Sexual strategies in plants III. A quantitative method for describing the gender of plants

Abstract

Gender (maleness or femaleness) is a quantitative phenomenon in plants. For accurate descriptions of gender, numerical estimates of the relative capabilities of plants as pollen and ovule parents have advantages over the verbal morphological descriptions of gender that are traditionally used. Full descriptions require the distribution of gender among subsets of a sexual class (e.g., among individual plants) to be indicated as well as the average condition. Estimates of gender can be based on the pollen and ovule or seed production of each subset independently (phenotypic gender), or else the pollen and ovule or seed production of the whole population can be taken into account to assess the functional gender of any subset. The functional gender of a plant estimates the proportions of its genes which are transmitted through pollen (its maleness) or through ovules (its femaleness). Prospective estimates of functional gender are based on the relative maternal and paternal investments before anthesis (gamete estimates) or until the maturation of seeds (seed estimates). These estimates provide an ideal measure of the gender strategies of plants as sexual parents, but do not necessarily correspond closely with the actual success of a plant in leaving descendants through its male and female gametes. Equations for obtaining estimates of functional gender are derived for populations with random cross-fertilisation and for those with varying frequencies of self-fertilisation and random cross-fertilisation. The cosexual nature of plants of monomorphic populations and the separate male and female sexes of dimorphic populations are discussed.