Subterranean Fishes of the World

3199

2013

AIM 2013

Populations of cave Astyanax have two main types of mitochondrial DNA whose differences are of such magnitude as to be derived from separate species. The commonly accepted view is that an old stock colonized the cave environment and a younger stock is currently found in the surface populations and in some of the cave populations. Hybridization has occurred between both stocks at multiple cave localities. It is often assumed that gene flow from this hybridization reduces the level of troglomorphy in cave population of Astyanax but, are there cases in which the hybridization of two surface species actually promoted the evolution of troglomorphy in cave fish? In the case of two populations of the Pennsylvania Grotto Sculpin, the northernmost cave adapted fish in the world, hybrid populations did show an increase in troglomorphic development. One of these populations, the Nippenose cave fish, has a suite of modifications that readily identify them as cave-adapted: Smaller eyes, elongated pectoral fins, more numerous and enlarged cephalic lateralis pores, and a broader head/mouth. On the contrary, the population of Tytoona cave fish do not have the aforementioned suite, with the exception of slightly enlarged cephalic pores. When looking at mitochondrial markers, the Tytoona cave population has a single haplotype, identical to C. bairdi from surrounding surface streams. Interestingly, the more troglomorphic Nippenose cave population shares mitochondrial haplotypes with two species of sculpin —Cottus cognatus and C. bairdi. Molecular data as well as morphology support that in the Nippenose grotto sculpin, a hybridization event of C. cognatus and C. bairdi generated an adaptationally distinct sculpin lineage, while in those localities where a single species colonizes the cave, gene flow may have prevented it. The case of the Pennsylvania Grotto Sculpin raises questions about our understanding of hybridization in Astyanax. It has become a paradigm that Astyanax cave forms were established by an ancestral surface-dwelling form that is either extinct, or no longer present in this region of Mexico. This may be incorrect since surface populations with mitochondrial haplotypes related to both old and young stocks are current inhabitants of the surrounding areas of Sierra de El Abra: Rascon, and Tamasopo for the old stock and Tampaon and Boquillas for the young stock. While questionable, it may be worth considering that in the evolution of cave Astyanax, a hybridization event could have been implicated in the successful colonization of the cave environment in light of the fact that in other cave-adapted fish, hybridization appears to have been important.

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