Mitochondrial DNA Col haplotype variation in sibling species of rough periwinkles

Three sibling species of rough periwinkles are currently recognized: Littor ina arcana, L. compressa and L. saxatilis. Certain forms of L. saxatilis ar e also argued by some to deserve species status, such as the barnacle-dwell ing 'L. neglecta' and the lagoonal 'L. tenebrosa'. Relationships between th ese taxa, and between and within representative populations, are investigat ed using sequence analysis and restriction fragment length polymorphism of a mitochondrial DNA fragment spanning the cytochrome oxidase I and cytochro me oxidase II gene boundary. These data show that there is some sharing of haplotypes between species, with L. arcana haplotypes paraphyletic with res pect to L. saxatilis haplotypes, and L. compressa haplotypes paraphyletic t o both L. arcana and L. saxatilis haplotypes. Such sharing of mtDNA haploty pes could be a consequence of either persistent hybridization or episodes o f hybridization, or incomplete lineage sorting of ancestral polymorphisms. On the balance of evidence it is suggested that the latter, rather than hyb ridization events, is the more likely causal agent of the observed distribu tion. Intraspecific variation is extensive and it is suggested that the pat terns of intraspecific polymorphism are explainable by a combination of his torical factors (the impact of the Pleistocene ice-age) and contemporary re strictions to gene flow. It is argued that Littorina haplotypes evolved in at least two separate glacial refugia and became scattered by the subsequen t range expansion around most of the coastline. Recent factors such as rest ricted gene flow and bottlenecks would then be capable of affecting the hap lotype distribution, resulting in the pattern observed.