Photo: Siddharth Kankaria/Research Matters
Evolutionary changes are manifested in natural populations over many generations, which makes it difficult to observe evolution in real-time. Nevertheless, there are many evolutionary biologists who are keen to understand the intricacies of this process and Prof. N G Prasad from the Indian Institute of Science Education and Research (IISER), Mohali is one of them. Recently, Prof. Prasad and his colleagues published a study based on how fruit flies respond to crowding during larval stages of development.
This study, led by Dr. Vinesh Shenoi, a former PhD student of Prof. Prasad, investigated how an environmental stress factor like larval crowding, can shape sexual behaviour in laboratory maintained fruit fly populations. Due to their short life span and relative ease of maintenance, fruit flies are an ideal model system in understanding evolution.
The biological fitness or Darwinian fitness of an organism is measured in terms of its' contribution towards the gene pool of subsequent generation. In other words, fitness is a function of progeny produced and its longevity. It is accepted among evolutionary biologists that there exists a trade off between reproduction and survivorship. Since both reproduction and survival are costly, individuals have to allocate available resources judiciously between the two. Thus an increased investment in survival leads to reduced investment in reproduction and vice-versa. For instance organisms with higher reproductive output are likely to have reduced lifespan.
In one of his previous studies Dr. Shenoi found that the populations, which are maintained at higher densities during larval stage for over hundred generations, have evolved to have a higher lifespan. Hence, he set out to investigate if there is a trade-off between lifespan and reproduction. He directed his efforts to look at the reproductive behaviour in these populations and compare it against their ancestral populations with no crowding history. Interestingly, the study found that the crowded populations have evolved higher courtship activity compared to their ancestors.
“We chose to quantify male courtship activity as it is an active process and requires a lot of energy. Previous studies have shown that an increase in courtship activity reduces the lifespan of males. Persistent courtship and mating can reduce average progeny produced and lifespan in females. We were surprised not only to know that increased energy investment in lifespan was not reflected as a deficit in reproductive output in terms of behaviour, but also not to find any effect on mating success,” says Dr. Shenoi.
The researchers found increased courtship activity among flies selected for crowding. But further investigations into dynamics of mating showed that the crowded flies evolved no significant reproductive advantage compared to the ancestral population.
“Trade-offs across traits that are important for an organism’s Darwinian fitness have been suggested to limit the kind of trait combinations that can evolve. Our results indicate that trade-offs can themselves evolve. It is also possible that trade-offs are context specific, somewhat like the Cheshire cat, appearing and disappearing based on the environment. This has major implications for our understanding of the evolutionary process” says Prof. Prasad.
Larval crowding is a complex selection pressure and can affect both males and females of a population in multiple ways. There have been many studies on larval crowding but yet there are many areas to be addressed. Prof. Prasad’s lab is currently tracking the evolution of the male ejaculate and the females in the crowded populations. Drosophila males transfer an ejaculate containing more than 100 proteins along with sperms into the body of females during mating. Some of these proteins affect the fertility of females while others reduce her lifespan. Living in crowded conditions can affect the quality and quantity of the proteins and the sperms. Thus it is no surprise that the lab is interested in this question. Hopefully this and similar studies will give us further insights into the mechanisms of evolutionary processes.