Fig 8. Multi-step model for neo-Y evolution in D. albomicans.

A. The sequential fusions of the neo-sex chromosomes during speciation are depicted in the schematic. D. albomicans chromosomes have black outlines, to differentiate from D. nasuta chromosomes. Chromosomes that can recombine with each other are encompassed by gray boxes. Following the neo-X fusion, the neo-Y emerged via fusion with a chromosome that harbors a block of D. nasuta haplotype (no black outlines). B. Prior to the cessation of male recombination, the diversity of neo-Y is primarily the product of recombination with the neo-X and Chr. 3. During this time, the D. nasuta haplotype on the neo-Y is broken down by recombination with the neo-X. After males stop recombining in the population (maroon bar), the diversity is rapidly lost leading to the fixation of different haplotypes in different populations and deleterious mutations (blue bars) begin to accumulate on the neo-Y. The population in which male recombination stopped earlier (Pop. A) is expected to have more degenerate neo-Ys as compared to the population in which recombination stopped later (Pop. B). Bottom graphs track the accumulation of deleterious mutations and reduction in diversity in the two populations over time. Cessation of male recombination in the two populations are marked by red arrows.