Complex phylogeography and historical hybridization between sister taxa of freshwater sculpin (Cottus).

Species ranges that span different geographic landscapes frequently contain cryptic species- or population-level structure. Identifying these possible diversification factors can often be accomplished under a comparative phylogeographic framework. However, comparisons suffer if previous studies are limited to a particular group or habitat type. In California, a complex landscape has led to several phylogeographic breaks, primarily in terrestrial species. However, two sister taxa of freshwater fish, riffle sculpin (Cottus gulosus) and Pit sculpin (Cottus pitensis), display ranges based on morphological identifications that do not coincide with these breaks. Using a comprehensive sampling and nuclear, mitochondrial and microsatellite markers, we hypothesized that proposed species ranges are erroneous based on potential hybridization/gene flow between species. Results identified a phylogeographic signature consistent with this hypothesis, with breaks at the Coast Range Mountains and Sacramento/San Joaquin River confluence. Coastal locations of C. gulosus represent a unique lineage, and 'true' C. gulosus were limited to the San Joaquin basin, both regions under strong anthropogenic influence and potential conservation targets. C. pitensis limits extended historically throughout the Sacramento/Pit River basin but currently are restricted to the Pit River. Interestingly, locations in the Sacramento River contained low levels of ancestral hybridization and gene flow from C. gulosus but now appear to be a distinct population. The remaining population structure was strongly correlated with Sierra Nevada presence (high) or absence (low). This study stresses the importance of testing phylogeographic breaks across multiple taxa/habitats before conservation decisions are made, but also the potential impact of different geographic landscapes on evolutionary diversification.

Neither philopatric nor panmictic: microsatellite and mtDNA evidence suggests lack of natal homing but limits to dispersal in Pacific lamprey.

Most species with lengthy migrations display some degree of natal homing; some (e.g. migratory birds and anadromous salmonids) show spectacular feats of homing. However, studies of the sea lamprey (Petromyzon marinus) indicate that this anadromous species locates spawning habitat based on pheromonal cues from larvae rather than through philopatry. Previous genetic studies in the anadromous Pacific lamprey (Entosphenus tridentatus) have both supported and rejected the hypothesis of natal homing. To resolve this, we used nine microsatellite loci to examine the population structure in 965 Pacific lamprey from 20 locations from central British Columbia to southern California and supplemented this analysis with mitochondrial DNA restriction fragment length polymorphism analysis on a subset of 530 lamprey. Microsatellite analysis revealed (i) relatively low but often statistically significant genetic differentiation among locations (97% pairwise F(ST) values were <0.04 but 73.7% were significant); and (ii) weak but significant isolation by distance (r(2) = 0.0565, P = 0.0450) but no geographic clustering of samples. The few moderate F(ST) values involved comparisons with sites that were geographically distant or far upstream. The mtDNA analysis--although providing less resolution among sites (only 4.7%F(ST) values were significant)--was broadly consistent with the microsatellite results: (i) the southernmost site and some sites tributary to the Salish Sea were genetically distinct; and (ii) southern sites showed higher haplotype and private haplotype richness. These results are inconsistent with philopatry, suggesting that anadromous lampreys are unusual among species with long migrations, but suggest that limited dispersal at sea precludes panmixia in this species.