Phylogeography of the longnose dace (Rhinichthys cataractae) species group in northwestern North America — the Nooksack dace problem

The longnose dace (Rhinichthys cataractae (Valenciennes, 1842); Cyprinidae) is one of the most widespread freshwater fishes in North America, and across its range there have been several divergent forms described that are of uncertain taxonomic status. One of these forms, the Nooksack dace, is found in southwestern British Columbia and adjacent portions of western Washington, and is distinguished from longnose dace by a lower number of lateral-line scales. We sequenced a total of approximately 1400 base pairs (bp) of mitochondrial DNA (mtDNA) and noted that the longnose dace found west of the Continental Divide and Nooksack dace constituted reciprocally monophyletic clades that differed from each other by between 2% and 3% sequence divergence. Sequence analysis at two nuclear loci (the S7 ribosomal protein intron 1 (S7) and recombination activation gene 1 (RAG1)), however, showed no consistent difference between longnose dace and Nooksack dace and several alleles were shared between them. By contrast, consistent differences at both mtDNA and nuclear DNA loci were resolved between R. cataractae samples from east and west of the Continental Divide. The Nooksack dace does not appear to warrant separate taxonomic status from the longnose dace, but the mtDNA differences support its recognition as an important component of the evolutionary and biogeographic legacy of R. cataractae.

Glacial cycles as an allopatric speciation pump in north-eastern American freshwater fishes

Allopatric speciation may be the principal mechanism generating new species. Yet, it remains difficult to judge the generality of this process because few studies have provided evidence that geographic isolation has triggered the development of reproductive isolation over multiple species of a regional fauna. Here, we first combine results from new empirical data sets (7 taxa) and published literature (9 taxa) to show that the eastern Great Lakes drainage represents a multispecies suture zone for glacial lineages of freshwater fishes with variable levels of genetic divergence. Second, we performed amplified fragment length polymorphism analyses among four pairs of lineages. Results indicate that lineages with relatively deep levels of mtDNA 5' COI (barcode) sequence divergence (>2%) developed strong reproductive barriers, while lineages with lower levels of divergence show weaker reproductive isolation when found in sympatry. This suggests that a threshold of 2% sequence divergence at mtDNA could be used as a first step to flag cryptic species in North American freshwater fishes. By describing different levels of divergence and reproductive isolation in different co-occurring fishes, we offer strong evidence that allopatric speciation has contributed significantly to the diversification of north-eastern American freshwater fishes and confirm that Pleistocene glacial cycles can be viewed as a 'speciation pump' that played a predominant role in generating biodiversity.