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Abstract
BackgroundThe Mus musculus musculus/M. m. domesticus contact zone in Europe is characterised by sharp frequency discontinuities for sex chromosome markers at the centre of wider clines in allozyme frequencies.ResultsWe identify a triangular area (approximately 330 km2) where the musculus Y chromosome introgresses across this front for up to 22 km into domesticus territory. Introgression of the Y chromosome is accompanied by a perturbation of the census sex ratio: the sex ratio is significantly female biased in musculus localities and domesticus localities lacking Y chromosome introgression. In contrast, where the musculus Y is detected in domesticus localities, the sex ratio is close to parity, and significantly different from both classes of female biased localities. The geographic position of an abrupt cline in an X chromosome marker, and autosomal clines centred on the same position, seem unaffected by the musculus Y introgression.ConclusionWe conclude that sex ratio distortion is playing a role in the geographic separation of speciation genes in this section of the mouse hybrid zone. We suggest that clines for genes involved in sex-ratio distortion have escaped from the centre of the mouse hybrid zone, causing a decay in the barrier to gene flow between the two house mouse taxa.
Highlights
The Mus musculus musculus/M. m. domesticus contact zone in Europe is characterised by sharp frequency discontinuities for sex chromosome markers at the centre of wider clines in allozyme frequencies
Virtually no introgression was found for the Y chromosome in Bulgaria [19], northern Germany [20], and Denmark [21,22]. These observations have built up a remarkably consistent picture of wider autosomal clines versus more narrow sex linked clines spanning more than 2500 kilometres of the contact front between the subspecies in Europe, remarkable because it seems the dynamics of secondary contact are broadly similar over a very large geographic range relative to the dispersal of house mice [14,23]. This geographic repeatability and the fact that a model organism is involved, gives the musculus-domesticus contact zone the potential to play a central role in understanding the general features of secondary contact and the nature of barriers to gene flow between taxa, and in particular the action and nature of 'speciation' genes associated with sex chromosomes
A survey focused on the distribution of autosomal and sex-limited diagnostic markers across the Czech and Slovak Republics revealed an unexpectedly gradual transition of the Y chromosome compared to an X locus, and even to five of the autosomal loci analyzed [24], contradicting the results from other parts of the hybrid zone and causing us to question the generality of our understanding of the outcome of house mouse contact
Summary
The Mus musculus musculus/M. m. domesticus contact zone in Europe is characterised by sharp frequency discontinuities for sex chromosome markers at the centre of wider clines in allozyme frequencies. Virtually no introgression was found for the Y chromosome in Bulgaria [19], northern Germany [20], and Denmark [21,22] These observations have built up a remarkably consistent picture of wider autosomal clines versus more narrow sex linked clines spanning more than 2500 kilometres of the contact front between the subspecies in Europe, remarkable because it seems the dynamics of secondary contact are broadly similar over a very large geographic range relative to the dispersal of house mice [14,23]. A survey focused on the distribution of autosomal and sex-limited diagnostic markers across the Czech and Slovak Republics revealed an unexpectedly gradual transition of the Y chromosome compared to an X locus, and even to five of the autosomal loci analyzed [24], contradicting the results from other parts of the hybrid zone and causing us to question the generality of our understanding of the outcome of house mouse contact
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