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Riddle of the sexless rotifer solved, biologists say

Jan. 28, 2010
Courtesy Science
and World Science staff

An en­dur­ing mys­tery of bi­ol­o­gy has been solved, ac­cord­ing to sci­en­tists.

Re­search­ers had won­dered why the bdel­loid ro­ti­fer, a mi­cro­scop­ic fresh­wa­ter an­i­mal, has­n’t died off af­ter re­pro­duc­ing with­out sex for mil­lions of years.

One that did­n't make it: This bdel­loid ro­ti­fer was killed by a fun­gal par­a­site, Ro­tif­er­oph­thora an­gu­sti­spora, re­search­ers say. Spore-bearing elon­ga­tions or hy­phae formed by the par­a­site sprout out from the di­gested corpse. Al­though this one failed to dry up in time to kill off its tor­men­tor, the oth­er mem­bers the clonal roti­ifer pop­u­la­tion can still find time to dry up and es­cape, sci­en­tists say. (Im­age cour­te­sy Kent Loef­fler, Kath­ie T. Hodge and C.G. Wil­son)


By re­ly­ing ex­clu­sively on asex­u­al or sex­less re­pro­duc­tion, each ro­ti­fer cre­ates ge­net­ic­ally iden­ti­cal off­spring, clones of it­self. This would seem to shut off most pos­si­bil­i­ties of change or adapta­t­ion over genera­t­ions, com­pared to sex­u­al re­pro­duc­tion, in which each off­spring re­com­bines its par­ents’ genes in new ways. 

There­fore, an en­tirely sex­less crea­ture would pre­sumably lose the “arms race” that is the strug­gle for sur­viv­al. Adapt­a­ble crea­tures such as par­a­sites, known to af­flict bdel­loid ro­ti­fers in par­tic­u­lar, would even­tu­ally fin­ish them off.

Re­search­ers have now come up with a so­lu­tion. It seems that the bdel­loid ro­ti­fer has al­ready evolved an an­ti-par­a­site de­fense so ruth­lessly ef­fec­tive that it has closed off any room for a vi­a­ble counter-maneuver by the par­a­sites, at least so far.

Chris­to­pher G. Wil­son of Cor­nell Uni­vers­ity in New York and col­leagues raised popula­t­ions of the ro­ti­fers in a lab­o­r­a­to­ry, and no­ticed that the asex­u­al in­ver­te­brates could rid them­selves of a deadly fun­gal par­a­site by dry­ing them­selves up com­pletely and blow­ing away with the wind to new territo­ry. 

The ro­ti­fers be­came so dry that their par­a­sites could­n’t sur­vive the pun­ish­ing con­di­tions. They were then able to ride the breeze and start afresh in new, pre­sumably par­a­site-free pas­tures.

The re­search ap­pears in the Jan. 29 is­sue of the re­search jour­nal Sci­ence.


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An enduring mystery of biology has been solved, according to scientists. Researchers had wondered why the bdelloid rotifer, a microscopic freshwater animal, hasn’t died off after reproducing without sex for millions of years. By relying exclusively on asexual or sexless reproduction, each rotifer creates genetically identical offspring, clones of itself. This would seem to shut off most possibilities of change or adaptation over generations, compared to sexual reproduction, in which each offspring recombines its parents’ genes in new ways. Therefore, an entirely sexless creature would presumably lose the “arms race” that is the struggle for survival. Adaptable creatures such as parasites, known to afflict bdelloid rotifers in particular, would eventually finish them off. Researchers have now come up with a solution. It seems that the bdelloid rotifer has already evolved an anti-parasite defense so ruthlessly effective that it has closed off any room for a viable counter-maneuver by the parasites, at least so far. Christopher G. Wilson of Cornell University in New York and colleagues raised populations of the rotifers in a laboratory, and noticed that the asexual invertebrates could rid themselves of a deadly fungal parasite by drying themselves up completely and blowing away with the wind to new territory. The rotifers became so dry that their parasites couldn’t survive the punishing conditions. They were then able to ride the breeze and start afresh in new, presumably parasite-free pastures. This research appears in the Jan. 29 issue of the research journal Science.