Linking functional molecular variation with environmental gradients: Myosin gene diversity in a crustacean broadly distributed across variable thermal environments

To investigate the molecular basis of temperature adaptation in natural populations we used the candidate gene approach, targeting the myosin heavy chain (MyHC) gene. The functional effects of genetic variation in MyHC have been well characterised, and changes in the flexibility of the surface loops 1 and 2, caused by modulations in length, amino acid composition and charge can play an important role in thermal acclimation in fish. However, the extent that MyHC diversity is influenced by natural thermal gradients is largely unknown. Sequence variation in MyHC cDNA was examined in 7 species of gammarid amphipod with broad latitudinal distributions and differing intertidal thermal habitats in the NE Atlantic and Arctic Oceans. A high degree of diversity was detected in the loop 1 nucleotide sequences, although not all are likely to be functional transcripts, and their deduced amino acid sequences indicated no differences in the length and charge of loop 1 and associated binding kinetics. Four isoforms for loop 2 were detected which differed in sequence length and charge distribution, suggesting functional differences in sliding velocities and ATPase activities. While all species, and indeed most individuals, expressed multiple loop 2 isoforms, analysis of the two species with the greatest number of sequenced clones revealed that G. duebeni, a high-shore species with the highest thermal tolerance, expressed a greater diversity of forms than G. oceanicus, a low intertidal species more sensitive to temperature change. Latitude further influenced MyHC loop 2 diversity in G. duebeni, as the number of isoforms increased in the northern populations. Species-specific variations in MyHC diversity were observed, irrespective of phylogenetic associations revealed by analysis of the mitochondrial cytochrome oxidase 1 (CO1) gene. Overall, it appears that the temporal temperature variations associated with higher intertidal habitat may be a greater selective agent for MyHC isoform diversity in gammarid muscles than broad spatial changes with latitude.