Year of Graduation


Level of Access

Open Access Thesis

Embargo Period


Department or Program


First Advisor

David Carlon


Hybrid zones provide natural laboratories to study how specific genes, and interactions among genes, may influence fitness. On the east coast of North America, two separate populations of the European green crab (Carcinus maenas) have been introduced in the last two centuries. An early invasion from Southern Europe colonized New England around 1800, and was followed by a second invasion from Northern Europe to Nova Scotia in the early 1980s (Roman 2006). As these populations hybridize, new combinations of genes potentially adapted to different ends of a thermal spectrum are created in a hybrid zone. To test the hypothesis that mitochondrial and nuclear genes have effects on thermal tolerance, I measured response to cold stress in crabs collected from locations between southern Maine and northern Nova Scotia, and then genotyped the mitochondrial CO1 gene and two nuclear SNPs. Three mitochondrial haplotypes, originally from Northern Europe, had a strong effect on the ability of crabs to right themselves at a temperature of 4.5ºC. Crabs carrying these three haplotypes were 20% more likely to right compared to crabs carrying the haplotype from Southern Europe. The two nuclear SNPs, which were derived from transcriptome sequencing and were strong outliers between Northern and Southern European C. maenas populations, had no effect on righting response at low temperature. These results add C. maenas to the short list of ectotherms in which mitochondrial variation affects thermal tolerance, and suggests that natural selection is shaping the structure of the hybrid zone between the northern and southern populations This discovery of linkage between mitochondrial genotype and thermal tolerance also provides potential insight into the patterns of expansion for invasive populations of C. maenas around the world.