Ornithine metabolism and the osmotic stress response in mytilid congeners

ABSTRACT Previous transcriptomic studies have suggested that ornithine decarboxylase (ODC) expression in Mytilids (Mytilus spp.) may be a critical component of the osmotic stress response. Ornithine is catabolized into polyamines by ODC or into glutamate or proline by ornithine aminotransferase (OAT). To better understand how ornithine is metabolized under low salinity conditions, we examined species-specific variation in OAT and ODC expression for three species that vary in their salinity tolerance (M. trossulus, M. edulis, and M. galloprovincialis). We found a consistent decrease in ODC expression during hypoosmotic exposure in all three species but pronounced species-specific increases in OAT expression. During hyperosmotic stress, the patterns of expression of these genes reversed, suggesting that proline or glutamate synthesis is important during low salinity exposure, while polyamine synthesis may be more important during hyperosmotic exposure. These responses were most pronounced in M. galloprovincialis, with an 18-fold increase in OAT during hypoosmotic exposure.