Botanic Gardens Trust, Sydney, Australia

Orchidaceae tribe Diurideae

Phylogeny and its implications for the evolution of pollination systems

Dr Peter H. Weston - Senior Principal Research Scientist

2012 is the 150th anniversary of the publication of Charles Darwin’s first book on orchids, 'On the Various Contrivances by which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing'. This book was the immediate follow-up to Darwin’s magnum opus on evolution, 'On the Origin of Species'. This anniversary was commemorated at last year’s International Botanical Congress by a symposium on orchid pollination, in which I and several colleagues were invited to present a talk on the evolution of pollination systems in the predominantly Australian tribe Diurideae. The presentations from that symposium have now been converted into chapters for a book entitled 'Darwin, Then and Now', to be published by University of Chicago Press.

In our chapter, we have analysed the phylogenetic relationships between genera of diurids using all of the comparative evidence that we could assemble: morphological, anatomical and embryological characters, and alignments of nuclear and chloroplast DNA sequences. In doing so, we constructed the most highly resolved and strongly supported estimate of the phylogeny of the tribe that has been produced so far. Data mapped on the presence or absence of floral nectaries, pollinator orders, superfamilies and families, pollination systems and functional numbers of flowers per plant was then mapped onto the best tree to test several evolutionary hypotheses. Discussions of the evolution of pollination systems in orchids have tended to assume that nectar production is ancestral in the family and that deceptive pollination systems have evolved repeatedly from such rewarding ancestors. However, my colleagues and I have found that the most recent common ancestor of the Diurideae was most probably pollinated by bees that were deceptively induced to visit its nectarless flowers by their resemblance to flowers that provided nectar and/or pollen rewards. Lineages that attracted pollinators by offering nectar or the false promise of sex, or that dispensed with cross-pollination altogether, evolved multiple times from such an ancestor. These results are con-sistent with those found for other tribes of orchids in recently published analyses, all of which have pointed to food mimicry appearing very early in the evolution of the orchids. 





Diuris sulphurea is pollinated by native bees that gather small quantities of nectar secreted from the base of the labellum near the centre of the flower, unlike most of its close relatives, which are nectarless. Nectar production had been thought to be ancestral in the orchid tribe Diurideae but work conducted by our research group has shown that the most recent common ancestor of this tribe was probably a food mimic pollinated by bees that were deceptively induced to visit its flowers.

Photo: Peter Weston