Freezing temperatures present a lethal barrier that limits the geographic distribution of many organisms; this research will illuminate the evolution and significance of freeze tolerance. The genes underlying freeze-tolerance will be isolated from a marsh snail (Melampus bidentatus) found in freezing regions atypical for its genus. Tests for natural selection for freeze tolerance in northern species will be made through comparison of DNA sequences of these genes from freeze-tolerant species and their tropical relatives. Expression of freeze-tolerance genes also will be measured and compared among winter populations of freeze-tolerant and non-tolerant species to determine if the pattern of gene expression contributes importantly to survival of freezing temperatures. Collectively, the results will test for the importance of evolution of freeze-tolerance in allowing range expansion from tropical to temperate regions. A tropical maximum in species diversity has been recognized for over a century, and the rarity of evolution of the ability to avoid freezing may underlie the pattern. This research will directly test for correlation between the evolution of freeze tolerance and a poleward range expansion. The research will identify the genetic and molecular basis of a key ecological trait, freeze tolerance, and will lead to a better understanding of what determines the range of a species. The work will test a key mechanism hypothesized to underlie a major global diversity pattern and will provide a detailed case study of one species? evolutionary responses to global climatic change.
