Hybrid speciation in sparrows II: a role for sex chromosomes?

Abstract

Homoploid hybrid speciation in animals is poorly understood, mainly because of the scarcity of well‐documented cases. Here, we present the results of a multilocus sequence analysis on the house sparrow ( Passer domesticus ), Spanish sparrow ( P. hispaniolensis ) and their proposed hybrid descendant, the Italian sparrow ( P. italiae ). The Italian sparrow is shown to be genetically intermediate between the house sparrow and Spanish sparrow, exhibiting genealogical discordance and a mosaic pattern of alleles derived from either of the putative parental species. The average variation on the Z chromosome was significantly reduced compared with autosomal variation in the putative parental species, the house sparrow and Spanish sparrow. Additionally, divergence between the two species was elevated on the Z chromosome relative to the autosomes. This pattern of variation and divergence is consistent with reduced introgression of Z‐linked genes and/or a faster‐Z effect (increased rate of adaptive divergence on the Z). F ST ‐outlier tests were consistent with the faster‐Z hypothesis: two of five Z‐linked loci ( CHD1Z and PLAA ) were identified as candidates for being subject to positive, divergent selection in the putative parental species. Interestingly, the two latter genes showed a mosaic pattern in the (hybrid) Italian sparrow; that is, the Italian sparrow was found to be fixed for Spanish sparrow alleles at CHD1Z and to mainly have house sparrow alleles at PLAA . Preliminary evidence presented in this study thus suggests that sex chromosomes may play a significant role in this case of homoploid hybrid speciation.

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