Compelling reasons for choosing Daphnia as a model species

This resident of lakes and ponds throughout the globe has been the subject of nearly three centuries of study by taxonomists, physiologists and limnologists. As primary grazers of algae and primary forage of fish, Daphnia play a central role in freshwater ecology and are considered a 'keystone' species. Its reproductive cycle is ideal for experimental genetics. Generation time in the lab rivals that of almost all other model eukaryotic systems. Daphnia are easily cultured and reach maturity within 5 - 10 days, making it possible to contrast genomic responses throughout their ontogeny.
Reproduction is normally clonal (allowing the maintenance of constant genetic lineages), but sex can be induced environmentally (allowing the production of inbred or outbred lineages). The clonal nature of the organism provides an exceptional opportunity for separating genetic (among-clone) responses to ecological challenges from environmental and developmental noise (within-clone).
Distinct lineages have colonized radically different environments (deep permanent lakes vs. shallow temporary ponds) on multiple occasions, providing an exceptional opportunity for evaluating whether similar selective challenges are countered in genetically consistent ways within independent lineages. Population boundaries are often clearly defined and they support huge numbers of individuals. Zooplankton are especially sensitive to environmental stress and specific assemblages typify certain contaminants. D. pulex is widely used for environmental and toxicity testing, with hundreds of papers on this subject being published each year. Molecular phylogenies of Daphnia provide the necessary foundation for comparative genomics. Genomic work is greatly facilitated by the expansive data base (including the complete genome sequence) of its fellow arthropod, the fruitfly. Daphnia are transparent throughout life, thus providing a very useful feature for studies of tissue-specific gene expression. They have a relatively small genome size, approximately twice the size of Drosophila, making it one of the smallest crustacean genomes measured to date. The sexually produced diapausing eggs can be stored viably for considerable periods of time. In nature, these propagules accumulate yearly in lake sediments and their densities can exceed 1,000,000 eggs per square meter of stratum. Moreover, they can also be hatched from lake sediments up to a century old, allowing the past products of evolution to be resurrected and evaluated against their current descendants in a controlled setting.

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