Abstract
The fluid nature of the ocean, combined with planktonic dispersal of marine larvae, lowers physical barriers to gene flow. However, divergence can still occur despite gene flow if strong selection acts on populations occupying different ecological niches. Here, we examined the population genomics of an ectoparasitic snail, Coralliophila violacea (Kiener 1836), that specializes on Porites corals in the Indo‐Pacific. Previous genetic analyses revealed two sympatric lineages associated with different coral hosts. In this study, we examined the mechanisms promoting and maintaining the snails’ adaptation to their coral hosts. Genome‐wide single nucleotide polymorphism (SNP) data from type II restriction site‐associated DNA (2b‐RAD) sequencing revealed two differentiated clusters of C. violacea that were largely concordant with coral host, consistent with previous genetic results. However, the presence of some admixed genotypes indicates gene flow from one lineage to the other. Combined, these results suggest that differentiation between host‐associated lineages of C. violacea is occurring in the face of ongoing gene flow, requiring strong selection. Indeed, 2.7% of all SNP loci were outlier loci (73/2,718), indicative of divergence with gene flow, driven by adaptation of each C. violacea lineage to their specific coral hosts.
Highlights
Genome-wide single nucleotide polymorphism (SNP) data from six sympatric populations of C. violacea revealed two clearly differentiated clusters that were largely concordant with coral host, consistent with results from mitochondrial DNA (Simmonds et al, 2018)
As with insects (Jean & Jean-Christophe, 2010; Simon et al, 2015), this genome-wide differentiation supports the conclusion of ecological divergence based on host association and adds to a small but growing literature on ecological divergence in marine environments (Fritts-Penniman et al, 2020 ; Potkamp & Fransen, 2019; Titus, Blischak, & Daly, 2019)
While SNP data reveal significant divergence between host-specific lineages of C. violacea, divergence was substantially lower in genome-wide SNPs compared to mtDNA (FST = 0.047 vs. ΦCT = 0.561)
Summary
While ecological speciation has been documented for almost three decades across a wide variety of organisms on land Reef-building corals have tight ecological associations with a wide variety of invertebrate taxa (Zann, 1987), including ~900 named species of sponges, copepods, barnacles, crabs, shrimp, worms, bivalves, nudibranchs, and snails (reviewed by Stella et al, 2010). This wide array of symbiotic relationships creates tremendous potential for host shifting and the development of host specificity that can lead to sympatric speciation. We use genome-wide single nucleotide polymorphisms (SNPs) to investigate the possibility of ecological divergence with gene flow in populations of a corallivorous gastropod, C. violacea, from the Coral Triangle. QB3 Genomics at the University of California, Berkeley performed quality checks (qPCR, BioAnalyzer) and sequencing, multiplexing 10–20 snails per lane in 5 lanes of a 50 bp Single-End run on an Illumina HiSeq 2000 sequencer
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