Fruit fly mum's gift protects larvae through development

As if life wasn't hard enough for insect-kind, with intensive agriculture and insecticides obliterating their numbers, the planet is also warming at an unprecedented rate and it is not clear how well these ectotherms will fare in a warmer future. Brent Lockwood from the University of Vermont, USA, explains that the majority of our understanding of how these animals will adapt is based on research carried out on adult insects, with little account taken of the effects that higher temperatures may have on earlier life stages. ‘For terrestrial insects, this oversight has been particularly problematic’, he says, explaining that, unlike their parents, developing insects are essentially immobile during the earliest life stages and unable to evade the hottest conditions. In addition, recently fertilised eggs are unable to activate their own protective genes following a sudden heat wave, depending instead on the pool of protective RNA molecules bequeathed by their mothers to them. Lockwood and Kristi Montooth from the University of Nebraska, USA, suspected that one such maternal molecule – Hsp23 mRNA – might encode a protein that protects against cellular damage caused by overheating, so they decided to investigate how well Drosophila eggs that were well provisioned by their mothers with Hsp23 mRNA survived during a heat wave.

Stimulating fruit fly mums to produce eggs with higher levels of the protective Hsp23 mRNA, Lockwood and Cole Julick then allowed the females to mate with males and lay their eggs on a nutritious fruit gel before heating the eggs to temperatures ranging from 22 to 40°C for 45 min. ‘Our temperature treatments mimic the sudden temperature changes that frequently occur in nature where the temperature of rotting fruit can increase rapidly on a hot day’, says Lockwood. They then transferred the eggs to a comfortable 22°C before monitoring their survival and development over a 15 day period.

Impressively, the eggs that had received a larger dose of Hsp23 mRNA from their mothers were significantly more robust than eggs that had just received the regular dose. ‘For example, a brief exposure to 34°C killed approximately 50% of normal embryos, but killed only 10% of embryos with higher levels of the Hsp23 mRNA provided by their mothers. This is a huge effect caused by the action of only a single gene’, says Lockwood. And when the team of three tracked the development of the normal eggs, which had low levels of the protective molecule, through the larval stage to pupation, they were surprised to see how seriously the brief heat wave affected their development many days later: larvae from the overheated normal eggs were unable to climb as well as larvae from overheated eggs that were packed with their mother's additional supply of Hsp23 mRNA.

‘These results demonstrate that single genes can have big effects on the whole organism, and maternal effects are not only important for the physiology of early embryos but also have effects that last through larval development’, says Lockwood, who is keen to find out whether populations of fruit flies that experience warmer climates have developed specialised strategies to tolerate toasty conditions compared with populations from cooler climes.