Correlation of morphological diversity with molecular marker diversity in the rough periwinkle Littorina saxatilis (Olivi)

Two morphological varieties of Littorina saxatilis, widespread around the U nited Kingdom, are a thin-shelled, high-shore morph (L. saxatilis H) and a thick-shelled, mid-shore animal (L. saxatilis M). Mitochondrial DNA (mtDNA) analysis by PCR-RFLP was used to test whether gene flow between these morp hs is restricted. At Galloway, Scotland. replicated sampling (different yea rs and different transects over a distance of 800 in) has been undertaken. One mtDNA haplotype is predominant in H and a different haplotype common in the M animals. Repeatability in space and time suggests a real HIM differe ntiation. A similar pattern of mtDNA haplotype variation is seen in a singl e sample of the Swedish E and S morphs of Littorina saxatilis. However, thi s pattern is not evident everywhere. Variation in the mtDNA and four nuclea r DNA loci was examined within and between L. saxatilis H and M morphs from the south coast of England. Because shape variation in this region additio nally separates into three shape groupings (regions identified from multiva riate morphometric analysis where shape is more homogeneous within, than be tween groups), genetic variability was examined within and between these gr oupings as well as between H and M. On the south coast, an apparent associa tion of shape and mtDNA haplotype is identified, but AMOVA analysis shows n o support for the association being with shape grouping or H and M morphs. Although the nature of this shape-genotype association is unknown, a mtDNA haplotype and an allele at the nuclear CAL-2 locus are confined mainly to o ne shape group. Analysis of association of mtDNA haplotype with H and M mor phology suggests a strong correlation can be found in some areas (Galloway, Mumbles (south Wales), and between similar morphs in Sweden) yet no associ ation is seen at others (Ravenscar, UK, Ballynahown, Ireland, and the south coast of England). Thus, unravelling the basis of the H and M forms will r equire more detailed studies, with replication, as at Galloway, and also wi th additional molecular markers.