Speciation and mitochondrial DNA diversification of the diving beetles Agabus bipustulatus and A. wollastoni (Coleoptera, Dytiscidae) within Macaronesia

Diversification of populations of two morphologically similar diving beetles within the Agabus tristis group, A. wollastoni and A. bipustulatus, was investigated, with partial mtDNA cytochrome b (Cyt b) sequences, allozymes and landmark-based morphometrics. The Madeiran endemic A. wollastoni was collected from 11 localities. Population genetic and morphological variation was compared to Scandinavian localities of the widespread west Palearctic A. bipustulatus, recorded also from the Azores. Agabus wollastoni and European A. bipustulatus specimens representing eight and 13 localities respectively, were used in evaluating their phylogenetic relationship. Maximum parsimony analysis of the Cyt b sequences showed that both the A. bipustulatus and A. wollastoni specimens form well-supported monophyletic groups. Three lines of evidence suggest that Agabus wollastoni has speciated through a few founders: (1) a well-supported mtDNA line; (2) the mean heterozygosity of A. wollastoni is lower when compared to A. bipustulatus on the mainland; and (3) several uncommon alleles of A. bipustulatus are missing in A. wollastoni. The Azorean A. bipustulatus population was drastically affected by the colonization event, since several loci have become fixed with a resulting lower mean heterozygosity. The colonization was relatively recent, as the mtDNA lineage represented in the Azores is deeply nested within the A. bipustulatus clade. Population structure shows moderate inbreeding of A. wollastoni, and extensive substructuring at all localities with moderate gene-flow between them. Morphological variation in A. wollastoni showed significant differentiation among several populations. Island colonizations, population structure of A. wollastoni, and an observed pattern of variation of the α-glycerophosphate dehydrogenase locus are discussed.