A study of fruit flies bred in the lab shows that they may actually evolve to modulate their reproductive traits based on the extent of competitiveness of their competing males. This study of 150 generations of fruit flies, spanning about six years, shows how the environment can influence evolution of reproductive traits.
It is a well-known fact that males of different species compete for the attention of females to mate with. In species where the female mates with many males, as in Drosophila (fruit fly) the sperms extend this competition to the post-copulatory domain. This is known as post-copulatory sexual selection.
This study shows that over several generations, males actually evolve to modulate their reproductive investment based on their perception of the number and quality of the competition. The study was published in the journal Evolution.
It is significant because it shows how male fruit flies change the expression of their reproductive traits depending on the changes in the environment. “This study shows the evolution of plasticity according to the change in the socio-sexual environment,” says N.G. Prasad of Indian Institute of Science Education and Research, Mohali, who led the research. “In general, the rapidly changing environment can have a great impact on an organism’s behaviour. Acquiring plasticity enables an individual to cope with the fluctuating environment and maintain fitness in adverse conditions,” he adds, in an email to The Hindu.
The researchers maintained the study for over 150 generations. “The flies are maintained on a 14-day life cycle every generation. Therefore, one generation corresponds to 14 days, and 150 generations corresponds to 2,100 days, which is approximately six years,” explains Dr. Prasad.
In general, post-copulatory sexual selection can be competition between the various sperm in the female reproductive tract to fertilise the egg, and also sperm choice mediated by the female reproductive tract. To understand this, the group studied three reproductive traits for males of male-biased and female-biased regimes. These were mating latency, copulation duration and sperm defence ability. Of these, mating latency is a measure of female receptivity and male courtship efficiency and intensity. Sperm defence ability is the ability of the experimental male sperm to displace another from a rival male. While mating latency and copulation duration were manually observed, the sperm defence ability was inferred from the (inherited) eye colour of the offspring.
The experiments demonstrated that first, males can plastically modulate their reproductive investment in response to change in socio-sexual environment experienced in early-life. Secondly, evolution under different operational sex ratios led to divergence of male reproductive investment patterns. Also, the identity of early-life competitors can influence the pattern of reproductive investment.
In order to eliminate the potential of parental effects to influence the results of the study, the flies were passed through one generation of standardisation. “During this process, flies were maintained in ancestral conditions and selection was not imposed on them. Then all the experimental conditions were also retained same for both the populations (male-biased and female-biased) and all the treatments,” he says.