Fishy odor just like dad’s

PORTLAND, Ore. — A tendency for daughters to fall for guys that are like their dads helps keep two species of fish from interbreeding.

Two distinct species of the threespine stickleback dart about in several lakes of British Columbia, where the two fishes could easily mate with each other. But they don’t; the slimmer ones, which feed on plankton in open water, mate with their own kind, while the larger, bottom-feeding ones mate with theirs.

Experiments now show that early in life, females of both kinds pick up some cue from their fathers, probably his odor, that provides a guide later on when it comes time to choose a mate, according to Genevieve Kozak of the University of Wisconsin–Madison. The experiments suggest that this process, known as imprinting, may help the stickleback species stay separate even though they live in the same lakes, Kozak said June 26 at the Evolution 2010 meeting.

“One of the coolest talks I’ve seen,” said evolutionary biologist Daphne Fairbairn of the University of California, Riverside. Just how new species form and stay separate while sharing space remains a lively topic in biology, and for some creatures, such as the extraordinarily diverse cichlid fish in African lakes, biologists are still looking for a good explanation. “I think the cichlid people are going to jump on this,” Fairbairn said.

The open-water sticklebacks, called limnetics, and the bottom-dwellers, or benthics, both descend from a marine ancestor that moved into fresh water when the most recent glaciers receded in Canada. Each lake’s benthic and limnetic forms have become distinct species, Kozak said, although biologists have not yet sorted out the species’ names.

Fathers do the childcare in these sticklebacks, as in many other fish species. After laying the eggs, mom swims off, but dad hovers over the developing embryos, defending them from predators and making sure fresh water circulates.

To see what influence the dad care has, Kozak and Jenny Boughman, now at Michigan State University in East Lansing, switched limnetic egg clutches to the care of benthic fathers and vice versa. While the switched daughters grew up to prefer the odor of their foster fathers’ species, daughters raised with no father around showed no preference in species of their mates. Apparently, there’s just something about a dad.

That something seems to be his odor, Kozak said. She exposed developing eggs to a father at several different periods during development and then checked for imprinting later on.

Embryos that were around adult males only during days two and three after fertilization, a time before their olfactory or visual systems have formed, didn’t grow up to show any preference. Yet embryos aged four to five days did imprint. That’s when the olfactory system develops, but before the visual system does.

Demonstrating a mating effect due to paternal imprinting would be unusual as far as he knows, says fish morphologist Peter Wainwright, of the University of California, Davis, who was not at the meeting. Much more prominent among fish are cases of imprinting by environmental cues, like salmon preferring the chemistry of their native streams.