Nectar spur evolution in the Mexican lobelias (Campanulaceae: Lobelioideae)

Phylogenetic studies are often hampered by the independent evolution of characters that may potentially obscure relationships. The adaptive significance of the nectar spur and its evolution within the Mexican lobeliads (Campanulaceae) is considered here. The taxonomic delimitations of Heterotoma from the Mexican species within the genera Lobelia and Calcaratolobelia were tested. Independent molecular data were gathered to determine whether the Mexican spurred lobeliads should be treated as distinct genera. The internal transcribed spacer (ITS) region from 18-26S nuclear rDNA and chloroplast DNA from the 3' trnK intron were sequenced from 14 representative species. Our data suggest that Heterotoma, as originally conceived, is a good evolutionary unit within Lobelia and that the presence of a nectar spur is an important morphological character that can be used in defining phylogenetic position. This study also suggests that morphological changes associated with hummingbird pollination have evolved more than once in the Mexican lobeliads, from small blue-flowered, insect-pollinated relatives.