Search for: All records

Abstract Phenotypic variation is common along environmental gradients, but it is often not known to what extent it results from genetic differentiation between populations or phenotypic plasticity. We studied populations of a livebearing fish that have colonized streams rich in toxic hydrogen sulphide (H2S). There is strong phenotypic differentiation between adjacent sulphidic and non-sulphidic populations. In this study, we varied food availability to pregnant mothers from different populations to induce maternal effects, a form of plasticity, and repeatedly measured life-history and behavioural traits throughout the ontogeny of the offspring. Genetic differentiation affected most of the traits we measured, in that sulphidic offspring tended to be born larger, mature later, have lower burst swimming performance, be more exploratory, and feed less effectively. In contrast, maternal effects impacted few traits and at a smaller magnitude, although offspring from poorly provisioned mothers tended to be born larger and be more exploratory. Population differences and maternal effects (when both were present) acted additively, and there was no evidence for population differences in plasticity. Overall, our study suggests that phenotypic divergence between these populations in nature is caused primarily by genetic differentiation and that plasticity mediated by maternal effects accentuates but does not cause differences between populations. 

Abstract Animal genitalia evolve rapidly because of coevolution between male and female traits. However, how the ecological context in which mating occurs might modulate the evolution of genital traits remains poorly understood. We investigated how a change in the sensory environment (the absence of light upon cave colonization) impacted the expression of genital traits in a live-bearing fish, Poecilia mexicana (Poeciliidae), with populations in adjacent cave and surface habitats. Quantifying characteristics of the female urogenital aperture and the male gonopodium (a modified anal fin used for copulation), we found significant differences in genital traits of both sexes. Females from cave populations exhibited larger and more rounded genitalia. Males from cave populations exhibited a significantly enlarged palp, a fleshy gonopodial appendage that has been hypothesized to have sensory functions. Our results suggest that genital traits can diverge rapidly among closely related populations exposed to different environmental conditions. The absence of light could impact genital evolution directly, if some genital structures have sensory functions that compensate for the lack of visual information during copulation, or indirectly, if the absence of light impacts dynamics of sexual conflict or cryptic female choice that arise through the interaction between the sexes.