Evolution of flower morphology, pollen dimorphism, and nectar composition in Arcytophyllum, a distylous genus of Rubiaceae

A phylogenetic study of Arcytophyllum based on ITS was conducted and compared with an earlier study based on cpDNA. The position of the widespread A. thymifolium as sister to all other species was confirmed and several well-supported clades could be retrieved. The Central American A. lavarum is well embedded between exclusively or predominantly South American species. To understand the expression of heterostyly in the genus, we analyzed inter- and intraspecific variation in floral morphology, nectar, pollen–ovule (P/O) ratio and seed set of ten species in 11 populations. Stigma and anther levels differed significantly between the morphs in the species/populations investigated, except for A. filiforme, in which anther levels did not differ significantly between the two morphs. Different expressions of heterostyly in Arcytophyllum seem independent of phylogenetic relationships. Nectar sugar composition was similar between the morphs. Nectar of most species presented a larger proportion of hexoses than of sucrose; only the most derived species, A. macbridei and A. vernicosum, have higher sucrose proportions. There is a significant positive correlation between corolla tube length and the proportion of sucrose. Pollen dimorphism, with regard to both number (long-styled>short-styled) and size (short-styled>long-styled), was observed in all taxa investigated except A. filiforme. According to the P/O ratios, the breeding systems range from facultative autogamy to facultative xenogamy. The lowest P/O ratios were found in A. filiforme, and the highest in A. rivetii. Hymenoptera, Diptera and Coleoptera were observed as flower visitors. Seed production did not differ significantly between the morphs in eight of the 11 species/populations investigated. There is, however, a tendency in all species/populations (except in A. macbridei Peru) for the short-styled morph to have a higher percentage of seeds per ovule, indicating that the short-styled morphs display higher female reproductive success.