One of the fundamental questions in biology is: Why have separate sexes? Having separate sexes avoids the worst type of inbreeding: self-fertilization. In animal-pollinated plants it is hypothesized that separate sexes evolve in certain areas (e.g., mountain-tops, deserts) because the less specialized pollinators there predominantly self-fertilize these plants. An alternative hypothesis is separate sexes evolve because resources (e.g., water) needed for sexual function are scarce, and can be allocated more efficiently if the sexes are separated. Claret-cup cacti present a unique opportunity to concurrently test these two hypotheses because they possess both hermaphroditic (sexes together) and dioecious (sexes separated) populations. This species is pollinated by less-specialized bees in and around dioecious populations, which supports the inbreeding-avoidance hypothesis. However, dioecious populations also tend to occur in arid areas. This supports the resource-allocation hypothesis. Genetic analyses at the plant, population and landscape scale will determine if the rate of self-fertilization produced by bees is sufficient to produce dioecy. The direct and indirect effects of pollinators, rainfall and other environmental factors on the evolution of dioecy will be assessed using Structural Equation Modeling.
Funding of this research will provide the first test of the inbreeding-avoidance hypothesis in natural populations. Developing the first microsatellite markers for cacti for this project has led to collaborations across three institutions and training of minority undergraduates in modern genetic techniques.
