill xtSQR1 in sulfide-rich habitat, further supporting the specific role of xtSQR1 paralog in environmental H₂S detoxification in the gill. Gill SQR protein level as measured by Western blot, and gill SQR enzyme activity were also higher in sulfide-rich habitat. Immunohistochemical staining further showed that SQR expression was co-localized with Na⁺/K⁺-ATPase-positive epithelial and pillar cells of the gill filament. This is the first evidence of duplicate SQR genes in crustaceans. Overall, our study suggests that the subfunctionalization of duplicate xtSQR genes may play an important role in sulfide detoxification to maintain the sulfide homeostasis in X. testudinatus, providing an ecophysiological basis for its adaptation to the high-sulfide hydrothermal vent environment.

Keywords: Hydrothermal vent adaptation; Paralog subfunctionalization; Sulfide detoxification; Sulfide homeostasis; Sulfide: Quinone oxidoreductase; Xenograpsidae crabs.