Out of Africa: evidence of the obligate mutualism between long corolla tubed plant and long-tongued fly in the Himalayas

Mutualism between long corolla tubed plants and their potential pollinators, long-tongued flies, is a classic example of coevolution, but to date, has only been reported from the regions of southern Africa. Many plant species from the Himalayas also show botanical characteristics that could be consistent with pollination by long-tongued flies. Here, we seek the evidence of the long-tongued-long tubed fly/flower mutualism out of Africa, in a different continent and climatic region, the Himalayas. Floral traits of Himalayan region endemic alpine genus, Roscoea, indicate possible mutualism with long-tongued flies for pollination success; however, effective pollinators of this genus are yet unknown. This study investigates whether long-tongued flies and Roscoea purpurea in Nepal Himalayas show exclusive mutualism for their survival/reproduction. We made extensive observations of floral visitors of R.purpurea and food source of Philoliche longirostris across their wide ranges of populations in Nepal Himalayas for three consecutive years (2012-2014). To confirm the obligate reliance of R.purpurea upon P.longirostris for pollination success, manipulated pollination experiments were conducted at two populations for 2years. Similarly foraging behavior, visitation frequency, and pollination efficiency of P.longirostris were assessed at two populations for 2years, and its contribution for the reproductive success of R.purpurea was evaluated. Our results indicate that R.purpurea is self-compatible but lacks autonomous selfing and obligatorily relies on P.longirostris for reproductive success. Across all populations, P.longirostris was observed as an exclusive and highly efficient pollinator of R.purpurea, while P.longirostris exclusively depends up on R.purpurea for food source. Out of Africa, this study provides the first evidence of long-tongued fly pollination system and indicates the possibility of additional instances of such a rare phenomenon in the Himalayas. Finding of specialized pollinator of Roscoea only at its evolutionary center indicates that Roscoea species are originally pollinated by long-tongued flies. Spatial mismatch with specialized pollinators may have induced the evolution of autonomous selfing in North Indochinese clades of Roscoea. This finding thus substantiates how geographic disjunction causes the shifting of pollination mechanism in closely related plant species.