No one knows when the European invader arrived, probably in the early twentieth century, but by the time anyone noticed in the 1980s, it was too late. The Mediterranean blue mussel (Mytilus galloprovincialis) had already driven its close relative, Mytilus trossulus, from its warm home in Southern California, leaving trossulus the cooler waters it occupied in Northern California and around the Pacific basin to Japan. George Somero has been fascinated by the physiological adaptations that organisms make to their environments and was intrigued by the galloprovincialisinvasion. He knew that galloprovincialis had left the Pacific for the warmer Mediterranean a mere 3.5 million years ago, but what adaptations had the mussel made during its brief European sojourn that allowed it to thrive and drive trossulus out? Peter Fields, returning to Somero's lab for a brief sabbatical period, decided to take up the challenge by investigating one of the mussel's key metabolic enzymes: cytosolic malate dehydrogenase(p. 656).
According to Fields, malate dehydrogenase is a well-characterised, and essential, component of several metabolic pathways, but the enzyme is also temperature sensitive, making it an ideal candidate for adaptation to warmer conditions. However, sequencing the gene from mussels proved trickier than Fields had hoped. Little is known about mollusc genomes, so designing the DNA primers essential for the sequencing process proved challenging. Fortunately Somero's colleague, Andy Gracey, was on hand to guide Fields through the complex sequence databases needed to design the primers, before Fields ran the sequencing reactions on both blue mussels and another California local, Mytilus californianus, that lives in the mild waters where galloprovincialis's and trossulus's territories overlap.
Having sequenced the three genes, Fields realised that one position in the gene lit up as a mutation hot spot; all three species had completely different amino acids at position 114, the hinge of a loop region essential for the enzyme's catalytic activity. Fields was astonished. He explains that he'd investigated related enzymes in cold adapted species and never seen mutations at this location before. What effect would this mutation have on the enzyme's function?
Instead of extracting the enzyme directly from each of the three mussels,Fields' student, Emily Rudomin, expressed all three proteins in bacteria,allowing her to produce larger quantities of each enzyme than she could extract from the mussels. Measuring each enzyme's kinetics, Fields realised that each of the enzymes were well adapted to the water temperature where the mussels lived. He explains that enzymes that function well in warm conditions have low substrate turnover rates and bind their substrate more tightly than enzymes adapted to cold conditions, which process their substrate faster and bind substrate more loosely. Sure enough, the warm adapted galloprovincialis' malate dehydrogenase behaved like a warm adapted enzyme, while the cold adapted trossulus enzyme functioned best at low temperatures. Mytilus californianus, found on the beach outside Somero's Pacific Grove lab, was optimised for a mild, intermediate temperature.
The mutated amino acid located in the hinge region had dramatically affected the enzyme's function, and is one of a raft of adaptations giving the impostor, Mytilus galloprovincialis, the upper hand over Mytilus trossulus in hot water.
