Ordospora colligata

Ordospora colligata is an intracellular parasite belonging to the Microsporidia. It is an obligatory gut parasite with the crustacean Daphnia magna as its only host. So far it has been reported from Europe and Asia. The life cycle of O. colligata consists of two different stages, the merogonial stage with immature spores and the sporogonial stage with mature spores. Ordospora colligata is transmitted horizontally from living hosts. Vertical transmission has not been observed. The spores are released with the faeces and float in the water until a new host ingests them while filter feeding. With this direct waterborne transmission the parasite easily infects other hosts, therefore, the prevalence is typically high and can reach up to 100% in natural populations. When female D. magna are infected with O. colligata, their reproductive success is about 20% lower, and the normal lifespan of 60 days is shortened to about 50 days. In comparison with other endoparasites of Daphnia, O. colligata is rather avirulent and does not drive infected host populations to extinction. The infections are difficult to detect due to the lack of external signs. The complete development of this intracellular parasite takes place in the gut epithelium. Therefore, the host needs to be dissected and its gut epithelium examined under a phase contrast microscope, to confirm infection. A magnification of 200-400x is necessary for visualizing the spores. O. colligata lives in epithelial gut cells in a large parasitophorous vacuole, which is formed by the host at the time of infection. The host nucleus is shifted to a more lateral position. The vacuoles are filled with mature spores in the centre and immature presporal stages at the periphery. When these epithelium cells with parasitophorous vacuoles are shed in the gut, the parasite spores leave the host together with the faeces. All life cycle stages of the parasite have isolated nuclei and the merogonial (immature) and sporogonial (mature) stages occur together in the vacuole. The immature presporal stages, which are called merozoites, have a round or ovoid shape and measure 2.8 µm in diameter. During merogony, small plasmodia with four nuclei are produced and the shape changes to an elongated form with 1.6 µm width. The fact that merogonial and sporogonial stages co-occur suggests the existence of repeated merogonial divisions. The last division of merozoites results in sporonts, but meiosis has not been observed. In comparison to the merozoites the sporonts have a less dense cytoplasm. Electron-dense secretions create a thick layer on the surface and tubular material (63-65 nm in diameter) is produced. The earliest sporonts are elongated oval cells with two nuclei and these sporogonial stages occur often pairwise. This pairwise structure leads to the interpretation of two sporoblast mother cells, which have failed to separate during the first division of the sporont. After another nuclear division, each sporoblast mother cell turns into a chain of four sporoblasts. Since the separation of the initial division has failed to complete, the two chains are still connected, leading to chains of eight mature spores. These eight spores are not only externally linked but also via cytoplasmic bridges.