Annals of Botany 121: 651–663, 2018 doi:10.1093/aob/mcx196, available online at www.academic.oup.com/aob Pollination by fungus gnats and associated floral characteristics in five families of the Japanese flora Ko Mochizuki* and Atsushi Kawakita Center for Ecological Research, Kyoto University, Otsu, Shiga 520-2113, Japan *For correspondence. E-mail: [email protected] Downloaded from https://academic.oup.com/aob/article-abstract/121/4/651/4781678 by guest on 22 May 2020 Received: 6 November 2017 Editorial decision: 13 November 2017 Accepted: 13 December 2017 Published electronically 29 December 2017 • Background and aims Pollination by fungus gnats (Mycetophilidae and Sciaridae) is uncommon, but is nevertheless known to occur in 20 genera among eight angiosperm families. Because many fungus gnat-pollinated plants possess a dark red floral display, we hypothesized that fungus gnat pollination is more widespread among plants with similar floral display than currently known. We thus studied the pollination biology of flowers with dark red pigmentation in five families, focusing particularly on plants having small, flat, actinomorphic flowers with exposed nectaries and short stamens, because these floral characteristics mirror those of a known fungus gnat-pollinated genus (Mitella). • Methods We observed daytime and night-time floral visitors for a total of 194.5 h in Aucuba japonica (Garryaceae), Euonymus spp. (Celastraceae), Disanthus cercidifolius (Hamamelidaceae), Micranthes fusca (Saxifragaceae) and Streptopus streptopoides (Liliaceae). Visitors were categorized into functional groups, and a pollination importance index (PII) was calculated for each functional group based on visitation frequency, pollen load and behaviour on flowers. • Key results Fungus gnats were dominant among the 1762 insects observed (36–92 % depending on the plant species) and were the most important pollinators among all plants studied (PII: 0.529–1). Fungus gnat visits occurred during the daytime and, more frequently, at dusk. Most often, pollen grains became clumped on the ventral side of the head and/or thorax as the short-proboscid fungus gnats foraged on nectar and came into contact with anthers located close to the flower base. • Conclusions Pollination by fungus gnats is probably more common than previously thought, especially in habitats similar to those of the plants studied (moist forest understorey, streamside or subalpine meadow) where fungus gnats are abundant year-round. Our results further suggest that there may be a previously unnoticed association between fungus gnat pollination and dark red coloration, and a shared overall floral architecture among the plants studied. Keywords: Aucuba japonica, dark red flower, Diptera, Disanthus cercidifolius, Euonymus, forest environment, fungus gnat, Micranthes fusca, Mycetophilidae, pollination syndrome, Sciaridae, Streptopus streptopoides. INTRODUCTION on mushrooms, fungal mycelia in rotten wood, decaying plant material and, more rarely, bryophytes or other insects Dipteran insects are among the most common visitors to angio- (Vockeroth, 1981; Steffan, 1981; Søli et al., 2000; Jakovlev, sperm flowers and often play key roles as pollinators in many 2011, 2012). Fungus gnats are usually considered as inef- plant communities (Kearns, 2001; Larson et al., 2001; Ssymank fective pollen carriers due to their small bodies, inconstancy et al., 2008; Lefebvre et al., 2014; Inouye et al., 2015; Orford and weak flight ability (Mesler et al., 1980; Proctor et al., et al., 2015; Ollerton, 2017). However, the importance of dip- 1996; Larson et al., 2001; Willmer, 2011). Nevertheless, teran pollination is possibly still underestimated, because pollination by fungus gnats is known to occur in eight research continues to uncover plants that depend strongly on angiosperm families: Orchidaceae, Liliaceae, Asparagaceae, dipterans for pollination, some of which attract dipteran pollina- Araceae, Aristolochiaceae, Polygonaceae, Apocynaceae and tors by previously unsuspected mechanisms (Stökl et al., 2011; Saxifragaceae (Vogel, 1973; Jones, 1974; Ackerman and Mesler, Ren et al., 2012; Song et al., 2014; Oelschlägel et al., 2014; 1979; Mesler et al., 1980; Sugawara, 1988; Fuller, 1994; Vogel Heiduk et al., 2016). Small dipteran insects, such as fungus and Martens, 2000; Goldblatt et al., 2004; Okuyama et al., 2008; gnats, gall midges and drosophilid flies, are particularly difficult Yamashiro et al., 2008; Barriault et al., 2010; Duque-Buitrago to observe in the field, and thus their roles as pollinators are et al., 2014; Ollerton et al., 2017; Suetsugu and Sueyoshi, 2017). probably not fully appreciated (Larson et al., 2001; Goldblatt Some of these plants are believed to mimic fungal oviposition et al., 2004; Woodcock et al., 2014). sites because pollinator eggs are often observed on the flowers Fungus gnats, or flies in Mycetophilidae, Sciaridae and (Vogel, 1978; Sugawara, 1988; Scanlen, 2006), whereas others related Sciaroidea families, are small, weak-flying insects com- employ sexual deception (Blanco and Barboza, 2005; Phillips mon in moist forest and riparian habitats. Their larvae feed et al., 2014), offer nectar reward (Ackerman and Mesler, 1979; © The Author(s) 2017. