Latitudinal variation in protein expression after heat stress in the salt marsh mussel Geukensia demissa

Individuals of a broadly distributed species often experience significantly different environmental conditions depending on location. For example, the mussel Geukensia demissa occurs intertidally from the Gulf of St. Lawrence to central Florida; within this range, northern populations are exposed to temperatures cold enough to freeze the tissue, whereas southern populations can experience temperatures approaching the species' upper lethal limit. Thus, G. demissa provides an ideal system with which to study physiological variation in conspecifics occurring across a broad latitudinal range. We collected G. demissa at five sites from Maine to Florida, encompassing a range of 1900 km, and have used a proteomic approach to describe how protein expression varies in individuals from the different locations. We acclimated individuals from each site to common conditions (18°C) for 4 weeks, and exposed a subset of these to acute heat stress (40°C). We separated gill proteins using two-dimensional gel electrophoresis and quantified abundances of the resulting protein spots. Among mussels acclimated to 18°C protein, expression profiles were more similar among individuals from the same site than among sites, but there was no discernible correlation with latitude. In contrast, after acute heat stress, protein expression among mussels from different locations varied substantially, with 31 of 448 proteins changing in abundance in the northernmost (Maine) group, compared with 5-11 proteins in the four southern groups. Identification of 27 of these proteins revealed five functional clusters: chaperones, cytoskeletal proteins, oxidative stress proteins, regulatory proteins, and a translation initiation factor. Across these functional categories, the two northernmost groups, Maine and New York, showed the greatest number of proteins that changed significantly in abundance, as well as the greatest fold-change in abundance for many of the proteins. We conclude that the northern populations of G. demissa are physiologically distinct from the southern groups, and that the differences in protein-expression profiles are consistent with greater sensitivity to heat stress to the north.