GENDER VARIATION AND THE EVOLUTION OF DIOECY IN THYMELAEA-HIRSUTA (THYMELAEACEAE)

Phenotypic gender in the evergreen shrub Thymelaea hirsuta was studied at regular intervals over 3-6 years in five natural populations in th e western desert of Egypt. Quantitative estimates of gender were calcu lated for each plant, each year over the entire study period. Plants w ere also classified using a typological framework, allowing discrimina tion between plants that followed different flowering phenologies (e.g ., dichogamy) but that may have had the same quantitative values of ph enotypic gender. Two contrasting patterns of gender were revealed. App roximately one-third of all plants were stable in their gender express ion (i.e., they produced the same floral pattern and quantitative gend er on every occasion); the remainder had labile gender phenotypes, sho wing varying degrees and patterns of gender lability. The relative fre quency of gender phenotypes differed significantly between sites. Stab le unisexual forms were most prevalent at the depression sites, where the water table is closer to the soil surface. Patterns of gender vari ation in T. hirsuta can be interpreted in terms of a complex evolution ary pathway to dioecy, represented by several intermediate stages, inc luding dichogamous, monoecious, and subdioecious individuals. Alternat ively, there may be no directionality to the array; gender phenotypes may reflect localized selection that confers differential fitness on t he phenotypes, to the extent that such phenotypes may be heritable.