Because of selection,both fruit fly(host) and bacterium (pathogen) evolve having the maximum fitness

The natural world is rife with pairs of antagonists . Plants and viruses ,insects and pathogens , bacteria and their phages , and so on. In these systems it is an interesting question to study how the resistance to a pathogen, in the case of the host , and virulence towards the host,in the case of a pathogen, evolve. Towards understanding this better , Indian Institute of Science Education and Research Mohali researchers have taken up the system of the fruit fly (Drosophila melanogaster) and a bacterial pathogen that affects the fruit fly, sometimes even causing death – Pseudomonas entomophilia – have been co- evolved to study the pathway of evolution taken by the system of antagonists . In this case , they find that being surrounded by enemies actualy makes the organism stronger , or fitter , to combat the enemy .

Co – evolving systems

How does one set up a co evolving system experimentally ? A population of flies are infected by the pathogen and the infection is allowed to take its course . Among the infected flies ,only those that survive the infection, namely the ones that have the best immune systems to combat the pathogen , are taken to breed the next generation. Similarly, bacteria are collected from the flies that die due to the infection. These are the bacteria that have the virulence sufficiently strong to cause death in the present population. These bacteria are taken to breed the and also infect the flies in the next generation. Thus , both the host (fruit fly) and the pathogen (bacteria) are selected for having the maximum fitness .

Four types

The methodology of the experiment is like this : Four types of populations were bred in the lab . One in which , as described above , the host and pathogen both co-evolved . The second was a population in which only the host was selected from the flies that did not die due to the infection. Every generation, infection was done from a stock of ancestral bacteria which was not evolving. The third and fourth were two types of control populations. This methodology allowed the researchers to compare the evolution process in hosts that were co-evolved against their pathogen and the hosts that were adapted against a static , non evolving pathogen. They found that the former category evolved higher survivorship against the co-evolved pathogen than the hosts that was adapted against a non -evolving , static pathogen . Further they also found that the co-evolved hosts showed higher survivorship with respect to ancestral , unevolved pathogens than their counterparts who have been pitted against static pathogens .