A major controversy exists regarding the adaptive significance of large floral displays in plants with consistently low fruit-set: why produce so many the extra flowers are there to produce pollen which will successfully convey male genes to other flowers. This "pollen donation hypothesis" has been a difficult issue to address. While it is reasonably easy to measure fruit or seed production, it is technically much more difficult to quantify pollen success. Recently, plant population biologists have turned to paternity exclusion analysis as a possible solution to the problem. In principle, the technique is identical to the process of using blood types to determine human paternity. The proposed research will test the pollen donation hypothesis in milkweeds (Asclepias). Genetic markers from protein electrophoresis will be used to perform a paternity exclusion analysis. This will identify seeds sired by individual plants in the population. Inflorescence size will be manipulated by removing flowers in experimental populations to determine the relationship between inflorescence size and numbers of seeds sired and numbers of seeds produced. This study will yield important basic information about the forces that shape the evolution of inflorescence size in plants. It will also provide data regarding levels of gene flow within and between populations. Finally the development of paternity analysis in plants will provide a basis for investigating breeding systems of both native and crop plants. Furthermore, should the need arise again (as in World War II) for milkweeds to be used as substitute sources of kapok and rubber, we will understand better the forces that limit fruit-set and will have new insights into how to design effective breeding programs.
