Research
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Daphnia
Daphnia magna,
and its pathogenic bacterial microparasite Pasteuria ramosa combine a number of features which makes them an
ideal for laboratory and field studies of host-parasite ecology and evolution.
First, Daphnia are rather small (1-2mm) and have a generation time of about one week, which results in 15 to 20 generations per growing season, so that the evolution of populations can be observed within a reasonable timeframe. The small size and ease of handling of Daphnia also permits the collection of large samples from the wild, and the maintenance of thousands of individuals in the lab.
Second, because Daphnia have a clear carapace, the parasite spore mass in the haemolymph or ovaries of infected individuals is highly visible, and thus the infection status of hosts in field samples, as well as those in experiments, can be classified with the naked eye. This aspect greatly expedites the processing of field samples and the scoring of individuals during experiments.
Third, females infected in field collections can be treated with antibiotics, after which they resume reproduction. This makes it possible to perform experiments that include host genotypes derived from naturally-infected subset of populations (this would normally not be possible because infected hosts do not reproduce). Using this technique, it has been possible to show that susceptibility in the field correlates strongly with susceptibility to laboratory infection (see below).
Fourth, reproduction in Daphnia can be controlled so that it is exclusively clonal, where females produce daughters that are genetically identical to their mother, and thus it is possible to truly replicate a host population sample by splitting the offspring of collected individuals into separate replicates.
Fifth, clonal reproduction of hosts and the ability to freeze P. ramosa transmission spores are especially valuable aspects of this system. The genetic structure of both host and parasite populations collected from the field can be ‘frozen’ until they are used in an experiment. Sixth, field studies using allozymes on Daphnia populations have shown that clonal frequencies may fluctuate wildly. The cause of these fluctuations has not been established. Finally, there is now a Daphnia genome project, and there is huge potential to exploit the Drosophila and Anopheles genome projects.
Importantly, and in contrast to the situation with Drosophila, a substantial amount is known about the natural parasites (and field ecology) of Daphnia. More is known about Anopheles parasites, but we know of no directly transmitted parasites of Anopheles which do not represent a major biosecurity risk to other users of our Anopheles insectaries.
For other reasons why Daphnia is a good model, and for further info in the Daphnia genome project, click here.
For further info on Daphnia parasites, click here and here.