Haven’t we all heard of the saying “no pain, no gain”, referring to trade-offs -- an inevitable part of life? Trade-offs are seen in every field, be it physics, biology, evolution, business or work. Nature has numerous examples of such trade-offs. Cheetah, the fastest animal on land, had to give up bone-strength for longer and sleeker bones to run with such speeds, while the lion had to strengthen the bone to tackle huge prey, compromising on speed. In fact, trade-offs are pivotal to evolutionary theory. Charles Darwin, in his ‘Origin of Species’ mentions that animals must evolve as ‘integrated wholes’ and “the whole organism is so tied together that when slight variations in one part occur, and are accumulated through natural selection, other parts become modified”.
Evolutionary biology states that one trait in an organism cannot increase without a decrease in another. This is due to the finite availability of resources, energy, space and time. Yet, surprisingly, a study by Prof. N G Prasad and his team from the Indian Institute of Science Education and Research, Mohali, shows that fruit flies (Drosophila melanogaster), may have defied this rule! In their study, the researchers found no evidence of a ‘trade-off’ with important traits in these flies, during the development of immunity. Existing, important traits like longevity and fertility remained intact, even though immunity increased.
To understand evolutionary trade-offs, the researchers infected 30 successive generations of fruit flies with fatal, pathogenic bacteria -- Pseudomonas entomophila. For the next 15 generations, they let the flies alone without infecting them with bacteria, and measured their immunity at the end, hoping that the absence of selection (the bacterium, in this case) might result in the loss of a newly developed trait (immunity). To their surprise, they found that the newly acquired immunity against the pathogen, at least in the sample of fruit flies studied, did not result in any significant trade-off with other traits like lifespan, producing more offspring (fecundity), hatching many eggs (egg hatchability), development time for an embryo, or resistance to starvation and body weight.
This study is significant because it changes our understanding of the importance of trade-offs in evolution. This study reiterates that trade-offs are like the Cheshire Cat of Lewis Carol’s Alice in Wonderland -- appearing and disappearing in different contexts. Trade-offs between immunity and other traits are context-specific say the researchers.
However, the researchers speculate that it is possible for these ‘missing’ trade-offs to manifest in different conditions or in different but non-apparent immune mechanisms that has not been a scope of this study. “Most of the trade-offs are interpreted to be a consequence of resource availability being limiting. However, flies bred in the laboratory have access to ad libitum food. Therefore, even if immunity is costly, it is likely that this effect gets masked because of already abundant resources. Alternately, our results question the premise of immunity being a costly trait”, says Ms. Vanika Gupta, the main author of the study.
The researchers also argue that the cost of evolving immunity should have been very less, otherwise, the newly developed trait would be immediately reversed as soon as the selection pressure (bacteria) was removed. They also cite examples where the expected trade-offs between immunity and other traits are not observed; arguing that development of immunity could be context specific.
Undoubtedly, this study has opened a Pandora’s box of questions for future research. But the striking finding is that an important trait like developing immunity, absolutely essential for the survival of a population during an epidemic, is not a costly affair after all!