THE ORIGIN OF DIOECISM FROM HETEROSTYLY IN NYMPHOIDES (MENYANTHACEAE)

Heterostyly refers to a relatively rare combination of morphological, physiological, and genetic mechanisms which promotes outbreeding in certain angiosperms. Under conditions where the selective forces favoring this degree of outbreeding are reduced or removed, one or all of the components of this character complex may break down with the result that inbreeding partially or almost wholly replaces outbreeding. There are numerous examples of such a breakdown of heterostyly in the direction of increased selfpollination in taxa belonging to various plant families. Because the complex combination of floral mechanisms and incompatibility characteristic of heterostylous plants is apparently generally effective in promoting outbreeding, the breakdown of heterostyly in the opposite direction, namely in the direction of increased outbreeding, is uncommon. Nevertheless, there is evidence that in at least two angiosperm families (Rubiaceae and Menyanthaceae) heterostyly has been replaced by an even more effective outbreeding mechanism in the form of dioecism. The purpose of this paper is to discuss the breeding systems of various distylous and dioecious members of the menyanthaceous genus Nymphoides and to present an account of the probable origin of dioecism in this genus. Nymphoides has an almost cosmopolitan distribution, occurring primarily in tropical and subtropical areas but also in certain temperate regions of both the Northern and Southern hemispheres. The plants are aquatic or subaquatic, and are typically limited to the still water of ponds, lakes, and slow-moving streams. In all Nymphoides species examined, maturation of floral buds is underwater. In the morning of anthesis the buds are raised above the water level and flowering occurs. After a few hours the corolla withers, the pedicel deflects, and subsequent fruit development is underwater. DIOECIOUS SPECIES Dimorphic heterostyly is widespread in the genera of Menyanthaceae, but dioecism seems to have been previously unrecorded for this family (Gilg, 1895). The two species of Nymphoides indigenous to the United States are dioecious. These are N. cordata, an inhabitant of ponds and slow streams ranging from eastern Canada to Florida and Louisiana, and N. aquatica, a species of similar habitats ranging from New Jersey to Florida and Texas. The two species are closely related but are distinguishable on foliar, root, and floral characters. The floral morphology of these dioecious species is very similar. The ovary of pistillate flowers is capped by a short style and a large fleshy stigma. The vestigial stamens of such flowers are relatively large and conspicuous, consisting of white anthers and short filaments. Alternating with these non-polliniferous stamens are staminode-like clusters of yellow trichomes (Fig. 1). The yellow corolla bases and additional tufts of yellow trichomes at the basal petal margins provide additional color contrast to the white corollas. Staminate flowers have well-developed stamens that produce copious pollen. The pistil of such flowers essentially lacks a style or stigma, but the ovary is large and is filled with ovules of approximately the size and number of those produced by pistillate flowers (Fig. 1). Nevertheless, despite repeated polli-