Mechanisms generating microgeographic clines in shell traits in intertidal snail populations

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My research program uses experiments, genomics, and modelling to test the hypothesis that genetic divergence within populations of direct-developing intertidal snails is maintained by spatial variation in selection by predators and by the risk of dislodgement from waves. My long-term field predator invasion experiments on Vancouver Island, Canada test the prediction that transplanting predatory shore crabs into wave-exposed areas where they are normally rare will cause evolution of a microgeographic cline in shell thickness in the surrounding populations of Littorina subrotundata. I mimicked an invasion of non-indigenous predators by building concrete refuges for a native predatory purple shore crab that allows access to prey in two wave-exposed intertidal areas where there are few natural predators on snails (Littorina). Our tethering experiments to quantify selection by predatory shore crabs for thicker shell armour in L. subrotundata near my experimental predation gradient allow us to parameterize my spatial model of a quantitative trait. During this grant cycle we will validate my model by culturing snails from our experimental populations in a common environment to see if increases in thickness have occurred. We will then parameterize my model for two ecotypes of L. saxatilis present on wave-exposed rocky shores in NW Spain that show extreme genetically-based morphological divergence for size, shell thickness, ridging and colouration and partial reproductive isolation through size-assortative mating. My Ph.D. student has developed a new inexpensive method using Next Generation Sequencing (NGS) and a shared computer network to simultaneously find and genotype thousands of DNA markers (Single Nucleotide Polymorphisms (SNPs)) in a size-selected sample of individual snail genomes. My undergraduate students have created backcrosses between the upper- and lower-shore ecotypes of L. saxatilis and are rearing the full-sibling families in the University of Guelph's AquaLab. My students will look for associations between complex traits (shell size, shape, and mass) and SNPs. We will then map the SNPs associated with a particular trait onto the draft L. saxatilis genome. We will use these same SNP markers to find outlier loci in genomic regions contributing to genetic differentiation between the two ecotypes and identify candidate genes involved in biomineralization of shell armour. Once this model system for ecological speciation is better characterized, we will use the same NGS methodology on our experimental populations of L. subrotundata. Our work contributes to predicting the effects of increased rates of predator invasions attributable to climate change. Our work also has applications to understanding the effects of ocean acidification on predator-prey interactions between calcified invertebrates. **