Abstract
Genetic‐environment associations are increasingly revealed through population genomic data and can occur through a number of processes, including secondary contact, divergent natural selection, or isolation by distance. Here, we investigate the influence of the environment, including seasonal temperature and salinity, on the population structure of the invasive European green crab (Carcinus maenas) in eastern North America. Green crab populations in eastern North America are associated with two independent invasions, previously shown to consist of distinct northern and southern ecotypes, with a contact zone in southern Nova Scotia, Canada. Using a RAD‐seq panel of 9,137 genomewide SNPs, we detected 41 SNPs (0.49%) whose allele frequencies were highly correlated with environmental data. A principal components analysis of 25 environmental variables differentiated populations into northern, southern, and admixed sites in concordance with the observed genomic spatial structure. Furthermore, a spatial principal components analysis conducted on genomic and geographic data revealed a high degree of global structure (p < .0001) partitioning a northern and southern ecotype. Redundancy and partial redundancy analyses revealed that among the environmental variables tested, winter sea surface temperature had the strongest association with spatial structuring, suggesting that it is an important factor defining range and expansion limits of each ecotype. Understanding environmental thresholds associated with intraspecific diversity will facilitate the ability to manage current and predict future distributions of this aquatic invasive species.
Highlights
The contemporary study of “seascape genetics” (Riginos & Liggins, 2013) is increasingly revealing fine-scale local structuring through the use of genomic data, allowing robust investigations of the forces influencing spatial genetic variation
Invasive species where hybridization occurs among invasive and native species or waves of invasion represent an unique natural history experiment (e.g., Fitzpatrick et al, 2010; Saarman & Pogson, 2015; Viard, David, & Darling, 2016) and allow the examination of both how secondary contact may influence the distribution of genomic variation as well as how hybridization and introgression maybe be influenced by factors such as environmental variation
Additional genomic resources from green crab, other decapods, or crustaceans in general, including annotated genomes or transcriptomic data, could lead to the identification of other gene regions that show relationships between allele frequency and environment, shedding light on the processes which have led to this observed population structure and any reproductive barriers associated with constraints in gene flow
Summary
The contemporary study of “seascape genetics” (Riginos & Liggins, 2013) is increasingly revealing fine-scale local structuring through the use of genomic data, allowing robust investigations of the forces influencing spatial genetic variation. Fine-scale population structure in marine species (Benestan et al, 2015; Bradbury et al, 2010; Harrisson et al, 2017) may be driven by a number of processes, including secondary contact among lineages or closely related species (e.g., Abbott et al, 2013; Hewitt, 1996, 2000; Mayr, 1942), recent adaptation along an environmental gradient (e.g., Endler, 1977; Sanford & Kelly, 2011), or some combination of physical isolation, genetic drift, and selection (Harrison & Larson, 2016). Cold winters in the Gulf of Maine have previously been linked to increased adult mortality whereas warm winters have been linked to increased recruitment (Yamada & Kosro, 2010) In both the native and introduced ranges, the mean high temperature at which cardiac function fails in adult crabs is consistently higher in southern populations which are adapted to overall warmer sea surface temperatures (Tepolt & Somero, 2014). Our results will provide insight into how secondary contact dynamics can be correlated with environmental heterogeneity potentially constraining the success of coastal marine invaders, and will help to understand and predict future range expansions and contractions of green crab ecotypes
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