ION TRANSPORTERS GENE EXPRESSION IN GILLS AND INTESTINE OF MALE AND FEMALE THREESPINED STICKLEBACKS DURING FRESHWATER ADAPTATION

In euryhaline fish species, including the three-spined stickleback, a key physiological response to freshwater adaptation aimed at maintaining osmotic homeostasis is the enhancement of ion uptake from the environment and the reduction of ion loss. The hormone prolactin, a central regulator of this process, primarily targets the gills and intestine. Our previous work demonstrated that, in a model of freshwater adaptation in sticklebacks, prolactin expression and the sensitivity of osmoregulatory tissues to prolactin differ between males and females. In the present study, we measured the expression levels of genes encoding the α1a and α3a subunits of Na⁺/K⁺-ATPase, as well as ion transporters NKCC1a, NKCC2, NCC, and NHE2, in the gill and intestinal tissues of male and female three-spined sticklebacks (Gasterosteus aculeatus L.) under conditions of acute (24 h) and chronic (72 h) freshwater adaptation, relative to control conditions. During freshwater adaptation, females – but not males – exhibited increased intestinal expression of nhe2 and atp1a3 (as well as the atp1a1/atp1a3 expression ratio), and nkcc1a, along with decreased expression of nkcc2. In contrast, only males showed an increase in ncc expression in the intestine. In both sexes, freshwater exposure led to a significant decrease in nkcc1a expression in the gills. These findings support our hypothesis of sex-dependent plasticity in osmoregulatory function in sticklebacks, with females exhibiting a more pronounced response. This pattern further aligns with previously reported stronger activation of the prolactin axis in females under freshwater adaptation conditions.

freshwater adaptation, threespined stickleback Gasterosteus aculeatus L., Na⁺/K⁺-ATPase, NKCC, NCC, prolactin