In both Cotula dioica and C. dispersa there are dioecious populations and simple-monoecious populations (in which all heads of all plants are bisexual, with numerous male and female florets.) In addition, in C. dioica and also in C. rotundata, there are complex-monoecious populations in which all plants have both male heads and bisexual heads with one or a few female florets and many male florets; the plants resemble inconstant males of dioecious populations in their sex expression, secondary sex characters and genetical behaviour, but they are not associated with females and are therefore equisexual (contribute equally via male and female gametes during sexual reproduction). In C. dispersa, there are also ‘unisexual male’ and ‘unisexual female’ populations resembling one or other sex of dioecious populations. ‘Unisexual’ populations sometimes have a small proportion of florets of the opposite sex and hence may produce a few seeds; when they reproduce sexually, the plants are functionally equisexual. One population of C. dispersa has a single patch of males in a large area of females.
It is postulated that ‘unisexual female’ populations arise by the loss of males from a dioecious population. Rare seeds produced on these females (the heterogametic sex in dioecious populations) may lead to the reappearance of males, producing a dimorphic population. Further male × female crosses regenerate normal dioecy by equalizing the numbers of males and females. Following the loss of females from a dioecious population, ‘unisexual male’ populations may restore seed production by selection for increased proportions of female florets. In this way, dioecy reverts to simple-monoecy via ‘unisexual male’ populations and complex-monoecy. Reversion to monoecy has probably occurred independently in the three species.