Gene Flow and Divergence Across the Equatorial Tropical Marine Barrier: Past, Present and Future

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The marine communities of the northern and southern hemispheres are kept distinct by warm water and strong currents near the equator. Nevertheless, many taxa have established "antitropical" distributions, occurring in temperate zones on either side of the tropics but not within the warmest tropical regions. Although paleontological studies and molecular phylogenies provide information about the timing and direction of trans-tropical colonization, the existence of single species with antitropical distributions provides an opportunity to begin to investigate several fundamental questions about the recent history of and future prospects for biological connectivity between the northern and southern hemispheres. This project focuses on characterizing connectivity across the tropics in the antitropical gooseneck barnacle Pollicipes elegans. This eastern Pacific species is found in both hemispheres but is absent from the warmest waters immediately to the north of the equator. Previously-collected data show that mtDNA haplotype lineages are shared between northern and southern hemisphere populations, and that the separation between north and south has been relatively recent, within the last 160,000 years. This research has two main goals. First, the investigators will use multi-locus sequence data and microsatellite loci to estimate the time of separation of northern and southern populations and how much gene flow has occurred since that split, both over evolutionary timescales and across recent generations. The second goal is to characterize larval thermal tolerance and performance as a means to test the hypothesis that temperature is a barrier to larval dispersal across the warmest tropical regions in P. elegans. Although several other hypotheses can potentially explain what keeps P. elegans out of the tropics, such as the isolating effects of the equatorial boundary currents or post-settlement processes, assessing the temperature sensitivity of the dispersive larval stage will provide the first important physiological test of how temperature limits connectivity. Understanding the history of gene flow and the temperature tolerance of dispersive larvae will provide insight into the impacts of recent and projected increases in tropical sea surface temperatures on recent and future patterns of connectivity between the hemispheres.This project will provide financial support and training in fieldwork, population genetics, and larval ecology for two graduate students and two postdoctoral researchers. Undergraduate students will be involved with aspects of the research through a hands-on lab class taught in Molecular Population Biology and recruitment from courses taught at Clemson. The investigators will also continue to engage a local school district (LaFrance, South Carolina) via classroom visits by scientists (PI, graduate students, and postdocs). The investigators publish actively in leading scientific journals and written invited contributions for the fields of biogeography and life history evolution. Research in the their labs is disseminated via web pages and previous work has reached the public through a wide variety of media outlets.