Faculty adviser Dr. Beryl Simpson and graduate student Jan Saunders will study the tropical tree genus Waltheria and related species, using herbarium specimens and new field collections made in Brazil. Because the genus is poorly known, work will focus on improved species descriptions, maps of geographic distribution, identification keys, and inferences about evolutionary relationships of the estimated 50 species, 25 of which occur in Brazil. The resultant taxonomic monograph will also resolve the nomenclature of the 120 scientific names currently applied to these plants worldwide. In addition, the unusual breeding system of distyly (two floral forms, on separate plants) will be studied. Pin plants bear flowers with long styles and short stamens, whereas thrum plants bear flowers with short styles and long stamens. The floral dimorphism typically is linked by a supergene to a diallelic sporophytic incompatibility system which reduces the amount of inbreeding that can occur within a plant population. In this study, the morphological differences between pin and thrum forms in several species will be analyzed extensively, especially the pollen. Possible stages in the evolution of distyly will also be studied, in the course of comparative analyses of Waltheria and related genera of the Sterculiaceae family.