Genomic evidence for a large-Z effect.
The 'large-X effect' suggests that sex chromosomes play a disproportionate role in adaptive evolution. Theoretical work indicates that this effect may be most pronounced in genetic systems with female heterogamety under both good-genes and Fisher's runaway models of sexual selection (males ZZ, females ZW). Here, I use a comparative genomic approach (alignments of several thousands of chicken-zebra finch-human-mouse-opossum orthologues) to show that avian Z-linked genes are highly overrepresented among those bird-mammalian orthologues that show evidence of accelerated rate of functional evolution in birds relative to mammals; the data suggest a twofold excess of such genes on the Z chromosome. A reciprocal analysis of genes accelerated in mammals found no evidence for an excess of X-linkage. This would be compatible with theoretical expectations for differential selection on sex-linked genes under male and female heterogamety, although the power in this case was not sufficient to statistically show that 'large-Z' was more pronounced than 'large-X'. Accelerated Z-linked genes include a variety of functional categories and are characterized by higher non-synonymous to synonymous substitution rate ratios than both accelerated autosomal and non-accelerated genes. This points at a genomic 'large-Z effect', which is widespread and of general significance for adaptive divergence in birds.
Ellegren
Hans
H
Department of Evolutionary Biology, Uppsala University, Norbyvägen 18D, 752 36 Uppsala, Sweden. hans.ellegren@ebc.uu.se
D000220:Adaptation, Biological / Q000235:genetics*; D000818:Animals; D002645:Chickens / Q000235:genetics; D019143:Evolution, Molecular; D005260:Female; D046369:Finches / Q000235:genetics; D018628:Gene Dosage; D016678:Genome; D023281:Genomics; D006801:Humans; D008297:Male; D051379:Mice; D008957:Models, Genetic; D009893:Opossums / Q000235:genetics; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D012730:Sex Chromosomes; D019849:Sex Determination Processes