Population Genomics and Biogeography of the Northern Acorn Barnacle ( Semibalanus balanoides ) Using Pooled Sequencing Approaches

The northern acorn barnacle (Semibalanus balanoides) is a robust system for the study of evolutionary processes in the intertidal. S. balanoides has a well-characterized ecology, a wide circumboreal distribution, and a life history characterized by tractable environmental stressors at various ecological scales. In this chapter, we discuss a variety of topics concerning the development of S. balanoides as a model in ecological genomics as well as inferences of demography and historical phylogeography. In addition, we introduce two novel genomic tools for S. balanoides: the complete mtDNA sequence and the second draft of the nuclear genome (Sbal2). Using these tools, we conducted a reanalysis of previously described mtDNA haplotypes, a and b, as well as genome-wide levels of variation and population structure across the North Atlantic using pooled sequencing approaches. Analyses of sequence data from older and more recent Illumina platforms revealed the effects of technical bias in the estimates of population genomic metrics. We found concordant levels of nuDNA and mtDNA genetic variation with no evidence of demographic bottlenecks. We observed low genome-wide FST values across the Atlantic, suggesting a large number of ancestral polymorphisms and shared standing variation across the basin. Comparisons of genome-wide estimates of FST with those derived from a discriminant analysis of principal components uncovered population-structure-informative SNPs. This suggests the existence of latent population structure across broad scales, despite the capacity for extensive planktonic dispersal. Noticeably, our samples collected in Iceland displayed higher similarity to North American populations than to the rest of Europe. We hypothesize this is consistent with a periglacial refugium in Iceland concomitant with a barrier to gene flow caused by the North Atlantic current. Lastly, we discuss challenges and opportunities for the improvement of genomic tools in barnacles. Our reflections in this area are easily generalizable to most natural populations.