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The main goal of this study was to evaluate sex expression at geographic and individual levels in Ochradenus baccatus, an Israeli desert shrub. Although this species was formerly thought to be strictly dioecious, in a survey of 24 Ochradenus populations along a latitudinal rainfall gradient, we found sex expression to be extremely variable. While females reproduced only by seeds, males ranged across a continuum from ’’pure males’’ that produced only pollen to ’’inconstant males,’’ which produced seeds in addition to pollen. For sexual dimorphism to evolve from cosexuality, it is believed that the sex morphs must be compensated for their loss of one sex function (e.g., pollen production) by an increase in the remaining sex function (i.e., seed production). In our study we found that females produced 9-20 times as many seeds as males, and we showed in a controlled laboratory experiment that their offspring had higher germination and seedling growth rates. Pure males invested more biomass in stamens than did inconstant males.

Previously published models concerned with the evolution of sexual dimorphisms have predicted that: (1) the frequency of inconstant males should be inversely related to the frequency of females, and (2) if the degree of male inconstancy is influenced by environmental factors, then seed production in males should decline along with habitat quality. We found support for both of these predictions. Although the sex ratio did not differ significantly from 1:1 in 19 of the 24 populations surveyed, the frequency of male inconstancy did vary geographically. The fraction of males that produced fruit in a population declined on a north-south gradient that reflected a three-fold decline in rainfall. Among populations, the frequency of females was negatively correlated with the frequency of inconstant males. We postulate that the advantages of conditional fruit production in males (i.e., ability to produce seeds during periods of unpredictable pollinator service) will result in gynodioecy and male inconstancy remaining a stable feature of populations of O. baccatus.