The many elusive pollinators in the genus Amorphophallus

The genus Amorphophallus encompasses some 230 species and is one of the largest genera of the Araceae family. Most species release scents, smelling of carrion, faeces, dung and similar nauseating odours for pollinator attraction and are therefore considered to have evolved a deceptive pollination syndrome. Some of the most iconic members of the genus, such as the A. titanum and A. gigas, are considered to be carrion mimics. Copro-necrophagous insects, beetles and flies in particular, are attracted by these scents and are therefore assumed to act as pollinators. However, many reports and observations on Amorphophallus pollinators are anecdotal in nature or do not distinguish between legitimate pollinators and non-pollinating visitors. Moreover, some published observations are not readily accessible as they are many decades old. Therefore, the available data and information about insect visitors and/or pollinators in the genus Amorphophallus is compiled, reviewed and discussed.

Specialization for Tachinid Fly Pollination in the Phenologically Divergent Varieties of the Orchid Neotinea ustulata

Despite increased focus on elucidating the various reproductive strategies employed by orchids, we still have only a rather limited understanding of deceptive pollination systems that are not bee- or wasp-mediated. In Europe, the orchid Neotinea ustulata has been known to consist of two phenologically divergent varieties, neither of which provide rewards to its pollinators. However, detailed studies of their reproductive biology have been lacking. Our study aimed to characterize and understand the floral traits (i.e., morphology, color, and scent chemistry) and reproductive biology of N. ustulata. We found that the two varieties differ in all their floral traits; furthermore, while Neotinea ustulata var. ustulata appears to be pollinated by both bees (e.g., Anthophora, Bombus) and flies (e.g., Dilophus, Tachina), var. aestivalis is pollinated almost entirely by flies (i.e., Nowickia, Tachina). Tachinids were also found to be much more effective than bees in removing pollinaria, and we show experimentally that they use the characteristic dark inflorescence top as a cue for approaching inflorescences. Our results thus suggest that while both N. ustulata varieties rely on tachinids for pollination, they differ in their degree of specialization. Further studies are, however, needed to fully understand the reproductive strategy of N. ustulata varieties.

Evidence for a food-deceptive pollination system using Hylaeus bees in Caladenia hildae (Orchidaceae)

Numerous orchid species are pollinated by food deception, where rewardless flowers attract foraging pollinators through the mimicry of other flowers or the use of non-specific floral signals. Here we investigate the pollination of Caladenia hildae, a member of a diverse Australian genus containing species pollinated by sexual deception, and species pollinated by food foraging pollinators. Despite eight bee species occurring at the main study site, only food foraging bees of a single species of Hylaeus (Colletidae) were observed to remove and deposit pollen of C. hildae. Spectral reflectance of C. hildae flowers differed from co-flowering rewarding species in terms of both the wavelengths of light reflected, and the pattern of colouration. As such, there was no evidence that C. hildae uses a pollination strategy based on floral mimicry. However, the attraction of only a single bee species at this site suggests that C. hildae may use a deceptive strategy that exploits sensory biases or behaviours that differ between Hylaeus sp. and the remainder of the bee community. While Hylaeus have been recorded visiting orchid flowers in several parts of the world, C. hildae may represent the first documented case of an orchid species specialised on pollination by Hylaeus bees.

A specialized deceptive pollination system based on elaborate mushroom mimicry

Despite its potential effectiveness for outcrossing, few examples of pollination via mushroom mimicry have been reported. This may be because the conditions under which the strategy can evolve are limited and/or because demonstrating it is challenging. Arisaema is a plant genus that has been suggested to adopt mushroom mimicry for pollination, although no compelling evidence for this has yet been demonstrated. Here, we report that Arisaema sikokianum utilizes mostly a single genus of obligate mycophagous flies (Mycodrosophila) as pollinators, and that the insect community dominated by Mycodrosophila is strikingly similar to those found on some species of wood-decaying fungi. Comparative chemical analyses of Arisaema spp. and various mushrooms further revealed that only A. sikokianum emits a set of volatile compounds shared with some mushroom species utilized by Mycodrosophila. Meanwhile, other closely related and often sympatric Arisaema species do not possess such typical traits of mushroom mimicry or attract Mycodrosophila, thereby likely achieving substantial reproductive isolation from A. sikokianum. Our finding indicates that mushroom mimicry is an exceptional and derived state in the genus Arisaema, thus providing an unprecedented opportunity to study the mechanisms underlying the coordinated acquisition of mimicry traits that occurred during a recent speciation event.

Fluctuating selection across years and phenotypic variation in food-deceptive orchids

Nectarless flowers that deceive pollinators offer an opportunity to study asymmetric plant-insect interactions. Orchids are a widely used model for studying these interactions because they encompass several thousand species adopting deceptive pollination systems. High levels of intra-specific phenotypic variation have been reported in deceptive orchids, suggesting a reduced consistency of pollinator-mediated selection on their floral traits. Nevertheless, several studies report on widespread directional selection mediated by pollinators even in these deceptive orchids. In this study we test the hypothesis that the observed selection can fluctuate across years in strength and direction thus likely contributing to the phenotypic variability of this orchid group. We performed a three-year study estimating selection differentials and selection gradients for nine phenotypic traits involved in insect attraction in two Mediterranean orchid species, namely Orchis mascula and O. pauciflora, both relying on a well-described food-deceptive pollination strategy. We found weak directional selection and marginally significant selection gradients in the two investigated species with significant intra-specific differences in selection differentials across years. Our data do not link this variation with a specific environmental cause, but our results suggest that pollinator-mediated selection in food-deceptive orchids can change in strength and in direction over time. In perennial plants, such as orchids, different selection differentials in the same populations in different flowering seasons can contribute to the maintenance of phenotypic variation often reported in deceptive orchids.