Phenotypic and transcriptomic responses to salinity stress across genetically and geographically divergent Tigriopus californicus populations

Striking convergent evolution is found in the properties of the organic osmotic solute (osmolyte) systems observed in bacteria, plants, and animals. Polyhydric alcohols, free amino acids and their derivatives, and combinations of urea and methylamines are the three types of osmolyte systems found in all water-stressed organisms except the halobacteria. The selective advantages of the organic osmolyte systems are, first, a compatibility with macromolecular structure and function at high or variable (or both) osmolyte concentrations, and, second, greatly reduced needs for modifying proteins to function in concentrated intracellular solutions. Osmolyte compatibility is proposed to result from the absence of osmolyte interactions with substrates and cofactors, and the nonperturbing or favorable effects of osmolytes on macromolecular-solvent interactions.

Local adaptation in the sea was regarded historically as a rare phenomenon that was limited to a handful of species with exceptionally low dispersal potential. However, a growing body of experimental studies indicates that adaptive differentiation occurs in numerous marine invertebrates in response to selection imposed by strong gradients (and more complex mosaics) of abiotic and biotic conditions. Moreover, a surprisingly high proportion of the marine invertebrates known or suspected of exhibiting local adaptation are species with planktonic dispersal. Adaptive divergence among populations can occur over a range of spatial scales, including those that are fine-grained (i.e., meters to kilometers), reflecting a balance between scales of gene flow and selection. Addressing the causes and consequences of adaptive genetic differentiation among invertebrate populations promises to advance community ecology, climate change research, and the effective management of marine ecosystems.