Fig 1. Outline of model features.

Panel A: Recombination rate parameters between the ancestral sex-determining locus (X, here assumed to have alleles X and Y), a locus under selection (A, with alleles A and a), and a new sex-determining locus (M, with alleles M and m). Panel B: Haploid selection is often sex limited, occurring during haploid production or competition in one sex (shown here in males by dashed circles). If X or Y alleles are linked with alleles that experience haploid selection in males (r < 1/2), then zygotic sex ratios can become biased because either X- or Y-bearing male gametes/gametophytes will be more abundant after haploid selection. Similarly, zygotic sex ratio biases can arise if haploid-selected alleles are linked with new sex-determining alleles (R < 1/2). However, the zygotic sex ratio is not biased by male haploid selection in ZW sex-determining systems. Panel C: During cis-GSD transitions (XY to XY or ZW to ZW), a neo-Y allele (m) spreads until all males bear the neo-Y, and the ancestral Y allele is lost. Panel D: During trans-GSD transitions (XY to ZW or ZW to XY), a neo-W allele (m) spreads until all females bear the neo-W, and the ancestral X allele is lost. Neo-W alleles allow Y-associated alleles into females, which may impede or aid their spread. GSD, genetic sex determination.