Data from: Positive selection drives faster-Z evolution in silkmoths

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 genetic
drift. Empirical results to date suggest that, while the overall pattern
across taxa is complicated, systems with male-heterogamy show a faster-X
effect attributable to more efficient selection, while the faster-Z effect
in female-heterogametic taxa is attributable to increased drift. To test
the generality of the faster-Z pattern seen in birds and snakes, we
sequenced the genome of the Lepidopteran silkmoth Bombyx huttoni. We show
that silkmoths experience faster-Z evolution, but unlike in birds and
snakes, the faster-Z effect appears to be attributable to more efficient
positive selection. These results suggest that female-heterogamy alone is
unlikely to explain the reduced efficacy of selection on the bird Z
chromosome. It is likely that many factors, including differences in
overall effective population size, influence Z chromosome evolution.