Diversification of sympatric Sapromyza (Diptera: Lauxaniidae) from Madeira: six morphological species but only four mtDNA lineages.

A series of recent studies on speciation of insects within the Canary Islands have indicated considerable within-island diversification, similar to that described in the Hawaiian islands. Little work has yet been carried out on the neighboring Madeiran archipelago, which is also volcanic. This study examines relationships among all known Lauxaniid flies of the genus Sapromyza from Madeira (including six newly described morphological species) based on mitochondrial gene trees constructed from cytochrome c oxidase (subunit I) and 16S rRNA partial sequences. Phylogenies based on maximum likelihood distances, a Bayesian method based on Markov chain Monte Carlo sampling from the posterior probability distribution, and maximum parsimony show that eight of the nine Madeiran species comprise a single monophyletic group. This clade is also split into two subclades representing black- and yellow/orange-bodied forms. The latter mtDNA clade corresponds to only two species (Sapromyza imitans and Sapromyza indigena) which are not reciprocally monophyletic. Monophyly is strongly supported within four of the six black-bodied species but not for the species pair (Sapromyza inconspicua, Sapromyza laurisilvae). We discuss the double occurrence (at least) of introgressive hybridization/incomplete lineage sorting within this group and suggest that recent speciation is the most likely explanation. The remaining species on the island, Sapromyza madeirensis, is very divergent from the aforementioned group, occupying a more basal position in the tree than the other Atlantic island and continental Sapromyza that were included in the analysis. At least two speciation events for Madeiran Sapromyza appear to correspond to quite ancient periods relative to the age of the island, while others are more recent. This suggests that a combination of island colonization and within-island sympatric and/or vicariance-mediated speciation may explain the observed diversity.