Trans-oceanic host dispersal explains high seabird tick diversity on Cape Verde islands.

Parasites represent ideal models for unravelling biogeographic patterns and mechanisms of diversification on islands. Both host-mediated dispersal and within-island adaptation can shape parasite island assemblages. In this study, we examined patterns of genetic diversity and structure of Ornithodoros seabird ticks within the Cape Verde Archipelago in relation to their global phylogeography. Contrary to expectations, ticks from multiple, geographically distant clades mixed within the archipelago. Trans-oceanic colonization via host movements probably explains high local tick diversity, contrasting with previous research that suggests little large-scale dispersal in these birds. Although host specificity was not obvious at a global scale, host-associated genetic structure was found within Cape Verde colonies, indicating that post-colonization adaptation to specific hosts probably occurs. These results highlight the role of host metapopulation dynamics in the evolutionary ecology and epidemiology of avian parasites and pathogens.

Sympatric speciation by allochrony in a seabird.

The importance of sympatric speciation (the evolution of reproductive isolation between codistributed populations) in generating biodiversity is highly controversial. Whereas potential examples of sympatric speciation exist for plants, insects, and fishes, most theoretical models suggest that it requires conditions that are probably not common in nature, and only two possible cases have been described for tetrapods. One mechanism by which it could occur is through allochronic isolation-separation of populations by breeding time. Oceanodroma castro (the Madeiran or band-rumped storm-petrel) is a small seabird that nests on tropical and subtropical islands throughout the Atlantic and Pacific Oceans. In at least five archipelagos, different individuals breed on the same islands in different seasons. We compared variation in five microsatellite loci and the mitochondrial control region among 562 O. castro from throughout the species' range. We found that sympatric seasonal populations differ genetically within all five archipelagos and have ceased to exchange genes in two. Population and gene trees all indicate that seasonal populations within four of the archipelagos are more closely related to each other than to populations from the same season from other archipelagos; divergence of the fifth sympatric pair is too ancient for reliable inference. Thus, seasonal populations appear to have arisen sympatrically at least four times. This is the first evidence for sympatric speciation by allochrony in a tetrapod, and adds to growing indications that population differentiation and speciation can occur without geographic barriers to gene flow.

Lack of host-dependent genetic structure in ectoparasites of Calonectris shearwaters.

We compared patterns of mitochondrial DNA (mtDNA) differentiation in three host-specific lice (Halipeurus abnormis, Austromenopon echinatum and Saemundssonia peusi) and one generalist flea (Xenopsylla gratiosa), parasitizing 22 colonies of Cory's and Cape Verde shearwater (Calonectris). The shearwater hosts show distinct phylogeographic structure corresponding to the three taxa Calonectris d. diomedea, C. d. borealis, and C. edwardsii. The host-specific lice appeared undifferentiated among the three Calonectris taxa, whereas the more generalist flea displayed significant levels of population differentiation. Neither genetic distances among host populations, nor their spatial distribution explained the patterns of genetic variability observed in the ectoparasites. The lack of differentiation among lice is unexpected, given that previous work has found evidence of cospeciation between procellariiform seabirds and their lice, and lice typically have an elevated rate of mtDNA evolution with respect to their hosts. Our results suggest that either rates of evolution in seabird lice are not always as high as previously thought, or that the magnitude of movement of lice between seabird hosts has been substantially underestimated.

Phylogeography of the Calonectris shearwaters using molecular and morphometric data.

We investigated phylogenetic relationships and the biogeographic history of the Calonectris species complex, using both molecular and biometric data from one population of the Cape Verde shearwater Calonectris edwardsii (Cape Verde Islands), one from the streaked shearwater C. leucomelas (western Pacific Ocean) and 26 from Cory's shearwater populations distributed across the Atlantic (C. d. borealis) and the Mediterranean (C. d. diomedea). The streaked shearwater appeared as the most basal and distant clades, whereas the genetic divergences among the three main clades within the Palearctic were similar. Clock calibrations match the first speciation event within Calonectris to the Panama Isthmus formation, suggesting a vicariant scenario for the divergence of the Pacific and the Palearctic clades. The separation between the Atlantic and Mediterranean clades would have occurred in allopatry by range contraction followed by local adaptation during the major biogeographic events of the Pleistocene. The endemic form from Cape Verde probably evolved as a result of ecological divergence from the Mediterranean subspecies. Finally, one Mediterranean population (Almeria) was unexpectedly grouped into the Atlantic subspecies clade, both by genetic and by morphometric analyses, pointing out the Almeria-Oran oceanographic front (AOOF) as the actual divide between the two Cory's shearwater subspecies. Our results highlight the importance of oceanographic boundaries as potentially effective barriers shaping population and species phylogeographical structure in pelagic seabirds.