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Marine Ecology Progress Series

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MEPS 227:195-204 (2002)  -  doi:10.3354/meps227195

A GTT microsatellite repeat motif and differentiation between morphological forms of Littorina saxatilis: speciation in progress?

Craig S. Wilding*, John Grahame, Peter J. Mill

Centre for Biodiversity and Conservation, School of Biology, University of Leeds, Leeds LS2 9JT, United Kingdom

ABSTRACT: A size (and sequence) variable microsatellite has been identified in the gastropod Littorina saxatilis following sequencing of products obtained through randomly amplified polymorphic DNA amplification. Size frequency distributions for this GTT repeat motif have been produced from a total of 439 L. saxatilis. Although there is evidence for a high prevalence of null alleles, consistent, largely significant, differences are found between the average size of this repeat in high shore, thin shelled morphs (L. saxatilis H) compared to mid shore, thick shelled animals (L. saxatilis M), with the former having consistently larger allele sizes. These findings are evident on both a large scale (around the coast of Britain) and a microgeographic scale (replicated sampling of a single shore). Unusually large alleles (>400 bp) are also considerably more prevalent in L. saxatilis H, and sequencing of these, including DNA flanking the GTT repeat, indicates that they do not represent independent expansion events. The significant difference in GTT repeat size, and increased prevalence of such related alleles in the H form compared to the M morph suggests that either gene flow is greater within, than between, morphs or that selection is acting on this locus or a closely linked locus. We argue for the latter scenario and discuss why this indicates that these forms are diverging across their range in the British Isles.

KEY WORDS: Speciation · Littorina saxatilis · Microsatellite · GTT · Repeat · Gastropod

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