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Evolutionary Genetics of Cave-Dwelling Fishes of the Genus Astyanax

John C. Avise and Robert K. Selander
Evolution
Vol. 26, No. 1 (Mar., 1972), pp. 1-19
DOI: 10.2307/2406978
Stable URL: http://www.jstor.org/stable/2406978
Page Count: 19
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Evolutionary Genetics of Cave-Dwelling Fishes of the Genus Astyanax
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Abstract

Allozymic variation in 11 proteins encoded by 17 loci was analyzed in 393 individuals of the characid fish Astyanax mexicanus, representing populations in three caves and six surface rivers and arroyos in northeastern Mexico. Coefficients of genic similarity among cave and surface populations averaged 0.82, a value similar to those reported for conspecific populations of other vertebrates. This degree of similarity presumably reflects a history of periodic connection between temporarily isolated populations and a consequent reduction in opportunity for genetic divergence. The genetic information does not support the assignment of surface and cave populations to different genera (Astyanax and "Anoptichthys") or to different species. Levels of genic variability are high in surface populations, with individuals having 11.2% of their loci in heterozygous state, on the average. In cave populations, variability is absent (Pachon Cave), severely reduced (Los Sabinos Cave, 3.2%), or less than the average for surface populations (Chica Cave, 7.7%). At three of the 17 loci, populations in Pachon and Los Sabinos caves are monomorphic or nearly so for different alleles; and, at a fourth locus, both are monomorphic for an allele not detected in surface populations. A correlation between degree of eye development and allozymic genotype for the sample from Chica Cave suggests that individuals from the surface are entering the cave and interbreeding with the eyeless forms. Reduced variability in the small populations of Astyanax occurring in cave pools is attributed primarily to genetic drift. The eyeless, unpigmented condition is believed to have evolved in whole or part prior to the present-day subdivision of the populations, which presumably resulted from recession of flood waters or shifting drainage patterns. According to a model of periodic interconnection of caves and intermittent gene flow among cave populations, selection and other deterministic evolutionary processes could affect most or all of the cave populations as a unit. Owing to the subdivision of aquatic troglobites into small populations, it is not possible to distinguish the effects of drift and stabilizing or other forms of selection in reducing genetic variability. An examination of a terrestrial troglobitic beetle (Rhadine subterranea) in two caves near Austin, Texas, demonstrated that the presumably uniform, stable environment of caves does not necessarily impose low degrees of genetic variability in populations.

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