Selective pollination by fungus gnats potentially functions as an alternative reproductive isolation among five Arisaema species

Fungus gnat pollination in Arisaema urashima: the interplay of lethal traps and mutualistic nurseries

While most flowering plants engage in mutualistic interactions with their pollinators, Arisaema species employ a unique, seemingly antagonistic strategy by imprisoning and causing the pollinators to perish within their spathes. Recent studies have revealed that Arisaema thunbergii primarily relies on a fungus gnat, Leia ishitanii, with some individuals possibly escaping female spathes after oviposition. We investigated interactions between A. urashima and its pollinating fungus gnats, given that A. urashima is closely related to A. thunbergii. Specifically, we tested whether decaying A. urashima serve as brood-sites for some pollinators and whether these pollinators can escape seemingly lethal floral traps. We retrieved A. urashima spathes together with adult insect corpses trapped within the spathes and incubated the spathes to see if conspecific insects emerged. In addition, under laboratory conditions, we observed the escape behaviour of Sciophila yokoyamai, whose next-generation adults most frequently emerge from the decaying spathes. Our findings indicate that S. yokoyamai almost always escapes from the female spathe after oviposition while using the inflorescence as a nursery. In contrast, other pollinators of A. urashima, including Mycetophila spp., remain trapped and perished within the spathes. This study demonstrates that A. urashima spathes can function both as lethal traps and mutualistic nurseries, with outcomes differing among pollinator species. Our results also suggest that the contribution of certain pollinators to Arisaema reproduction is underestimated or even neglected, given that information on their pollinator assemblages has been based on floral visitors trapped within the inflorescences.

When is lethal deceptive pollination maintained? A population dynamics approach

BACKGROUND AND AIMS

Not all plant-pollinator interactions are mutualistic, and in fact deceptive pollination systems are widespread in nature. The genus Arisaema has a pollination system known as lethal deceptive pollination, in which plants not only attract pollinating insects without providing any rewards, but also trap them until they die. Many Arisaema species are endangered from various disturbances, including reduction in forest habitat, modification of the forest understorey owing to increasing deer abundance, and plant theft for horticultural cultivation. We aimed to theoretically investigate how lethal deceptive pollination can be maintained from a demographic perspective and how plant and pollinator populations respond to different types of disturbance.

METHODS

We developed and analysed a mathematical model to describe the population dynamics of a deceptive plant species and its victim pollinator. Calibrating the model based on empirical data, we assessed the conditions under which plants and pollinators could coexist, while manipulating relevant key parameters.

KEY RESULTS

The model exhibited qualitatively distinct behaviours depending on certain parameters. The plant becomes extinct when it has a low capability for vegetative reproduction and slow transition from male to female, and plant-insect co-extinction occurs especially when the plant is highly attractive to male insects. Increasing deer abundance has both positive and negative effects because of removal of other competitive plants and diminishing pollinators, respectively. Theft for horticultural cultivation can readily threaten plants whether male or female plants are frequently collected. The impact of forest habitat reduction may be limited compared with that of other disturbance types.

CONCLUSIONS

Our results have emphasized that the demographic vulnerability of lethal deceptive pollination systems would differ qualitatively from that of general mutualistic pollination systems. It is therefore important to consider the demographics of both victim pollinators and deceptive plants to estimate how endangered Arisaema populations respond to various disturbances.

Post-pollination barriers contribute to coexistence of partially pollinator-sharing Arisaema species (Araceae)

Reproductive isolation plays an important role in maintaining the species integrity of sympatric close relatives. For sympatric Arisaema species, interspecific gene flow is expected to be effectively prevented by pre-pollination barriers, particularly strong pollinator isolation mediated by fungus gnats. However, due to the lack of quantitative studies combining multiple pre- and post-pollination barriers, it is not known whether pre-pollination isolation is complete, and whether post-pollination barriers also contribute to reproductive isolation among some Arisaema species. In this study, we quantified the individual strengths and absolute contributions of four pre- and post-pollination barriers (phenological isolation, pollinator isolation, hybrid fruit formation, and hybrid seed formation) among three sympatric Arisaema species (A. bockii, A. lobatum, and A. erubescens). Although phenological isolation and pollinator isolation reduced the frequencies of interspecific pollen transfer among these species, the partial overlap of flowering times and pollinator assemblages resulted in incomplete pre-pollination isolation. Post-pollination barriers also contributed to reproductive isolation at the hybrid fruit and seed formation stages. We propose that, although pre-pollination barriers are expected to contribute more to total isolation than post-pollination barriers in Arisaema, pre-pollination barriers may not completely prevent interspecific pollen transfer among some Arisaema species. Post-pollination barriers, which are generally ignored, may also have contributed significantly to reproductive isolation in Arisaema.

