PMC

Results of StarBeast phylogenetic analysis of 662 single-copy orthogroup sequences. Species with female heterogamety are indicated by red boxes, and male heterogamety by blue boxes. Female heterogamety in Silene borysthenica, and S. otites, and male heterogamety in S. colpophylla, were confirmed or inferred from the molecular data presented here, and the male heterogamety in S. pseudotites and female heterogamety in S. wolgensis from results of interspecific crosses. The values indicated at the nodes are posterior probabilities. Branch colours indicate the most probable scenario for the evolution of the sex-determining system in section Otites. Branches and nodes where female heterogamety is inferred are coloured red, while the branches with male heterogamety are in blue. The branch to S. baschkirorum is coloured green as its heterogametic state is not known. Silene sibirica is not dioecious.

A general representation of the substitutions involved in changing the heterogametic system, both for model 1 transitions (A) and for model 2 transitions (B). In both models, numerical subscripts reveal regularities in the progression of mutations whose fixations switch the system of heterogamety. (A) For model 1, because the sex chromosome locus does not change, we have used the usual sex chromosome labels X and Y. Transitions between heterogametic systems that involve the arrival and fixation (“substitution”) of new dominant sex determining mutations are labeled a and c. a is from male to female heterogamety, and involves the substitution c is from female to male heterogamety, and involves the substitution Transitions that involve the substitution of new recessive sex determining mutations are labeled b and d. b is from female to male heterogamety via the substitution d is from male to female heterogamety, via the substitution (B) For model 2, we have used the arbitrary letters A and B to denote the two separate chromosomes/unlinked loci: one switches to a sex chromosome, and the other switches to an autosome, each time the heterogametic system changes. Again, a and c are used to label heterogametic transitions involving substitution of new dominant sex-determining mutations; a from male to female heterogamety via substitution of the mutation on a previous autosome, and c from female to male heterogamety via substitution of the mutation on a previous autosome. b and d refer to the reverse transitions, involving substitution of new recessive sex determining mutations; b from female to male heterogamety via substitution of the mutation on a previous autosome, and d from male to female heterogamety via substitution of the mutation on a previous autosome. We find in both models that, when all sexual genotypes are equally fit, dominant sex-determining mutations substitute at a higher rate than recessive sex-determining mutations—with reference to the present figure, we find a bias toward rightward transitions of the heterogametic system.

Schematic view of the pattern of sex chromosomal inheritance in organisms with (a) male heterogamety (XY sex determination) and (b) female heterogamety (ZW sex determination).

Examples of sex chromosome systems in plants. (a) XY: male heterogamety (Silene latifolia), (b) ZW: female heterogamety (Salix suchowensis) and (c) UV: haplo-diploid system (Marchantia polymorpha), showing maternal (pink) and paternal (blue) sex chromosomes.