HIGH OUTCROSSING RATES MAINTAIN MALE AND HERMAPHRODITE INDIVIDUALS IN POPULATIONS OF THE FLOWERING PLANT DATISCA-GLOMERATA

MODELS for the maintenance of androdioecy (the presence of male and hermaphrodite individuals in a breeding population) in plants predict that males must have a fertility at least double the male fertility of hermaphrodites in order to be maintained by selection1-3. An even greater advantage is required in partially self-fertilizing populations1-3 as the gain in fitness through increased pollen production is least when few ovules are available for outcrossing. Because such stringent theoretical requirements make the evolutionary stability of this breeding system highly unlikely, functional androdioecy is thought to be rare in plants, and indeed the only documented instance occurs in populations of Datisca glomerata (Datiscaceae)4. As such, these populations provide a unique opportunity to test predictions concerning the evolution of androdioecy in plants. Here we report high outcrossing rates (65-92%) in two androdioecious populations of D. glomerata using random amplified polymorphic DNA markers. These outcrossing rates, when analysed with respect to existing evidence concerning pollen production and inbreeding depression in this species, are sufficiently high to satisfy theoretical requirements for the maintenance of androdioecy.