Maintenance of sympatric floral tube-length variation in a Cape irid

Transition zones between morphologically different individuals of the same species provide insights into the evolution and maintenance of gene-flow barriers. Here we investigate Tritoniopsis revoluta, which has geographically variable tube lengths, thought to be adaptations to insects with different proboscis lengths. We found a narrow transition zone between plants differing by three-fold in perianth tube length. We determined whether strong gene-flow barriers result from assortative mating arising from different pollinators, a high prevalence of selfing, or post-pollination incompatibilities between plants with different tube lengths. We found that there was little evidence to support assortative mating through different pollinators. Both short-and long-tubed plants were mainly visited by bees with short proboscides. Selfing is unlikely to contribute significantly to seed set, plants with different tube lengths were interfertile and hybrid plants were fertile. We conclude that the contact zone is unstable because these ecotypes have not accrued enough allopatric differences to translate into strong gene-flow barriers, or, alternatively, bimodality is not a consequence of secondary contact but the result of a novel mutation for short tubes spreading through a long-tubed population. (C) 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104, 129-137.