Microscale genetic differentiation along the vertical shore gradient in White Sea snails Littorina saxatilis (Olivi) assessed by microsatellite markers

摘要

The intertidal zone represents a unique type of habitat, where several important environmental characteristics (especially temperature and degree of aerial exposure) change concord-antly and drastically over a small spatial scale along the vertical shore gradient. Studies on a model gastropod, Littorina saxatilis (Olivi), have demonstrated considerable morphological, physio logical and genetic differentiation in populations of this species along this gradient. Selection has been suggested to play an important role in genetic differentiation of L. saxatilis, leading to significant differences at a few (presumably selectively important) loci between high and low shore subpopulalions despite high genetic homogeneity for most other (presumably neutral) loci. Stable physiological and heritable morphological differences between high and low shore subpopulations of L. saxatilis are consistent with adaptative responses to differential selection at different shore levels. In some populations, partial reproductive isolation has been observed between high and low-shore ecotypes of L. saxatilis, suggesting possible specia-tion in statu nascendi. All these findings have firmly established high and low-shore ecotypes of L. saxatilis as a valuable model-system for the study of the roles of selection and gene flow in phenotypic differentiation and microevolution. Study of fine-scale metapopulation structure and microevolution in this species requires further development of high-resolution genetic markers, which might be useful in revealing the genetic differentiation between closely related groups such as subpopulations of L. saxatilis. In this study, we use microsatellite DNA markers for the first time in this model organism. Using this technique, we examined microscalc genetic differentiation along the vertical gradient of the intertidal zone in a White Sea population of L. saxatilis. In order to test whether genetic differentiation along the vertical shore gradient is higher than would be expected from the distance alone, we assayed the variability in allele frequencies for five microsatellite loci in L. saxalilis from high and low shore levels, and between equidistant sub-populations within the same shore level.