  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 of
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 while the only female-heterogamy taxon
studied to date (birds) shows a faster-Z effect primarily attributable to
increased drift. 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.
