Drosophila yeast lodgers are more than dietary supplement Free

Yeast extract spreads are popular across the planet: Australians go nuts for Vegemite, Brits are super-keen on Marmite and Drosophila larvae are partial to a yeast snack. ‘Many Drosophila species… feed and lay eggs in yeast-rich decaying fruits’, says Brent Sinclair, from University of Western Ontario, Canada, adding that the insects benefit from the vitamins and protein provided by yeasts. But every creature, including Drosophila and their larvae, also depends on the symbiotic microbial communities that reside within their guts, with most research interest focusing on the bacterial lodgers. Yet, little was known about the relationship between insects and the yeasts that reside within their guts. Do insects depend on yeasts simply as dietary supplements, or do yeasts actively contribute to the wellbeing of their fly hosts? Sinclair and his colleagues decided to investigate how two yeast microbial lodgers, baker's yeast, Saccharomyces cerevisiae, and Lachancea kluyveri could affect the development of their Drosophila larval hosts.

But first, Sinclair and colleagues had to breed fruit fly larvae that were completely free of internal microbial lodgers. ‘Making axenic [microbe-free] flies is hard’, says Sinclair, describing how yeast expert Marc-André Lachance worked with Yanira Jiménez-Padilla (both at University of Western Ontario) to perfect production of fruit fly eggs with no microbes on the surface, ready to develop into microbe-free larvae; ‘Yanira has magic hands’, exclaims Sinclair. Jiménez-Padilla then took some of the microbe-free eggs and allowed them to hatch and develop, while providing others with the microbe cocktail that the flies carry naturally, allowing them to develop the internal microbial community that usually inhabits the guts of developing larvae. When other fly eggs hatched, Jiménez-Padilla and undergraduate researcher Babafemi Adewusi (University of Western Ontario) provided the microbe-free larvae with one or other of the yeasts to dine on.

Then they monitored how long all of the larvae took to develop into adult flies, and it was clear that developing without microbial lodgers took its toll. The larvae required ∼13 days to develop into flies. However, the eggs that had been bathed in the natural microbial cocktail carried in the guts of fruit flies developed fully into adults in just 11.5 days. Impressively, the larvae that had been provided with living yeast when young developed as fast as the larvae with the full microbial cocktail. The yeast alone restored the larvae's ability to develop into adult flies at the natural speed, and when the team took a peek inside the guts of the developing larvae, the insects were packed with healthy developing yeast.

But Jiménez-Padilla was curious: were the flies depending on the yeast for its nutritional value, digesting the microbes, or were the live yeasts inside the insects contributing somehow to the larvae's wellbeing? This time, she and Adewusi provided microbe-free larvae with a diet of dead yeast – which could not set up home in the larvae's guts after consumption – while feeding other larvae on a dietary supplement providing the same nutrients as the dead yeast, to find how fast they developed. Although the supplemented larvae developed faster than if they had no microbial lodgers, they only developed as fast as the larvae provided with dead yeast to dine on. Live yeast in the larvae's guts definitely provided an advantage, allowing them to develop faster than if the yeast was just providing nutrition.

The team suspects that the yeast lodgers could help their Drosophila hosts either by regulating their growth, driving the larvae to eat more, or by modifying the gut to absorb nutrition more efficiently. But one thing is sure, developing fruit fly larvae need live gut yeast lodgers for the best start in life.