The researchers, at Harvard University, the University of Massachusetts and the US Geological Survey, report the finding in the current issue of Proceedings of the Royal Society B. They compared female salmon, male salmon that will eventually undertake the well-known journey from their river birthplaces to oceans ? and then migrate heroically back upstream one to three years later to spawn ? and males of the same age known as "sneakers" that mature at greatly reduced size without leaving freshwater.
"The finding that hundreds of the nearly 3,000 genes we studied were expressed differently in the brains of sneakers and other male salmon came as a surprise," says Nadia Aubin-Horth, a postdoctoral researcher in the Bauer Center for Genomics Research in Harvard's Faculty of Arts and Sciences. "Since these males of the same species in the same wild environment differed only in their life history, we did not expect the expression of so many of their genes to differ."
Aubin-Horth and her colleagues were also surprised by some of the 17 separate classes of genes demonstrating differing activity levels.
"It makes sense that growth genes are suppressed in sneakers and genes associated with reproduction are expressed more, since these fish essentially trade bodily size for faster reproductive maturity," she says. "However, it was unexpected, for instance, that genes associated with learning and memory would be expressed at higher levels in the brains of sneakers. It's not yet clear why disparities like this would arise."
Aubin-Horth says it is