Genetic and ecomorphological divergence between sympatric Astyanax morphs from Central America

Intraspecific ecological and morphological polymorphism can promote
ecological speciation and the build-up of reproductive isolation. Here, we
evaluate correlations among morphology, trophic ecology, and genetic
differentiation between two divergent morphs (elongate and deep-body) of
the fish genus Astyanax in the San Juan River basin in Central America, to
infer the putative evolutionary mechanism shaping this system. We
collected the two morphs from three water bodies and analyzed: 1) the
correlation between body shape and the shape of the premaxilla, a relevant
trophic morphological structure, 2) the trophic level and niche width of
each morph, 3) the correspondence between trophic level and body and
premaxillary shape, and 4) the genetic differentiation between morphs
using mitochondrial and nuclear markers. We found a strong correlation
between the body and premaxillary shape of the morphs. The elongate-body
morph had a streamlined body, a premaxilla with acuter angles and a
narrower ascending process, and a higher trophic level, characteristic of
species with predatorial habits. By contrast, the deep-body morph had a
higher body depth, a premaxilla with less acute angles, and a broader
trophic niche, suggesting generalist habits. Despite the strong
correlation between morphological and ecological divergence, the morphs
showed limited genetic differentiation, supporting the idea that morphs
may be undergoing incipient ecological speciation, although alternative
scenarios such as stable polymorphism or plasticity should also be
considered. This study provides support for the role of ecological factors
promoting diversification in lake and stream-dwelling freshwater fish.