Roscoea

Abstract: The family Zingiberaceae is generally treated as consisting of two subfamilies, the Zingiberoideae with distichous phyllotaxy and the leaf-sheath open, and the Costoideae with spiral phyllotaxy and the leaf-sheath closed and tubular. (These subfamilies are sometimes treated as distinct families). Roscoea is clearly a member of the Zingiberoideae yet, as Spearing (1977) noticed, closed, tubular leaf-sheaths occur in all the species of Roscoea and one species of Cautleya Royle. There is however some difference between the details of the sheath in these two genera. In Roscoea the sheaths are closed to the very top and on the opposite side the leaf-keel is decurrent as a ridge onto the sheath. In Cautleya the sheath is not always closed right to the top and the leaf-keel does not continue beyond the petiole, the sheath thus not ridged, and the false stem terete.

Abstract: According to the concept of pollination syndromes, floral traits reflect specialisation to a particular pollinator or set of pollinators. However, the reproductive biology of endemic, and often specialised, plants may require increased attention as climate change accelerates worldwide. Species of Roscoea endemic to the Himalayan region have striking orchid-like flowers with long corolla tubes, suggesting pollination by long-tongued insects. Until now, the reproductive biology of species of Roscoea has been poorly documented. We investigated the floral biology, breeding system and pollination ecology of R. cautleoides and R. humeana, from Hengduan Mountains, a global biodiversity hotspot in southwest China. We also tested whether floral longevity increases pollination success. Pollination experiments showed that the two species were self-compatible and depended on insects for fruit production. Over several flowering seasons we did not observe any potential pollinators with long tongues that matched the corolla tube visiting flowers in centres of distribution. The principal pollinators observed were pollen-collecting generalist bees, with low visitation frequencies. In general, members of the ginger family are characterised by short-lived (usually 1 day) flowers, but flowers of R. cautleoides and R. humeana last 8 and 6 days, respectively. Removing stigmas decreased fruit set in both study populations. Our results suggest that the original pollinators may have been long-tongued insects that are now absent from the Chinese Himalayas because habitats have responded to climate change. However, long-lived and self-compatible flowers, coupled with the presence of generalist pollinators, are traits that have allowed these gingers to reproduce and continue to persist in the alpine habitats.

Abstract: Multiple barriers may contribute to reproductive isolation between closely related species Understanding the relative strength of these barriers can illuminate the ecological factors that currently maintain species integrity and how these factors originally promoted speciation 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, earlier for R tumjensis and later for R purpurea, and complete fidelity of visitation by different pollinator species, bumblebees for R tumjensis and a long-tongued fly for R purpurea Furthermore, the nectar of R tumjensis flowers is available to the shorter tongued bumblebees while R purpurea nectar is less accessible, requiring deep probing from long-tongued flies Although flowering phenology is a strong current barrier that seemingly obviates any need for pollinator discrimination, this current pattern need not reflect selective forces occurring at the initial divergence of R tumjensis There has been considerable pollinator switching during the radiation of the Himalayan Roscoea, and the association of flowering time with type of pollinator in these sympatric species may have originated among the earliest or latest flowering individuals or populations of an ancestor to exploit either bumblebee activity early in the breeding season or long-tongued fly abundance later in the season These two sympatric Roscoea species add to accumulating evidence of the primacy of prezygotic pollination traits in speciation among angiosperms even in the absence of postzygotic incompatibility