A 34-million-year-old fossil from the Isle of Wight is the world’s oldest example of a fig wasp, according to a report in the journal Biology Letters today.
The report was written by a team of scientists including those at the Natural History Museum, led by Dr Steve Compton at the University of Leeds.
The ancient fig wasp is almost identical to modern species, proving that this tiny insect has remained virtually unchanged during this time.
Scanning electron microscope images of pollen grains from the 34-million-year-old fossil fig wasp (top) and the modern fig (bottom) both at x1500 magnification.
As the name suggests, there is a close connection between these wasps and fig trees. In fact, the around 800 fig tree species worldwide depend on more than 800 different fig wasps to pollinate them. This new research reveals evidence that this mutually dependent partnership was already in place 34 million years ago.
‘What makes this fossil fascinating is not just its age, but that it is so similar to the modern species,’ says fig wasp expert Dr Steve Compton. ‘This means that the complex relationship that exists today between the fig wasps and their host trees developed more than 34 million years ago and has remained unchanged since then.’
The identity of the Isle of Wight fig wasp was only revealed after a span of many years, beginning with the specimen being wrongly labelled as an ant, when it was first discovered in the 1920s.
The specimen was looked after in the Natural History Museum’s collections and later the mistake was spotted by the late Dr Mikhail Kozlov. Dr Compton then began to study the fossil.
Fig wasps have body shapes and features that let them crawl into fig plants to reach the flowers. However, adults are only 1.5mm in length so scientists needed a lot of magnification to study the details.
The team analysed the specimens using the Museum’s state-of-the-art microscopy facilities, comparing the ancient fig wasp fossil with a modern fig wasp, and also with a 20-million-year-old specimen preserved in Dominican amber.
Dr Alex Ball, manager of the Museum’s Electron Microscope Unit, scanned the specimens and produced scanning electron microscopy (SEM) images of the insects, with magnifications from 200 to 1500 times. As well as being striking, the details on the images shows that the fossil insects had the same body shape and features as modern species.
Scanning electron microscope images of pollen pockets, shown by arrows, on the 34-million-year-old fig wasp (at top x600), in a modern fig wasp (in the middle x400), and in the 20-million-year-old amber specimen (at the bottom x500). The last 2 show the guard hairs around the pollen.
Dr Ball says, 'Having such good instrumentation allows us to produce cutting edge results on such a wide range of specimens and is one of the Museum’s strengths as a research institution.'
'Our labs are very busy, we have nearly 300 hundred researchers using them every year and we spend about a third of the year training people to use the instruments, so our research time is very precious, but this was a really nice research project.
'The story is fascinating and the specimens are beautifully preserved, but imaging them was really challenging. If we had just the specimens in the insect limestone, the story would have been interesting enough, but having the specimen in Dominican amber added a whole new dimension to the study.'
Dr Ball describes how the amber specimens could not be photographed with the SEM. 'SEM only produces images of the surface of the specimen. To obtain the amber images we used a confocal microscope, which uses a laser beam to make the sample fluoresce.
'We can then capture a series of images and reconstruct the fine detail. We used the confocal microscope to compare the modern, fossil and amber specimens and the specimens are virtually identical.'
The team found evidence for something called active pollination in the ancient fig wasp that is also found in some modern wasps.
Active pollination is where fig wasps collect pollen in pockets on the underside of their bodies, and then move to another tree and pull out and spread the pollen on the flowers before laying their eggs. Fig wasp larvae do better when feeding from a pollinated flower so this benefits the wasp as well as the tree.
The team found these pollen pockets and grains of pollen using scanning electron microscopy and they say it proves that active pollination was already achieved over 34 million years ago, remaining unchanged to this day.
Another clue about a sinister method of fig tree pollination was revealed in the research. The organ used to lay eggs is called the ovipositor. The length of the ovipostor in the ancient wasp showed that the wasp could not lay eggs in figs of the tree that it fed on.
All figs would strip wings off wasps as they entered the figs, to stop them flying elsewhere, and in figs like this, the wasp is trapped, with no chance to reproduce. This method is found in the trees that produce the edible figs in Europe we eat today, which is why there are no wasps inside when we eat them!
Figs are an extremely important fruit. More birds and mammals feed on figs than any other fruit. Understanding how the fig tree and wasp react to environmental changes over time could be crucial. The team says it is reassuring to know that these plants and their pollinators have responded successfully to previous episodes of climate change.