Abstract:
Caballo Moro, a karst window cave in northeastern Mexico, supports a mixed population of cave Astyanax mexicanus: eyed and eyeless. The relationships of these sub-populations to one another and to other populations of Mexican tetras were examined using RAPD DNA fingerprint markers. The eyed tetras of Caballo Moro Cave are genetically closer to blind tetras from Caballo Moro and other caves in the region than they are to eyed tetras from the surface. The two forms are not genetically identical, however, and may represent distinct sub-populations. Eyed and eyeless fish have a distributional bias in the cave, with eyed fish preferentially in the illuminated area and blind fish in the dark zone. Aggression of eyed towards blind fish in the illuminated area contributes to this bias and may serve to stabilize the eye-state polymorphism. We considered four hypotheses for the origin of Caballo Moro eyed cave fish. The RAPD data rule out that the mixed population represents a transitional stage of evolution, or that the eyed fish are unmodified surface immigrants. We cannot rule out that the eyed fish are the direct descendants of surface fish that have acquired markers from blind fish by hybridization, although the apparent distinctness of the two sub-populations suggests otherwise. An alternative hypothesis, that the eyed fish of the cave are direct descendants of blind cave fish that re-acquired eyes with the opening of the karst window, is consistent with the data and tentatively accepted.