PMC

Major sex chromosome changes associated with divergence of mammal lineages. De novo evolution of SRY and the XY sex chromosome system (green star) occurred early in therian evolution and may have triggered divergence of Theria from Prototheria. At the same time, the ancient ZW system was bolstered by translocation with an autosome (blue cross), which may also have promoted divergence of Prototheria from Theria. Two further translocations occurred before divergence of platypus and echidna; different final translocations may have triggered divergence of Families Ornithorhynchidae (platypus) and Tachyglossidae (echidna). In the Theria, fusion of the XY with an autosome (denoted by +) occurred before eutherian radiation, and may have triggered divergence of Metatheria and Eutheria. Fusions are evident in some marsupial lineages, and many changes in sex determination (symbols in other colours) are observed in different rodent lineages which may signify a recent burst of sex chromosome turnover and speciation.

Early steps in sex chromosome evolution. The Mucorales sex locus supports the hypothesis posited by Ohno []: a key transcription factor for sex arises on an autosome and one allele undergoes gene inversion followed by gene divergence resulting in divergent alleles (P. blakesleeanus sex locus) (upper). A repetitive element is integrated at the sex locus and elsewhere on the same chromosome (P. blakesleeanus sex locus). The multiple copies of the repetitive element sequence provide templates for recombination and inversion and thereby facilitate the expansion of the sex locus to form a proto-sex chromosome (sex loci of P. blakesleeanus, R. oryzae, and S. megalocarpus). However, allelic sex genes in the same orientation could be an ancestral form, and therefore gene inversion might follow gene divergence (M. circinelloides sex locus) (lower).

Sex ratios and penetrance by sex determination system. a Sex ratio by sex determination system. Dashed line indicates a 1:1 sex ratio, with values lower than the line indicating a female-biased sex ratio, and those above the line a male-biased sex ratio. (LG13 ZW, n = 8 families, 63 males and 115 females total; LG5-14 XY, n = 10 families, 115 males and 83 females total; χ ² test for divergence from expected 1:1 sex ratio: *, p = 0.023, χ ² = 5.2; ***, p < 0.0001, χ ² = 15.2; LG13 ZW and LG5-14 XY family sex ratio differ from each other, p = 0.0053, Welch’s unequal variances t-test). b Penetrance by sex determination system, not significant

Phylogeny showing the relative divergence times of the evolution of dioecy and the sex chromosomes as well as the lineage split between poplars and willows.