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Tiny flea boasts most genes known in an animal

Feb. 4, 2011
Courtesy of Indiana University Bloomington
and World Science staff

The an­i­mal with the most genes of any known is the nearly mi­cro­scop­ic wa­ter flea, sci­en­tists have found: about 31,000, com­pared with 23,000 for hu­mans.

The freshwa­ter crea­ture, known sci­en­tif­ic­ally as Daph­nia pulex, is the first crus­ta­cean to have its ge­nome se­quenced, or de­cod­ed. Crus­taceans are a class of aquat­ic an­i­mals that in­clude crabs or lob­sters, char­ac­ter­ized by hard shells and seg­mented bod­ies.

Daph­nia pulex (wa­ter flea) with a brood of ge­net­i­cal­l iden­ti­cal fu­ture off­spring. (Cred­it: Paul D.N. Hebert, Uni­ver­si­ty of Guelph )


The find­ings are part of a re­port in this week's is­sue of the re­search jour­nal Sci­ence by mem­bers of the Daph­nia Ge­nomics Con­sor­ti­um, a net­work of sci­en­tists led by the Cen­ter for Ge­nomics and Bioin­for­mat­ics at In­di­ana Uni­vers­ity Bloom­ing­ton and the U.S. De­part­ment of En­er­gy's Joint Ge­nome In­sti­tute.

“Daph­ni­a's high gene num­ber is largely be­cause its genes are mul­ti­ply­ing, by cre­at­ing cop­ies at a high­er rate than oth­er spe­cies,” said proj­ect lead­er John Col­bourne, ge­nomics di­rec­tor at the cen­ter. That rate is “30 per­cent great­er than that of hu­mans.”

Sci­en­tists have stud­ied Daph­nia for cen­turies be­cause of its im­por­tance in aquat­ic food webs and for its trans­forma­t­ional re­sponses to en­vi­ron­men­tal stress. Threats prompt some of the an­i­mals to pro­duce ex­ag­ger­at­ed spines, neck-teeth or hel­mets in self-de­fense. And like the vir­gin nymph of Greek my­thol­o­gy that shares its name, Daph­nia thrives with­out males—by cloning it­self, un­til harsh en­vi­ron­men­tal con­di­tions fa­vor sex­u­al re­pro­duc­tion.

Daph­nia in a col­orized im­age de­signed to clar­i­fy sur­face struc­tures. The an­i­mal has a trans­lu­cent body and a com­pound eye. (Cred­it: Jan Mich­els, Chris­t­ian-Al­brechts-Uni­ver­si­taet zu Kiel)


“More than one-third of Daph­ni­a's genes are un­doc­u­mented in any oth­er or­gan­ism… they are com­pletely new to sci­ence,” said Don Gil­bert, co­au­thor of the re­port and a bi­ol­o­gist at the uni­vers­ity. 

If more an­i­mals turn out to be like that, “in­forma­t­ion from tra­di­tion­al mod­el spe­cies used only in lab­o­r­a­to­ry stud­ies may be in­suf­fi­cient to dis­cov­er the roles for a con­si­der­able num­ber of an­i­mal genes,” Col­bourne said.

Daph­nia is emerg­ing as an im­por­tant mod­el or­gan­ism in its own right for a new field of sci­ence, en­vi­ron­men­tal ge­nomics, re­search­ers said. The field aims to bet­ter un­der­stand how the en­vi­ronment and genes in­ter­act, and how to apply this un­der­standing to man­ag­ing wa­ter re­sources and pro­tect­ing hu­man health against pol­lu­tion. 

James E. Klau­nig, an ex­pert in en­vi­ron­men­tal health at the uni­vers­ity, pre­dicts the new work will lead to more real­is­tic and sci­en­tif­ic risk evalua­t­ions: “the Daph­nia sys­tem is an ex­quis­ite aquat­ic sen­sor, a po­ten­tial high-tech and mod­ern ver­sion of the mi­ne­shaft ca­nary.”


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The animal with the most genes of any known is the nearly microscopic water flea, scientists have found: about 31,000, compared with 23,000 for humans. The freshwater creature, known scientifically as Daphnia pulex, is the first crustacean to have its genome sequenced, or decoded. Crustaceans are a class of aquatic animals that include crabs or lobsters, characterized by hard shells, several pairs of legs and segmented bodies. The findings are part of a report in this week's issue of the research journal Science by members of the Daphnia Genomics Consortium, a network of scientists led by the Center for Genomics and Bioinformatics at Indiana University Bloomington and the U.S. Department of Energy's Joint Genome Institute. “Daphnia's high gene number is largely because its genes are multiplying, by creating copies at a higher rate than other species,“ said project leader John Colbourne, genomics director at the center. That rate is “30 percent greater than that of humans.“ Scientists have studied Daphnia for centuries because of its importance in aquatic food webs and for its transformational responses to environmental stress. Threats prompt some of the animals to produce exaggerated spines, neck-teeth or helmets in self-defense. And like the virgin nymph of Greek mythology that shares its name, Daphnia thrives without males—by cloning itself, until harsh environmental conditions favor sexual reproduction. “More than one-third of Daphnia's genes are undocumented in any other organism… they are completely new to science,“ said Don Gilbert, coauthor of the report and a biologist at the university. If more animals turn out to be like that, “information from traditional model species used only in laboratory studies may be insufficient to discover the roles for a considerable number of animal genes,“ Colbourne said. Daphnia is emerging as an important model organism in its own right for a new field of science, environmental genomics, researchers said. The field aims to better understand how the environment and genes interact, and how to apply this understanding to managing water resources and protecting human health against pollution. James E. Klaunig, an expert in environmental health at the university, predicts the work will lead to more realistic and scientific risk evaluations: “the Daphnia system is an exquisite aquatic sensor, a potential high-tech and modern version of the mineshaft canary.“