Early developmental stages of two actinosporeans, Raabeia and Aurantiactinomyxon (Myxozoa), as detected by light and electron microscopy

A new model for myxosporean (Myxozoa) development explains the endogenous budding phenomenon, the nature of cell within cell life stages and evolution of parasitism from a cnidarian ancestor

The phylum Myxozoa is composed of endoparasitic species that have predominately been recorded within aquatic vertebrates. The simple body form of a trophic cell containing other cells within it, as observed within these hosts, has provided few clues to relationships with other organisms. In addition, the placement of the group using molecular phylogenies has proved very difficult, although the majority of analyses now suggest that they are cnidarians. There have been relatively few studies of myxozoan stages within invertebrate hosts, even though these exhibit multicellular and sexual stages that may provide clues to myxozoan evolution. Therefore an ultrastructural examination of a myxozoan infection of a freshwater oligochaete was conducted, to reassess and formulate a model for myxozoan development in these hosts. This deemed that meiosis occurs within the oligochaete, but that fertilisation is not immediate. Rather, the resultant haploid germ cell (oocyte) is engulfed by a diploid sporogonic cell (nurse cell) to form a sporoplasm. It is this sporoplasm that infects the fish, resulting in the multicellular stages observed. Fertilisation occurs after the parasites leave the fish and enter the oligochaete host. The nurse cell/oocyte model explains previously conflicting evidence in the literature regarding myxosporean biology, and aligns phenomena considered distinctive to the Myxozoa, such as endogenous budding and cell within cell development, with processes recorded in cnidarians. Finally, the evolutionary origin of the Myxozoa as cnidarian parasites of ova is hypothesised.

Transmission of freshwater myxozoans during the asexual propagation of invertebrate hosts

The phylum Myxozoa contains over 1350 species almost all of which are considered to be obligate parasites of aquatic animals. The phylum is composed of two classes, the Myxosporea and the Malacosporea, species of which are important pathogens responsible for severe economic losses in cultured fisheries. The life cycles of freshwater Myxozoa are believed to involve horizontal, indirect transmission, involving an invertebrate (oligochaetes or bryozoans) and a vertebrate host (fish or amphibians). Here, we describe myxozoan propagation through the fragmentation of invertebrate hosts to form new infected individuals. The two hosts examined are an oligochaete Lumbriculus variegatus infected with an unidentified myxosporean (Triactinomyxon sp.) and the bryozoan Fredericella sultana infected with the malacosporean Tetracapsuloides bryosalmonae which causes proliferative kidney disease, a major constraint of the European rainbow trout industry. Such intra-clonal propagation is a novel form of vertical transmission that is likely to be widespread within the Myxozoa and could form an important method by which some of these parasites maintain and proliferate within the aquatic environment.

Early developmental stages of two actinosporeans, Raabeia and Aurantiactinomyxon (Myxozoa), as detected by light and electron microscopy

The development of actinosporeans in their oligochaete host proceeding pansporocyst formation is relatively well documented, however, phases preceding it are not as well known. The initial stages in the development of two actinosporeans, Raabeia type 1 of Oumouna et al. [Parasitol. Res. 2002] and Aurantiactinomyxon pavinsis (Ormières, 1968) Marquès [Languedoc, Universite des Sciences et Techniques, Dissertation, 1984] from schizogony to gametogony and sporogony are described. Both actinosporeans begin their development as multinucleate stages near the basal lamina of the oligochaete intestine. Proximal to these stages and between the host epithelium cells are uninucleate cells whose nuclei divide to produce binucleate cells. These divide mitotically to produce cells with four nuclei which then undergo plasmotomy to yield a tetracellular stage and the first phase in pansporocyst formation. From the uninucleate stage to the tetranucleate stage, the cell membrane of the parasite is associated closely via finger-like projections with the intestinal epithelial and glandular cells of the host.