Adaptation to pollination by fungus gnats underlies the evolution of pollination syndrome in the genus Euonymus

BACKGROUND AND AIMS

Dipteran insects are known pollinators of many angiosperms, but knowledge on how flies affect floral evolution is relatively scarce. Some plants pollinated by fungus gnats share a unique set of floral characters (dark red display, flat shape and short stamens), which differs from any known pollination syndromes. We tested whether this set of floral characters is a pollination syndrome associated with pollination by fungus gnats, using the genus Euonymus as a model.

METHODS

The pollinator and floral colour, morphology and scent profile were investigated for ten Euonymus species and Tripterygium regelii as an outgroup. The flower colour was evaluated using bee and fly colour vision models. The evolutionary association between fungus gnat pollination and each plant character was tested using a phylogenetically independent contrast. The ancestral state reconstruction was performed on flower colour, which is associated with fungus gnat pollination, to infer the evolution of pollination in the genus Euonymus.

KEY RESULTS

The red-flowered Euonymus species were pollinated predominantly by fungus gnats, whereas the white-flowered species were pollinated by bees, beetles and brachyceran flies. The colour vision analysis suggested that red and white flowers are perceived as different colours by both bees and flies. The floral scents of the fungus gnat-pollinated species were characterized by acetoin, which made up >90 % of the total scent in three species. Phylogenetically independent contrast showed that the evolution of fungus gnat pollination is associated with acquisition of red flowers, short stamens and acetoin emission.

CONCLUSIONS

Our results suggest that the observed combination of floral characters is a pollination syndrome associated with the parallel evolution of pollination by fungus gnats. Although the role of the red floral display and acetoin in pollinator attraction remains to be elucidated, our finding underscores the importance of fungus gnats as potential contributors to floral diversification.

Pre- and post-pollination barriers between two exotic and five native Sagittaria species: Implications for species conservation

Anthropogenic introduction of species has resulted in a breakdown of geographical barriers and hybridization in previously allopatric species. Thus, examining hybridization proneness of exotic species contributes to revealing its potential threat. Moreover, reproductive barriers may be strengthened or weakened due to long-term geographical isolation for these newly sympatric species. However, few studies have evaluated multiple barriers between alien and native species. In this study, we quantified the importance of four pre-pollination barriers (phenological, floral traits, pollen production, and floral constancy) and four post-pollination barriers (pollen-pistil incompatibility, seed set, seed viability, and seedling survival) between two introduced and five native Sagittaria species. Results showed that introduced S. platyphylla was cross-compatible with two native species, whereas introduced S. montevidensis was incapable of hybridizing with any native species. Different barriers were asymmetric within species pairs and multiple barriers acted in concert to maintain species boundaries. Post-pollination barriers contributed more to total reproductive isolation in native species, whereas pre-pollination barriers played a stronger role in total reproductive isolation for two introduced species. Seed set was the only barrier that was positively correlated with genetic distance. Our results provide a perspective to better understand reproductive barriers for secondary contact species. We highlight the importance of monitoring hybridization events before human introduction and the possible conservation strategies to remove invasive species with hybridization proneness.

Fly Pollination of Kettle Trap Flowers of Riocreuxiatorulosa (Ceropegieae-Anisotominae): A Generalized System of Floral Deception

Elaborated kettle trap flowers to temporarily detain pollinators evolved independently in several angiosperm lineages. Intensive research on species of Aristolochia and Ceropegia recently illuminated how these specialized trap flowers attract particular pollinators through chemical deception. Morphologically similar trap flowers evolved in Riocreuxia; however, no data about floral rewards, pollinators, and chemical ecology were available for this plant group. Here we provide data on pollination ecology and floral chemistry of R. torulosa. Specifically, we determined flower visitors and pollinators, assessed pollen transfer efficiency, and analysed floral scent chemistry. R. torulosa flowers are myiophilous and predominantly pollinated by Nematocera. Pollinating Diptera included, in order of decreasing abundance, male and female Sciaridae, Ceratopogonidae, Scatopsidae, Chloropidae, and Phoridae. Approximately 16% of pollen removed from flowers was successfully exported to conspecific stigmas. The flowers emitted mainly ubiquitous terpenoids, most abundantly linalool, furanoid (Z)-linalool oxide, and (E)-β-ocimene—compounds typical of rewarding flowers and fruits. R. torulosa can be considered to use generalized food (and possibly also brood-site) deception to lure small nematocerous Diptera into their flowers. These results suggest that R. torulosa has a less specific pollination system than previously reported for other kettle trap flowers but is nevertheless specialized at the level of Diptera suborder Nematocera.