THE BREAKDOWN OF TRIMORPHIC INCOMPATIBILITY IN OXALIS SECTION CORNICULATAE.

Abstract

Among the most complex outcrossing systems in flowering plants are those in which floral heteromorphism is associated with a physiological incompatibility system. Such heteromorphic incompatibility systems can arise only as a result of strong selective pressures exerted over a long period of time. There is excellent evidence that when the selective pressures regulating the maintenance of these systems are relaxed or altered in their direction, the integrated syndrome of morphological and physiological traits characteristic of heteromorphic incompatibility may rapidly break down, usually in the direction of increased inbreeding. There are several evolutionary routes which the breakdown of heteromorphic incompatibility may follow. The diverse breeding systems of the American representatives of Oxalis section Corniculatae (Oxalidaceae) are most probably derived from an ancestral trimorphic incompatibility system. This paper presents a study of the breeding systems of several members of this group and briefly discusses the genetic and ecological circumstances leading to the development of these derivative systems. Oxalis section Corniculatae recently has been the subject of a taxonomic monograph by Eiten (1963), whose nomenclature will be followed here. As circumscribed by Eiten, the section contains 14 species of caulescent, non-bulbous, yellowor orangeflowered herbs with trifoliate leaves. Members of this section occur mostly in North and South America and the Greater Antilles, but two species are believed to have a natural amphi-Pacific range in North America and eastern Asia. TRISTYLOUS INCOMPATIBILITY AS AN ANCESTRAL CONDITION