Molecular Ecology (2005) 14, 1407–1419 doi: 10.1111/j.1365-294X.2005.02492.x © 2005 Blackwell Publishing Ltd Blackwell Publishing, Ltd. Genetic continuity of brood-parasitic indigobird species KRISTINA M. SEFC,*§ ROBERT B. PAYNE,† and MICHAEL D. SORENSON * *Boston University, Department of Biology, 5 Cummington Street, Boston MA 02215, USA, †University of Michigan, Museum of Zoology & Department of Ecology and Evolutionary Biology, 1109 Geddes Ave., Ann Arbor, MI 48109–1079, USA Abstract Speciation in brood-parasitic indigobirds (genus Vidua) is a consequence of behavioural imprinting in both males and females. Mimicry of host song by males and host fidelity in female egg laying result in reproductive isolation of indigobirds associated with a given host species. Colonization of new hosts and subsequent speciation require that females occasionally lay eggs in the nests of novel hosts but the same behaviour may lead to hybrid- ization when females parasitize hosts already associated with other indigobird species. Thus, retained ancestral polymorphism and ongoing hybridization are two alternative explanations for the limited genetic differentiation among indigobird species. We tested for genetic continuity of indigobird species using mitochondrial sequences and nuclear microsatellite data. Within West Africa and southern Africa, allopatric populations of the same species are generally more similar to each other than to sympatric populations of dif- ferent species. Likewise, a larger proportion of genetic variation is explained by differences between species than by differences between locations in alternative hierarchical AMOVAS, suggesting that the rate of hybridization is not high enough to homogenize sympatric populations of different species or prevent genetic differentiation between species. Broad sharing of genetic polymorphisms among species, however, suggests that some indigobird species trace to multiple host colonization events in space and time, each contributing to the formation of a single interbreeding population bound together by songs acquired from the host species. Keywords: ancestral polymorphism, brood parasitism, hybridization, mimicry, speciation, Vidua Received 18 August 2004; revision received 15 December 2004; accepted 12 January 2005 Introduction Population subdivision as a consequence of host fidelity is perhaps the most plausible mechanism for sympatric differentiation. In phytophagous insects, rearing environ- ment influences subsequent habitat choice and in turn promotes assortative mating that isolates populations associated with different hosts (e.g. Bush 1994; Berlocher & Feder 2002; Nason et al . 2002; Diegisser et al . 2004). Similar mechanisms are responsible for speciation in brood para- sitic indigobirds (genus Vidua , Payne 2004), in which host–parasite associations are established and maintained through behavioural imprinting. Species-specific parasites of various estrildid finches, both male and female indigobirds learn the songs of their respective hosts. Adult males combine mimicry of host song with indigobird vocaliza- tions and thereby attract female indigobirds reared by the same host species (Nicolai 1964; Payne 1985; Payne et al . 1998, 2000), whereas females preferentially parasitize nests of the species that reared them (Payne et al . 2000). Thus, behavioural imprinting results in prezygotic isolation of indigobirds associated with different hosts and promotes rapid speciation after a new host is colonized (Payne et al . 2000; Sorenson et al . 2003). Both morphological and genetic data are consistent with a recent diversification of indigobird species through host colonization rather than a more ancient process of host– parasite cospeciation (Klein & Payne 1998; Sorenson & Payne 2002; Sorenson et al . 2004). Although morpholo- gically similar, indigobird species are distinguished by consistent differences in adult male plumage colour and iridescence, and for certain species, by differences in body size and colours of the feet, legs and bill (Payne 2004). Correspondence: Michael D. Sorenson, Fax: 617-353-6340; E-mail: msoren@bu.edu §Present address: Institute of Zoology, University of Graz, Universitätsplatz 2, A-8010 Graz, Austria.