IISER researchers have been able to show that fruit flies selected for dispersal for as long as 69 generations have evolved traits — increased aggression, exploration and activity — which together is more likely to make them more invasive. In response to climate change and habitat destruction, there is an increasing possibility for the evolution of these traits in organisms selected for dispersal. This will lead to native species becoming more invasive in the same ecosystem.

Invasive species are normally considered to come from outside a given ecosystem. But a study by researchers from the Indian Institute of Science Research and Education (IISER) Pune has shown that selection process for dispersal in fruit flies (Drosophila melanogaster) can lead to evolution of traits related to invasive potential. So a native species can become invasive in the same ecosystem.

Though the team led by Sutirth Dey from the Institute’s Biology Division did not specifically study for invasion potential of the flies, the experiments they conducted did show the flies acquiring three important traits — increased aggression, exploration and activity — which together is more likely to make them more invasive.

 “Faced with vagaries of climate change and habitat destruction, there is an increasing possibility for the evolution of these traits in many mobile organisms,” says Prof. Dey. If dispersal to safer environments is the “first line of defence” for organisms to cope with adversities, becoming an invasive species even within the same ecosystem might be the ugly fallout.

In a study published in June last year in the journal Oikos, the researchers subjected four populations, each containing about 2,400 flies, to artificial selection for increased dispersal over 33 generations. This was done by placing the flies in a bottle and connecting it to another using a long pipe. Only the first 50% of flies that moved to the second bottle were allowed to breed. Thus there was a direct selection for dispersal propensity — ability to leave the source area. And every 3-4 generations, the distance between bottles was increased by using a longer tube. This was done to mimic increased habitat fragmentation.

Changes in dispersal traits

Within 10 generations of selection, the dispersal propensity and ability to travel long distances was significantly greater in the selected flies than the controls. The selected fruit flies displayed the similar characteristics after 20 generations.

“We checked for dispersal propensity in the 10^th and 20^th generation without food and water, which was similar to the selection procedure. The absence of food and water acted as drivers and led to increased dispersal propensity in the flies,” says Dr. Sudipta Tung. “Then we became more ambitious and wanted to check whether the flies dispersed even in the absence of an external driver for dispersal. Even in the presence of food and water, we found that after 33 generations, the selected flies still displayed greater propensity and ability to disperse.”

“Even when there was no reason for selected flies to disperse [due to presence of food], they were still dispersing. So dispersal had become constitute,” he says.

The propensity to disperse was same in both sexes but males dispersed or travelled to longer distances than females. They also found that the selected population had larger number of flies that travelled long distances.

Body size, fecundity, longevity

When organisms evolve in response to selection for greater dispersal the body size tends to become bigger. But in a paper published on August 3 in the journal Evolution, the researchers found that selection did not lead to any increase in body size in both sexes. The studies were carried out using flies from 49-69 generations.

When flies are selected for dispersal, there is increased likelihood that it would be at the expense of fecundity and longevity to accommodate the increased energy demands of dispersal. But there was no change in either fecundity or longevity in the selected flies.

Changes in behavioural traits

“But we observed distinct changes in three behavioural traits — activity, exploration and aggression. And these three traits are closely related to the invasive potential of a species,” says Dr. Tung. “Other research groups have shown that these behaviours are found associated with invasive potential in some species.”

The selected flies exhibited greater activity than the controls and this increased activity was seen in the absence and presence of food. After 24 hours, the activity of the selected flies was significantly more than the control males but the time spent resting and quality of rest/sleep were the same in the selected flies and controls.

The selected flies also exhibited greater exploratory tendency. Greater exploratory tendency helps the species to survive in a new environment. In many species, there is a strong relationship between exploration and invasion.

Like exploration, the selected flies exhibited greater aggression. Aggression helps species to retain resources and mates or gain new ones. But in the experiments carried out by the researchers, the flies exhibited elevated aggression even when there was no obvious fitness advantage. The selected flies did not have to compete with native flies either. “In our experiment, aggression has likely evolved as a correlated response of dispersal evolution,” they write.

Role of neurotransmitters

In collaboration with Dr. Kavita Dorai from the Department of Physical Sciences at IISER Mohali, the researchers looked at what was responsible for the change in behavioural traits. “Since dispersal is a complex process, it is difficult to figure out the genes responsible for it. So we looked at the changes in the levels of the underlying metabolites,” says Prof. Dey.

Octopamine, a neurotransmitter, was significantly elevated — 2.2 fold higher — in the selected flies than the controls. “The elevated octopamine level explains the underlying mechanism behind the observed changes in behavioural traits in the selected flies,” Prof. Dey says.

Dispersal is an energy intensive process. And the selected flies showed increased levels of cellular respiration to meet this energy demand. There was an increase in the levels of Adenosine triphosphate (ATP), which is the energy currency for cells. Other energy-related molecules too were elevated in the selected flies.

Published in The Hindu on August 4, 2018