Phylogeography, genetic structure, and gene flow in the endemic freshwater shrimp Palaemonetes suttkusi from Cuatro Ci,negas, Mexico

The valley of Cuatro Ci,negas in Mexico has the highest degree of local endemicity of any habitat in North America. A large number of endemic aquatic species occur both in the Cuatro Ci,negas basin and in the upper parts of the Rio Salado de los Nadadores drainage, located immediately to the east of the valley. No natural surface flow occurs between these basins but artificial canals connect both areas. It is not clear whether the sharing of endemics between the Cuatro Ci,negas basin and the Rio Salado is due to migration through canals. We conducted a phylogeographic study of mitochondrial haplotypes of the endemic shrimp Palaemonetes suttkusi to determine the evolutionary distinctiveness of the population found in Rio Salado. We discovered that P. suttkusi likely has occurred naturally in both basins well into the distant past. Based on molecular clock analyses of the COI gene, the populations in the Rio Salado and much of the Cuatro Ci,negas basin likely began diverging from each other between 1.9 and 11.2 Myr ago. The general levels of divergence are substantial but our results suggest there has also likely been recent gene flow between these basins. This is consistent with migration through human-made canals, but also consistent with the occurrence of natural gene flow during intermittent wet periods in the past million years. We also found significant differentiation of the Pozas Azules area from the rest of the Cuatro Ci,negas basin, a finding that is concordant with several phylogeographic studies on other aquatic endemics in Cuatro Ci,negas. We recommend that the upper parts of the Rio Salado, the Pozas Azules area, and the rest of the Cuatro Ci,negas basin should each be considered independent evolutionarily significant units for conservation, and that migration of species through human-made canals should be monitored and controlled.