Gene flow across a major biogeographic barrier is not increasing under climate change for the barnacle Catomerus polymerus

Abstract

On the south-east coast of Australia, the impermeability of the Southeast Australian Biogeographic Barrier (SEABB) limits the dispersal of many intertidal taxa, but current flow across the SEABB is expected to be increasing under climate change. The surf barnacle Catomerus polymerus has extensive populations on either side of the barrier that both mitochondrial cytochrome c oxidase subunit I (mtCOI) sequence data and surveys using 4-6 microsatellite loci imply have been separated for millennia (>167000 yr). Nevertheless, these data sets (based on collections from 2005-2012) also imply that there has been some recent dispersal across the SEABB. If trans-barrier migration is now possible, then in the absence of selection the differentiation of lineages should be eroded. Next-generation sequencing provides the opportunity to both better quantify levels of population differentiation attributable to the SEABB and to determine if there is regional selection. Our data (3801 single nucleotide polymorphisms) for individuals collected in 2019 within 3 eastern and 4 western sites support earlier reports of low population differentiation within eastern and western regions (FST < 0.021) and the strong regional separation of eastern and western lineages (FRT = 0.23). Moreover, in contrast to earlier studies, we did not detect any putative migrants, with all individuals assigning most strongly to their sampling region. Most strikingly, analysis using BAYESCAN implied that 47 loci (1.24%) of our surveyed loci show evidence of significant regional diversifying selection, which implies that even if C. polymerus larvae are able to cross the barrier, they will be strongly disfavoured by selection.