Gender specialization in heterodichogamous Grayia brandegei (Chenopodiaceae): evidence for an alternative pathway to dioecy

Abstract

We examined components of male and female reproductive success in protogynous and protandrous sexual morphs of the heterodichogamous and largely monoecious chenopod shrub Grayia brandegei. Percentage femaleness of flowering stalks ranged from 0 to 37.6% female ( = 15.5%) for protandrous plants and from 14 to 100% female ( = 55.8%) for protogynous plants. Functional gender estimates based on ovule production at two locations ranged from 23.0 to 31.8% female for the protandrous morph, and from 65.3 to 77.0% female for the protogynous morph. Realized gender estimates based on total seed production ranged in value from 3.6 to 16.8% female for the protandrous morph and from 76.5 to 96.4% for the protogynous morph, depending on location and year. Differences in reproductive success of the two morphs were largely due to a reduction in the female function of protandrous plants. Protogynous plants produced more female flowers per stalk and had a higher percentage of seed-filled fruits than did protandrous plants. Differences between sexual morphs were more pronounced in dry areas or years in which overall seed production was minimal. Differential seed production between morphs likely reflects temporal patchiness in environmental conditions, particularly in water availability. The significance of these findings in support of heterodichogamy as an evolutionary pathway to dioecy is discussed.