Intergradation between discrete lineages of Tevnia jerichonana, a deep-sea hydrothermal vent tubeworm

We describe a broad zone of intergradation between genetically differentiated, northern and southern lineages of the hydrothermal vent tubeworm, Tevnia jerichonana. DNA sequences from four genes, nuclear HSP and ATPs$\alpha$ and mitochondrial COI and Cytb were examined in samples from eastern Pacific vent localities between 13°N and 38°S latitude. Allelic frequencies at these loci exhibited concordant latitudinal clines, and genetic differentiation (pairwise $\Phi$ST's) increased with geographical distances between sample localities. Though this pattern of differentiation suggested isolation-by-distance (IBD), it appeared to result from hierarchical population structure. Genotypic assignment tests identified two population clusters comprised of samples from the northern East Pacific Rise (NEPR: 9–13°N) and an extension of the Pacific-Antarctic Ridge (PAR: 31–32°S) with a zone of intergradation along the southern East Pacific Rise (SEPR: 7–17°S). The overall degrees of DNA sequence divergence between the NEPR and PAR populations were slight and not indicative of lengthy isolation. Bayesian assignment methods suggested that the SEPR populations constitute intergrades that connect the NEPR and PAR populations. Though it typically is difficult to distinguish between primary and secondary intergradation, our results were consistent with parallel studies of vent-restricted species that suggest a high degree of demographic instability along the superfast-spreading SEPR axis. Frequent local extinctions and immigration from NEPR and PAR refugia probably shaped the observed pattern of intergradation.