Atmospheric hypoxia limits selection for large body size in insects

Area of Science:

• Evolutionary Biology
• Insect Physiology
• Paleoclimatology

Background:

• Phanerozoic atmospheric oxygen fluctuations correlate with insect body size.
• Higher oxygen levels may facilitate the evolution of larger insect sizes.

Purpose of the Study:

• To test the hypothesis that oxygen levels influence insect body size evolution.
• To investigate the genetic basis of size evolution in insects under varying oxygen conditions.

Main Methods:

• Selected Drosophila melanogaster for large body size across three oxygen partial pressures (aPO(2)): 10, 21, and 40 kPa.
• Maintained size selection for 11 generations, then returned all flies to normoxia (21 kPa aPO(2)) for one generation.

Main Results:

• Significant body size increase (15%) observed in flies selected at 21 and 40 kPa aPO(2).
• Flies selected at 10 kPa aPO(2) showed strongly reduced body sizes.
• After returning to normoxia, all selected flies exhibited similar enlarged sizes, indicating oxygen-independent genetic effects.

Conclusions:

• Hypoxia acts as a physical constraint on insect body size, even under strong selection for larger mass.
• This supports the hypothesis that Triassic hypoxia may have contributed to reduced insect sizes during that period.