Host adaptation to an invasive body-snatching parasite
In the Gulf of Mexico, the parasitic barnacle Loxothylacus panopaei tunnels into mud crabs, castrates them, and turns them into zombie nursemaids for the parasite's offspring. Fifty years ago, this body-snatching parasite invaded the Chesapeake and spread south to Florida. In its native Gulf range, the parasite infects just 1-5% of crabs, while in the invasive range infection rates are often over 70%. This project explores the parasite's impacts on its host's evolutionary trajectory, focusing on the idea of adaptive host resistence. By combining transcriptomics with field ecology and laboratory experiments, I am comparing crab populations across a mosaic of coevolutionary history that includes regions where the parasite is native, where it is absent, and where it was recently introduced. Fieldwork ranges from the bayous of Cajun country, through bays and estuaries of the Gulf and Atlantic coasts, to New Hampshire, where this parasite has never been found and the host crabs are naive. Back in the laboratory, I experimentally expose crabs from each field site to parasite larvae to determine if populations vary in their susceptibility to parasitism under the same environmental conditions, and take tissue samples to peer into the genome of the host for evolutionary fingerprints. By combining field, lab, and genomic data, I am exploring the process of how – and how quickly – parasites impact the evolution of their hosts.
Thermal adaptation in invasive green crabs
The European green crab (Carcinus maenas) has been highly successful across a wide gradient of thermal environments in its native and invasive ranges. As such, the species may provide a template for the mechanisms underpinning success in a rapidly changing climate. I combined macrophysiology and high-throughput sequencing to examine thermal tolerance, gene expression, and gene sequence across seven locations in the species' native European and invasive North American ranges. Even after common-temperature acclimation, crabs differed in their thermal tolerance between sites, suggesting local adaptation in the native range. This adaptation may have facilitated invasion across a wide thermal gradient on the East Coast of North America. Using mRNA-sequencing, I identified a suite of genes with expression changes that are associated with acclimation to hot and cold temperatures.These same mRNA-seq data were used to generate a panel of thousands of variable SNPs that detected subtle structure between sites. Outlier analyses identified several candidate genes likely under selection in their local environments; allele frequency at these genes was disproportionately correlated with temperature. Despite high gene flow, green crabs do appear to be locally adapted to their environments at the levels of physiology, gene expression, and gene sequence.