Phagomyxida

Introduction

This is a little-studied group of marine micro-organisms that parasitize filamentous brown algae and diatoms. Both the Phagomyxida (Phagomyxa species) and Maullinia have previously been regarded as belonging to the Plasmodiophorida, a group of micro-organisms that parasitize cells of higher plants, algae or oomycetes. However, recent work suggests that, although they are closely related to the Plasmodiophorida, they should be classified separately from this group.

Characteristics

Both Phagomyxa and Maullinia produce zoospores (swimming spores) propelled by two flagella that are heterokont (of unequal length). These infect host cells then their nuclei divide to produce a multinucleate plasmodium. This becomes a sporangium as it cleaves into zoospores which are released to initiate further infections. In these respects, Phagomyxa and Maullinia are identical to the Plasmodiophorida. In Maullinia there is also evidence for an unusual and characteristic ‘cruciform’ type of nuclear division which is a characteristic feature of the Plasmodiophorida. Maullinia also use a similar mechanism to the Plasmodiophorida to penetrate the cell wall using a specialised “beak” or “stiletto” shaped penetration device that injects the cytoplasm into that of the host.

However, unlike the Plasmodiophorida, neither of these two groups appear to form resting spores or cystosori, so parts of the plasmodiophorid life cycle appear to be absent. Phagomyxa species phagocytose food particles and digest them in a specialised food vacuole; this does not occur in the Plasmodiophorida.

Few taxa have been studied. Maullinia ectocarpi is a parasite of several marine filamentous brown algae including Ectocarpus spp.(Ectocarpales, Phaeophyceae). Phagomyxa algarum is also a parasite of marine filamentous brown algae. Two other Phagomyxa species are parasites of diatoms; Phagomyxa bellerocheae parasitizes Bellerochea malleus and P. odontellae parasitizes Odontella sinensis.

Discussion of Phylogenetic Relationships

Both Maullinia and Phagomyxa are closely related to, but distinct from the Plasmodiophorida. Evidence for this comes from morphological and biological studies and from analysis of ribosomal DNA sequences. Current classification places the Plasmodiophorida and the Phagomyxida as separate orders within the Phytomyxea. Recent evidence from ribosomal DNA studies suggests that Maullinia should be grouped with the Phagomyxida rather than the Plasmodiophorida.

References

1. Bulman, S. R., Kühn, S. F., Marshall, J. W. & Schnepf, E. (2001). A phylogenetic analysis of the SSU rRNA from members of the Plasmodiophorida and Phagomyxida. Protist 152, 43-51.

2. Karling, J. S. (1944). Phagomyxa algarum n. gen. n. sp., an unusual parasite with plasmodiophoralean and proteomyxan characteristics. American Journal of Botany 31, 38-52.

3. Maier, I., Parodi, E., Westermeier, R. & Müller, D. G. (2000). Maullinia ectocarpii gen. et sp. nov. (Plasmodiophorea), an intracellular parasite in Ectocarpus siliculosus (Ectocarpales, Phaeophyceae) and other filamentous brown algae. Protist 151, 225-238.

4. Schnepf, E. (1994). A Phagomyxa-like endoparasite of centric marine diatom Bellerochea malleus: a plasmodiophoromycete. Botanica Acta 107, 374-382.

5. Schnepf, E., Kuhn, S. F. & Bulman, S. (2000). Phagomyxa bellerocheae sp. nov. and Phagomyxa odontellae sp. nov., Plasmodiophoromycetes feeding on marine diatoms. Helgoland Marine Research 54, 237-242.

6. Ward, E., Motteram, J., Kanyuka, K. & Adams, M. J. (2006). The use of PCR methods for Polymyxa graminis to study intraspecific variation, phylogeny and inoculum levels. In Proceedings of the Sixth Symposium of the International Working Group on Plant Viruses with Fungal Vectors (Rush, C. M., ed.), pp. 100-103. American Society of Sugar Beet Technologists, Denver, USA.