Inaccuracy patterns and precise pollination among distylous Palicourea and Psychotria (Rubiaceae)

Abstract

Heterostyly is a polymorphism in which populations comprise two (distyly) or three (tristyly) floral morphs with reciprocal positioning of the height of the anthers and stigmas (reciprocal herkogamy). Such reciprocal herkogamy permits precise pollen placement on pollinators’ bodies and pollination success by promoting disassortative pollen transfer between floral morphs. Here, we aimed to understand how the different components of reciprocity relate to and differ from each other in distylous flowers, and whether factors, such as genus, morph-ratio variation, and corolla tube length influence the imprecisions in the sexual organs within populations. We gathered literature and original morphometrics data comprising 98 Palicourea and Psychotria populations from 44 species, the two largest distylous Rubiaceae genera, which differ in some floral features and pollination systems (hummingbirds vs. insects, respectively). We estimated reciprocity using new inaccuracy indices and standardization techniques, which allowed comparisons among a wide array of populations. Our results showed that maladaptive bias (i.e. departure from optimum reciprocity) of low organs was higher than the other decomposed inaccuracies indices tested. In addition, inaccuracy of low organs was higher than those of high organs. We did not find any relation of either genus, morph-ratio variation, or corolla tube length on general inaccuracy measurements. Apparently, pollinator-mediated selection on floral traits, imposed by different pollinators groups, did not differ markedly among the studied distylous Rubiaceae. This is the first study on inaccuracy components for such an ample array of populations and provides insights on the relative importance of morphological traits that optimize the functionality of distyly. We highlight the applicability of using standardized reciprocity indices for cross populational investigations, which may both support previous insights and reveal unknown patterns in distylous plants.