Pollination by sexual deception via pro-pheromone mimicry?

Abstract

Introduction

Most plant species worldwide depend on insects for pollination (Ollerton et al., 2011), with volatile organic compounds being pivotal for mediating pollinator attraction in many of these plants (Raguso, 2008; Dötterl & Gershenzon, 2023). Among plants, orchids are exceptional in their extraordinary range of pollinators, pollination strategies, and floral volatiles (Ackerman et al., 2023; Perkins et al., 2023). One of the most remarkable pollination strategies is that of sexual deception, where the flower imitates female insects to attract male pollinators, with sex pheromone mimicry typically being key to pollinator attraction (Schiestl 2005; Ayasse et al., 2011). While the chemical basis of the sexual mimicry and the extreme pollinator specificity has been confirmed by field bioassays with synthetic compounds for a growing number of sexually deceptive orchids (see Bohman et al., 2016a; Bohman et al., 2020a; Peakall et al., 2020), these examples represent just a tiny fraction of the hundreds of known cases of orchids employing this pollination strategy (Johnson and Schiestl 2016; Peakall, 2023).

Australia is home to a high proportion of sexually deceptive orchids, where several hundred species spanning 11 genera are now known to use this strategy (Gaskett, 2011; Peakall, 2023). Cryptostylis was the first Australian orchid genus discovered to be sexually deceptive (Coleman, 1927), with all five Australian species dependent on the same pollinator, the orchid dupe wasp, Lissopimpla excelsa Costa (Ichneumonidae) (Coleman, 1927, 1929, 1930a, 1930b; Nicholls, 1938). While attempted copulation (pseudocopulation) is not always necessary for pollination (Peakall, 2023), Cryptostylis represents an extreme amongst sexually deceptive plants as one of only two confirmed cases (the other being the beetle-pollinated Disa forficaria (Cohen et al., 2021)) where flowers induce ejaculation by some male pollinators (Coleman, 1930b; Gaskett et al., 2008). While it is almost 100 yr since Coleman conducted simple experiments with Cryptostylis revealing that wasps could locate hidden flowers, leading to her astute conclusion that scent and mimicry were involved in this case of pollination by sexual deception (Coleman, 1930a), the compounds responsible for pollinator attraction have only just started to be elucidated. In previous experiments with (S)-2-(tetrahydrofuran-2-yl)acetic acid from Cryptostylis ovata R.Br, only close approaches by L. excelsa have been observed (Bohman et al., 2019). As such, it is still unknown what induces attempted copulation in male L. excelsa, suggesting that additional chemical cues remain to be discovered.

While the Ichneumonidae is one of the most diverse families of Hymenoptera, with over 25 000 species known (Yu et al., 2016), sex pheromones released by females have been structurally elucidated for just three ichneumonid species (Bohman et al., 2019). In Itoplectis conquisitor, a blend of neral and geranial elicits male sexual activity (Robacker & Hendry, 1977). Eller et al. (1984) found that Syndipnus rubiginosus uses ethyl (Z)-9-hexadecenoate as its sex pheromone, while in Campoletis chlorideae, tetradecanal and 2-heptadecanone have been identified as the sex pheromones (Guo et al., 2022). Limited data for ichneumonids make it difficult to predict the likely compounds involved in inducing attempted copulation in L. excelsa with Cryptostylis.

Sex pheromones are typically female-produced volatile compounds that underpin the chemical sexual communication among conspecifics (Witzgall et al., 2010). Some insects, however, produce precursors (pro-pheromones) that are subsequently modified by external processes to become bioactive compounds. For example, relatively nonvolatile unsaturated long-chain hydrocarbon pro-pheromones have been shown to be oxidatively cleaved in air into attractive volatile aldehydes, as demonstrated in sawflies (Bartelt et al., 1982, 2002; Bartelt & Jones, 1983; Cossé et al., 2002; Staples et al., 2009), flies (Collignon, 2011; Lebreton et al., 2017), cockroaches (Hatano et al., 2020), beetles (Wickham et al., 2012) and wasps (Swedenborg & Jones, 1992; Xu et al., 2020; Faal et al., 2022). Recently, it has also been shown that in poplar and corn, instead of short-chain aldehydes being directly biosynthesised by the plant, unsaturated waxes are produced that are oxidatively cleaved to yield the bioactive aldehydes, such as nonanal (Chen et al., 2023). So far, no evidence of pro-pheromone mimicry by plants has been presented.

The objective of this study was to investigate whether pro-pheromone mimicry may be involved in the sexual attraction of the ichneumonid wasp pollinator L. excelsa to the orchid Cryptostylis ovata, thereby providing the first evidence of the involvement of pro-pheromone mimicry in pollination.