Female fruitfly fecundity

Developing egg chambers from the Drosophila ovary.

The frequency with which mating occurs has differing consequences for males and female fruitflies in terms of their fitness and lifespan. For males, the more mates they have, the better their chances of reproductive success. But for females, a shift to too much mating and reproduction may be costly in terms of lifespan, given the harmful effect of copulation and the energy needed to produce an egg. There’s a fine balance to be struck between mating and your own survival.

For the fruitfly Drosophila melanogaster, the female’s mating rate is highly dependent on how much food is available. Low amounts of food allow her to live longer, but with a lower rate of reproduction. The underlying mechanism behind this process was unknown, but new research suggests that a chemical pathway known as the insulin/insulin-like growth factor-like signalling pathway (IIS for short) is the reason behind it.

The IIS system has been studied in a variety of organisms and is known to be important in development and metabolism – the pathway allows an animal to know how much it’s eaten. In the fruitfly, manipulation of the genes in the IIS pathway can mimic the effects seen when restricting the amount of food eaten, leading to longer life. This an effect also seen in various other species such as nematode worms and even some monkeys.

Dr Stuart Wigby, from the University of Oxford, found that female fruitflies genetically manipulated to have a longer lifespan also showed reduced mating behaviour. But why should long life be associated with less mating? Essentially, it comes down to resource allocation.

When resources are scarce, they can be allocated to either survival, or reproduction. By allocating them towards survival it gives a fly the opportunity to overcome the hard times and reproduce again when there is plentiful food.

But for the genetically engineered flies, times aren’t really that hard – their bodies just think they are. Says Wigby, “By altering the IIS pathway, you’re tricking the flies into thinking they’re hungry, tricking the body into thinking its receiving less food than it really is. You’re affecting physiology in a similar way to when food is scarce.”

The flies’ genetics make them believe that they are hungry, so they remain in a perpetual state of survival – and their levels of mating go down accordingly.

Wigby now plans to study another pathway, the TOR pathway involved in sensing levels of available amino acids. TOR runs in parallel with the IIS system and the researchers hope that by studying the two together the genetic mechanisms linking diet and mating can be understood better.

Benjamin Thompson

Image credit: NIMR, MRC/Wellcome Images