Altitudinal differentiation of reproductive tactic plasticity despite the close proximity of two Aquarius remigis populations

In the water strider (Aquarius remigis), direct and delayed reproductive tactics are largely canalized in southern and northern areas of North America, respectively. A transition zone occurs in upstate New York, defined as the area where lowland striders plastically express either tactic based on photoperiod and food abundance cues. We tested whether these cues similarly influence tactics among higher altitude transition zone populations. We compared the proportion of direct reproducers from a high altitude stream (HAS) and a low altitude stream (LAS) reared under low and high food abundance, as well as a normal and late-solstice photoperiod. With sexes combined, higher proportions of direct reproducers were found among LAS striders, and among striders reared in the late solstice photoperiod and high food abundance treatments. Among males, a food abundance × stream interaction was significant, suggesting greater canalization of reproductive tactics among HAS males. Among females, photoperiod was a significant factor, but did not interact with stream of origin. Field measurements showed that water surface temperatures and food abundance were lower in HAS compared to LAS. These conditions, as well as shorter season lengths, likely select against direct reproduction among HAS striders, possibly resulting in the observed canalization of reproductive tactics.