Fig. 1: Sex chromosome evolution in a lineage with female heterogamety (a ZW system in which the females are ZW heterozygotes), diagramming understanding of bird ZW pairs from several taxa described in refs. ^{17,18,19,20,21,}–²².

A A young ZW chromosome pair with a centromere at the left-hand end (indicated by a filled circle) and a large pseudo-autosomal region (PAR) in which crossovers occur at the other end. A non-recombining W-linked region has evolved, in which W-specific variants have accumulated (symbolised by pink in the W diagram), but it is not highly diverged, and its size, gene content and arrangement are still similar to those of the Z, though it differs by the presence of a female-determining locus within the region. B The homologous W chromosome arm, after a subsequent recombination suppression event, creates a new stratum (the new pink region to the right of the dashed vertical line). The initial W-linked region has degenerated (grey indicates gene losses) and expanded by accumulating repetitive sequences (pink triangles), making the W longer than the Z. The new fully sex-linked region, Stratum 2, has gained some W-specific sequences but has not yet degenerated. This change leaves a smaller PAR at the chromosome end. C Another recombination suppression event has occurred across a new region, further shortening the PAR, and creating a 3^rd stratum that has not yet degenerated, while stratum 2 has started degenerating and expanding.