Effective pollination of Aeschynanthus acuminatus (Gesneriaceae) by generalist passerines, in sunbird-absent East Asia

Abstract

Aeschynanthus (Gesneriaceae), a genus comprising approximately 160 species in subtropical Southeast Asia, has red, tubular flowers, typical of a sunbird pollination syndrome. A. acuminatus, the species that is distributed extending to the northern edge of the genus, where the specialized nectarivorous sunbirds are absent, possesses reddish-green flowers and a wide-open corolla tube, flowering time shifts from summer to winter and the species achieves high fruiting success. This atypical flower led us to investigate the pollination biology of this species. Three species of generalist passerines, Grey-cheeked Fulvetta (Alcippe morrisonia, Sylviidae), White-eared Sibia (Heterophasia auricularis, Leiothrichidae) and Taiwan Yuhina (Yuhina brunneiceps, Zosteropidae), were recorded visiting A. acuminatus flowers. Pollination effectiveness was quantified via conspecific pollen presence on stigmas and natural fruit set. The significantly high natural fruit set (60%) and conspecific pollen transfer rate (94%) indicate high reproductive success facilitated by the accurate pollen placement on the birds. The existence of copious (61 µL) and highly diluted (7%) hexose-dominant nectar, together with a major reflectance peak of corolla lobe in the long-wavelength red color spectrum, is consistent with the pollination syndrome of generalist passerines. The high pollination effectiveness of A. acuminatus due to the recruitment of generalist passerines as pollinators, and the specializations of floral traits to match generalist bird pollination, appear crucial in the successful colonization on islands such as Taiwan that lack specialized bird pollinators.

Variable evidence for convergence in morphology and function across avian nectarivores

“…Studies have shown, however, that the tendency to hover or perch while feeding depends primarily on plant architecture, not on energetic limitations due to body mass (Miller, 1985;Wester, 2013;Westerkamp, 1990). Most hummingbirds perch on any structure that they can reach with their short feet while extracting nectar from a flower (e.g., Feinsinger & Colwell, 1978;Stiles, 2008), and sunbirds and honeyeaters can hover for brief periods (e.g., <1 min; Geerts & Pauw, 2009), but it occurs facultatively when a plant does not supply suitable perches (Anderson et al, 2005;Chang et al, 2013;K.-H. Chen, Lu, et al, 2019;Ford & Paton, 1977;Geerts & Pauw, 2009;Janeček et al, 2011;Padyšáková & Janeček, 2016;Pyke, 1980;Sejfová et al, 2021;Wester, 2013). To accomplish hovering, sunbirds and honeyeaters use a typical asymmetric flight stroke (lift generated on downstroke only; Norberg, 1990) and generate sufficient lift by increasing wingbeat frequency (Zimmer, 1943).…”

Limited-Distance Pollen Dispersal and Low Paternal Diversity in a Bird-Pollinated Self-Incompatible Tree

“…The blooming B. ceiba attracted many tourists and when B. ceiba starts blooming, local farmers transplanted rice seedlings in nearby paddies, resulting in decrease in bird visitation frequency and duration; (2) A massive number of flowers on one tree allows birds to meet their nectar needs without moving frequently between trees, which was proved by the restricted pollen dispersal distance (Figure 2 and Figure 3). The high efficiency of pollen removal and low combing behavior of birds (Chen et al, 2019;Song et al, 2019) also made the birds carry a large amount of pollen grains from few pollen donors on their bodies and, thus, have the potential to pollinate multiple ovules in a single visit. One of our observations, where 30 randomly seeds of > 300 seeds of a plant were all from the same father, supported this possibility (Figure 2 and Table 3); and (3) The investigated plant population is not a completely closed population (Figure 2) and some pollen donors might from unlabeled individuals within study population or nearby populations, which might cause underestimation of multiple paternity in our results.…”

“…However, increasing number of studies for almost two decades show that generalist birds act as alternative pollinators in Asia (Roguz et al, 2018). Remarkably, generalist passerines have been demonstrated as the most effective pollinators for winter flowering plants in regions where sunbirds are absent, especially in dry seasons and in the islands (Chen et al, 2019). These include species such as Fritillaria imperialis (Roguz et al, 2021), Firmiana spp.…”

“…Under extreme environmental conditions such as dry weather (Stiles, 1978), on island habitats (Chen et al, 2019), and due to increasing human interference (Byrne et al, 2007), a better understanding of the mating system of generalist-pollinated species and of how birds influence mating patterns is further warranted. Identifying the underlying behavioral mechanisms of bird pollinators responsible for pollen movement would deepen the understanding of evolutionary and ecological consequences of bird pollination for plant mating.…”

Transcriptomic Analysis Suggests Auxin Regulation in Dorsal-Ventral Petal Asymmetry of Wild Progenitor Sinningia speciosa

“…Zygomorphy has evolved independently in many angiosperm lineages [1]. Within the Lamiales, the flowers of many outcrossing zygomorphic lineages in the Gesneriaceae show diversification in size, shape, and color that are associated with pollination specializations [2][3][4]. Species in the genus Sinningia (subfamily Gesnerioideae, tribe Gesnerieae) are remarkably diverse, with flowers that have features consistent with bee, hummingbird, bat, and moth pollination syndromes [5].…”