ORIGINS AND DIVERSITY OF PERIPHERAL POPULATIONS OF RIO GRANDE SUCKER (PANTOSTEUS PLEBEIUS) IN THE SOUTHWESTERN UNITED STATES

Rio Grande sucker (Pantosteus plebeius) is continuously distributed in parts of its range, but occurs in geographically isolated populations at the periphery. We used 10 microsatellites and DNA sequence data from the mitochondrial cytochrome b gene to characterize genetic diversity and reconstruct evolutionary relationships of five peripheral populations in the southwestern United States. An isolated population in Rio Bonito (Pecos River drainage) is most closely related to a central Rio Grande population in Alamosa Creek, and part of the monophyletic Rio Grande lineage. Another disjunct population, Bluewater Creek in the western Rio Grande drainage, has a Mimbres lineage mtDNA haplotype that likely originated via interbasin transfer. Both peripheral populations are important for conservation of the species as a whole. Three other isolated populations are closely related to mainstem "core" populations in the Mimbres and Gila rivers that colonized these intermittently connected tributaries. One peripheral population in Allie Canyon, Mimbres River, has persisted longer in isolation than Rocky Canyon or Trout Creek in the Gila system. In general, a process of range expansion during relatively cool and wet periods, followed by retreat to wetted refugia during warm and dry periods, leads to genetic differentiation of peripheral populations of Rio Grande sucker at local and across-basin scales.