Physiological mechanisms underlying a species invasion [electronic resource] : adaptive differences between invasive and native mussels (genus mytilus) on the California coast
Keywords: Biology.

The focus of this dissertation is to identify physiological mechanisms that facilitate the success of invasive versus native marine mussels (genus Mytilus) on the California coast. Such mechanisms are the result of a recent adaptive divergence between two species due to the disparate environmental conditions in the Mediterranean Sea (invasive home range) and the North Pacific Ocean (native home range). I have discovered candidate genes and biological pathways that are likely to be important for the evolution of (i) tolerance to environmental stress and (ii) enzyme function in different thermal habitats. Invasive and native blue mussels (genus Mytilus) on the California coast: the role of physiology in a biological invasion (Lockwood & Somero 2011 Journal of Experimental Marine Biology and Ecology). In physiological comparisons, invasive Mytilus galloprovincialis was found to be more warm-adapted than native Mytilus trossulus. I measured higher activities of enzymes involved in ATP generation to show that the native M. trossulus is better adapted to colder conditions than M. galloprovincialis. Higher metabolic capacity (temperature compensation) in the native species may lead to higher metabolic costs at sites where the two species co-occur. These data predict that M. galloprovincialis will continue to be the dominant blue mussel species along the warmer range of the California Current. Transcriptomic responses to acute heat stress in invasive and native blue mussels (genus: Mytilus): molecular correlates of invasive success (Lockwood et al. 2010 Journal of Experimental Biology). We developed an oligonucleotide microarray to elucidate possible differences in stress-induced gene expression between these congeners. In acute heat-stress experiments, 1531 genes showed temperature-dependent changes in expression that were highly similar in the two congeners. By contrast, 96 genes showed species-specific responses to heat stress, functionally characterized by their involvement in oxidative stress, proteolysis, energy metabolism, ion transport, cell signaling and cytoskeletal reorganization. The gene that showed the biggest difference between the species was the gene for the molecular chaperone small heat shock protein 24, which was highly induced in the invasive species and showed only a small change in the native. These different responses to acute heat stress may help to explain -- and predict -- the success of this invasive mussel species in a warming world. Transcriptomic responses to salinity stress in invasive and native blue mussels (genus: Mytilus) (Lockwood & Somero 2011 Molecular Ecology). To determine the extent to which interspecific differences in physiological tolerance to low salinity might explain limits to the invasive species' biogeography, I used an oligonucleotide microarray to compare the transcriptional responses of these two species to an acute decrease in salinity. Among 6777 genes, 117 showed changes in expression that were similar between species, and 12 genes showed species specific responses to salinity stress. Osmoregulation and cell cycle control were important aspects of the shared transcriptomic response to salinity stress, whereas the genes with species-specific expression patterns were involved in mRNA splicing, polyamine synthesis, exocytosis, translation, cell adhesion, and cell signaling. Forty-five genes that changed expression significantly during salinity stress also changed expression during heat stress, but the direction of change in expression was typically opposite for the two forms of stress. Temperature adaptation of orthologous enzymes correlates with biogeographic distributions of invasive vs. native marine mussels (genus Mytilus) on the California coast. Physiological comparisons indicate that M. galloprovincialis is more warm-adapted than M. trossulus, but the degree of divergence between these species at the protein level is largely unknown. To determine the extent to which temperature adaptation has shaped the evolution of orthologous proteins in these two species, we measured the effects of temperature on the kinetic properties of five enzymes that are important for ATP-generation and that represent distinct protein structural families. We surveyed phosphoglucomutase (PGM), phosphoglucose isomerase (PGI), pyruvate kinase (PK), phosphoenolpyruvate carboxykinase (GTP) (PEPCK), and isocitrate dehydrogenase (NADP) (IDH). Of these four enzymes, only IDH orthologs showed significantly different kinetics between the two species. The Michaelis-Menten constant (Km) of isocitrate of IDH from M. galloprovincialis was lower and more thermally stable than that of M. trossulus, and thus had higher substrate affinity at high temperatures. This suggests that the IDH of M. galloprovincialis is warm-adapted relative to the ortholog of M. trossulus. Taken together, these results indicate that while there has been significant temperature adaptation between IDH orthologs of M. galloprovincialis and M. trossulus, this type of biochemical adaptation may only occur in a fraction of the proteome.