Data from: Genomic evidence for ecological divergence against a background of population homogeneity in the marine snail Chlorostoma funebralis

The balance between natural selection, gene flow and genetic drift is
difficult to resolve in marine invertebrates with extensive dispersal and
fluctuating population sizes. The intertidal snail Chlorostoma funebralis
has planktonic larvae and previous work using mtDNA polymorphism reported
no genetic population structure. Nevertheless, recent studies have
documented differences in thermal tolerance and transcriptomic responses
to heat stress between northern and southern California, USA, populations.
To gain insight into the dynamics influencing adaptive divergence, we used
double-digest restriction site-associated DNA (ddRAD) sequencing to
identify 1861 genomewide, quality-filtered single-nucleotide polymorphism
(SNP) loci for C. funebralis collected from three northern and three
southern California sites (15 individuals per population). Considering all
SNPs, there was no evidence for genetic differentiation among populations
or regions (average FST = 0.0042). However, outlier tests revealed 34 loci
putatively under divergent selection between northern and southern
populations, and structure and SNP tree analyses based on these outliers
show clear genetic differentiation between geographic regions. Three of
these outliers are known or hypothesized to be involved in stress granule
formation, a response to environmental stress such as heat. Combined with
previous work that found thermally tolerant southern populations show high
baseline expression of stress response genes, these results further
suggest that thermal stress is a strong selective pressure across C.
funebralis populations. Overall, this study increases our understanding of
the factors constraining local adaptation in marine organisms, while
suggesting that ecologically driven, strong differentiation can occur at
relevant loci in a species with planktonic larvae.