Do pollinators isolate new species of plants?

It’s time for the last paper of the pollinator-driven speciation week. The previous posts have examined how pollinators can select and cause diversity among plants but, if they’re all the same species, could pollinators simply mix all the genes back up again?

Floral odour chemistry defines species boundaries and underpins strong reproductive isolation in sexually deceptive orchids by Rod Peakall and Michael R. Whitehead tackles the final part of the process of speciation. Once you have differences between plants, how do barriers to cross-pollination arise? Peakall and Whitehead examined orchids in the genus Chiloglottis, which appeared in the blog last month.

Chiloglottis orchids
Photographs of the four Chiloglottis study taxa showing their flowers and the chemical structures of the chiloglottone semiochemicals used to attract their respective pollinators. (A) C. valida (CVA) and pollinator Neozeleboria monticola, chiloglottone 1; (B) C. aff. valida (CAV), chiloglottones 1 and 2; (C) C. pluricallata (CPL), chiloglottones 1 and 2; (D) C. aff. jeanesii (CAJ) and pollinator N. sp. (impatiens2), chiloglottone 3. Key to chiloglottones: 1 = 2-ethyl-5-propylcyclohexane-1,3-dione; 2 = 2-ethyl-5-pentylcyclohexane-1,3-dione; 3 = 2-butyl-5-methylcyclohexane-1,3-dione. Scale bar = 10 mm.

Photos: Peakall and Whitehead.

Chiloglottis orchids are found in eastern Australia. The flower doesn’t provide food, instead is appears to offer another reward. Part of the flower looks like a female wasp, and it has the scent to match. Any male attempting to mate will be disappointed not simply because the female is a fraud, but also because the orchid tags the wasp with pollinaria. These get carried to the next destination, which might well be another Chilogottis orchid.

Chilogottis appeals to thynnine wasps, but these aren’t all one species. Each wasp will be looking for specific mate. The way the wasp finds the mate is through the scent. Could variations in the chemistry of the flowers isolate species by attracting one kind of wasp, but not another?

The study initially showed this was plausible. Some of the orchids flowered around the same time of year in the same locations as their sister species, so clearly there was some barrier that wasn’t caused by geography. In addition, there was a lot of morphological overlap between many of the flowers, so there was no mechanical reason why they should be isolated.

It was also possible to cross-breed the flowers when pollinated by hand, and these produced viable seeds so this adds to the puzzle of what the barrier is.

The big observable difference what the chemical cocktails that the plants put out as their floral scent. If you categorise the flowers this way, then there are genetic differences. Peakall and Whitehead argue that what the Chilogottis orchids represent are a number of plants in the process of divergence. This helps give an overview of the divergence process. For example if you visited a forest, you could see trees in all states from saplings to fallen trunks and work out the life cycle of a tree without waiting hundreds of years. In a similar way, being able to identify different diverging branches of orchids at different stages means you get an overview of speciation without having to wait generations for the final result.

To some extent the idea of pollinator-driven speciation could be a puzzle. For many angiosperms, it’s pollinators that keep the species together, exchanging pollen from the flowers of one plant to the other. Peakall and Whitehead’s paper show how the Grant-Stebbins model works, with the shift in pollinators leading to the speciation of the orchids. What might look like a paradox is soluble after all.

You can pick up this paper from Annals of Botany.