Data from: Z chromosome divergence, polymorphism, and relative effective population size in a genus of lekking birds

Sex chromosomes contribute disproportionately to species boundaries as
they diverge faster than autosomes and often have reduced diversity. Their
hemizygous nature contributes to faster divergence and reduced diversity,
as do some types of selection. In birds, other factors (mating system and
bottlenecks) can further decrease the effective population size of
Z-linked loci and accelerate divergence (Fast-Z). We assessed Z-linked
divergence and effective population sizes for two polygynous sage-grouse
species and compared them to estimates from birds with various mating
systems. We found lower diversity and higher FST for Z-linked loci than
for autosomes, as expected. The πZ/πA ratio was 0.38 in Centrocercus
minimus, 0.48 in Centrocercus urophasianus and 0.59 in a diverged,
parapatric population of C. urophasianus, a broad range given the mating
system among these groups is presumably equivalent. The full data set had
unequal males and females across groups, so we compared an equally
balanced reduced set of C. minimus and individuals pooled from both C.
urophasianus subgroups recovering similar estimates: 0.54 for C.
urophasianus and 0.38 for C. minimus. We provide further evidence that
NeZ/NeA in birds is often lower than expected under random mating or
monogamy. The lower ratio in C. minimus could be a consequence of stronger
selection or drift acting on Z loci during speciation, as this species
differs strongly from C. urophasianus in sexually selected characters with
minimal mitochondrial divergence. As C. minimus also exhibited lower
genomic diversity, it is possible that a more severe demographic history
may contribute to its lower ratio.