Diversity and phylogeny of skuas from the Antarctica

Abstract

Among eight taxa of skuas in the world, brown (Catharacta lonnbergi) and south polar skuas (C. maccormicki) breed in the Antarctic region. Although the classic taxonomy of skuas has regarded brown and south polar skuas as separated species, recently taxonomic levels of two taxa are under dispute. The southern breeding range of brown skuas was overlapped with northern breeding range of south polar skuas in South Orkney Islands, South Shetland Islands, Antarctic peninsula and so on. Not only hybridization from mixed pairs, but also backcross between male hybrid and female brown skuas were observed there. Even though they can distinguished by morphological characteristics, incomplete reproductive isolation seems to be a worthwhile subject to reinvestigate their taxonomy and phylogeny. To study the diversity and phylogeny of skuas, we compared their body size and sequence data for mitochondrial cytochrome b (cytb), cytochrome c oxidase I (COI), and intron of nuclear Z-chromosome-linked chromo-helicase binding protein gene (CHD1Z) from both taxa and hybrids. All measured brown (n = 49) and south polar skuas (n = 91) were completely separated with body size each other. However, existence of hybrids, which had intermediate body size of both taxa, confused morphological classification. In addition, they were not distinguished with mitochondrial DNA sequence data. Interspecific variation (0-0.98 %) of cytb was similar to intraspecific variation (0-0.83%) and there were no fixed species-specific substitutions. Herbert and his colleagues suggested a threshold of 10ⅹ the mean intraspecific difference to identify bird species. If the mean interspecific variation is over the threshold, they can be classified as separated species. In our study, the mean interspecific differences (0.1-0.2%) were smaller than the threshold (1.47%), thereupon brown and south polar skuas could not be discriminated with the 10ⅹ rules. Consistent with mtDNA, we did not also detect significant interspe