The rates of introgression and barriers to genetic exchange between hybridizing species: sex chromosomes vs. autosomes

BioRxiv : the Preprint Server for Biology
Christelle Fraisse, H. Sachdeva

Abstract

Interspecific crossing experiments have shown that sex chromosomes play a major role in the reproductive isolation of many species. However, their ability to act as reproductive barriers, which hamper interspecific genetic exchange, has hardly been evaluated quantitatively in relation to autosomes. Yet, this genome-wide limitation of gene flow is essential for understanding the complete separation of species, and thus speciation. Here, we develop a mainland-island model of secondary contact between hybridizing species of a XY (or ZW) sexual system. We obtain theoretical predictions for the frequency of introgressed alleles, and the strength of the barrier to neutral gene flow of the two types of chromosomes carrying multiple interspecific barrier loci. Theoretical predictions are obtained for scenarios where introgressed alleles are rare. We show that the same analytical expressions apply for sex chromosomes and autosomes, but with different effective parameters. The specific features of sex chromosomes (hemizygosity and absence of recombination in the heterogametic sex) lead to reduced levels of introgression of the X (or Z) compared to autosomes. This effect is enhanced by certain types of dosage compensation or sex-biased fo...Continue Reading

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