Plants, algae, and bacteria have a clear advantage over the animal kingdom when it comes to their powers of photosynthesis. A certain species of sea slug can steal photosynthetic DNA from algae, but animals don't photosynthesize on their own. At least that's what we thought. New evidence has come to life suggesting that a species of aphid might be able to convert sunlight to energy.
The pea aphid (Acyrthosiphon pisum) is already a strange creature. Aphids reproduce by parthenogenesis and can be born pregnant, though males (who sometimes lack mouths) are born in colder weather. They're also the only animal that has been identified that can synthesize carotenoids, the pigments that appear in chloroplasts and chromoplasts that harness solar energy for use by the cells. Does that mean that pea aphids photosynthesize?
That's the question explored in a recent study, "Light-induced electron transfer and ATP synthesis in a carotene synthesizing insect," published this week in Nature. In 2010, Yale entomologist Nancy Moran discovered that aphids possess the gene for synthesizing carotenoids, meaning that the pigment is "home grown" rather than lifted from another photosynthetic species. Carotenoids are metabolically expensive chemicals to synthesize, prompting Alain Robichon, an entomologist at the Sophia Agrobiotech Institute, to wonder what purpose the carotenoids serve.
Robichon studied different colored aphids of the species: green aphids, which contain high levels of carotenoids and are born in colder lab conditions; orange aphids, which are born in optimal lab conditions; and white aphids, which contain little to no carotenoid pigment and are born when resources are limited. Robichon's team found that the green aphids make significantly higher levels of ATP than white aphids do, and that orange aphids make more ATP when placed in sunlight than when placed in darkness. This suggests that the pigments may be part of a system of photo-induced electron transfer that enables aphids to synthesize energy from sunlight.
Although study co-author Maria Capovilla cautions that further research is needed before we can determine whether aphids can photosynthesize as many non-animal species can, she notes that this ability could function as an emergency backup that helps aphids survive their treks from plant to plant. While feeding aphids already consume more sugar than they need to survive, perhaps a photosynthetic ability enables them to travel to a new host plant. And perhaps aphids are particularly well suited to survive periods of environmental stress, at least if they only last a single generation.
Pea aphid photo by Shipher Wu.