Exposing local adaptation: Synergistic stressors elicit population-specific lactate dehydrogenase-B (ldh-b) expression profiles in Australian barramundi, Lates calcarifer

The molecular response of fish to independently and/or concurrently applied ecological stressors (e. g. thermal and/or aerobic stress) can be quantified at the level of transcript abundance (i. e. gene expression). In temperate fish, the expression of the metabolic candidate gene lactate dehydrogenase-B (ldh-b) responds to both aerobic swimming challenge and extended acclimation to various ecologically relevant temperatures. We examined hepatic ldh-b expression in juvenile Lates calcarifer from two geographically, genetically and thermally distinct Australian populations to determine if similar environmental stressors also influence the transcription of this locus in a tropical fish. Hepatic ldh-b expression was quantified following 28-day acclimation to ecologically relevant temperatures (20, 25, 30 and 35°C). Expression was also quantified in L. calcarifer subjected to aerobic swimming challenge at these temperatures. Fish from southern (high latitude) and northern (low latitude) populations within this species' Australian distribution exhibited a significant increase in hepatic ldh-b expression following aerobic swimming challenge at native temperatures of 25 and 30°C, respectively (p < 0.001). Southern and northern fish also exhibited significant increase in hepatic ldh-b expression (p < 0.001 and p < 0.01, respectively) following 28-day acclimation to heat-stress (35°C). However, only southern fish exhibited significant increase in expression (p < 0.001) following 28-day acclimation to cold-stress (20°C). The novel evidence presented herein suggests that (a) transcription of hepatic ldh-b is responsive to both aerobic and thermal stress when applied independently, and (b) southern Australian L. calcarifer populations may be locally adapted to cooler seasonal water temperatures.