Oxygen levels can have a dramatic effect on body size. It is thought that the colossal insects that roamed the planet during the Carboniferous period were only able to grow large because of elevated oxygen levels. And low oxygen can have the opposite effect: some species that grow at high altitude are smaller than organisms living at sea level. Jon Harrison and his colleagues from Arizona State University, USA, are fascinated by the mechanisms that regulate body size in response to oxygen levels. ‘We were interested to see if there was a period during development during which this size reduction takes place,’ says Erica Heinrich, an undergraduate student working in Harrison's lab. So she joined Manoush Farzin and Jaco Klok to find out when developing Drosophila melanogaster are sensitive to oxygen levels ().
Designing a complicated array of experiments where Drosophila at various stages of development were exposed to periods of hypoxia (10% oxygen), the team then collected the fully-grown adults and measured their wing cell size, body mass and body cell size to find out how a limited oxygen supply had affected their growth.
Surprisingly, all of the adult insects that had experienced hypoxia during any developmental stage were on average 7% smaller than the insects that were exposed to normal oxygen levels. Even the insects that were exposed to brief – 24 h – hypoxic windows were small. And by carefully controlling when the flies experienced hypoxia, the team found that the insects were most sensitive to limited oxygen availability during the final stage of larval development and early in pupal development.
Curious to find out why the flies that were reared in hypoxic conditions were smaller than flies reared in 21% oxygen, the team looked at the insect's cell sizes and cell number. All of the flies that had experienced hypoxia had smaller wings than flies reared in normoxia and insects that had only experienced hypoxia as pupae had smaller wing cells.
Having found that the insects were more sensitive to hypoxia during the late larval and pupal stages, and that developmental stage also affected the size of the insect's wing cells after hypoxia exposure, the team says, ‘There may be at least two independent mechanisms by which hypoxia reduces adult body size.’ Explaining that hypoxia inducible factor and nitric oxide are known to modulate cell function in response to hypoxia, they are keen to find out if either play a role in the hypoxic fruit flies' size.
