UV Reflectance but No Evidence for Colour Mimicry in a Putative Brood-Deceptive Orchid Corybas Cheesemanii

Current Zoology 60 (1): 104–113, 2014 UV reflectance but no evidence for colour mimicry in a putative brood-deceptive orchid Corybas cheesemanii M. M. KELLY, A. C. GASKETT* School of Biological Sciences, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand Abstract Rewardless orchids attract pollinators by food, sexual, and brood-site mimicry, but other forms of sensory deception may also operate. Helmet orchids (Corybas, Nematoceras and related genera) are often assumed to be brood-site deceivers that mimic the colours and scents of mushrooms to fool female fungus gnats (Mycetophilidae) into attempting oviposition and polli- nating flowers. We sampled spectral reflectances and volatile odours of an endemic terrestrial New Zealand orchid Corybas cheesemanii, and co-occurring wild mushrooms. The orchid is scentless to humans and SPME GC-MS analyses did not detect any odours, but more sensitive methods may be required. The orchids reflected strongly across all visible wavelengths (300700nm) with peaks in the UV (~320nm), yellow-green (500600 nm) and red regions (650700 nm), whereas mushrooms and surrounding leaf litter reflected predominantly red and no UV. Rather than mimicking mushrooms, these orchids may attract pollinators by exploiting insects’ strong sensory bias for UV. Modelling spectral reflectances into a categorical fly vision model and a generic tetrachromat vision model provided very different results, but neither suggest any mimicry of mushrooms. However, these models require further assessment and data on fly spectral sensitivity to red wavelengths is lacking – a problem given the predominance of red, fly-pollinated flowers worldwide [Current Zoology 60 (1): 104113, 2014]. Keywords Diptera, Colour space, Fly pollination, Orchidaceae, Spectral reflectance, Visual modelling Flowers typically attract pollinators by advertising in the Orchidaceae, in which approximately one-third of rewards such as food (Chittka and Thompson, 2001; species do not reward their pollinators (Jersáková et al., Raguso, 2004). Advertising usually involves floral 2006). Most of these deceptive orchids exploit the fora- odours and colours that attract pollinators, enhance the ging behaviour of their pollinators, both male and fe- detectability of flowers, influence pollinator orientation male insects, by resembling rewarding flowers but fail- on flowers, and interact with pollinator learning to gene- ing to provide food (Jersáková et al., 2009). Sexual de- rate behaviours such as floral constancy (Hempel De ception, in which male insects are fooled into sexual Ibarra et al., 2000; Gegear and Laverty, 2001; Raguso, behaviour with flowers, is less common than food de- 2008; Wright and Schiestl, 2009). Floral odours and ception and generally involves specialisation to a spe- colours can even be tailored to pollinator physiology, cies-specific pollinator (Peakall et al., 2010; Gaskett, sensory systems, and behavioural patterns, creating 2011; but see Phillips et al., 2013). For some sexually ‘private channels’ or pollinator syndromes that select for deceptive orchids, mimicry of female insect scents, specific guilds or even single species of pollinators colours and shapes can even cause pollinators to ejacu- (Fenster et al., 2004; Schaefer et al., 2004; Johnson et late on flowers and waste their sperm (Blanco and Bar- al., 2006; Shuttleworth and Johnson, 2010). boza, 2005; Gaskett et al., 2008). A third and less In rewardless or deceptive pollination systems, col- well-studied form of deception by orchids is brood-site our and scent signals may interact differently with pol- deception, in which flowers exploit primarily female linator behaviour than in rewarding systems, including insects by mimicking oviposition sites (Shi et al., 2009; additional functions such as mimicry, or sensory traps Urru et al., 2011). This pollination mechanism may im- that exploit pollinators’ pre-existing sensory biases (for pose reproductive costs on female insects if they ovi- reviews see Jersáková et al., 2009; Schaefer and Ruxton, posit in flowers, and the eggs and larvae subsequently 2009; Gaskett, 2011; Schiestl and Dötterl, 2012; die (Kaiser, 2006). Schiestl and Johnson, 2013). Deceptive pollination has Brood-site mimicry is best-known for the Araceae, evolved in a range of plants, but is particularly common Aristolochiacea and Rafflesiaceae families, but is pre- Received July 5, 2013; accepted Dec. 2, 2013. Corresponding author. E-mail: [email protected] © 2014 Current Zoology KELLY MM, GASKETT AC: UV but no mimicry in Corybas cheesemanii 105 sent in several other angiosperm families including Or- or identity for the genus Corybas are for species that chidaceae (Raguso, 2004; Kaiser, 2006; Urru et al., have