Research sheds light on how crop-threatening insects can evolve to take advantage of new food sources
LINDELL BEACH, B.C. — An entomologist has discovered that the genes of the apple maggot fruit fly are capable of rapid evolutionary change, which allows it to take advantage of new food that provides a safer haven for its larvae.
Greg Ragland, assistant professor of entomology at Kansas State University, worked with a team of biologists at several universities to look at how and why insect species evolve to become a threat to economically important crops. One theory has been that strong environmental changes, such as drastic seasonal weather shifts, can rapidly encourage different sections of an organism’s genome to change.
They focused on the apple maggot fly, Rhagoletis pomonella, which historically laid its eggs in the fruit of the native hawthorn tree. However, in the 1850s, a small group of flies was seen laying their eggs in apples.
“Like many insects, Rhagoletis flies are heavily parasitized, mainly by parasatoid wasps that attack the larvae feeding in the fruit,” said Ragland.
“Just like the flies, the parasatoids cue on specific fruit odours. The flies do this to find fruit to lay their eggs in (while) the parasatoids do this to find the fly larvae. There is natural variation among flies in the odours that they ‘like,’ or seek out, and probably a few colonizing flies accidentally laid eggs in apples, which smell different than hawthorn fruit but are in the same plant family. The larvae seem to feed reasonably happily on apples, so the barrier really was probably changing the odour preferences of the fly.”
He said the parasatoids don’t like the smell of apples and so seldom find the larvae inside. This gave the larvae a safe place to feed while the parasatoids continued to feed on the larvae in hawthorn. Natural selection favoured fly development toward apples.
Ragland said apples also begin to develop fruit earlier than hawthorns. Flies had to evolve earlier seasonality so that the adults’ egg-laying time would coincide with the ripening of the apple fruit for their larvae.
Apples are the most valuable edible tree fruit in British Columbia. Sixty percent of all orchard land is planted with apple trees, and the province produces 24 percent of the apples grown in Canada. Market production averages 96,600 tonnes with a farmgate value of $36.7 million.
Apple maggots, which are a pest in most apple growing regions of Ontario, were first reported in the Pacific Northwest in 1979 and showed up in Alberta in 2005 and in B.C.’s Fraser Valley in 2006. They are also found on Vancouver Island and in the Prince George area. The pest has not been detected in the major fruit growing regions of the Okanagan, Similkameen or Creston valleys.
Adult flies are five millimetres long with a black body, yellowish head and legs and white bands on the thorax. Their wings have black bands, which are identification markers from other related fruit flies.
The research team sequenced the genomes of hawthorn and apple flies and then reared flies under short-term hawthorn-like and apple-like conditions.
They found immediate differences under the short-term conditions. The genetic changes in the short-term experiment lined up almost completely with the genetic changes in the hawthorn and apple groups. The results suggested that many regions of the genome can evolve toward change in just a few generations, driven by environmental differences.
Ragland said the differences in the flies between those synchronized with hawthorn fruit and those adapted to apples equate to two different genetic groups. Each generation of fly will build on those differences, which bring about the formation of a new species.
“We now have lots of evidence that evolution can bring about relatively drastic changes in the span of years or decades,” said Ragland.
Their study focused on seasonality and timing, adaptations that could be achieved in a short time span.
“(There is) much evidence for rapid evolution of seasonality in mosquitoes, squirrels, moths and birds, and more,” he said.
He said the research shows that species are not static. Instead, they are fluid in the environment in which they live. A change in the environment can drive genetic change over a short time scale and produce a hither-to benign fly into a serious crop pest.
The study was published in the scientific journal Ecology Letters.