Mom and dad may also be brother and sister.Timo Thünken

For one group of African fish, incest is not only a way of life — it may also be a boon.

Given a choice, the cichlid Pelvicachromis taeniatus, often found in aquariums, prefers to mate with siblings nearly three times out of four. And males who shacked up with their sisters spent more time guarding their fry and less time fighting with their mate than unrelated couples. The end result was happy families and healthy kids.

This runs against the usual view that inbreeding harms offspring by bringing harmful mutations together. While a normal copy of a gene can sometimes mask a mutation, offspring that inherit two mutated copies lack any such protection. This can make inbred offspring less able to survive or reproduce.

But Timo Thünken and his collaborators at the University of Bonn in Germany found that inbred and outbred P. taeniatus had the same growth and survival rates. These results, together with recent studies in birds and other fish, suggest that the popularity of inbreeding in the animal kingdom may have been underestimated, Thünken says.

Close to home

In the wild, single P. taeniatus females seek out males that have claimed a good cave to live in. After a mate is selected, couples are largely monogamous and both parents take turns guarding the cave against predators.

Thünken collected about 20 breeding pairs of P. taeniatus from the Moliwe River in Cameroon. When Thünken began his research, he wanted to know how the fish recognized their kin to avoid inbreeding. To his surprise, he found that rather than avoid siblings, the fish selects close relatives, he reports in Current Biology¹.

Precisely how cichlids recognize their kin remains a mystery — Thünken suspects that scent plays a role.

Although he hasn't yet ruled out the possibility that inbreeding has negative effects, Thünken believes that fathers who mated with their sisters were more invested in parenthood, because their kids shared more of dad's genes, boosting his genetic contribution to the population. That's a relatively well-known effect — what's surprising is that this benefit seems to outweigh detriments for this population.

The same phenomenon has been seen in some insects and other fish species says William Shields, an evolutionary biologist at the State University of New York in Syracuse. "We still deal with the overriding dogma that inbreeding is deleterious," says Shields, "but there's evidence from lots of organisms that inbreeding can have advantages."

Furthermore, it only takes three or four generations of inbreeding to purge the gene pool of many of the mutations that initially make it harmful, says Shields.

Thünken says it's possible that the cichlids he collected had already been inbreeding for long enough to dump their genetic baggage. That, together with limited availability of mates, could make inbreeding a more attractive choice, he says.

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• References

  1. Thünken T., Bakker T.C.M., Bakker T.C.M., Baldauf S. A.& Kullman H. I. Curr. Biol., 17.. 225 - 229 (2007).