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For some species, an upside to inbreeding

Feb. 5, 2007
Courtesy Cell Press
and World Science staff

Breed­ing be­tween close kin is thought to be fraught with ev­o­lu­tion­ary pit­falls: it tends to sad­dle off­spring with dan­ger­ous lev­els of ge­net­ic de­fects. 

But ev­o­lu­tion­ary the­o­ry pre­dicts that un­der some cir­cum­stances, in­breed­ing may have ben­e­fits that out­weigh the costs. Re­search­ers say have found ev­i­dence to back this idea. 

Pel­vi­ca­chro­mis tae­ni­a­tus. Na­tive to low­er Ni­ge­ria and Cam­e­roon in West Af­ri­ca, slight­ly over three inches long as an ad­ult, and va­r­i­a­ble in col­or and mark­ings, it is a har­dy and fast-re­pro­duc­ing fish.


In some spe­cies, “close in­breed­ing may be ex­plained by rel­a­tives be­ing bet­ter par­ents,” Timo Thün­ken and col­leagues of the Uni­ver­si­ty of Bonn, Ger­ma­ny, wrote in the Feb. 6 is­sue of the re­search jour­nal Cur­rent Bi­ol­o­gy.

Although they didn’t re­com­mend this parent­ing stra­te­gy for humans, the re­search­ers noted the be­ne­fi­cial ef­fect among an Af­ri­can cich­lid fish, Pel­vi­ca­chro­mis tae­ni­a­tus. These fish, in which both par­ents help care for young, pre­ferred un­fa­mil­iar, close kin rath­er as mates, the bi­ol­o­gists said.

Pa­ren­ting costs en­er­gy, and kin­ship tends to fa­vor co­op­er­a­tion. In­deed, ob­ser­va­tions of this fish showed that re­lat­ed par­ents “were more co­op­er­a­tive and in­vested more” in par­ent­ing, the sci­en­tists wrote. The rea­son, they added, is likely that a male mat­ing with his sis­ter en­sures that his young re­ceive ex­tra copies of his genes—from their mo­ther. That would pro­mote his own ev­o­lu­tion­ary suc­cess.

So in­breed­ing is­n’t uni­form­ly bad, Thün­ken and col­leagues wrote: ev­o­lu­tion­ar­ily speak­ing, “an in­di­vid­ual has to trade off the costs against the ben­e­fits” of it.

The main drawback is that in­breed­ing can br­ing to­geth­er cou­ples with the same gene de­fects. The mu­ta­tions are of­ten harm­less in them be­cause there are back-up cop­ies of the genes. But in­breed­ing par­ents can pool the mu­ta­tions in off­spring, over­whelm­ing the back­up sys­tems.

There was no ev­i­dence of this oc­cur­ring in the fish, Thünken and col­leagues wrote, for un­clear rea­sons. Per­haps good pa­ren­ting made up for bad genes, they spec­u­lat­ed. An­oth­er pos­si­bil­i­ty, they wrote, is a pos­si­ble self-cor­rect­ing mech­an­ism tied to in­breed­ing. The­o­rists be­lieve that re­peat­ed in­breed­ing may lead to die-offs of bad­ly mu­tat­ed in­di­vid­uals. That “purges” the pop­u­la­tion’s bad genes, stav­ing off ge­net­ic dis­in­te­gra­tion.


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Breeding between close kin is thought to be fraught with evolution ary pitfalls, largely as it tends to saddle offspring with dangerous levels of genetic defects. But evolution ary theory predicts that under some circumstances, inbreeding may have benefits that outweigh the costs. Researchers say have found evidence to back this idea. In some species, “close inbreeding may be explained by relatives being better parents,” Timo Thünken and colleagues at the University of Bonn, Germany, wrote in the Feb. 6 issue of the research journal Current Biology. They reported finding this effect among an African cichlid fish, Pelvicachromis taetiatus, in which both parents help care for young. The fish preferred mating with unfamiliar close kin rather than non-kin, the biologists said. Parental work takes up energy, and kinship generally favors cooperation. So a possible explanation for kin preference in breeding in this species is that it offers a benefit by fostering parental cooperation, the scientists said. Indeed, observations of this fish showed that related parents “were more cooperative and invested more” in parenting, the scientists wrote. The reason, they added, may be that “a male mating with his sister assures the transmission of his gene copies that are found in his sister to the next generation,” promoting his evolution ary success. So inbreeding isn’t uniformly bad, they wrote: evolution arily speaking, “an indi vidual has to trade off the costs against the benefits of in- or outbreeding.” The main problem with inbreeding is that it can bring together couples having the same gene defects. The mutations are often harmless in the parents, because they have back-up copies of the genes that work. But by inbreeding they can pool the mutations in offspring, overwhelming the backup systems. There was no evidence of this occurring in the fish, Thünken and colleagues wrote, for unclear reasons. It may be that good parental care made up for bad genes, they speculated. Another possibility, they wrote, is a possible self-correcting mechanism tied to inbreeding. Theorists believe that repeated inbreeding may lead to die-offs of badly mutated indi viduals. That “purges” the bad genes from a population, saving it from genetic deterioration.