Many eukaryotes have separate female and male individuals. In such species, one genome must implement two distinct developmental programs, producing two distinct phenotypes with different roles in reproduction. Sexually dimorphic development can be accomplished through either differential expression of genes transmitted through both sexes, or the expression of genes that are transmitted through only one sex and often grouped onto a nonrecombining sex chromosome 1, 2. The circumstances that lead one of these evolutionary outcomes to predominate remain unknown [3]. On a nonrecombining sex chromosome, a gene can specialize on female or male function, potentially producing a more optimal phenotype 4, 5. However, both adaptive evolution and purifying selection may be curtailed by a lack of meiotic recombination [6]. In addition, genes residing in a sex-limited region may generate genetic conflict between the sexes over transmission to the next generation [7]. Such arms-race dynamics can have profound influences on the structure of gene-regulatory networks, the genome architecture of sexual dimorphism, and the evolution of reproductive isolation 8, 9, 10. The variation in sex chromosome systems across the tree of life suggests that we need to take an explicitly comparative approach to identify the factors shaping their evolution [11]. Here, I highlight new data on the genetics of sex determination and sexual differentiation of bryophytes, showing that bryophyte models possess variation that may shed light on the causes and consequences of sex-biased autosomal variation and the evolution of sex chromosomes.