now been reclassified as Nematoceras, Corysan- 2011; for a brief review of brood-site deception in or- thes or Singularybas (Thomson, 1878; Miller, 1918; chids, see Kelly et al., 2013). In general, brood-site de- Jones, 1970; Fuller, 1994; for revised names, see ceptive flowers tend to be fly- or beetle-pollinated and Clements et al., 2007). correspondingly, mimic oviposition sites such as carrion, After recent revisions, New Zealand retains just one dung, fermenting fruit or fungi, mainly though olfactory helmet orchid in the genus Corybas, the diminutive, signals (Christensen, 1994; Jürgens et al., 2006; Kaiser, terrestrial, endemic spurred helmet orchid Corybas 2006). For example, across several flower families, car- cheesemanii. This species is nectarless and therefore rion and carnivore dung mimicry often involves produc- deceptive (Kelly et al., 2013), but as for most helmet tion of sulphide compounds such as dimethyl disulphide orchids, it is unclear whether pollination is via brood- and dimethyl trisulphide, whereas herbivore dung mimi- site mimicry and attraction of female insects that typi- cs produce phenol, indole and p-cresol (Jürgens et al., cally oviposit in fungi. Our recent attempts to identify 2006; Urru et al., 2011). Flowers can also attract flies by the pollinator(s) of C. cheesemanii were inconclusive as mimicking yeast and fungal odours (Stökl et al., 2010; a wide range of both male and female flies were at- Goodrich, 2012). For example, the orchid Dracula tracted to the orchids and co-occurring mushrooms, chestertonii emits classic fungal compounds such as mostly Mycetophilidae (Mycetophila colorata, M. fagi, 3-octenol, 3-octenone, 1 octen-3-ol, and 1-octen-3-one M. filicornis, M. nr. subspinigera and M. vulgaris sp. (Kaiser, 2006; De Pinho et al., 2008). group), Lauxaniidae (Sapromyza spp.) and Anisopodi- Brood-site deception is also likely to involve visual dae (Kelly et al., 2013). This suggests a broader pollina- signals. Many arums and stapeliads have red (“hematic”) tion strategy than previously assumed. Brood-deceptive colouration reminiscent of a carcass and sometimes a flowers vary in the extent of pollinator specialisation, white or translucent centre that may draw pollinators reflecting the range of insects attracted to the brood- into the flower (Raguso, 2004; Jürgens et al., 2006; sites they mimic. For example, pollinator specialisation Urru et al., 2011). The brood-deceptive orchid Dracula in flowers that mimic fungi will vary according to the chestertonii resembles the gilled cap of a mushroom host-specificity of the fungus gnats they attract (Myce- (Kaiser, 2006; Endara et al., 2010), and Paphiopedilum tophilidae; Jakovlev, 2012). True specialists attract only dianthum is patterned with small contrasting spots that one pollinator (e.g. Paphiopedilum barbigerum polli- resemble aphids; important prey for the larvae of its nated by the syrphid Episyrphus balteatus; Shi et al., hoverfly pollinator (Shi et al., 2009). 2009), whereas others specialise at the genus or family Helmet orchids from the terrestrial Asian-Pacific level and could be termed functional group specialists genera Corybas, Nematoceras, Corysanthes, and Anzy- (e.g. Dracula spp. attract drosophilids, especially Zy- bas (Diurideae) are often considered specialist brood- gothrica spp.; Endara et al., 2010). Generalists such as site deceivers that mimic the colours and scents of many stapeliads attract a broad diversity of fly families mushrooms to fool female fungus gnats (Mycetophili- (Jürgens et al., 2006). dae) into acting as pollinators (Jones, 1970; Fuller, 1994; Here we consider whether C. cheesemanii, exhibits Pridgeon et al., 2001, Van Der Cingel, 2001; Jones, visual and chemical signals consistent with pollination 2006; Scanlen, 2006; Clements et al., 2007; St George, by brood-site deception. We sample the volatile odours 2007; Vereecken and McNeil, 2010). Data on pollina- and spectral reflectance of C. cheesemanii and sympa- tors is drawn largely from fortuitous anecdotal observa- tric basidiomycetes (mushrooms). To analyse how the tions of Mycetophilids found on or inside flowers, or colours of the orchid and mushroom might be perceived carrying orchid pollinia, and insect eggs in flowers pre- by potential pollinators, we model the spectral reflec- sumably oviposited by duped pollinators (Scanlen, 1996; tances into two model visual systems incorporating Dip- Tyler, 2005; Scanlen, 2006; St George, 2007; Scanlen, teran spectral sensitivities. 2008). However, these reports often involve single ob- 1 Materials and Methods servations or small sample sizes, and may be of conside- rable antiquity (e.g. Thomson, 1878; Miller, 1918). C. cheesemanii is a somewhat common but largely Conclusions drawn from these scarce data

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