Theoretical sex ratios of dioecious and gynodioecious angiosperms

Abstract

SUMMARYThe following parameters affecting the frequency of male and female plants in sexually dimorphic Angiosperms are defined: relative fecundity (as seed parent), F, differential survival, S, and gamete success, G, all as male/female quotients, and the probability of fertilisation of ovules on female plants, in terms of the number of pollinator visits to the flowers, x.Four different modes of inheritance of gynodioecy and dioecy are examined: The equilibrium frequencies of females and (for three models) the rates of change in female frequency per generation at non-equilibrium frequencies are expressed in terms of the controlling parameters.The conditions necessary for sexual dimorphism and the changes in frequencies of the sex genotypes as the number of pollinator visits increase and the relative seed fecundity decreases towards zero (dioecy) are derived for each model.Differential survival of the sexes (and differential gamete success in the heterozygous male model) cause an increase at equilibrium in the sex surviving more frequently or the sex derived from the more successful gamete. In a dioecious population, the ratio of the sexes is equal to their relative survival or their relative gamete success.Populations at non-equilibrium frequencies return more rapidly to equilibrium with the heterozygous male model than with the heterozygous female and cytoplasmic models. In the extreme case when females are lost from a population, with the heterozygous female and cytoplasmic models extinction or loss of sexual dimorphism occurs, but with the heterozygous male and dual male models sexual dimorphism is regenerated. The ability of systems with heterozygous males to regenerate dimorphism may explain the prevalence of such systems in both gynodioccious and dioecious populations.

Outcrossing rates and inferred levels of inbreeding depression in gynodioecious Cucurbita foetidissima (Cucurbitaceae)

“…Levels of inbreeding depression inferred from genetic marker data are compared with experimental estimates Information on morph-specific outcrossing rates and on levels of inbreeding depression in gynodioecious species is of interest for a number of reasons. Models of gynodioecy in which male sterility is controlled by nuclear genes predict that females must gain at least a twofold advantage in female fitness relative to hermaphrodites to persist in natural populations (Lewis, 1941;Lloyd, 1974Lloyd, , 1975Charlesworth & Charlesworth, 1978). As it is unlikely that male sterility will result in a twofold increase in seed production, large fitness increases in the seeds produced by females relative to hermaphrodites are expected.…”

The evolution and maintenance of gynodioecy in sexually and vegetatively reproducing plants

“…We now wish to incorporate p into equation (5), so that the effects of the fitness parameters on the equilibrium frequency of females may be analysed. The usual way of doing this is to partition the seed fecundity parameter f into two components-ovule production (o) and fertilisation rate (e)-each of which may be expressed as a female:hermaphrodite ratio, and then to specify a function of p for e. Two frequency-dependent functions have been proposed for e, one by Lloyd (1974aLloyd ( , 1975 and the other by Charlesworth and Ganders (1979) and Charlesworth (1981).…”

Selection in plant populations of effectively infinite size: III. The maintenance of females among hermaphrodites for a biallelic model

“…Many authors have studied theoretically the maintenance of females among hermaphrodites (e.g., Lewis, 1941;Jam, 1968;Ross and Shaw, 1971;Valdeyron et a!., 1973;Lloyd, 1974;Charnov eta!., 1976;Charlesworth and Charlesworth, 1978), and some interesting experimental data are also available (e.g., Lewis and Crowe, 1956;Assouad et a!., 1978;Horovitz and Beiles, 1980). However, the theoretical studies are limited to special cases in that they do not allow all the parameters which may be of importance to vary (table 1).…”