A tiny species of wasp, just 1.5 millimetres long, can shuttle pollen between isolated fig trees in a desert landscape, flying up to 160km in a single night with the help of the prevailing winds.
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The Ceratosolen arabicus wasps are weak fliers that live for only up to two days, but they are the only insects capable of pollinating the Ficus sycomorus fig trees scattered along the arid Ugab valley in Namibia.
The wasps manage to keep the isolated fig trees together thanks to the strong winds that sweep the valley. ‘We thought we were looking at a fragmented tree population,’ says lead author Professor Philip Gilmartin, from Durham University. ‘But these trees are interbreeding, despite being many kilometres apart.’
‘The most amazing thing is the sheer distance covered by these wasps in one night.’
Prof Philip Gilmartin, Durham University
The sycamore fig tree and the Ceratosolen wasps ‘are utterly dependent on each other,’ says Dr Steve Compton, an entomologist from the University of Leeds who co-directed the study.
After mating inside a fig, the females leave the tree carrying a precious cargo of pollen. They are blown by the wind until they detect the specific smell of a suitable fig tree. ‘The females pollinate the flowers, lose their wings and antennae, lay their eggs inside a fig and die,’ says Compton. Six weeks later, a new generation emerges.
Gilmartin, Compton and their team wanted to find out how this cycle works in a desert environment where trees are separated by many kilometres. Compton and Leeds PhD student Sophia Ahmed trekked the Ugab valley from the river mouth in the Skeleton Coast inland for over 250km, mapping and sampling the 79 mature sycamore fig trees they found along the way and collecting seeds from the figs on the trees.
The team germinated fig tree seedlings back in the lab and analysed their DNA at the NERC Molecular Genetics Facility in Sheffield. ‘This allowed us to visualise the genetic differences and to identify the parents of each seedling,’ explains Gilmartin.
Since they were working in an arid valley where all fig trees can be accounted for, the team was able to calculate the distance covered by the pollinating wasps between the seedling’s ‘father’, where the wasp was born and collected the pollen, and the ‘mother’, where the wasp laid its eggs and died.
One tree near the mouth of the Ugab valley produced seedlings with pollen from another fig tree located 160km inland.
‘The most amazing thing is the sheer distance covered by these wasps in one night,’ says Gilmartin. ‘They are very weak fliers, but they can travel at a remarkable rate between trees, thanks to the wind.’
The findings, reported yesterday in the Proceedings of the National Academy of Sciences, show that the ‘father’ trees are always located upstream in the valley. This suggests that the pollinating wasps travelled from east to west according to the easterly winds that prevail during winter – the time when samples were collected – which means that western trees closer to the sea will struggle to be pollinated.
But during summer, winds in the Ugab valley are predominantly from the west. This annual change of wind direction helps the trees overcome their geographical isolation.
Gilmartin and colleagues found that wasps can travel up to 160km, but they also uncovered evidence that the tiny pollinators can actually do much better than that. ‘Some of our seedlings have unknown fathers,’ he says. ‘They have DNA that we cannot trace back to any of the trees we sampled in the Ugab.’ This possibly means that these seedlings were fathered by trees located even further afield.