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You are what you eat!
Biologists at Indiana University (IU) have been observing how gene pathways control different physical traits in horned beetles — genus Onthophagus. They found that variation in physical traits within the same species can depend solely on nutritional changes during fetal development.
In many animals, nutrition — not genetic differences — controls the appearance of certain physical traits. Ants and bees, for example, grow into workers or queens based upon the food they are fed as larvae.
But the exact genetic mechanisms connecting developmental fate to nutritional conditions were poorly understood. Now, IU biologists Armin Moczek and Teiya Kijimoto, his former postdoctoral research associate and lead author on the paper, have pinpointed genetic pathways controlling some decisions.
Their work is published in the May 4th issue of the Proceedings of the National Academy of Sciences.
The genes reported on in the study are part of the hedgehog signaling pathway, best known to biologists for playing a role in specifying front and back location for body structures — such as a leg or wing. In horned beetles, Kijimoto and Moczek identified that this pathway inhibits the growth of structures when an insect experiences low nutrition during development.
Previously, Moczek found a separate signaling pathway, the "sex determination pathway," possessed a complimentary function, promoting growth only under high nutritional conditions.
Specific genes in the hedgehog signaling pathway are called "hedgehog," "patched" and "smoothened." Together, they form a "relay" system transmitting information from outside the nucleus, such as levels of nutrition, to influence which genes get turned on or off (expressed) inside the nucleus.
Biologists are interested in "developmental thresholds" as they allow members of the same species to adapt in response to changing levels of nutrition. In tunneling dung beetles (Onthophagus taurus), high nutrition initiates males to develop horns and become aggressive fighters, jousting for access to females. However, low nutrition stops male beetles from developing horns forcing them to mate by stealth — sneaking between a rival and his willing mate, to copulate and run away before attack. Tunneling dung beetle males only turn into horned fighters if larva receive enough nutrition to reach over 5 millimeters in size as adults.
Thresholds also evolve and shift. Over the last 50 years, horn size in Onthophagus taurus males in the United States has lowered for lack of competition from other males. But in Western Australia, male on male competition is so intense — only the largest males succeed as fighters and thus mate more frequently.
Other than nutrition, many organisms react to external cues to reach similar developmental thresholds. Temperature seasonally cues butterflies to appear different in the spring and fall; cues the winged and wingless stages of aphids; and the winter and summer fur coats of arctic foxes. Underlying thresholds allow organisms to generate alternative versions of themselves, despite sharing a single genome between them.
Looking forward, Moczek plans to investigate:
This research was funded in part by the National Science Foundation. Kijimoto is currently on faculty at West Virginia University.
Two horned beetles — genus Onthophagus. Despite having the same genome, these beetles