Heterostyly (i.e., reciprocal placement of anthers and stigmas between two or three floral morphs) is hypothesized to enhance outcrossing and reduce selfing. However, few studies have documented reciprocity among individual plants; instead, mean anther and stigma heights for floral morphs are usually reported, masking interindividual variation. We measured eight floral dimensions for individuals in five populations of three heterostylous Rubiaceae. The three methods used to quantify reciprocity yielded different conclusions regarding the degree to which populations conformed to expectations for heterostylous plants. Only Psychotria poeppigiana had stigma and, to a lesser degree, anther heights in discrete classes. Variation among plants of Bouvardia ternifolia and Psychotria chiapensis yielded a continuum of anther and stigma heights across populations. Comparison of distances between stigma and anthers indicated that only flowers of B. ternifolia had, as expected, a constant value for this distance. Finally, regression relationships between anther and stigma heights and corolla length showed that only in one population each of B. ternifolia and P. poeppigiana, and in P. chiapensis, was distance between anthers and stigmas the same across the range of corolla sizes for both floral morphs. Variation among these species in expression of heterostyly was not clearly linked to phylogenetic relationship or pollinator syndromes. Two approach herkogamous (AH) species were studied for comparison. Flowers of Psychotria brachiata were consistently AH, but flowers of P. pittieri were highly variable. Determining fitness consequences of population-level variation in sexual systems requires studies linking floral morphology to pollinator behavior and pollen transfer.

Alternative ontogenetic pathways among heterostylous species of Rubiaceae may reflect differences in their evolutionary histories. In this study, measurements were taken at different developmental stages on a series of longstyled (LS) and short-styled (SS) buds of the heterostylous taxa Psychotria chiapensis, P. poeppigiana, and Bouvardia ternifolia (all Rubiaceae). Results indicated that modifications in growth rates of stamens relative to corollas in all three species led to differences in anther heights between LS and SS flowers. Distinct style heights for LS and SS flowers of P. chiapensis and P. poeppigiana originate in the earlier stages of bud development and are maintained as styles elongate at equal rates. This contrasts with B. ternifolia, which has differences in style heights resulting from unequal relative growth rates between floral morphs. The ''approach herkogamous'' floral morphology, defined by having stigmas positioned above anthers, has been proposed as a potential evolutionary precursor for heterostylous taxa. To examine this hypothesis, floral development of two species with approach herkogamous morphologies, Psychotria pittieri and P. brachiata, was compared to that of the three heterostylous taxa. Differences in the relative rates of style elongation for flowers of approach herkogamous versus heterostylous species predict additional steps in the original model for the evolution of heterostyly from an approach herkogamous ancestor. The diversity of heterostylous ontogenies found within Rubiaceae provides insight into potential evolutionary pathways for this sexual system in other angiosperm families.

Abstract. Alternative ontogenetic pathways among heterostylous species of Rubiaceae may reflect differences in their evolutionary histories. In this study, measurements were taken at different developmental stages on a series of longstyled (LS) and short-styled (SS) buds of the heterostylous taxa Psychotria chiapensis, P. poeppigiana, and Bouvardia ternifolia (all Rubiaceae). Results indicated that modifications in growth rates of stamens relative to corollas in all three species led to differences in anther heights between LS and SS flowers. Distinct style heights for LS and SS flowers of P. chiapensis and P. poeppigiana originate in the earlier stages of bud development and are maintained as styles elongate at equal rates. This contrasts with B. ternifolia, which has differences in style heights resulting from unequal relative growth rates between floral morphs. The ''approach herkogamous'' floral morphology, defined by having stigmas positioned above anthers, has been proposed as a potential evolutionary precursor for heterostylous taxa. To examine this hypothesis, floral development of two species with approach herkogamous morphologies, Psychotria pittieri and P. brachiata, was compared to that of the three heterostylous taxa. Differences in the relative rates of style elongation for flowers of approach herkogamous versus heterostylous species predict additional steps in the original model for the evolution of heterostyly from an approach herkogamous ancestor. The diversity of heterostylous ontogenies found within Rubiaceae provides insight into potential evolutionary pathways for this sexual system in other angiosperm families.Key words. Distyly, floral development, herkogamy, heterostyly, ontogeny, Psychotria, Rubiaceae.Received May 7, 1999. Accepted November 30, 1999 Studies of floral ontogeny have played a key role in our understanding of plant sexual system evolution (Diggle 1992) including studies of gynodioecy (Gibson and Diggle 1998), cleistogamy and chasmogamy (Lord 1982;Lord and Hill 1987), autogamy (Hill et al. 1992;Fenster et al. 1995), and heterostyly (Richards and Barrett 1984, 1992Richards and Koptur 1993). Convergent evolution of floral or reproductive form among closely related taxa has often been revealed by differences in developmental patterns (Guerrant 1982;Hufford 1988 Hufford , 1997Kellogg 1990;Friedman and Carmichael 1998).One goal of this study is to compare the heterostylous ontogeny of two congeneric and a more distantly related species to progress toward defining the degree of developmental variation among heterostylous species within a family (or evolutionary unit). Heterostyly is a sexual system in which plants of the same species produce one of two (distyly) or three (tristyly) floral morphs (Darwin 1877;Ganders 1979;Barrett 1990Barrett , 1992b. In a distylous species, individual plants produce either long-styled (LS) flowers with the stigma positioned above the anthers or short-styled (SS) flowers with the stigma positioned below the anthers (see Fig. 1), both of which occur ...

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