Parallel shifts in trout feeding morphology suggest rapid adaptation to alpine lake environments.

Eco-evolutionary interactions following ecosystem change provide critical insight into the ability of organisms to adapt to shifting resource landscapes. Here we explore evidence for the rapid parallel evolution of trout feeding morphology following eco-evolutionary interactions with zooplankton in alpine lakes stocked at different points in time in the Wind River Range (Wyoming, USA). In this system, trout predation has altered the zooplankton species community and driven a decrease in average zooplankton size. In some lakes that were stocked decades ago, we find shifts in gill raker traits consistent with the hypothesis that trout have rapidly adapted to exploit available smaller-bodied zooplankton more effectively. We explore this morphological response in multiple lake populations across two species of trout (cutthroat trout, Oncorhynchus clarkii, and golden trout Oncorhynchus aguabonita) and examine the impact of resource availability on morphological variation in gill raker number among lakes. Furthermore, we present genetic data to provide evidence that historically stocked cutthroat trout populations likely derive from multiple population sources, and incorporate variation from genomic relatedness in our exploration of environmental predictors of feeding morphology. These findings describe rapid adaptation and eco-evolutionary interactions in trout and document an evolutionary response to novel, contemporary ecosystem change.

Dispersal of a defensive symbiont depends on contact between hosts, host health, and host size.

Symbiont dispersal is necessary for the maintenance of defense mutualisms in space and time, and the distribution of symbionts among hosts should be intricately tied to symbiont dispersal behaviors. However, we know surprisingly little about how most defensive symbionts find and choose advantageous hosts or what cues trigger symbionts to disperse from their current hosts. In a series of six experiments, we explored the dispersal ecology of an oligochaete worm (Chaetogaster limnaei) that protects snail hosts from infection by larval trematode parasites. Specifically, we determined the factors that affected net symbiont dispersal from a current "donor" host to a new "receiver" host. Symbionts rarely dispersed unless hosts directly came in contact with one another. However, symbionts overcame their reluctance to disperse across the open environment if the donor host died. When hosts came in direct contact, net symbiont dispersal varied with both host size and trematode infection status, whereas symbiont density did not influence the probability of symbiont dispersal. Together, these experiments show that symbiont dispersal is not a constant, random process, as is often assumed in symbiont dispersal models, but rather the probability of dispersal varies with ecological conditions and among individual hosts. The observed heterogeneity in dispersal rates among hosts may help to explain symbiont aggregation among snail hosts in nature.