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Low Mitochondrial Dna Sequence Variation in the Microendemic Cuatro Ciénegas Platyfish Xiphophorus gordoni

ABSTRACT. 
 The desert spring ecosystem of the Cuatro Ciénegas basin, Mexico, hosts an extraordinary but increasingly imperiled assemblage of native and endemic aquatic organisms. Among the more vulnerable to extinction is the microendemic Cuatro Ciénegas platyfish Xiphophorus gordoni, which is at particular risk due to its highly restricted distribution and small population size. Given the growing demands on water resources in this arid region, we conducted a survey of mitochondrial DNA sequence variation in the cytochrome b gene to establish a baseline for long-term genetic monitoring of this endangered species. Fifty-eight specimens were collected from the 2 subpopulations known to persist, Laguna Santa Tecla (27) and Teclitas (31). Three haplotypes were observed across all samples, with 3 and 2 haplotypes observed at Laguna Santa Tecla and Teclitas, respectively. Sequence divergence ranged from 0.02% to 0.03% among variants and averaged 0.317 differences between populations. Haplotype and nucleotide diversities were moderately low in Laguna Santa Tecla (0.467 and 0.0008, respectively) and severely low in Teclitas (0.065 and 0.0001, respectively). Exact tests revealed significant heterogeneity in haplotype distributions between sites (P = 0.006), as did the pairwise estimate of &PHgr;ST (0.135; P = 0.005). Tests of deviation from neutrality were nonsignificant. Microendemism of X. gordoni and marginal substitution differences between the few extant haplotypes suggest that low genetic variation is a historical characteristic of this species; however, population declines associated with recent habitat loss have likely further compromised standing levels of genetic variation. Development of a long-term population and genetic monitoring program that incorporates multilocus nuclear DNA markers is advised and should be tied to a more thorough characterization of the basic biology and life history of this species, establishment of captive or refuge populations, and further improvement of habitat.