Apparent 'sympatric' speciation in ecologically similar herbivorous beetles facilitated by multiple colonizations of an island

Coexistence of recently diverged and ecologically similar sister species in complete sympatry represents a particularly compelling case for sympatric speciation. This study investigates the possible sympatric origin of two coexisting bark beetle taxa that utilize the same host plant on the island of La Palma in the Canary Islands. Aphanarthrum subglabrum and Aphanarthrum glabrum ssp. nudum breed inside dead twigs of Euphorbia lamarckii plants and are closely related to the allopatric A. glabrum ssp. glabrum in Tenerife, El Hierro and La Gomera. We tested the various speciation hypotheses in a genealogical context, using mitochondrial gene fragments from Cytochrome Oxidase I and 16S, and nuclear gene fragments from Enolase, Elongation Factor 1 alpha and Histone H3. Phylogenetic analyses of the combined nuclear DNA data strongly supported a sister relationship between two sympatric and reproductively isolated taxa in La Palma. However, network analyses of subdivided nonrecombinant segments of the Enolase locus indicated a closer relationship between the two allopatric A. glabrum subspecies, suggesting multiple colonizations of this island. A bimodal distribution of mtDNA haplotypes in La Palma further documented the independent colonization of this island, with asymmetric introgression of mtDNA between two lineages. Consequently, the sympatric origin of the La Palma species is concluded to have involved allopatric phases before the parallel colonization of this island and subsequent introgression at some loci. The clear genetic and morphological evidence for reproductive isolation between these species suggests that the sympatric completion of divergence was either due to initial genetic incompatibility, morphological character displacement in male genitalia, or a combination of these factors.