Floral tubes are often thought to be a consequence of adaptive specialization towards pollinator morphology. We explore floral tube length evolution within
Tritoniopsis revoluta
(Iridaceae), a species with considerable geographical tube length variation. We ask whether tube lengths of
T. revoluta
populations are associated with pollinator proboscis lengths, whether floral divergence occurs in the presence of different pollinators and whether floral convergence occurs between distantly related populations pollinated by the same pollinator. Finally, we ask whether tube length evolution is directional. Shifts between morphologically different pollinators were always associated with shifts in floral morphology, even when populations were very closely related. Distantly related populations had similar tube lengths when they were pollinated by the same pollinator. Shifts in tube length tended to be from short to long, although reversals were not infrequent. After correcting for the population-level phylogeny, there was a strong positive, linear relationship between floral tube length and pollinator proboscis length, suggesting that plants are functionally specialized on different pollinators at different sites. However, because tube length evolution in this system can be a bidirectional process, specialization to the local pollinator fauna is unlikely to result in evolutionary or ecological dead-ends such as canalization or range limitation.
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.
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