Gene flow among Modoc sucker and Sacramento sucker populations in the upper Pit River

The Modoc sucker (Catostomus microps) is a species that received legal protection in the United States based partially on concerns that anthropogenic environmental changes had restricted migration among populations and catalyzed hybridization with a congener, the Sacramento sucker (C. occidentalis). We applied eight microsatellite markers to samples of both species collected from throughout the range of Modoc suckers: two tributaries in the Pit River sub-basin and one tributary in the Goose Lake sub-basin. Modoc sucker populations in these three tributaries appeared to be largely isolated from one another: migration among Pit River tributaries appeared no greater than migration among sub-basins. Sacramento sucker populations appeared relatively more connected with one another, especially within the sub-basins. Interspecific hybrids were detected in all three tributaries. Tributaries in the Pit River sub-basin, which have retained typical habitats for both species, yielded relatively low...