Dataset for: Reproductive isolation in alpine gingers: how do co-existing Roscoea (R. purpurea and R. tumjensis) conserve species integrity?

Two Himalayan alpine gingers, Roscoea purpurea and R. tumjensis, occur sympatrically in central Nepal and have such similar morphology that it is not clear whether or how they maintain a distinct identity. Our quantitative measurements of the components of reproductive isolation show that they are, in fact, completely isolated by a combination of phenological displacement of flowering and complete fidelity of visitation by different pollinator species, a long-tongued fly for R. purpurea and bumblebees for R. tumjensis. The disjunction of pollinators seems surprising until the floral traits are analysed from the pollinators’ perspective. Models of both dipteran and hymenopteran colour vision indicate that flowers of the two species would be easily discriminable. Furthermore, the nectar of R. purpurea is less accessible, requiring deep probing from long-tongued flies, while the nectar of R. tumjensis flowers is available to the shorter-tongued bumblebees. Although current barriers need not reflect evolutionary history, two possible pathways to isolation seem likely depending on whether displacement of flowering time or novel floral signals arose first. These two sympatric Roscoea species add to accumulating evidence of the primacy of early acting, pollination traits in speciation among angiosperms even in the absence of post-zygotic incompatibility.