Deconstructing a floral phenotype:: do pollinators select for corolla integration in Lavandula latifolia?

An experimental field study was undertaken to assess whether correlational selection generated by pollinators can account for observed phenotypic integration of the two corolla lips in the zygomorphic flowers of the insect-pollinated Mediterranean shrub, Lavandula latifolia (Labiatae). The experiment consisted of manipulating the upper and lower corolla lips according to a two-way factorial design with interaction, and involved the complete (lower lip) or extensive (distal two thirds of upper lip) excision of one or the two lips. Per-flower male (proportion of pollen grains removed) and female (number of pollen tubes down the style) pollination success, and maternal fecundity (seed set), of experimental flowers were evaluated after exposure to natural pollination in the field. Artificially induced corolla variation had no significant effects on either male or female pollination success, and two aberrant floral morphologies, characterized by lack of corolla integration, had a maternal fecundity advantage over normal two-lipped phenotypes. These results do not support a predominant role of correlational selection by pollinators in maintaining corolla integration in L. latifolia, but are instead consistent with the alternative view that integration is mainly the consequence of genetic and/or developmental factors.