A new pollination system: brood-site pollination by flower bugs in Macaranga (Euphorbiaceae)

Abstract

Background and aims: Macaranga (Euphorbiaceae) is a large genus of dioecious trees with approx. 260 species. To date, only one pollination study of the genus has reported brood-site pollination by thrips in M. hullettii. In this study, the pollination system of Macaranga tanarius is reported.

Methods: The study was conducted on Okinawa and Amami Islands, Japan. Flower visitors on M. tanarius were collected and their pollen load and behaviour on the flowers examined, as well as inflorescence structure and reward for the pollinators.

Key results: The most abundant flower visitors found on the male and female inflorescences were Orius atratus (Anthocoridae, Hemiptera), followed by Decomioides schneirlai (Miridae, Hemiptera). Pollen load on O. atratus from flowering pistillate inflorescences was detected as well as from staminate flowers. Orius atratus and D. schneirlai are likely to use the enclosed chambers formed by floral bracts as breeding sites before and during flower anthesis, and feed on nectar on the adaxial surface of flower bracts. The extrafloral nectary has a ball-shaped structure and the contained nectar is not exposed; the hemipterans pierce the ball to suck out the nectar.

Conclusions: The results indicate that the plant is pollinated by flower bugs breeding on the inflorescences. This study may be the first report of pollination systems in which flower bugs are the main pollinators. Similarity of pollination systems between M. hullettii and M. tanarius indicates that the two brood-site pollination systems have the same origin. The pollinator species belongs to a predacious group, whose major prey includes thrips. The pollination system might represent a unique example of evolution from predatory flower visitors feeding on the pollinators (thrips) to the main pollinators.

Fig. 1.

Macaranga tanarius and pollinator: (A) overview of pistillate inflorescences (scale bar = 2 cm); (B) overview of staminate inflorescences (scale bar = 3 cm); (C) flowering pistillate inflorescence in which the outer bracts are extended to show the lobed stigma (scale bar = 5 mm); (D) flowering staminate inflorescences visited by Orius atratus (scale bar = 2 mm); (E) Orius atratus sucking nectar on the adaxial surface of a bract (scale bar = 1 mm); (F) scanning electron micrograph of nectaries on the adaxial surface of a bract (n, mature nectary before sucking; *, nectary already sucked and deflated; scale bar = 100 µm).

Fig. 2.

Number of hemipterans collected from five branchlets on staminate (A) and pistillate (B) inflorescences. Columns show an adult Orius atratus, an adult Decomioides schneirlai and hemipteran larvae as indicated. Vertical bars represent s.e.