The evolution of freeze tolerance in a historically tropical snail

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Geographic range differences among species may result from differences in their physiological tolerances. In the intertidal zone, marine and terrestrial environments intersect to create a unique habitat, across which physiological tolerance strongly influences range. The snail Melampus bidentatus occurs in coastal salt marshes in the western Atlantic and Gulf of Mexico. I have used sequence data from one mitochondrial (CO1) and two nuclear markers (histone H3 and a mitochondrial carrier protein, MCP) to identify three cryptic species within Melampus bidentatus, and to infer their relationships to other Melampus. To identify microhabitat differences between two cryptic species, I modeled their distributions using both marine and terrestrial environmental data. Temperature largely explained their range differences, but other environmental components (precipitation, salinity, and tidal height) explained facets of the range that temperature cannot. To test for phylogenetic conservation in freeze tolerance, I tested the mean lower lethal temperature (LT50) of three temperate and three tropical species. Cryptic species of M. bidentatus are significantly more freeze tolerant than their tropical relatives, although there was variation among locales within species, most likely due to microhabitat variation. The temperate species M. floridanus was also freeze tolerant, but without testing the LT50 of its closest relatives in the Pacific, I cannot determine this represents an independent evolution of freeze tolerance, or if this trait is more widely shared among Melampus. Nonetheless, the lack of freeze tolerance in the most basal species that I have tested (M. bullaoides), and the predominantly tropical distributions of most ellobiids, suggests that the evolution of freeze tolerance has allowed for the invasion of the temperate zone by Melampus. Using massively parallel sequencing, I have isolated > 500,000 expressed sequence tags and assembled these into ~20, 000 seasonally expressed transcripts. A comparison of these transcripts has revealed 2 candidate markers to test for their association with freeze tolerance in M. bidentatus, and many more markers that can be used for further phylogenetic analyses in Melampus. Further work to examine variation in both the sequence and expression of these proteins is needed to determine if they underlie adaptive differences among species.