Abstract
Among Accipitriformes sensu stricto, only a few species have been reported to form hybrid zones; these include the red kite Milvus milvus and black kite Milvus migrans migrans. M. milvus is endemic to the western Palearctic and has an estimated total population of 20–24,000 breeding pairs. The species was in decline until the 1970s due to persecution and has declined again since the 1990s due to ingestion of rodenticide-treated baits, illegal poisoning and changes in agricultural practices, particularly in its core range. Whereas F1 M. milvus × M. migr. migrans hybrid offspring have been found, F2 and F3 hybrids have only rarely been reported, with low nesting success rates of F1 hybrids and partial hybrid sterility likely playing a role. Here, we analyzed the mitochondrial (CO1 and CytB) and nuclear (Myc) DNA loci of 184 M. milvus, 124 M. migr. migrans and 3 F1 hybrid individuals collected across central Europe. In agreement with previous studies, we found low heterozygosity in M. milvus regardless of locus. We found that populations of both examined species were characterized by a high gene flow within populations, with all of the major haplotypes distributed across the entire examined area. Few haplotypes displayed statistically significant aggregation in one region over another. We did not find mitochondrial DNA of one species in individuals with the plumage of the other species, except in F1 hybrids, which agrees with Haldane´s Rule. It remains to be investigated by genomic methods whether occasional gene flow occurs through the paternal line, as the examined Myc gene displayed only marginal divergence between M. milvus and M. migr. migrans. The central European population of M. milvus is clearly subject to free intraspecific gene flow, which has direct implications when considering the origin of individuals in M. milvus re-introduction programs.
Highlights
Hybrid zones have recently received increasing attention, and several hybrid sterility genes have been identified, e.g., in house mouse Mus musculus musculus × Mus musculus domesticus hybrids [1,2] and in fruit flies Drosophila pseudoobscura pseudoobscura × Drosophila pseudoobscura bogotana [3]
Whereas M. migrans was represented by 12 haplotypes, all previously examined M. milvus individuals only possessed three haplotypes, with a single haplotype associated with a kite from West Africa identified originally as M. migrans [39]
Haplotypes CO1-4, CO1-3 and CO1-11 accounted for 81% of all of the M. migrans individuals
Summary
Hybrid zones have recently received increasing attention, and several hybrid sterility genes have been identified, e.g., in house mouse Mus musculus musculus × Mus musculus domesticus hybrids [1,2] and in fruit flies Drosophila pseudoobscura pseudoobscura × Drosophila pseudoobscura bogotana [3]. Most observed bird hybrids have been found to be fertile, their fertility was generally decreased compared to the parental types that produced them. When two populations hybridize on an ongoing basis but the parental types of female-inherited mitochondrial DNA remain segregated, female hybrids are usually sterile [4]. Among Accipitriformes sensu stricto, only a few species have been reported to form hybrid zones. These include the red and black kites (Milvus milvus × Milvus migrans migrans) analyzed in this study; eagles (Aquila clanga × Aquila pomarina) as shown by Helbig et al [6]; and marsh harriers (Circus aeruginosus × Circus spilonotus) as shown by Fefelov [7]
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