Breaking the rules: discovery of sexual deception in Caladenia abbreviata (Orchidaceae), a species with brightly coloured flowers and a non-insectiform labellum

Caladenia is exceptional among orchid genera in that it contains both species pollinated by sexual deception and species pollinated by food-foraging insects. Whereas pollination strategies have been elucidated in some species complexes, others groups have received limited attention, hampering our ability to understand the evolution of the exceptional diversity of species and floral traits in Caladenia. Here, we conducted the first detailed pollination study of a member of the Caladenia filamentosa complex. On the basis of the brightly coloured and scented flowers exhibited by many species, it has been assumed that most members of the group are food-deceptive. However, we show that Caladenia abbreviata Hopper & A.P.Br. is pollinated by an undescribed species of Rhytidothynnus thynnine wasp via sexual deception. Floral dissections showed that the terminal glands on the sepals and petals are the primary source of the sexual attractant, supporting observations from intact flowers that pollinators frequently copulate with these floral parts. Interestingly, contact with the reproductive structures of the plant was not associated with attempted copulation. Our findings highlighted that sexual deception can evolve in lineages without dull-coloured flowers and insectiform labellum structures. Given that the orchid is apparently dependent on a single pollinator species, efforts to conserve this rare species will be dependent on the availability of the pollinator in suitable orchid habitat.

Sharing of Pyrazine Semiochemicals between Genera of Sexually Deceptive Orchids

It has recently been discovered that novel di-, tri- and tetra- substituted pyrazines are semiochemicals in Drakaea, an orchid genus that secures pollination by the sexual deception of male thynnine wasps. We examined if similar pyrazines were also present in the distantly related Caladenia barbarossa, a sexually deceptive orchid that is also pollinated by a thynnine wasp. Here we report for the first time the occurrence of two pyrazines, (3,5,6-trimethylpyrazin-2-yl)methyl 3-methylbutanoate (1) and 3-(3-methylbutyl)-2,5-dimethylpyrazine (2) in the orchid genus Caladenia. The former is known as a semiochemical involved in pollinator attraction in Drakaea livida. This convergence of floral odour between distantly related plants provides an exciting opportunity to understand the evolution and molecular basis of this sophisticated chemical mimicry.

Pollinator specificity, cryptic species and geographical patterns in pollinator responses to sexually deceptive orchids in the genus Chiloglottis: the Chiloglottis gunnii complex

Australian sexually deceptive orchids are typically highly pollinator specific, each species having a single unique hymenopteran pollinator species. Pollinator specificity in six of the nine described species in the Chiloglottis gunnii Lindl. complex was investigated by using field pollinator-choice tests, with Chiloglottis taxa translocated within and among biogeographical regions. Specific pollinators revealed the existence of five undescribed cryptic taxa in the C. gunnii complex, three within C. pluricallata D.L.Jones and two within C. valida D.L.Jones, in addition to the six described species. Of the 11 Chiloglottis taxa, 10 had a single thynnine-wasp pollinator throughout their sometimes large distributions, whereas one, C. valida, had a second pollinator in parts of its distribution. Eleven pollinators belonged to the genus Neozeleboria and one to Eirone. Pollinator-choice testing showed that cross-attraction of pollinators occurs between three geographically isolated Chiloglottis taxa on the New South Wales (NSW) New England Tableland and taxa in the South Eastern Highlands of NSW and Victoria. The data suggested there is sharing of chemical attractants and supported the recognition of at least five odour types within Chiloglottis, each encompassing one to three orchid taxa and their pollinators. The following two broad generalisations are made: (1) there is no cross-attraction of pollinators among sympatric Chiloglottis species, i.e. sympatric orchid taxa do not share attractant odours; and (2) all Chiloglottis species have different specific pollinators, although they may share attractant odours allopatrically. Some 28 thynnine-wasp species were attracted as minor non-pollinating responders to Chiloglottis taxa; five of these were pollinators of other Chiloglottis species. These wasps were much more taxonomically diverse than the pollinators, belonging to six genera, and suggest that some orchid-odour components are widely shared within the sex pheromones of the Thynninae.

Implications of pollination by food and sexual deception for pollinator specificity, fruit set, population genetics and conservation of Caladenia (Orchidaceae)

Caladenia is very unusual in that it contains species that attract pollinators by two different strategies, food and sexual deception. Among the sexually deceptive species, baiting for pollinators has shown that within populations orchid species are typically pollinated by a single species of thynnine wasp. However, some wasp species can be pollinators of more than one species of orchid usually when their ranges do not overlap. There is a trend for closely related orchids to exploit wasps from the same genus, with different lineages of orchids often pollinated by different genera. Very little is known about pollination of food-deceptive Caladenia species, although it is evident they attract a suite of generalist food-seeking insects. Food-deceptive species have a higher pollination rate than do sexually deceptive species. Studies of population genetics and pollen movements are few, although they suggest a pattern of fine-scale genetic structuring within populations, owing to predominantly restricted seed dispersal and low genetic differentiation among populations as a consequence of rare long-distance seed-dispersal events. Both evolutionary and ecological research of Caladenia will greatly benefit from a better understanding of the insect species involved in pollination, their ecological requirements and the ecological and genetic consequences of food and sexual deception.