Meeting Abstract
72.6 Monday, Jan. 6 09:15 Examining Physiological Evolution in Derived Freshwater Threespine Sticklebacks DIVINO, J.N.*; MCCORMICK, S.D.; SCHULTZ, E.T.; Univ. of Connecticut; US Geological Survey; Univ. of Connecticut jeffrey.divino@uconn.edu
Marine taxa capable of invading freshwater habitats experience new ecological opportunities that have often resulted in species diversification and adaptive radiation. However, rates of trait change in the novel environment will depend on the intensity and mode of selection, e.g., positive or relaxed. The threespine stickleback (Gasterosteus aculeatus) system provides a unique glimpse into rapid evolutionary change following salinity transitions. In the Pacific Northwest there are numerous derived freshwater populations that range in age, including some recent introductions that are only a few generations old. We adopt a common garden approach to investigate potential evolutionary changes in this fish’s osmoregulatory physiology and performance. Embryos from ancestral marine and derived freshwater populations were reared under identical laboratory conditions and then challenged at more extreme salinities representing both environments. We compared survival, sodium content, and various parameters associated with ion transporters in the gill. Although all populations exhibited euryhalinity, the ancestral stock and recent land-locked descendent each differed in survival, favoring their respective native salinities. Interestingly, the inter-population difference in survival rate between salinity treatments was asymmetric and suggests that a loss of seawater tolerance may occur at a slower pace than enhancement of freshwater tolerance. Molecular evidence suggests an osmoregulatory basis for these differences. We conclude that passage to an exclusively freshwater environment strongly selects for hyperosmoregulatory competency, but not necessarily against seawater tolerance, thus resulting in a halotolerance shift that involves both positive and relaxed selection.
