Differences in timing of migration and response to sexual signalling drive asymmetric hybridization across a migratory divide

Ruegg, Kristen; Anderson, Eric C.; Slabbekoorn, Hans

doi:10.1111/j.1420-9101.2012.02554.x

Cited by 26 publications

(30 citation statements)

References 78 publications

(173 reference statements)

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“…The large overlap in arrival time of the males of mixed genetic ancestry combined with females presumably arriving substantially later than the males, suggested that differential arrival time played a limited role in assortative mating in willow warblers. This is in contrast to the pattern found in populations of European blackcaps, where two populations with distinctly different migratory strategies breed in sympatry, but mate assortatively based on timing of arrival to the breeding grounds [[14]] as has also been observed in, Swainson’s thrushes [[26]] across their migratory divide. In both cases differences in arrival time at the breeding grounds is a major force for assortative mating.…”

Section: Discussioncontrasting

confidence: 65%

“…This is unlike, e.g. Swainson’s thrushes, where both co-occurring subspecies occupy distinct ecological niches [[26]], or blackcaps, where a recent study provides support for differences in habitat choice but not arrival time being associated with alternative migratory strategies in the populations studied [[29]]. Both willow warbler subspecies breed in similar terrain with trees and shrubs across their range, thus habitat is unlikely to act as barrier in promoting assortative mating in our system.…”

Section: Discussionmentioning

confidence: 99%

“…Also, even if slight song differences do exist but escaped our detection, it would not be possible to evaluate how these differences would result in assortative mating as we have no assay to score female preferences for song in the field. European blackcaps show a similar pattern of symmetrical responses to song playbacks [[29]], but Swainson’s thrushes show an asymmetrical response pattern where one subspecies responds more aggressively to homotypic song whereas the other subspecies’ response pattern to homotypic and heterotypic songs is indistinguishable [[26]].…”

Section: Discussionmentioning

confidence: 99%

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No evidence for assortative mating within a willow warbler migratory divide

et al. 2014

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IntroductionIn contact zones, genetic mixing of two taxa can be restricted by prezygotic (e.g. assortative mating) or postzygotic (lower fitness of hybrid offspring) barriers, or a combination of the two. A hybrid zone between two willow warbler subspecies (Phylloscopus trochilus trochilus, P. t. acredula) with distinctive migratory strategies occurs in central Sweden. These subspecies exhibit differences in migratory direction and distance, resulting in geographically distinct wintering areas in Africa. The subspecies may have diverged from a common refuge after the last ice age, and neutral genetic markers are homogeneous across their range. By contrast, several phenotypic traits and genetic markers of two chromosomal regions previously identified show steep clines across the divide. The evolutionary forces that maintain this migratory divide remain unknown. Here we use plumage colour, morphology, genetic markers and feather stable nitrogen-isotopes (δ15N) to assess if assortative mating between migratory phenotypes could be acting as a possible mechanism for keeping the two forms genetically separate and maintaining the migratory divide. We colour-ringed a willow warbler breeding population in the central part of the hybrid zone and observed the breeding population to assess phenotypic and genotypic traits of social pairs.ResultsOur data suggest that wintering area and genetic ancestry had an effect on male arrival time to the breeding grounds which could contribute to assortment. However, evidence for assortative mating could not be detected based on a comparison of plumage colour, morphology and δ15N between social mates.ConclusionThis finding was strengthened by analyses of subspecies-specific genetic markers, which allowed us to identify the presence of a large proportion of potential hybrids and backcrosses at the study site. Our results supported the hypothesis that pre-mating isolation in willow warblers is weak, resulting in extensive hybridisation across the migratory divide.

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Section: Discussioncontrasting

confidence: 65%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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No evidence for assortative mating within a willow warbler migratory divide

et al. 2014

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“…Ruegg et al . ; Delmore & Irwin ), given that Z. l. pugetensis is migratory and Z. l. nuttalli is a year‐round resident. However, migratory behaviour as an isolating mechanism is not mutually exclusive with our hypothesis that song is a behavioural barrier between the subspecies.…”

Section: Discussionmentioning

confidence: 99%

Behavioural response to song and genetic divergence in two subspecies of white‐crowned sparrows (Zonotrichia leucophrys)

et al. 2017

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Divergence in sexual signals may drive reproductive isolation between lineages, but behavioural barriers can weaken in contact zones. Here, we investigate the role of song as a behavioural and genetic barrier in a contact zone between two subspecies of white-crowned sparrows (Zonotrichia leucophrys). We employed a reduced genomic data set to assess population structure and infer the history underlying divergence, gene flow and hybridization. We also measured divergence in song and tested behavioural responses to song using playback experiments within and outside the contact zone. We found that the subspecies form distinct genetic clusters, and demographic inference supported a model of secondary contact. Song phenotype, particularly length of the first note (a whistle), was a significant predictor of genetic subspecies identity and genetic distance along the hybrid zone, suggesting a close link between song and genetic divergence in this system. Individuals from both parental and admixed localities responded significantly more strongly to their own song than to the other subspecies song, supporting song as a behavioural barrier. Putative parental and admixed individuals were not significantly different in their strength of discrimination between own and other songs; however, individuals from admixed localities tended to discriminate less strongly, and this difference in discrimination strength was explained by song dissimilarity as well as genetic distance. Therefore, we find that song acts as a reproductive isolating mechanism that is potentially weakening in a contact zone between the subspecies. Our findings also support the hypothesis that intraspecific song variation can reduce gene flow between populations.

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“…The reason for the barrier to gene flow remains unclear, but the two forms differ in a number of traits known in birds to influence reproductive isolation (Price ): differences in arrival times on the breeding grounds at the centre of the hybrid zone, plumage coloration and song (Ruegg et al . , ; Ruegg ).…”

Section: Introductionmentioning

confidence: 99%

A role for migration‐linked genes and genomic islands in divergence of a songbird

Ruegg

Anderson

Boone³

et al. 2014

Molecular Ecology

145

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Next-generation sequencing has made it possible to begin asking questions about the process of divergence at the level of the genome. For example, recently, there has been a debate around the role of 'genomic islands of divergence' (i.e. blocks of outlier loci) in facilitating the process of speciation-with-gene-flow. The Swainson's thrush, Catharus ustulatus, is a migratory songbird with two genetically distinct subspecies that differ in a number of traits known to be involved in reproductive isolation in birds (plumage coloration, song and migratory behaviour), despite contemporary gene flow along a secondary contact zone. Here, we use RAD-PE sequencing to test emerging hypotheses about the process of divergence at the level of the genome and identify genes and gene regions involved in differentiation in this migratory songbird. Our analyses revealed distinct genomic islands on 15 of the 23 chromosomes and an accelerated rate of divergence on the Z chromosome, one of the avian sex chromosomes. Further, an analysis of loci linked to traits known to be involved in reproductive isolation in songbirds showed that genes linked to migration are significantly more differentiated than expected by chance, but that these genes lie primarily outside the genomic islands. Overall, our analysis supports the idea that genes linked to migration play an important role in divergence in migratory songbirds, but we find no compelling evidence that the observed genomic islands are facilitating adaptive divergence in migratory behaviour.

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Differences in timing of migration and response to sexual signalling drive asymmetric hybridization across a migratory divide

Cited by 26 publications

References 78 publications

No evidence for assortative mating within a willow warbler migratory divide

No evidence for assortative mating within a willow warbler migratory divide

Behavioural response to song and genetic divergence in two subspecies of white‐crowned sparrows (Zonotrichia leucophrys)

A role for migration‐linked genes and genomic islands in divergence of a songbird

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