Abstract

Aim: Accurate characterization of evolutionary units (species or populations) underlies all ecological and evolutionary studies and is crucial to conservation planning. Seascapes have long been thought to be highly permeable to gene flow, yet over the last decade building evidence has shown that barriers to gene flow in marine environments are much more common than previously thought. Here, we precisely characterize two barriers to gene flow in the tunicate Pyura chilensis across 26° of latitude in the southeastern Pacific, assess their magnitude and explore their congruence with current biogeographical patterns of this region. Location: The southeastern Pacific (SEP), from Ilo, Perú (17°S) to Chiloé, Chile (43°S). Taxon: Pyura chilensis. Methods: We used a combination of highly polymorphic microsatellite markers and a 540 bp fragment of the Cytochrome Oxidase subunit I (COI) to compare individuals sampled at 26 localities spanning approximately 2500 km of the SEP. Genetic diversity was analysed using Bayesian clustering, haplotype networks, Isolation by Distance and cline models. A Coalescent simulator was used to estimate migration rates. Results: The results from both the microsatellite and COI markers indicate the presence of two genetic discontinuities: one at 34°S and one at 39°S which coincide with genetic breaks reported for other species. Interestingly, we were able to determine that genetic transitions occur abruptly and within short geographical distances (~30 km) compared with previous studies of this tunicate. Coalescent simulations indicate the 34°S break is less permeable than the 39°S break, and gene flow appears to be mostly from north to south. Main conclusions: Our results support other studies that show that seascapes are complex, and also highlight the importance of accurately sampling distribution ranges when making conclusions about gene flow. Overall, the two main biogeographical barriers to gene flow characterized in the southeastern Pacific are not homogeneously permeable and can be narrow (<30 km). These results are relevant for the management of fisheries in this region and specifically for this commercially important species.