Pollination and reproduction of Psychotria homalosperma, an endangered distylous tree endemic to the oceanic Bonin (Ogasawara) Islands, Japan

Abstract

Distyly is usually rare or not observed in species thriving on oceanic islands. The rarity of this breeding system is probably because of the difficulty of colonization for distylous plants and the paucity of pollinators on oceanic islands. However, the endemic and endangered tree Psychotria homalosperma has maintained its distylous nature in the oceanic Bonin Islands, Japan. To understand how the distylous breeding system of P. homalosperma has been maintained on these islands and to characterize the reproduction systems, we studied the pollination and reproductive biology of this species. Specifically, we observed current flower visitors and estimated their effects on plant reproduction. We also examined the floral traits and floral volatiles of P. homalosperma to infer its original pollinators, because plant-pollinator relationships in the Bonin Islands have recently been disrupted by anthropological activities. Finally, we examined the fruit set and pollen tube growth in the stigmas under hand and open pollination. Although original pollinators were presumed to be moths with long proboscises, the introduced honeybee, Apis mellifera, was the most common flower visitor. The honeybee carried pollen grains only unidirectionally, from the short-to long-styled morphs, because it could not reach the hidden stigmas of the short-styled flowers, and long-styled flowers set fruits 1.7-38 times more than short-styled ones. This case study indicates that the instability of pollinator fauna can cause distylous species to be rare on oceanic islands.

Plant-pollinator interactions in East Asia: a review

“…The flower visitors of L. grayi on the main island were dominated by A. mellifera, whereas diverse native visitors were observed on this species' flowers on the satellite islands (Tsuda et al 2018). On the Bonin Islands, many plants present pollination syndromes toward particular types of pollinators, but virtually no candidate pollinators visit these flowers (Abe 2006;Watanabe et al 2018). For example, some flowers present a hawkmoth pollination syndrome, but these flowers were observed to receive no or only very rare visits by moths (Abe 2006;Watanabe et al 2018).…”

“…On the Bonin Islands, many plants present pollination syndromes toward particular types of pollinators, but virtually no candidate pollinators visit these flowers (Abe 2006;Watanabe et al 2018). For example, some flowers present a hawkmoth pollination syndrome, but these flowers were observed to receive no or only very rare visits by moths (Abe 2006;Watanabe et al 2018).…”

Spiradiclis tubiflora (Rubiaceae), a new cave-dwelling species from southern China

“…It is known that being distylous is a unique phenotype in plants to ensure reproduction by avoiding self-pollination and to increase male fitness and outcrossing rates by reducing sexual interference between male and female functions (Watanabe et al 2017). In this study, however, all the individuals with observed flowers are with stamens inserted at the middle of the corolla tube and stigmas located near the throat of the corolla tube, this being the long-styled form.…”

Plant Species Biology – toward the future

“…Two papers were selected as the Best Paper Award 2018, by the Editorial Committee of Plant Species Biology . One paper is “Pollination and reproduction of Psychotria homalosperma , an endangered distylous tree endemic to the oceanic Bonin (Ogasawara) Islands, Japan” contributed by Watanabe, Kato, Kuraya and Sugawara (, pp. 16–27).…”