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 the western desert of Egypt. Quantitative estimates of gender were calculated for each plant, each year over the entire study period. Plants were also classified using a typological framework, allowing discrimination between plants that followed different flowering phenologies (e.g., dichogamy) but that may have had the same quantitative values of phenotypic gender. Two contrasting patterns of gender were revealed. Approximately one-third of all plants were stable in their gender expression (i.e., they produced the same floral pattern and quantitative gender on every occasion); the remainder had labile gender phenotypes, showing varying degrees and patterns of gender lability. The relative frequency of gender phenotypes differed significantly between sites. Stable unisexual forms were most prevalent at the depression sites, where the water table is closer to the soil surface. Patterns of gender variation in T. hirsuta can be interpreted in terms of a complex evolutionary pathway to dioecy, represented by several intermediate stages, including dichogamous, monoecious, and subdioecious individuals. Alternatively, there may be no directionality to the array; gender phenotypes may reflect localized selection that confers differential fitness on the phenotypes, to the extent that such phenotypes may be heritable.