The way that fig wasps pollinate fig trees has remained unchanged for at least 34 million years, the oldest known fig wasp fossils reveal.
Fig wasps emerging from a fig.
The fossils – from the Isle of Wight in England – show that the fossilised fig wasps used exactly the same parts of their bodies to carry pollen and to get inside figs as today's fig wasps.
'What this points to is that the way the insect and the plant have interacted has remained stable for a very long time,' explains Dr Steve Compton from the University of Leeds, lead author of the research.
Scientists have estimated that fig wasps and fig trees have been dependent on each other for around 65 million years by working back from fig and fig wasp family trees and genetics.
34 million year old fig wasp fossil which was mistaken for an ant when first described in the 1920's. The pocket where the pollen is stored has been arrowed. Magnification 200x.
But until now the oldest fossils that showed any evidence of this long relationship were just 16 to 23 million years old and came from amber found in the Dominican Republic.
'These Isle of Wight fossils are the oldest known fig wasp fossils by a long way,' says Compton.
SEM of a modern fig wasp. The pocket where the pollen is stored has been arrowed. Magnification 400x.
Fig trees need wasps to transfer their pollen to other trees, while the wasps need figs to lay their eggs inside. Each of the more than 800 species of fig tree needs a specific wasp – just 1.5 millimetres long – to pollinate it.
The fossils aren't new finds. One was sitting in the Natural History Museum for years and was wrongly identified as an ant when it was first described in the 1920s. Wasps and ants are closely related, so it was an easy mistake to make.
A Russian scientist discovered the mistake during recent research at the museum and called in Compton, who's a fig wasp expert, to investigate.
'What's so surprising is how similar to modern wasps these ancient wasps were.'
Dr Steve Compton, University of Leeds, Faculty of Biological Sciences
After noticing an orange patch on the side of one of the wasp's bodies and suspecting it was pollen, he decided to take a closer look.
Compton and his colleagues describe in Biology Letters how they used high-powered electron and confocal microscopes – that have a deeper depth of field – to look in more detail at the wasp fossil and two other Isle of Wight wasp fossils from Cambridge's Sedgwick Museum.
'It was fairly clear the specimens were fig wasps,' says Compton. 'They had the same unique mandibles and flattened heads, and were noticeably similar to modern wasps.'
Fossil pollen grains - each one is about 2 microns in length (1/500th of a millimetre). Magnification 1500x.
The researchers found ancient fig tree pollen, its microscopic granules shaped like a rugby ball, concentrated in one area of the wasp – in specialised pollen pockets on the underside of its body. This is exactly where modern fig wasps store their pollen.
'Not only is it unusual to find such an old fig wasp fossil, but it's really unusual to find ancient fig tree pollen. Because it's produced inside the fig not much is needed. This is the opposite to plants that rely on wind pollination – so much is produced by these trees that it's fairly easy to find ancient windborne pollen,' explains Compton.
The researchers say it's clear that these ancient fig wasps had already evolved pollen pockets 34 million years ago, and were actively pollinating fig trees.
'What's so surprising is how similar to modern wasps these ancient wasps were,' adds Compton.