Freshwater-to-marine transitions may explain the evolution of herbivory in the subgenus Mollienesia (genus Poecilia, mollies and guppies)

Abstract

Abstract The ability of organisms to cross ecosystem boundaries is an important catalyst of evolutionary diversification. The genus Poecilia (mollies and guppies) is an excellent system for studying ecosystem transitions because species display a range of salinity and dietary preferences, with herbivory concentrated in the subgenus Mollienesia. We reconstructed ancestral habitats and diets across a phylogeny of the genus Poecilia, evaluated diversification rates and used phylogenetically independent contrasts to determine whether diet evolved in response to habitat transition in this group. The results suggest that ancestors of subgenus Mollienesia were exclusively herbivorous, whereas ancestral diets of other Poecilia included animals. We found that transitions across euryhaline boundaries occurred at least once in this group, probably after the divergence of the subgenus Mollienesia. Furthermore, increased salinity affiliation explained 24% of the decrease in animals in the gut, and jaw morphology was associated with the percentage of animals in the gut, but not with the percentage of species occupying saline habitats. These findings suggest that in the genus Poecilia, herbivory evolved in association with transitions from fresh to euryhaline habitats, and jaw morphology evolved in response to the appearance of herbivory. These results provide a rare example of increased diet diversification associated with the transition from freshwater to euryhaline habitats.