ZYGOMORPHY AND HETERANTHERY IN SOLANUM IN A PHYLOGENETIC CONTEXT

The majority of species in the large genus Solanum (ca. 1500 species) have five-merous, radially symmetrical flowers with equal stamens. However, some Solanum species and groups are characterized by four-merous and/or zygomorphic flowers, unequal stamens and enantiostyly (styles deflected to one side of the flower). Previous workers have used flower and seed coat morphology in these unusual Solanum species as a guide to interpreting their evolutionary relationships. However, the phylogenetic position of the zygomorphic and heterantherous solanums is only beginning to be examined using molecular data and cladistic methodology. DNA sequence data from both the chloroplast and nuclear genomes are used to infer the phylogenetic position of Solanum taxa with variously modified flowers. Zygomorphy and heteranthery have evolved multiple times within Solanum, and most frequently within the spiny solanums (Solanum subg. Leptostemonum). The phylogenies shed light on the disparate morphologies and geographical distributions encountered in the zygomorphic and heterantherous species and pinpoint the likely relatives of these taxa among the actinomorphic Solanum species with equal stamens.