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For permissions, please e-mail: [email protected]. 652 Mochizuki and Kawakita – Fungus gnat pollination and shared floral characters Mesler et al., 1980; Goldblatt et al., 2004; Okuyama et al., and small, with dark red petals (Figs 1B and 2A, B). Aucuba ja- 2004) or even sacrifice developing ovules to seed-feeding pol- ponica is one of the first woody plants to flower in early spring linator larvae to accomplish the pollination (Song et al., 2014). (March–May). Despite the diversity of mechanisms used by these plants to The spindle tree genus Euonymus comprises about 130 spe- attract fungus gnats, several fungus gnat-pollinated plants share cies with a broad distribution range, including North America, a characteristic floral display: petals or spathes with dark red Europe, Asia, Australia and Madagascar (Ma, 2001). Most spe- pigmentation (Supplementary Data, Table S1). Dark red flow- cies are small trees or shrubs growing in a variety of habitats ers are otherwise common in sapromyiophilous plants (Fægri including the forest edge, forest floor and rocky coast. The and van der Pijl, 1979; Willmer, 2011; Johnson and Schiestl, flowers are small and typically greenish or yellowish; how- 2016; Shuttleworth et al., 2017) and are also known in sexu- ever, roughly 30 species have red, purple or chocolate-coloured ally deceptive orchids (Gaskett et al., 2017) and some fly- and flowers (Ma, 2001). To date, 18 Euonymus species have been beetle-pollinated plants (Young, 1985; Sakai and Inoue, 1999; recorded in Japan. These are typically small trees occurring Downloaded from https://academic.oup.com/aob/article-abstract/121/4/651/4781678 by guest on 22 May 2020 Teichert et al., 2012; Policha et al., 2016; Luo et al., 2017); on forest edges or within forests, whereas some are shrubs or however, knowledge of an association between such a floral shrublets on the forest floor. Four endemic species,E. lanceo- display and any particular pollination system remains frag- latus, E. melananthus, E. tricarpus and E. yakushimensis, have mentary. This observation motivated us to examine the floral red flowers and prefer the shaded forest floor or, occasionally, visitors of dark red-flowered Aucuba japonica (Garryaceae) subalpine/boreal meadows. Inflorescences of Euonymus are in 2014. We found that A. japonica flowers were heavily vis- cymes of up to ten flowers, developing on leaf axils. The flower ited by fungus gnats and noticed that the flowers ofAucuba possesses a prominent nectary disc that surrounds the pistils resemble those of the known fungus gnat-pollinated genus and stamens. Filament length varies among species; however, Mitella (Okuyama et al., 2004) in several ways: they are small species with dark red flowers invariably have a very short to (5–8 mm in diameter), flat and actinomorphic, with exposed almost no filament. nectaries, short stamens and dark red floral displays. We thus Euonymus lanceolatus is an evergreen dwarf shrub that grows examined if plants in other families with floral characteris- in shaded forests on the Sea of Japan side of the Japanese main- tics similar to those of Mitella and Aucuba are also fungus land, where there is heavy snow in winter. The plants are 0.5 gnat-pollinated. m high and have flexible stems that are pressed flat against the We searched for plants that share the above set of floral char- ground under snow (Ma, 2001; Fig. 1C). During June and July acteristics in the Japanese flora. Consequently, we studied the (rainy season), each plant produces a few to dozens of flowers pollinators of the following six shrubs and herbs: Aucuba ja- that open downwardly. ponica (Garryaceae), three Euonymus species (Celastraceae), Euonymus melananthus is a small, deciduous shrub (up Disanthus cercidifolius (Hamamelidaceae) and Micranthes to 1 m) growing on the shaded riparian forest floor, having a fusca (Saxifragaceae) (Figs 1 and 2). Additionally, we studied wide distribution in cool–temperate areas of Japan, but occur- Streptopus streptopoides (Liliaceae) because, although its flow- ring only infrequently. The plants resemble lianas, growing ers are mostly greenish and have only small areas of dark red