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Dita B. Vizoso


Institute of Zoology
Evolutionary Biology
University of Basel
Vesalgassesse 1
4051 Basel

> the ebert group
> the schärer group
Daphnia magna (Crustacea)

A small (4-8 mm when adult) cladoceran that inhabits fresh-water bodies (from lakes to tiny rock-pools). It filters the water using its modified legs as a fan (left side on the picture) and feeds primarily on planktonic algae that are retained by this filtering apparatus. It reproduces both parthenogenetically (see the eggs in the brood chamber on the right side of the picture) and sexually. Sexual reproduction is believed to occur as a response to environmental conditions. Females produce parthenogenetic males that will fertilize the same or other female, leading to the production of a resting egg (ephippia) that can endure extreme environmental conditions, and hatch in 'better times'.

Uninfected, adult Daphnia magna female from Finnland

Hamiltosporidium tvaerminnensis
previously known as Octosporea bayeri (Microsporidea)

An intracellular, obligate parasite of D. magna that is able to infect its host horizontally (via spores, shown on the right) and vertically (from mother to offspring, both parthenogenic and sexual). The spores are released into the water after the host is dead, and remain viable under severe environmental conditions (freezing temperatures, draught, heat). After a spore receives a still unknown cue (indicating that it is inside a Daphnia), it germinates, uncoiling a microtubule that penetrates a host cell and through which the sporoplasm is transferred. When a Daphnia becomes infected, either horizontally or vertically, the infection spreads mainly to the fat cells surrounding the ovary. After some weeks, the parasite can occupy most of the body of its host, as shown in the picture below. (Vizoso, Lass & Ebert, 2005)
In the clones studied so far, vertical transmission to the parthenogenetic eggs occurs with 100% efficiency. Daphnia's sexual eggs also get infected, but with a lower efficiency! This phenomenon has consequences to both host and parasite. [more on sex]

Spores of H. tvaerminnensis, in phase-contrast, 400x

Two female D. magna: one infected with H. tvaerminnensis (on the left) and one uninfected (on the right).
The opaque, yellowish, mass spreading througout the body in the infected one is the accumulated spores of the parasite.
Notice the presence of eggs in both females.

Vizoso, D. B., Lass, S. & Ebert, D. 2005. Different mechanisms of transmission of the microsporidium Octosporea bayeri: a cocktail of solutions for the problem of parasite permanence. Parasitology. 130: 501-509. [pdf]

Daphnia links

A thorough description of D. magna can be found by clicking on the magna in the Daphnia taxonomy page of U.Guelph

Everything you always wanted to know about parasitism in Daphnia

Microsporidia links

The Handbook of Microsporidia

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last uptaded: 27.10.2007