Flies, genes & garbage (and in that order)
As a Scientist with a keen interest in the ecology of animal behaviour I have this romantic idea about conducting field work in exotic and colourful places, yet, in the latter part of my PhD my choice of field work involves sifting through people's garbage in search of flies and genes that may play an important role in how animals move and colonize new places.
To understand this change in perspective we have to step back to work done in the 1970'ies on Drosophila larval foraging behaviour. Then a graduate student, Dr. Marla Sokolowski discovered a remarkable difference in how larval fruit flies forage through rotten fruit. On a single piece of fruit some of the larvae moved much more actively while foraging than others. Intrigued by this behaviour, Sokolowski went on to develop a simple behavioural assay that would not only characterize this complex behaviour, but also lead to the discovery of a gene largely responsible for larval foraging behaviour. Briefly, more active larvae (rover flies) carry a version of a gene known as foraging that is different from the version that less active larvae (sitter flies) carry. Today, this system remains one of the best understood examples of genes that influence behaviour in animal systems and because of these discoveries scientists have been able to look for similar genes in other organisms such as ants and honey bees that explain individual differences in behaviour.
When I was a young graduate student interested in migratory behaviour I was unaware of the foraging story. In my search for mechanisms responsible for differences in migration I emersed myself into the bird literature where people like Jospeph Grinnell and Angus Woodbury in the 1930'ies were working on the evolution of migration. Grinnell meticulously pieced together distances that individual birds would travel while foraging and compared those to the distances of individuals during migration. Surprisingly, Grinnell reported that during a single day of movement the two activities achieved similar distances and proposed that migratory behaviour simply evolved as an extension of behaviour that birds already possess. Woodbury, went a little further and suggested that although the two behaviours involve different motivational and sensory systems, a common mechanism likely is responsible for both.
This is a really neat idea because it suggests that by understanding genes involved with foraging behaviour we might be able to learn something about the mechanisms that drive long distance movement in animals.
This ultimately lead me to Sokolowski's fruit flies and the foraging gene and in 2010 my colleagues and I demonstrated in the field how more active larvae dispersed faster and further as adults than less active larvae. The experiment involved setting up a large field with food traps placed at regular distances along the four cardinal directions. However, In nature food is never regularly distributed and to understand how individuals with different versions of the foraging gene handle environments with food distributed irregular we designed an experimental trapping grid in the field that varied from many food patches with short distances to few patches with longer distances and released 10 000 flies, half with one version of the foraging gene and the other half with the other version of the gene.
To recognize rovers from sitters we marked each type with a different flourescent pigment.
We prepared to record flies at each trap four times a day, but after about two days it became clear that we were missing a large proportion of rover flies (more active as larvae). Prior to the field experiment, we did some experiments in the lab showing that rover and sitter flies did not differ in their preference for the food we used, so we knew it wasn't because of the food that we were missing the rover flies.
Meanwhile, the word around the field reserve was that reddish coloured fruit flies were showing up at garbage cans next to the housing facilities. We produced traps and strategically placed them around the reserve, both at orchards and garbages, and after a few hours inspected our captures.
Much to our excitement, all orchards yielded fruit flies, yet none of them were our marked flies. The garbage on the other hand was a different story. Although lots of other unmarked flies were found in the garbage many of the flies we recovered carried the red pigment of our rover flies.
While we had carried out our experiment rover flies had moved across forest patches and colonized garbages and were well on their way to establish new populations whereas so far none of the sitter flies have been detected outside of the experimental grid.
Although this finding is anecdotal, it suggests that by studying this gene we may be able to understand mechanisms underlying long distance movements in animal systems and arthopods species in particular. More importantly, it also suggests that we need to consider how habitat loss and increasing distances between available habitat may influence the composition of genetic diversity within populations. For fruit flies though, the situation is perhaps not so dire as the availability of human garbage is likely quite stable.
