Zebrafish choose adult companions based on experience as juveniles

May 24, 2004

AUSTIN, Texas—Scientists at The University of Texas at Austin have used fish whose color patterns are easy to alter to discover that early social interactions—not genetics—drive their choice of buddies in adulthood.

Graduate student Raymond Engeszer and integrative biologists David M. Parichy and Michael J. Ryan came to this conclusion after finding that “wild-type” striped zebrafish preferred unstriped counterparts as adult company if they grew up around them and vice versa. The only genetic difference between the fish was a change in a single unit of a gene that prevented dark pigment cells from developing in the unstriped fish.

“Even these little, small-brained fish are learning their social preference, which is fascinating since this sort of preference should have been under strong evolutionary selection and you might think it would be hard-wired at birth,” said Engeszer. He is a graduate student in ecology, evolution and behavior in the School of Biological Sciences, and lead author of an article on the findings that will be published May 25 in the journal Current Biology.

Zebrafish (Danio rerio) are common in many fish tanks, and have become a favored model in the past decade among scientists studying genetics related to human health and disease. But the genetic information on zebrafish means scientists also have an opportunity to work out the environmental and genetic factors underlying the behaviors of these fishes. This behavioral understanding could have a ripple effect by deepening the understanding of processes such as the formation of new species.

“In zebrafish, you can go all the way from the level of gene action to how that affects the way the animal looks to how it behaves, and to how these factors influence evolution,” said Parichy, the senior author from the College of Natural Sciences and Institute for Cellular and Molecular Biology.

To study zebrafish preferences, Engeszer mated the blue-and-gold striped wild-type zebrafish to the mutant zebrafish. The mutants are called nacre, a French word describing their “mother-of-pearl” appearance. Zebrafish eggs are see-through, allowing him to separate nacre offspring from ones with brown pigment that would become striped adults. About 40 of each variant were placed alone in tanks, and three similar zebrafish or three with the alternate color pattern were added.

Once test fish reached sexual maturity in a few months, they were moved to the middle chamber of a three-chambered, 55-gallon tank. On one end of the tank were two male and two female nacre fish, and on the other, four native counterparts. Layers of plastic divided the three chambers so that chemical and vocal cues couldn't pass between the fish.

The test fish invariably hung out near the end of the tank with fish that resembled their juvenile buddies.

“Some of the results were absolutely stunning. Out of 600 seconds of observation, they’d spend 595 with this group,” said Engeszer, noting that this schooling behavior is known to provide fish with protection from predators, greater mating opportunities and other advantages.

The scientists have begun looking at juvenile nacre and wild-type zebrafish to determine whether there is a specific time during development when the social preference takes hold, or imprints. They also plan to identify whether certain components of visual patterns in zebrafish are more important for recognition.

They have developed more than a dozen genetic variants in the lab that can be used for this, including ones with pale or bold stripes, vertical bars, spots and other color patterns. In addition, the scientists have about 40 other natural variants to develop variants from.

“We can delete or add different parts of the pattern and compare the responses of wild-type fish to see what they really key in on,” Parichy said.

Note: This research was supported by funding from the National Institutes of Health and National Science Foundation. For a copy of the research paper, contact Heidi Hardman at 617-397-2879.

For more information contact: Barbra Rodriguez, College of Natural Sciences, 512-232-0675.