Halloween genes help kissing bugs lay better eggs

Abstract

Getting a ‘kiss’ on the face from an insect isn't something most people would enjoy; but you'd enjoy it even less if that insect is a kissing bug (Rhodnius prolixus), because they are transmitters of Chagas disease. While not always life-threatening, Chagas disease can lead to life-long health conditions, making it important to understand everything we can about kissing bugs. With this in mind, Samiha Benrabaa, Ian Orchard and Angela B. Lange, all from the University of Toronto, Canada, decided to investigate which genes are involved in making fully developed eggs in this species. As the team already knew that the moulting hormone, called ecdysone, is responsible for these processes in other insects such as silkworms and fruit flies, they decided to start there. First, the trio needed to identify the genes in kissing bugs responsible for producing the hormone – which is achieved by a group of genes called ‘Halloween’ genes in other species – as well as the genes responsible for the hormone receptor. Benrabaa and colleagues found five Halloween genes and two receptor genes in the insects, but then needed to see if the kissing bugs’ ovaries actually expressed these genes. They took samples from the ovaries and found that the mRNA levels for the Halloween genes and the moulting hormone receptor were highest in the part of the ovary that contains immature or developing eggs.Kissing bugs, like mosquitos, require a blood meal for the eggs to fully develop; so, the team fed the bugs before sampling mRNA from the ovaries for 6 days afterwards. Unsurprisingly, the mRNA levels of most of the Halloween genes and hormone receptors were higher after the kissing bugs ate. But just because the mRNA levels were high, didn't mean that the hormone was being produced. The team therefore next measured the amount of hormone in the bug's blood after it ate and compared this with the levels of hormone before its meal. The amount of hormone increased dramatically after a meal, suggesting that the high levels of Halloween gene mRNA were producing more moulting hormone. This raised a new question for Benrabaa: what happens to the eggs if you block some of this mRNA?To answer this, the team interfered with hormone levels by injecting the kissing bugs with a double-stranded form of RNA to knock out one of the hormone receptors or a Halloween gene called shade, and then fed the bugs. Three days later (when the yolk would normally start to develop) and 6 days later (when the eggs would usually be laid), the trio looked at the number of developing and ready-to-lay eggs in the ovaries. The injected kissing bugs produced far fewer eggs, most of which had deformities in the shape or in the outer shell. The bugs went on to lay very few of these eggs, and even fewer of these hatched, which suggested to Benrabaa that the hormone receptors and shade might interact with other genes involved in producing the egg yolk or shell. The team then tested the mRNA levels of four genes (two involved in yolk production and two involved in making the shell) in these same bugs and found that blocking mRNA for the hormone receptors or shade also caused a drop in the mRNA levels of the genes involved in egg yolk and shell production.Benrabaa explains, ‘It may be possible to use RNA-interference-based pest management using these findings’. Whether or not we could use this technology to help rid the world of Chagas disease remains to be seen, but this is certainly one step forward in the process.