THE ECOLOGY OF SEX EXPRESSION IN A GYNODIOECIOUS ISRAELI DESERT SHRUB(OCHRADENUS-BACCATUS)

The main goal of this study was to evaluate sex expression at geograph ic and individual levels in Ochradenus baccatus, an Israeli desert shr ub. Although this species was formerly thought to be strictly dioeciou s, in a survey of 24 Ochradenus populations along a latitudinal rainfa ll gradient, we found sex expression to be extremely variable. While f emales reproduced only by seeds, males ranged across a continuum from ''pure males'' that produced only pollen to ''inconstant males,'' whic h produced seeds in addition to pollen. For sexual dimorphism to evolv e from cosexuality, it is believed that the sex morphs must be compens ated for their loss of one sex function (e.g., pollen production) by a n increase in the remaining sex function (i.e., seed production). In o ur study we found that females produced 9-20 times as many seeds as ma les, and we showed in a controlled laboratory experiment that their of fspring had higher germination and seedling growth rates. Pure males i nvested more biomass in stamens than did inconstant males. Previously published models concerned with the evolution of sexual dimorphisms ha ve predicted that: (1) the frequency of inconstant males should be inv ersely related to the frequency of females, and (2) if the degree of m ale inconstancy is influenced by environmental factors, then seed prod uction in males should decline along with habitat quality. We found su pport for both of these predictions. Although the sex ratio did not di ffer 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 g radient that reflected a three-fold decline in rainfall. Among populat ions, the frequency of females was negatively correlated with the freq uency of inconstant males. We postulate that the advantages of conditi onal 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.