A phenotypically plastic magic trait promoting reproductive isolation in sticklebacks?

Abstract

This study identifies one possible mechanism whereby gene flow is interrupted in populations undergoing evolutionary divergence in sympatry; this is an important issue in evolutionary biology that remains poorly understood. Variation in trophic morphology was induced in three-spined stickleback by exposing them from an early age either to large benthic or to small pelagic prey. At sexual maturity, females given a choice between two breeding males, showed positive assortative mate choice for males raised on the same diet as themselves. The data indicate that this was mediated through a preference for males with trophic morphology similar to that of fish with which the females were familiar (from their pre-testing holding tanks). In trials where the female did not choose the most familiar male, the evidence suggests that either she had difficulty discriminating between two similar males or was positively choosing males with more extreme morphologies (more benthic-like or pelagic-like). This study has shown for the first time that expression of a plastic trait induced at an early age, not only results in specialisation for local foraging regimes but can also play a significant role in mate choice. This is equivalent to an environmentally induced, plastic version of the “magic traits” that promote ecologically-driven divergence in sympatry, hence the proposed descriptor “plastic magic trait”.