Reproductive and phylogenetic divergence of tidepool copepod populations across a narrow geographical boundary in Baja California

Aim Previous work on the tidepool copepod Tigriopus californicus revealed a curious case of incipient speciation at the southern end of the species' range in Baja California, Mexico. The present study expands on the geography of this pattern and tests for congruence between reproductive and phylogenetic patterns. Location The Pacific coast of North America, from central Baja California to south-eastern Alaska (27-57 degrees N), including the full range of T. californicus. Methods Primary techniques included mating experiments (> 4000 crosses), phylogeny reconstruction (mitochondrial cytochrome c oxidase subunit I) and screening of single nucleotide polymorphisms (SNPs, 42 loci). Analyses used > 8000 copepods for the mating experiments, 86 copepods for the phylogeny and 41 copepods for the SNP assays. Phylogenies were constructed using Bayesian, maximum likelihood and maximum parsimony methods. Results Populations were found to fall into three reproductive groups: northern and southern groups that were reproductively isolated from each other, and an intermediate group that could serve as a conduit for gene flow. The northern and intermediate populations fell into one clade while all southern populations fell into a second clade. These two clades are now separated by less than 12 km at latitude 29.35 degrees N. Nuclear SNP data for a subset of locations confirmed striking divergence between populations on either side of this boundary. The second (southern) clade was further subdivided into two clades separated by the lagoon region of Guerrero Negro (latitude 28 degrees N). Main conclusions Reproductive assays and molecular data (both mitochondrial and nuclear) reveal a sharp break at 29.35 degrees N, a region with no obvious barriers to dispersal, with no evidence for mixing across this narrow transition zone. Results also showed a milder break at the Guerrero Negro Lagoon (28 degrees N), a location where breaks have been reported for other taxa.