Figure 3. The neo-X chromosome.
a, D. pseudoobscura neo-X chromosome formation cartoon. Muller A, D and E are attached to different centromeres (filled circles) in D. pseudoobscura relative to flanking species. b, Bar plot showing changes to D. pseudoobscura Muller D neo-X chromosome (orange) and Muller E autosomal arm (green), based on the predicted expression pattern of the ancestral autosomes. Categories are: switches in sex-biased expression (F, female-biased; M, male-biased), not found (absent), only found in D. pseudoobscura (new), and genes moving from (move off) and to (move on) the indicated arm. Significant differences (chi-squared test) between Muller D and Muller E are noted (**P<10−4; *P<0.05). The ancestral condition of the neo-X chromosome was inferred by examining chromosome linkage and expression on homologous arms in species that diverged from the Drosophila lineage before and after D. pseudoobscura.