Genotypes of cave, surface and hybrid Astyanax mexicanus

Genetic divergence in the presence of gene flow has been well documented,
but there is little information on the specific factors maintaining
divergence. The present study investigates this in the Mexican tetra
(Astyanax mexicanus), an excellent model for studying this question
because surface and cave populations differ markedly in phenotype and
genotype, but are interfertile.  Previous population studies
documented significant gene flow among cave and surface populations but
they focused on analyses of neutral markers, whose evolutionary dynamics
likely differ from those of genes involved in cave adaptation. The present
study advances our understanding of this question by focusing specifically
on the genetics responsible for eye and pigmentation reduction, signature
traits of cave populations. Direct observations of two cave populations
over the course of 63 years verify that surface fish frequently move into
the caves and even hybridize with the cavefish. Importantly, however,
historical records show that surface alleles for pigmentation and eye size
do not persist, but are rapidly eliminated from the cave gene pool. It has
been argued that the regression of eyes and pigmentation was driven by
drift, but the results of this study suggest that strong selection
actively eliminates surface alleles from the cave populations.