First steps towards the identification of evolutionarily significant units in Mexican native trout: An assessment of microsatellite variation

In the remote high-elevation streams and rivers in the northwest of Mexico, there is an endemic and highly diverse group of native trout related to the rainbow trout lineage (Oncorhynchus mykiss). Currently, only two species have been formally described from this trout complex, the Mexican golden trout, O. chrysogaster, and the San Pedro Martir rainbow trout, O. m. nelsoni, but only the former occurs in the Sierra Madre Occidental (SMO). South and north of the O. chrysogaster range, there are other undescribed trout that have been recently discovered. Given the various human impacts on aquatic systems occupied by these Mexican native trout, the lack of taxonomic descriptions and the imminent risk of losing populations without their even being documented, we sought to infer a hierarchical population structure of the Mexican native trout, determine the geographic patterns of genetic diversity and gene flow among them, and identify evolutionarily significant units for a prompt conservation strategy. The allelic diversity across eleven microsatellite loci was used to assess genetic divergences of wild trout collected throughout its range in Mexico. Our results reject the old hypothesis that many of the trout in Mexico are introduced or translocated hatchery-reared rainbow trout and strongly support the new hypothesis of distinct genetic populations and lineages in the northwest of Mexico. According to the different statistical analysis of the genetic data, we distinguish between species and subspecies (O. chrysogaster, O. mykiss, and O. m. nelsoni) of the other regional lineages of undescribed trout (northern and southern) inhabiting the SMO. We also found a strong genetic substructure within these large clusters. We propose the presence of seven Evolutionarily Significant Units in the drainage basins of the SMO (two occur in the northern region, four in the southern, and one for the area of distribution of O. chrysogaster) and one in north-western Baja California. Based in microsatellite data a minimum gene flow among populations is evidenced, resulting the presence the highly divergent populations.