Anomalous Variation in Mitochondrial Genomes of White-crowned (Zonotrichia leucophrys) and Golden-crowned (Z. atricapilla) Sparrows: Pseudogenes, Hybridization, or Incomplete Lineage Sorting?

The White-crowned Sparrow (Zonotrichia leucophrys) is a common breeding bird in scrubby habitat of the Pacific coast, montane, and boreal regions of North America; five subspecies are recognized (Fig. 1A).The Golden-crowned Sparrow (Z.atricapilla) has a more restricted breeding range and is not subdivided into subspecies (Fig. lB).The full species status of these two sparrows is undisputed by ornithologists.Despite considerable breeding season sympatry, there are few hybrids known.Because a previous restriction site analysis (Zink et al. 1991) of mitochondrial DNA (mtDNA) suggested that these two species are very closely related, we compared mtDNA sequences and allozyme data from those species and the other North American congeners (Z.querula, Z. albicollis).Partial sequences from two mitochondrial genes in Golden-crowned and White-crowned sparrows were virtually identical.Neither species is reciprocally monophyletic in a haplotype tree, and two haplotypes are shared between several White-crowned and Golden-crowned sparrows.Considered together, Z. leucophrys and Z. atricapilla mtDNA sequences possess less variation than that found in single populations of passeriform bird species.In contrast, one fixed allozyme difference and several frequency differences (Zink 1982) indicated that Z. leucophrys and Z. atricapilla have been evolving independently for a considerable period of time.In this paper, we evaluate four possible explanations that could account for that anomalous lack of mtDNA differentiation between Z. leucophrys and Z. atricapilla: (1) accidental amplification of a nuclear pseudogene, (2) hybrid origin of either Z. leucophrys or Z. atricapilla, (3) incomplete lineage sorting, or (4) past introgressive hybridization.Materials and Methods.--Wesequenced 985 base pairs (bp) from the mtDNA genome in each of 13 Golden-crowned and 22 White-crowned sparrows 3