Positive selection drives faster-Z evolution in silkmoths

Positive selection drives faster-Z evolution in silkmoths
Timothy B. Sackton (1), Russell B. Corbett-Detig (1), Javaregowda Nagaraju (2), R. Lakshmi Vaishna (2), Kallare P. Arunkumar (2), Daniel L. Hartl (1) ((1) Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, USA, (2) Centre of Excellence for Genetics and Genomics of Silkmoths, Laboratory of Molecular Genetics, Centre for DNA Fingerprinting and Diagnostics, Hyderabad, India)
(Submitted on 29 Apr 2013)

Genes linked to X or Z chromosomes, which are hemizygous in the heterogametic sex, are predicted to evolve at different rates than those on autosomes. This faster-X effect can arise either as a consequence of hemizygosity which leads to more efficient selection for recessive beneficial mutations in the heterogametic sex, or as a consequence of reduced effective population size on the hemizygous chromosome, which leads to increased fixation of weakly deleterious mutations due to random genetic drift. Empirical results to date have suggested that, while the overall pattern across taxa is complicated, in general systems with male-heterogamy show a faster-X effect primarily attributable to more efficient selection, whereas systems with female-heterogamy show a faster-Z effect primarily attributable to increased drift. However, to date only a single female-heterogamic taxa has been investigated. In order to test the generality of the faster-Z pattern seen in birds, we sequenced the genome of the Lepidopteran insect Bombyx huttoni, a close outgroup of the domesticated silkmoth Bombyx mori. We show that silkmoths experience faster-Z evolution, but unlike in birds, the faster-Z effect appears to be attributable to more efficient positive selection in females. These results suggest that female-heterogamy alone is unlikely to be sufficient to explain the reduced efficacy of selection on the bird Z chromosome. Instead, it is likely that a combination of patterns of dosage compensation and overall effective population size, among other factors, influence patterns of faster-Z evolution.

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