Female Drosophila melanogaster standing on a white piece of paper photographed with a USB microscope. Photo credit: Hannah Davis, CC BY-SA 4.0, via Wikimedia Commons.
Female Drosophila melanogaster standing on a white piece of paper photographed with a USB microscope. Photo credit: Hannah Davis, CC BY-SA 4.0, via Wikimedia Commons.
Anyone sharing a home with teenagers will testify that puberty can be bewildering, but imagine entering the time when you morph into an adult and your entire body changes. From being a limbless larva, you emerge as a fully fledged fly or beetle, complete with wings and legs. But the factors that trigger this profound metamorphosis were unclear. ‘This transition is ostensibly initiated at the attainment of a critical weight in the final larval instar’, says Jon Harrison from Arizona State University, USA, adding that the insect's network of breathing tubes may simply be unable to provide sufficient oxygen once larvae reach a certain size, initiating the dramatic life change. Intrigued that an oxygen supply failure when larvae outgrow their breathing tube network might elicit this extraordinary transformation, Harrison and colleagues from Arizona State University, North Dakota State University, USA, and University of Illinois Chicago, USA, began monitoring the molecular mechanism that protects tissues from low oxygen as Drosophila larvae proceeded through the critical life stage.
Collecting recently hatched Drosophila larvae, the team allowed the larvae to develop until they reached the final stage of development, just before they pupate ready to change into flies. Then, the researchers transferred some of the youngsters to a low oxygen chamber (10 kPa O2), while others remained at normal oxygen levels (21 kPa O2), and monitored whether the larvae began to suffer from an insufficient oxygen supply as they grew larger. The larvae that were growing in low oxygen clearly struggled, activating the cellular mechanism that protects tissues from damaging low oxygen levels. However, at no stage did the larvae living in normal oxygen show signs of oxygen deprivation as they grew. Lack of oxygen as the larvae reach a critical size and outgrow their breathing tube network supplying oxygen to every tissue in their body is not the signal triggering the dramatic transition as they metamorphose into adults.
So, what else might trigger this transformation? Knowing that the prothoracic glands located in the front section of the thorax release the essential hormones that govern metamorphosis, Harrison and colleagues wondered whether a localised reduction in the glands’ oxygen supply as the larvae grow larger might trigger them to begin metamorphosis. However, inactivating some of the protective components that are triggered when internal oxygen levels fall in the prothoracic glands did not alter the larvae's development. So, again, a fall in the internal oxygen levels in the prothoracic glands does not appear to trigger Drosophila larvae metamorphosis.
But when the team monitored the effect that growing up in low oxygen had on the larvae, the prothoracic gland definitely had an impact: switching off the cellular mechanism that protects the youngsters from low oxygen slowed their development and increased their mortality rate. This led Harrison and colleagues to conclude that sensing low oxygen levels in the prothoracic glands allows the larvae to develop faster to escape from locations where the oxygen levels have fallen dangerously low.
Although naturally occurring low oxygen levels in the body do not seem to trigger Drosophila metamorphosis, the team suspects that may not be the case for other insects. ‘Insects are notoriously variable and it would not be surprising if the mechanisms determining size vary among flies and butterflies’, says Harrison's colleague at Arizona State University, Jacob Campbell, still leaving the team on the hunt for the Holy Grail of what triggers insect metamorphosis.
