Breeding System Polymorphism in a Heterostylous Species

Heterostylous breeding systems have been the subject of continued interest since the work of Hildebrand (1866, 1871, 1887) and Darwin (1865, 1877) revealed the association of floral heteromorphism with an incompatibility system enforcing outcrossing. However, little is known of the selective factors that favor the development of heterostyly, or the selective forces which act to cause modifications of heteromorphic breeding systems. The evolutionary relationship of distyly and tristyly where they occur together in the same family has remained unclear. In Oxalis section Corniculatae, where both reproductive systems occur together, the derivation of distyly from tristyly has been suggested (Ornduff, 1972). The occurrence of distylous and tristylous populations of Oxalis alpina (section Ioenoxalis), a species of the southwestern United States, Mexico and Guatemala, suggested that studies of this species could provide further information on the evolutionary relationship of distyly and tristyly, and indicate how changes in heterostylous breeding systems have occurred. In trimorphic populations of Oxalis alpina the stigmas of one floral form occupy the lowest position in the flower, and the two sets of anthers occupy the middle and upper positions (Fig. 1). In a second floral