Population genetic structure of the endemic fish Gambusia marshi from the Cuatro Cienegas basin and its outflow in Coahuila, Mexico

The valley of Cuatro Cienegas in Coahuila, Mexico, has the highest degree of local endemism of any habitat in North America. Despite the arid desert climate, the basin harbours an extensive system of permanent streams, wetlands, and spring-fed pools, and is divided into two sub-basins by the central Sierra de San Marcos y Pinos. Microsatellite loci were surveyed to infer the genetic diversity and population structure of Gambusia marshi, a poeciliid fish endemic to the Cuatro Cienegas basin and the Rio Salado de Nadadores in north-east Mexico. Bayesian clustering analysis revealed four genetic populations within G. marshi and a major east-west division corresponding to the Sierra de San Marcos y Pinos. Most sample sites exhibited high levels of genetic differentiation, although there is evidence for recent gene flow between some of these locations. The population of G. marshi in Poza Anteojo is extremely divergent, and appears to represent a remnant of a different historical system. The G. marshi in Poza Anteojo should be considered an independent management unit. Documenting the number and distribution of distinct populations of G. marshi provides additional justification for protecting the Cuatro Cienegas basin from additional water withdrawals and further homogenization via the construction of additional canals. By analysing the population structure of a widespread species within a region containing many range-restricted and endemic species, these results shed new light on historical connections among aquatic ecosystems and raise awareness of the possibility that units of conservation concern may exist in other, more range-restricted taxa in the Cuatro Cienegas basin and encourage the assessment of their conservation status. The results also have implications for species in arid regions in other parts of the world that may also harbour as yet unrecognized isolated populations worthy of conservation.