Flowers that tell lies
The extraordinary tricks used by orchids to lure pollinating insects
Orchids are far more common that many people might think. In fact they account for about one in ten of all flowering plant species. Some of the most famous ones grow epiphytically on rainforest trees but there are many others that simply grow on the forest floor amongst the other terrestrial plants.
Like many flowering plants, most orchids rely on insects to carry pollen from one plant to another. However, whilst most flowering plants offer a food reward to their pollinators, almost a third of orchid species resort to deception. In many cases this is a simple lie; the flowers look and smell like they would bear nectar but contain none at all. However, in other orchids, this deception can take quite bizarre forms such as sexual deception. To achieve this, the orchid produces strange flowers with dark globular growths on one petal that may bear a superficial resemblance to a female insect. However, in the insect world, good looks alone are not enough, the female's pheromone scent being the most important attractant for males. Accordingly, to complete the deception, the orchid flower produces molecules that mimic these pheromones. The scent, and to a lesser extent the flower's appearance, entice the male insects to land on it. When it does so, the orchid's unusually large pollen mass becomes attached to its body. Some orchids even go so far as to temporarily trap the male insect by closing their petals once it lands. To escape the insect has to squeeze past the sticky pollen bundle.
Of course given the diversity and abundance of sexually deceptive orchids, there has to be a mechanism by which they can ensure that the pollen from a particular orchid finds its way to another plant of the same species. Generally, this is achieved by each species of orchid targeting a single species of local insect, which in turn has its own specific pheromone.
Professor Rod Peakall from the ANU School of Botany and Zoology is one of the world's leading authorities on orchid pollination. Together with his research group, he's currently conducting a study of Australian terrestrial orchids that use sexual deception to lure pollinating insects.
"When we began our study of some 30 orchid species growing in the southern half of Australia, we imagined that we might find 30 different combinations of pheromone used to attract the pollinating insects." He says. "In fact so far it has turned out to be just four communication channels." This raises the question, how do orchids ensure that they don't attract the pollinator insects of different orchid species which could lead to hybridisation?
The key to avoiding such hybridisation turns out to be locality. If two species live in geographically distant habitats, there is no danger of inter-pollination so they can both use the same pheromone mimic to attract the same insect species at the same time. "It's a bit like FM radio. Many different stations can transmit on the same frequency providing they are in widely separated parts of the country." Dr Peakall explains. "And of course conversely, if the two orchids share the same location, then they have to use a different pheromone mimic.
Apart from shedding light on the behaviour of orchids, this work has wider implications for evolutionary biology and especially for our understanding of speciation; the process by which populations of living things diverge into separate species that are no longer able to effectively interbreed. One of the most important mechanisms driving speciation is geographical isolation. Some members of a population become separated by large distances perhaps as the result of environmental changes or simply by chance. Each sub population then further evolves to suit its new habitat and eventually, the two groups become so genetically distant they form separate species.
However there is another more contentious speciation mechanism called sympatric speciation in which some new species form without this geographical separation. Dr Peakall believes that some of the orchid data point strongly to this possibility. In this case, it's the highly specific relationship orchids have with their insect pollinators that could drive the speciation. A small mutation can subtly change the orchid's pheromone mimicking molecule, for example it may become an isomer of the original (same chemical composition but different structure). Insect response to pheromones is highly specific, and such a change in the flower may change the pollination insect species it attracts. From that point on the two orchid populations would become effectively isolated from each other even though they still inhabit the same physical place. If this turns out to be the case, it will be one of the very few cases of sympatric speciation that has been directly observed.