Inflorescence and Early Flower Development in Loteae (Leguminosae) in a Phylogenetic and Taxonomic Context

Abstract

Molecular phylogeny shows that the temperate legume tribe Loteae is close to the mostly tropical Robinieae and monogeneric Sesbanieae, but comparative morphological studies of these groups are limited. Unusual patterns of inflorescence symmetry and calyx development have been described in some Loteae, but taxon sampling was low. We studied these features with scanning electron microscopy in 25 species of Loteae plus in three Robinia species. Phylogenetic trees of Loteae based on nuclear ribosomal ITS sequences and 77 morphological characters are constructed. Our data show that whorled flower arrangement is a synapomorphy of Loteae; joint initiation of the two adaxial sepals is a synapomorphy of a clade containing Hippocrepis, Scorpiurus, and Coronilla; floral buds bent backward early in development are a synapomorphy of Coronilla; bilateral umbel symmetry and the presence of a single whorl of flowers are probably primitive within Loteae. Inflorescences of Robinia show no special similarities with those of Loteae. Developmental data support homologies between sterile bracts in all Loteae. Even if the sterile bract is situated at the top of the peduncle, it is morphologically the first leaf on the peduncle. Monosymmetric umbels of Loteae (including the model legume Lotus japonicus) could be useful for investigating genetic control of symmetry in structures of hierarchic levels higher than flowers.