Fig. 3.

Comparison of the evolution of dioecy via one-locus- and two-locus-based models. (A) Model of the origin of dioecy from hermaphroditism via gynodioecy – two-locus model. Recessive male sterility alleles (ms) can be either represented by nuclear recessive mutations or can reflect the absence of the dominant fertility restorer in the case where the male sterility-causing cytoplasm is present in all plants in the population. In the next step, the dominant female-suppressing allele (Su^F) is recruited on the proto-Y chromosome. Occasional creation of asexual or hermaphrodite plants via recombination is possible. The cessation of the gynoecium is also influenced by genetic background and environment (sub-dioecy). Finally, the recombination between the female suppressor and the male sterility loci is arrested (as a consequence of divergence due to the accumulation of sexually antagonistic mutations and/or chromosomal inversions) and so the generation of asexual or hermaphrodite plants is avoided. This scheme is based on the original model by Charlesworth and Charlesworth (1978). (B) Alternative model of dioecy evolution from the monoecy – one-locus model. According to this model, the suppressors of gynoecium (Su^F) and stamen development (or fertility) (Su^M) are already present in the monoecious ancestor and their expression is controlled by intrinsic signals (e.g. hormonal signals connected with apical dominance). Because the strength and direction of the intrinsic signals varies along the plant axis, it results in the upper section carrying male flowers and the bottom part bearing female flowers. In this model, it is thought that there is a threshold value of stimuli separating male and female zones. In this situation, the mutations which are able to modify the effects of internal stimuli can cause an increase or decrease in the size of the male and female zones. Paradioecy (the presence of female flowers in ‘proto-males’ and male flowers in ‘proto-females’) can appear in the intermediate stage (not displayed in this scheme). In the extreme case, one sex can be completely absent in a given plant. Interestingly, if a single master sex-switching locus is recruited near the centromere or other non-recombining region, the sex chromosomes can originate in a single step. This scheme is based on the works by Lloyd (1972a, b, 1975, 1980, 1981), Webb (1999), Renner and Won (2001) and Renner (2016). Note: female flowers are shown in red; male flowers are shown in blue; and hermaphrodite flowers are shown as blue with a red middle part. Sex-linked non-recombining regions are shown as blue ovals.