Molecular phylogeniy of the genus Buteo

The cladogenesis of the genus Buteo is still unclear in many aspects. Several factors complicate the interpretation of the relationships among taxa. A broad spectrum of migratory behaviour is often found within a particular taxon, spanning from sedentary species to long distance migrants. The lack of clear-cut geographic separation harbours a potential for hybridization and introgression. On the other hand, several island populations appear isolated but their taxonomical status is debatable. Moreover, extreme intraspecific variability of morphological characters such as plumage colour sometimes impedes the unambiguous diagnosis of specimens. In a molecular approach to elucidate the unresolved phylogeny of the genus Buteo we decided to investigate the control region (CR), the most variable part of the mitochondrial genome. Preliminary experiments in our laboratory as well as recently published investigations of other groups (Mindell et al. 1998) indicate that the mitochondrial genome (mt genome) of Falconiformes has undergone one or perhaps several rearrangements including duplication of the CR. We realized that further molecular investigations of any falconiform genus on the basis of the CR could not be performed without prior clarification of the gene order and detailed characterization of the CR and CR-related sequences. Thus, our planned project will concentrate on two major topics: 1) rearrangements within the mt genome of Falconiformes (in particular the Accipitridae) and 2) phylogenetic analysis of the genus Ruteo. In a first step we shall clarify the gene order of the CR and related sequences within the genus Buteo. Subsequently, we shall isolate and sequence this region from representative species of the various falconiform families (Accipitridae, Cathartidae, Falconidae, Pandionidae, and Sagittariidae). Inter- and intraspecific sequence comparisons and analyses of the inner structural characteristics of the CR and its derivatives will serve as a basis for tracing the evolution of rearrangements within the mitochondrial genome of diurnal raptors. In the phylogenetic study of the genus Buteo (including subspecies and population level) we plan the following investigations: in a molecular analysis we shall select highly variable sections of the mitochondrial genome which will be isolated from the numerous taxa of B. buteo (PCR amplification, cloning, sequencing), especially those of uncertain taxonomical classification, e.g. the island populations. Furthermore the two subspecies of B. oreophilis will be included. In addition, we shall analyse the phylogenetic relationships between Old World and New World species of the genus Buteo. Morphological investigations will be carried out using external, skeletal and phenotypically relevant characters. In comparison with the molecular data, we shall evaluate the various morphological characters for their suitability as phylogenetically informative traits. Considering glacial and postglacial influences on distribution ranges, the results will serve to elucidate the cladogenesis of the genus Buteo.

The project focussed on the phylogenetic relationships within the genus Buteo, which were unclear in many aspects. Extreme intraspecific variability of morphological characters (e.g., different size and proportions, plumage characters) often impeded the unambiguous systematic assessments and evaluation of relationships. A molecular genetic study was initiated to elucidate these phylogenetic-systematic questions. For this purpose the analysis of the most variable section of the mitochondrial (mt) genome, the control region (CR), was the first choice. Yet, it turned out that the mt genome of the genus Buteo has undergone a rearrangement with respect to the "standard gene order" of birds. Thus the establishment of a molecular phylogeny of the genus Buteo on the basis of DNA sequences was possible only after isolation and detailed characterisation of the respective sections of the mt genome. In a first step the complete mitochondrial (mt) genome of B. buteo was sequenced and the mitochondrial gene order determined. Besides the CR, a second non-coding region was found which was designated pseudo control region. Sequence comparisons indicated that this section is most appropriate for the phylogenetic analysis of this genus. On the basis of this sequence we established the first comprehensive molecular phylogeny of the genus Buteo and other buteonine genera. Our results imply a neotropical origin of the genus Buteo. Moreover, the genus Buteo as currently defined does not represent a monophyletic group. Based on our DNA sequence data we propose several taxonomic consequences. The B. b. buteo-vulpinus complex, a group of presumed closely related Old World buzzards, was investigated morphologically as well as genetically. Our results suggest a rapid morphological differentiation in adaptation to different environments during range expansion after the last glaciation. This diversification happened rather recently at an evolutionary time scale and thus is not detectable with the genetic markers used. In contrast, the East Palearctic taxa B. b. japonicus, B. b. refectus, which have been treated as subspecies of B. buteo, are well differentiated genetically implying their classification as distinct species. As our results clearly demonstrate, speciation as a dynamic process cannot always be adequately dealt with by classical taxonomic approaches. We propose the "superspecies" concept as a practical way to overcome the problems imposed by incomplete separation of lineages and reticulated evolution in a recent radiation. The phylogenetic study of the genus Buteo underlined the importance of both the classical morphological approach and sequence analyses for resolving phylogenetic questions.