Hybridization but No Evidence for Backcrossing and Introgression in a Sympatric Population of Great Reed Warblers and Clamorous Reed Warblers

Abstract

Hybridization is observed frequently in birds, but often it is not known whether the hybrids are fertile and if backcrossing occurs. The breeding ranges of the great reed warbler (Acrocephalus arundinaceus) and the clamorous reed warbler (A. stentoreus) overlap in southern Kazakhstan and a previous study has documented hybridization in a sympatric population. In the present study, we first present a large set of novel microsatellite loci isolated and characterised in great reed warblers. Secondly, we evaluate whether hybridization in the sympatric breeding population has been followed by backcrossing and introgression.We isolated 181 unique microsatellite loci in great reed warblers. Of 41 loci evaluated, 40 amplified and 30 were polymorphic. Bayesian clustering analyses based on genotype data from 23 autosomal loci recognised two well-defined genetic clusters corresponding to the two species. Individuals clustered to a very high extent to either of these clusters (admixture proportions ≥0.984) with the exception of four previously suggested arundinaceus–stentoreus hybrid birds that showed mixed ancestry (admixture proportions 0.495–0.619). Analyses of simulated hybrids and backcrossed individuals showed that the sampled birds do not correspond to first–fourth-generation backcrosses, and that fifth or higher generation backcrosses to a high extent resemble ‘pure’ birds at this set of markers.We conclude that these novel microsatellite loci provide a useful molecular resource for Acrocephalus warblers. The time to reach reproductive isolation is believed to be very long in birds, approximately 5 Myrs, and with an estimated divergence time of 2 Myrs between these warblers, some backcrossing and introgression could have been expected. However, there was no evidence for backcrossing and introgression suggesting that hybrids are either infertile or their progeny inviable. Very low levels of introgression cannot be excluded, which still may be an important factor as a source of new genetic variation.

Rapid, complete reproductive isolation in two closely relatedZosteropsWhite‐eye bird species despite broadly overlapping ranges*

“…; Hansson et al. ). Since we did not find any hybrids, our results suggest that premating isolation may be more important in reproductive isolation between these two species.…”

“…Additionally, Olson et al (2010) found that hybrid chickadees (hybrids of Poecile atricapillus and Poecile carolinensis) had reduced metabolic efficiency compared to their parental species, and suggested that genetic incompatibilities between mismatched mtDNA and nuclear DNA could have severe physiological costs in hybrid birds. However, in species with strong selection against hybrids, it could be expected that hybrids would be detected but that backcrossed and introgressed individuals would not be found (as in Steeves et al 2010;Hansson et al 2012). Since we did not find any hybrids, our results suggest that premating isolation may be more important in reproductive isolation between these two species.…”

Phylogeny and biogeography of Primula sect. Armerina: implications for plant evolution under climate change and the uplift of the Qinghai-Tibet Plateau

“…Although introgression cannot occur without hybridization, hybridization followed by no backcrossing and introgression could still occur and such phenomenon has been detected in numerous studies ( e.g., [ 47 , 48 ]). Natural hybridization in Primula is common and has been confirmed by several studies [ 37 , 49 – 51 ], although, it is currently unclear to what degree species within sect.…”

Despite introgressive hybridization, North American birches (Betula spp.) maintain strong differentiation at nuclear microsatellite loci

“…The evolutionary consequences of hybridization can vary widely, depending on the strength of the reproductive isolating mechanisms. When hybrid fertility is reduced, backcrosses may rarely occur and introgression among the parental taxa may be limited (e.g., Hansson et al 2012). In other cases, hybrids may rapidly become reproductively isolated from the parental taxa through karyotypic and ecological divergence or polyploidization, thus limiting gene flow among parental species (Rieseberg and Willis 2007).…”