Out-of-the tropics or trans-tropical dispersal? The origins of the disjunct distribution of the gooseneck barnacle Polli

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RESEARCH

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Out-of-the tropics or trans-tropical dispersal? The origins of the disjunct distribution of the gooseneck barnacle Pollicipes elegans Sergio Marchant1* , Amy L. Moran2 and Peter B. Marko2

Abstract Background: Studying species with disjunct distributions allows biogeographers to evaluate factors controlling species ranges, limits on gene flow, and allopatric speciation. Here, we use phylogeographic and population genetic studies of the barnacle Pollicipes elegans to discriminate between two primary hypotheses about the origin of disjunct distributions of extra-tropical populations: trans-tropical stepping-stone colonization versus an out-of-the tropics origin. Results: Nucleotide diversity peaked in the centre of the species’ range in samples from El Salvador and was lower in samples from higher latitudes at Mexico and Peru. Haplotypes from El Salvador samples also had a deeper coalescent, or an older time to a most recent common ancestor. A deep phylogeographical break exists between Mexico and all samples taken to the south (El Salvador and Peru). Isolation-with-migration analyses showed no significant gene flow between any of the three regions indicating that the difference in genetic differentiation among all three regions is explained primarily by differences in population separation times. Approximate Bayesian Computation model testing found strong support for an out-of-the tropics origin of extra-tropical populations in P. elegans. Conclusions: We found little evidence consistent with a stepping-stone history of trans-tropical colonization, but instead found strong evidence for a tropical origin model for the largely disjunct distribution of P. elegans. Sea surface temperature and habitat suitability are likely mechanisms driving decline of populations in tropical regions, causing the disjunct distribution. Keywords: ABC, Climate, Coalescent, Demographic history, Diversity, Marine, Out-of-the tropics, Phylogeography, Sea surface temperature

Background Species with disjunct or geographically discontinuous ranges are important systems for understanding the factors controlling species’ distributions, population connectivity, and the process of allopatric speciation [1–10]. The two main biogeographic mechanisms that have been proposed to explain range disjunctions are dispersal across an uninhabitable region; or vicariance, the fragmentation of a species’ ancestral range through the formation of an * Correspondence: [email protected] 1 Department of Biological Sciences, Clemson University, 132 Long Hall, Clemson, SC 29634, USA Full list of author information is available at the end of the article

unsuitable or impassable region. In both cases, allopatric speciation can take place if the gap in a species’ range sufficiently limits gene flow [11]. Although vicariance and dispersal may not be mutually exclusive processes [12–14], distinguishing their roles remains central to the overall goal of biogeography [12–18], to characterize the primary mechanisms that create spatial and temporal