Animal With the Most Genes? A Tiny Crustacean
ScienceDaily (Feb. 3, 2011) — Complexity ever in the eye of its beholders, the animal with the most genes -- about 31,000 -- is the near-microscopic freshwater crustacean Daphnia pulex, or water flea. By comparison, humans have about 23,000 genes.Daphnia is the first crustacean to have its genome sequenced.
Daphnia pulex (water flea) with a brood of genetically identical future offspring. (Credit: Paul D.N. Hebert, University of Guelph)
The findings are part of a comprehensive report in the journal Science by members of the Daphnia Genomics Consortium, an international network of scientists led by the Center for Genomics and Bioinformatics (CGB) at Indiana University Bloomington and the U.S. Department of Energy's Joint Genome Institute. A bullet-point list of the Science paper's most important findings appears at the end of this release.
"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 and CGB genomics director John Colbourne. "We estimate a rate that is three times greater than those of other invertebrates and 30 percent greater than that of humans."
Daphnia Genomics Consortium projects can be found at
http://daphnia.cgb.indiana.edu, as well as a link to the nearly 40 companion papers based on the data reported in the Science paper.
Scientists have studied Daphnia for centuries because of its importance in aquatic food webs and for its transformational responses to environmental stress. Predators signal 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 in the absence of males -- by clonal reproduction, until harsh environmental conditions favor the benefits of sex.
Daphnia's genome is no ordinary genome.
"More than one-third of Daphnia's genes are undocumented in any other organism -- in other words, they are completely new to science," says Don Gilbert, coauthor and Department of Biology scientist at IU Bloomington.
Sequenced genomes often contain some fraction of genes with unknown functions, even among the most well-studied genetic model species for biomedical research, such as the fruit fly Drosophila. By using microarrays (containing millions of DNA strands affixed to microscope slides) that are made to measure the conditions under which these new genes are transcribed into precursors for proteins, experiments that subjected Daphnia to environmental stressors point to these unknown genes having ecologically significant functions.
"If such large fractions of genomes evolved to cope with environmental challenges, 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.
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Science 4 February 2011:
Vol. 331 no. 6017 pp. 555-561
DOI: 10.1126/science.1197761
RESEARCH ARTICLE
The Ecoresponsive Genome of Daphnia pulex
John K. Colbourne et al
*To whom correspondence should be addressed. E-mail: jcolbour@indiana.edu
ABSTRACT
We describe the draft genome of the microcrustacean Daphnia pulex, which is only 200 megabases and contains at least 30,907 genes. The high gene count is a consequence of an elevated rate of gene duplication resulting in tandem gene clusters. More than a third of Daphnia’s genes have no detectable homologs in any other available proteome, and the most amplified gene families are specific to the Daphnia lineage. The coexpansion of gene families interacting within metabolic pathways suggests that the maintenance of duplicated genes is not random, and the analysis of gene expression under different environmental conditions reveals that numerous paralogs acquire divergent expression patterns soon after duplication.Daphnia-specific genes, including many additional loci within sequenced regions that are otherwise devoid of annotations, are the most responsive genes to ecological challenges.
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