Genetic Differentiation and Phylogenetic Relationships of Bonelli's Warbler Phylloscopus bonelli and Green Warbler P. nitidus

Abstract

The mitochondrial genetic differentiation and molecular phylogenetic relationships of western and eastern Bonelli's Warblers (Phylloscopus b. bonelli, P. b. orientalis) and Green Warbler Phylloscopus nitidus were investigated. The cytochrome b gene was amplified by polymerase chain reaction and 1038 nucleotides were sequenced directly in these and several other Phylloscopus species plus Sylvia atricapilla and Acrocephalus scirpaceus. The mitochondrial genetic distance between eastern and western Bonelli's Warblers was as large (8.3-8.6%) as between each of them and the Wood Warbler P. sibilatrix. Compared to the amount of mtDNA differentiation between other closely related species and subspecies of birds, the divergence between the Bonelli's warblers is far greater than among typical subspecies. Together with the distinct differences in calls and structure of song elements the genetic data support full species status of the two taxa. The cytochrome b sequence of P. nitidus differed by 2.5-3.1% from P. trochiloides, to which it is most closely related. This divergence is also larger than between most subspecies of birds studied so far at this locus and is consistent with the placement of P. nitidus as an allospecies within the superspecies P. [trochiloides]. We discuss population-genetic scenarios for a potentially accelerated rate of mtDNA differentiation in relatively small, geographically isolated populations that might help to explain the large sequence divergence observed. A phylogenetic tree was constructed by maximum parsimony and neighbour-joining. Chiffchaff P. collybita and Willow Warbler P. trochilus are sister species and together form the sister group of Bonelli's and Wood Warblers. As expected, Arctic Warbler P. borealis was found to be the sister species of the Greenish Warbler complex (including nitidus), whereas the relationships of P. (inornatus) humei relative to the other species could not be resolved. Among the taxa studied, those without wing-bars belong to a phylogenetically older clade than those with wing-bars. MtDNA sequence data promise to be very useful in quantifying the genetic differentiation and phylogenetic relationships among closely related species, especially in morphologically poorly differentiated genera like Phylloscopus.