Comparative Biology of Chiloglottis (Orchidaceae) and its thynnine wasp pollinators (Tiphiidae)

A project undertaken at The Royal Botanic Gardens Sydney and supervised by P Weston

Orchids of the Australian genus Chiloglottis are pollinated through the sexual deception of male thynnine wasps mainly from the genus Neozeleboria. The orchids are remarkable for their mimicry of both the appearance and sex pheromones of wingless female thynnines. In this way, pollination is achieved without the production of costly floral rewards. Instead, the orchids are pollinated by enticing males into a ‘pseudocopulation’ of the labellum. One of the most fascinating aspects of this interaction is that orchid species tend to attract only a single pollinator species. Such strong specificity suggests the possibility that sexually deceptive orchids have co-evolved with their pollinators in a process dominated by co-speciation. In other words, the divergence of orchid species into new species might be associated with divergences occurring in the wasp pollinators themselves.

This Hermon Slade Foundation project took a novel evolutionary approach to understanding the evolutionary association between sexually deceptive orchids and their pollinators. Evidence from molecular genetics, morphology, pollinator responses, and the chemistry of insect pheromones was gathered to investigate how the interaction with pollinators influences patterns of plant diversification.

Firstly, we tested the idea of co-speciation by reconstructing and comparing the phylogenies of both Chiloglottis and its pollinators. If the orchids and wasps have co-evolved, we would expect their evolutionary histories to be matching. DNA sequence data provided a detailed resolution of the evolutionary relationships between wasp species but resolved only three main groups within Chiloglottis. Remarkably, each of the three main clades of Chiloglottis is pollinated (mostly) by a corresponding clade of Neozeleboria. However, at least two pollinator shifts are required to reconcile the orchid and wasp phylogenies.

By using a different and more variable molecular technique, called AFLPs, we obtained a fully resolved phylogeny of Chiloglottis. The results indicate that only one of the three orchid groups shows detailed phylogenetic congruence with the corresponding wasp phylogeny. Striking differences in the timing of orchid and wasp species divergences suggest that even this congruence should not be attributed to co-speciation. Finally, our AFLP study examined a complex of morphologically cryptic Chiloglottis forms that attract distinct pollinators. Our molecular results suggested that reproductive isolation can evolve rapidly through switches onto new pollinators. Remarkably, this process of divergence or recent speciation may be possible in sympatry.

We concluded that the evolution of Chiloglottis has involved the gradual "colonisation" of an already diverse group of insects, through a series of pollinator shifts. An expanded morphological analysis of Neozeleboria, including many non-pollinating species, supports this conclusion by showing that pollinator species comprise only a selection of a diverse thynnine lineage potentially available to the orchids. It is likely that the association with a diverse clade of thynnines has facilitated the dramatic diversification of Chiloglottis via minor changes in floral scent chemistry.

However, how does this seemingly random process of ‘pollinator shifting’ lead to the highly conservative association between Chiloglottis and Neozeleboria species? By examining the floral scent chemistry used to attract distinct species of wasps, we showed that related wasp species have very similar sex pheromone chemistry. Other traits, such as wasp emergence timing, are also evolutionarily conservative. Thus, the pollinator specialisation we see at the generic and infrageneric levels is probably the result of constraints imposed by the sex pheromones and emergence times of the pollinators themselves.

Publications and Presentations

Mant, J. G. and F. P. Schiestl (2003) Pollinator attraction and speciation in sexually deceptive orchids, Proc. European Orchid Conference, London, U.K.

Mant, J. G., P. H. Weston, F. P. Schiestl, R. Peakall (2003) Coevolution of Chiloglottis (Orchidaceae) and its thynnine wasp pollinators, Monocots III, 3rd International Conference on the Comparative Biology of the Monocotyledons, Los Angeles, U.S.A.

Mant, J.G., F. P. Schiestl, R. Peakall, P. H. Weston (2002). A phylogenetic study of pollinator conservatism among sexually deceptive orchids. Evolution. 56(5): 888-898.

Mant, J. G., F. P. Schiestl, R. Peakall, P. H. Weston, (2000) Phylogenetic perspectives on a highly specialised pollination system, Genetics Society of Australia, 47th Annual Meeting, Canberra, Australia

Mant, J. G., F. P. Schiestl, R. Peakall, P. H. Weston. (In prep) Recent origin of sexually deceptive orchids through pollinator-mediated reproductive isolation inferred from multilocus (AFLP) markers. American Journal of Botany.

Mant, J. G., C. C. Bower, R. Peakall, P.H. Weston. (In prep) Sympatric divergence of pollinator-specific, morphologically cryptic orchid races inferred from mulitlocus (AFLP) markers: a population based study. Molecular Ecology

Mant, J. G. and G. R. Brown. (In prep)  Historical contingency drives the diversification of sexually deceptive orchids pollinated by the thynnine wasp genus, Neozeleboria (Tiphiidae: Thynninae). Biological Journal of the Linnean Society

Schiestl, F. P., J. G. Mant, R. Peakall, F. Ibarra, W. Francke (2000) Pollinator attraction by pheromone-mimicking compounds in Australian and European sexually deceptive orchids. Proc. XXI Int. Congr. Entomol., Foz do Iguassu, Vol I: pp 183

Schiestl, F. P., J. G. Mant, R. Peakall, F. Ibarra, W. Francke (2000) Biologically active volatiles in Australian sexually deceptive orchids. 17th Annual Meeting Int. Soc. Chem. Ecol., Pocos de Caldas.

Schiestl, F. P., R. Peakall, J. G. Mant, (In press) Chemical communication in the sexually deceptive orchid genus Cryptostylis, Lindleyana.

 

Chiloglottis trilabra and its pollinator Neozeleboria proxima

The summer flowering Chiloglottis chlorantha

Phylogenetic trees of the orchid Chiloglottis and its wasp pollinators, produced from analyses of DNA sequence data. The wasp tree (left hand side) is based on sequences for two genes: mitochondrial 16S ribosomal RNA and nuclear wingless. The orchid tree is also based on two genes: the ITS region of the nuclear ribosomal RNA repeat unit and the chloroplast trnL-T intergenic spacer. The numbers above branches refer to bootstrap percentages – probabilistic indices that estimate the level of empirical support for the groups. Note that although there is some correspondence between wasp and orchid groupings the pattern of relative branch lengths is quite different between wasp and orchid trees. This strongly suggests that the orchids did not differentiate at the same time as the wasps but later.

The native bird orchid Chiloglottis trilabra. (photo: Colin Bower)