Amplified fragment length polymorphism analysis identifies hybrids between two subspecies of warblers

Abstract

In the western Pyrenees (Southwest France and Northwest Spain), a narrow hybrid zone exists between the common chiffchaff Phylloscopus (collybita) collybita and the Iberian chiffchaff Phylloscopus (c.) brehmii. In this zone, which is approximately 20 km wide, mixed matings and individuals singing the songs of both taxa occur at substantial frequencies (24 and 8.6%, respectively), suggesting frequent hybridization. Previous studies have shown very weak mitochondrial gene flow (Nm = 0.065), whereas four microsatellites suggested much higher nuclear gene flow (Nm = 4.9). In this study we used the amplified fragment length polymorphism (AFLP) method in order to identify hybrids and early backcrosses. We typed 91 birds from both allopatric and sympatric areas for 12 informative AFLP markers (of > 141 polymorphic fragments), obtained by screening 13 AFLP primer combinations. These individuals were previously typed for song (brehmii, collybita or mixed singers), mitochondrial DNA (mtDNA) haplotype and allelic genotypes at four microsatellite loci. Assignment tests demonstrated that in the zone of sympatry, a substantial number of intermediate genotypes existed among the birds previously believed to be pure collybita and brehmii, based on song and mtDNA haplotype. The majority of the mixed singers had intermediate genotypes. Our data suggest that the fraction of the adult population having a hybrid origin (hybrids or backcrosses) is in the order of 10%. With such a frequency of genetic hybrids, there would have been much more mtDNA introgression than observed, had female hybrids been perfectly fertile/viable. This result is consistent with male-biased gene flow and Haldane's rule.

Patterns of genetic, phenotypic, and acoustic variation across a chiffchaff (Phylloscopus collybita abietinus/tristis) hybrid zone

“…This has also been reported in another contact zone involving a different pair of chiffchaff species (Bensch, Helbig, Salomon, & Seibold, 2002). Our observation that one bird with tristis morphotype and >95% tristis nuclear SNPs actually harbored abietinus mtDNA indicates that female hybrids resulting from a cross between a male tristis and a female abietinus are fertile and can interbreed with tristis allowing for abietinus mitochondrial introgression into tristis .…”

A Comparison of Five Hybrid Zones of the Weta Hemideina Thoracica (Orthoptera: Anostostomatidae): Degree of Cytogenetic Differentiation Fails to Predict Zone Width

“…Differential introgression may reflect either the fact that cytoplasmic markers are independent of the nuclear genome and therefore are less likely to be linked to genes under selection, or greater selection against hybrid males (Haldane's Rule) because males are more typically the heterogametic sex (Barton 1993). However, Haldane's Rule results in narrow mitochondrial clines in hybrid zones of birds and moths in which the female is the heterogametic sex (Sattler and Braun 2000;Bensch et al 2002;Dasmahapatra et al 2002). In addition, differential dispersal or fitness of the two sexes can result in nonconcordant (different width) character clines or noncoincident clines (those with different centers; Carr et al 1986;Tegelströ m and Gelter 1990;Butlin and Neems 1994).…”

Are Iberian ChiffchaffsPhylloscopus (collybita) brehmiilong-distance migrants? An analysis of flight-related morphology

“…Among leaf warblers, song divergence seems to be the major pre-reproductive isolating mechanism (Helbig et al 2001, Irwin et al 2001. However, song differences are not sufficient for brehmii and collybita to remain isolated, as they are unable to completely discriminate each other by song alone (Helbig et al 2001, Bensch et al 2002. The high frequency of hybrids between brehmii and collybita in the contact zone suggests that post-reproductive isolating mechanisms are important, and our results support the idea that migratory divergence might contribute to the maintenance of the divide at the Pyrenees, despite considerable genetic interchange.…”

“…These two forms differ at the molecular level (4.6% divergence in the cytochrome b gene), and also have a different song (Helbig et al 1996). Remarkably, these differences have persisted over time despite noticeable gene flow, as shown by microsatellite analyses and direct identification of hybrids (Helbig et al 2001, Bensch et al 2002.…”