Form, function, and ontogeny of three heterostylous species of Rubiaceae

Abstract

The Rubiaceae, with a world-wide distribution, have a variety of pollinator syndromes and include the greatest number of heterostylous species of any angiosperm family. Variation in heterostyly among taxa of Rubiaceae is often attributed to potential incidences of independent evolution. I documented incompatibility systems, floral dimensions at anthesis, and floral development for three heterostylous taxa in Rubiaceae. I collected data from populations of hummingbird-pollinated Psychotria poeppigiana at La Selva Biological Station, Costa Rica and Soberania National Park, Panama, and from a population of hawkmoth-pollinated P. chiapensis also at La Selva. My study populations of hummingbird-pollinated Bouvardia ternifolia were in the Catalina and Santa Rita Mountains near Tucson, Arizona. To characterize incompatibility, I recorded inhibition sites of illegitimate pollen tubes. For B. ternifolia, locations of inhibition sites were similarly variable for each floral morph. In both species of Psychotria, incompatibility reactions of short-styled (SS) flowers typically occurred within stigmas whereas in long-styled (LS) flowers, incompatibility reactions most often occurred in styles. For all three species, I found significant differences in stigma and anther heights at anthesis corresponding to floral morph. However, because of extensive variation among individuals of both floral morphs, stigma and anther heights of LS and SS flowers are not strictly reciprocal. Mean anther height per plant forms a near continuum from LS to SS plants in most populations. Stigma heights of different floral morphs overlap in several plants of B. ternifolia and of P. chiapensis. Only for stigmas of P. poeppigiana did the organs of both floral morphs fall into distinct height categories. To study floral ontogeny, I measured buds representative of different developmental stages. For all species, height differences between anthers of LS and SS flowers at anthesis result from differences in relative growth rates. For B. ternifolia, a dimorphism in corolla size also contributes to anther height differences between floral morphs. Style heights of LS and SS flowers of B. ternifolia result from differences in relative growth rates between floral morphs. Contrasting with the Psychotrias, where style height differences are established early in development and maintained as styles elongate at the same relative rate.