A pair of studies led by IU researchers provide new evidence that when it comes to evolution, the testes might play a key role.
The research, led by Kimberly Rosvall, assistant professor in the IU College of Arts and Sciences’ Department of Biology, was conducted in two subspecies of dark-eyed junco, a type of American sparrow.
The white-winged junco, or Junco hyemalis aikeni, is found in the United States in the Black Hills of South Dakota. The slate-colored junco, or Junco hyemalis carolinensis, is from the Appalachian Mountains in the state of Virginia.
The first is larger and more aggressive, the other is smaller and more docile, according to an IU press release.
The first paper compares the subspecies in their expression of enzymes that make testosterone within the gonad. The second paper investigates how the subspecies’ gonads differ in the expression of stress hormone receptor genes, which are known to lower testosterone, according to a press release.
Rosvall and colleagues conducted their research by studying the testes of the two subspecies in the wild and in captivity.
In the case of the South Dakota juncos, the results suggested the brawnier birds’ gonads simply had more abundant testosterone-producing machinery.
When the researchers treated the birds with a testosterone-stimulating hormone, the South Dakota birds were able to make more testosterone faster and keep it elevated longer, according to a press release.
“An animal that keeps testosterone elevated for longer should have greater expression of these testosterone-mediated characteristics,” Rosvall said in a press release.
In the second study, Rosvall and colleagues found a supplementary cause for lower testosterone levels in the Virginia juncos.
The smaller birds’ testes showed higher levels of stress hormone receptors, known inhibitors of the production of testosterone, according to a press release.
The mechanism by which testosterone results in different traits involves hormone receptors found in the brain and body.
When testosterone binds to these sites, it affects the expression of genes that influence characteristics such as behavior, body size and plumage, according to a press release.
“At the genomic level, you can really see a significant difference between these big, sexy juncos and their smaller, less aggressive cousins,” Rosvall said in a press release. “Because we still saw some of these differences in males that we captured as juveniles, our research suggests that these differences in the gonad are genetic, or programmed very early in life.”
Leo Smith
