Surface and Cave Populations of Mexican Astyanax

Abstract

The Neotropic large-eyed and well-pigmented diurnal characid fish Astyanax has developed a series of cave populations in Northeastern Mexico. These divide morphologically into a group of strongly eye- and pigment-reduced (SEP) cave populations and another one characterized by variable eye size and pigmentation (VEP cave populations). Molecular and biogeographic data imply that they derive from the Neotropic Astyanax surface fish, which were able to invade North America up to the Rio Grande drainage after the closure of the Central American land bridge. Its recent distributional pattern is strongly influenced by Pleistocene climatic changes and is characterized by regional extinction and recolonization from the warmer south and/or survival in climatically buffered refuges. An example of this are the SEP cave fish populations, which according to cytochrome b analysis do not cluster with the surface fish from neighboring rivers and creeks but with fish from a remote location about 500 km away in the Central Mexican Plateau. In line with this, they do not group with either the VEP cave fish or with surface fish from the cave area, and based on microsatellites and SNP studies, they exhibit relation to populations from southern Mexico and Belize. The SEP cave fish and some relic surface fish populations from isolated locations all over Mexico derive from the oldest invasion. In contrast, based on cytochrome b studies, the VEP cave populations cluster with the recent surface fish from the cave area, which is widespread in Northern Mexico. The VEP cave populations derive from a more recent invasion of surface fish into Northern Mexico. In particular, the differing degree of eye reduction between SEP and VEP cave fish reflects the different times of cave entry. Cave colonization in VEP and SEP cave populations took place in parallel and resulted in multiple convergent evolutions.