Handbook Part 2 - International Mycological Association

S36IS3 - 0473
The Viral impact on thraustochytrids
Yoshitake Takao 1, Yuji Tomaru1, Yukari Sasakura 2, Keisuke Yamane 2, Keizo Nagasaki 1, Daiske Honda 2
1 National Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, Japan, 2-17-5
Maruishi, Hatsukaichi, Hiroshima 739-0452, Japan, 2 Konan University, 8-9-1 Okamoto,Higashinada, Japan
Thraustochytrids are cosmopolitan stramenopile “fungi”. They are distributed in saline lakes, marine, estuarine and
deep sea waters. The biovolume of thraustochytrids in coastal waters could reach ~43 % of the bacterial biovolume.
Their wide distribution and high abundance are of much ecological interest. In addition, thraustochytrids are known
to produce large amount of polyunsaturated fatty acids such as docosahexaenoic acid and docosapentaenoic
acid, which are considered important as food resources for higher organisms in marine systems. Because of these
distinctive features, the ecological importance of thraustochytrids in the coastal ecosystems has recently been
recognized; however, either their natural dynamics or ecological roles in situ have scarcely been understood. Viruses
and virus-like particles (VLPs) are the most abundant bioactive agents in marine environments; now, viral infection is
recognized as one of the important factors in controlling the biomass and clonal composition of bacterial and algal
populations. While, there are few reports concerning viruses infecting heterotrophic protists. In this study, we measured
the dynamics of viruses by MPN assays using 12 thraustochytrid stains as hosts, established and characterized viral
strains isolated from the western coast of Japan to discuss the ecological relationships between thraustochytrids and
their infectious viruses.
Based on the characteristics of viral strains isolated through the survey, we revealed they are most likely divided into
two groups: one is a small icosahedral single-stranded RNA virus (Schizochytrium single-stranded RNA virus: SssRNAV
[ø25nm]); the other is a large roundish (but not icosahedral) double-stranded DNA virus (Thraustochytrids DNA virus:
ThDNAV [ø140nm]). These two virus groups showed significantly different dynamics through the field survey conducted
in Hiroshima Bay, Japan in 2004-2005: SssRNAV showed a temporary increase in abundance following H. akashiwo
blooms; in contrast, ThDNAV remained at a relatively low concentration showing no drastic changes in abundance
through the survey. Considering the dynamics of each virus group should reflect the changes in abundance of its host,
there are at least two thraustochytrid groups coexisting in Hiroshima Bay that are ecologically different showing
dissimilar fluctuation patterns; one that utilizes on dying and dead algal cells and the other mainly functioning as a
decomposer for organic matters of land origin. It may be that the two host groups are dominant in the coastal
environments, and each of them is affected by a distinct type of virus; i.e., either SssRNAV or ThDNAV.
S36IS4 - 0284
The diversity of oomycete pathogens of nematodes and its implications to our understanding of oomycete
phylogeny.
G.W. Beakes, S.L. Glockling
1 University of Newcastle upon Tyne, Newcastle upon Tyne, United Kingdom, 2 University of Susex, Brighton, United
Kingdom
Most of the biflagellate holocarpic parasites of nematodes were traditionally classified in the Lagenidiales. However,
in his recent revision of oomycete systematics, Dick transferred most to a new order, the Myzocytiopsidales which also
encompassed a number of holocarpic marine parasites of crustacea. We have recently described the morphological
and ultrastructural development of a number of these holocarpic parasites of nematodes encompassing the genera,
Myzocytiopsis, Chlamydomyzium and Haptoglossa. We are also trying to use molecular markers to place these
organisms within context of the oomcyete phylogenetic tree, although it has proved a challenge to obtain such data
for these obligate parasites.
These apparently similar holocarpic organisms show varied types of zoosporogenesis, with some species showing
dictyuchoid and achlyoid aplanospore discharge, whilst others have either intra- and extra sporangial differentiation
of zoospores. Combined with significant differences in fine-structure between species, the evidence points to the
conclusion that the Myzocytiopsidales cannot be considered a natural assemblage. Families and species within this
order will undoubtedly need to be assigned elsewhere. It is also apparent that the systematics of these holocarpic
parasites needs to be based on both molecular and ultrastructural characters and that light microscopic morphology
alone is insufficient.
It does seem likely that these holocarpic parasites will prove pivotal to our understanding of the evolution of the
oomycetes. Mycelial saprotrophic or biotrophic groups within the Saprolegniaceae and Peronosporaceae probably
evolved from such holocarpic parasitic ancestors.
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