Elsevier

Protist

Volume 159, Issue 2, 23 April 2008, Pages 299-318
Protist

ORIGINAL PAPER
The Development, Ultrastructural Cytology, and Molecular Phylogeny of the Basal Oomycete Eurychasma dicksonii, Infecting the Filamentous Phaeophyte Algae Ectocarpus siliculosus and Pylaiella littoralis

https://doi.org/10.1016/j.protis.2007.11.004Get rights and content

The morphological development, ultrastructural cytology, and molecular phylogeny of Eurychasma dicksonii, a holocarpic oomycete endoparasite of phaeophyte algae, were investigated in laboratory cultures. Infection of the host algae by E. dicksonii is initiated by an adhesorium-like infection apparatus. First non-walled, the parasite cell developed a cell wall and numerous large vacuoles once it had almost completely filled the infected host cell (foamy stage). Large-scale cytoplasmic changes led to the differentiation of a sporangium with peripheral primary cysts. Secondary zoospores appeared to be liberated from the primary cysts in the internal space left after the peripheral spores differentiated. These zoospores contained two phases of peripheral vesicles, most likely homologous to the dorsal encystment vesicles and K-bodies observed in other oomycetes. Following zoospore liberation the walls of the empty cyst were left behind, forming the so-called net sporangium, a distinctive morphological feature of this genus. The morphological and ultrastructural features of Eurychasma were discussed in relation to similarities with other oomycetes. Both SSU rRNA and COII trees pointed to a basal position of Eurychasma among the Oomycetes. The cox2 sequences also revealed that the UGA codon encoded tryptophan, constituting the first report of stop codon reassignment in an oomycete mitochondrion.

Introduction

Eurychasma dicksonii is a unicellular, marine endoparasitic oomycete parasitizing brown seaweeds. It has been observed in the field by numerous investigators (e.g. Aleem, 1950a, Aleem, 1950b, Aleem, 1955; Jenneborg 1977; Konno and Tanaka 1988; Magnus 1905; Petersen 1905; Rattray 1885; Sparrow, 1934, Sparrow, 1960, Sparrow, 1969; Wright 1879), and it is one of the most widely reported heterotrophic parasites of brown seaweeds (Küpper and Müller 1999; Sparrow 1960). It infects a wide range of brown algae in the field (Jenneborg 1977; Sparrow 1960). Recently, laboratory-controlled inoculations with a Eurychasma culture demonstrated its ability to infect at least 45 species representative of all orders of brown algae (Müller et al. 1999). A SSU rRNA-based molecular phylogeny study showed that this parasite branched at the most basal position in the class Oomycetes, indicating the importance of this parasite in elucidating the phylogenetic origin and evolutionary development of this economically important lineage (Küpper et al. 2006).

However, in spite of its ecological and phylogenetic importance, no ultrastructural studies have been carried out on this organism so far. Sporangium development in Eurychasma has been described (Aleem 1950b; Petersen 1905; Sparrow, 1934, Sparrow, 1960) but there is still some uncertainty as to whether this organism is diplanetic. Some studies report that primary zoospores form internally within the sporangium before encysting on the inner sporangium wall to form the distinctive net sporangium (Sparrow, 1934, Sparrow, 1960). Secondary zoospores are then released internally into the sporangium cavity before escaping via the broad apical discharge tube.

In this study, the molecular phylogenetic position of two Eurychasma strains available in culture was investigated using the SSU rRNA gene and mitochondrial cox2 gene (as a translated COII amino acid sequence) as described recently in other marine oomycete genera (Sekimoto et al., 2007, Sekimoto et al., 2008). Continuous observations, made on individual parasite thalli under the inverted light microscope, were carried out in order to elucidate the time course and stages of thallus development. Samples were fixed for transmission electron microscopy so that spore differentiation could be compared with that described in other oomycete species. A general objective of this study was to obtain better insight into the developmental and cytological features of lower oomycetes.

Section snippets

Continuous Chamber Culture Observations (Fig. 1)

The typical time course of thallus/sporangial development of Eurychasma (strain Eury05, CCAP 4018/1) in Ectocarpus cell is summarized in Figure 1A–E. Documentation of Eurychasma development was started at the small, non-walled immature thallus stage (Figure 1, Figure 2). We estimate the duration of the whole developmental cycle of Eurychasma to be about 13–16 days. The time from the initial attachment of spores onto the host filaments (Fig. 2A) to the formation of the young non-walled thallus

Significance of the Observed Developmental Patterns of Eurychasma dicksonii (Fig. 10)

In this study we recorded the development of two isolates of E. dicksonii in dual cultures with Ectocarpus and Pylaiella and expanded on our light microscopic results with ultrastructural observations. A summary diagram of these observations is given in Figure 10. A similar overall pattern of thallus development was observed on both hosts (as well as other brown algal species, unpublished observations of Gachon, Müller and Küpper 2007), which suggests that this is the normal pattern of

Methods

Biological materials: The cultures of Eurychasma dicksonii (E. P. Wright) Magnus Eury96 (CCAP 4018/2) and its host Pylaiella littoralis (Linnaeus) Kjellman Pyl IRg (CCAP 1330/3) were established as described previously (Küpper et al. 2006; Müller et al. 1999). A Eurychasma-infected Ectocarpus siliculosus (Dillwyn) Lyngbye individual was collected at Le Caro (Brittany, France) in April 2005. The Eury05 strain (CCAP 4018/1) was isolated from this material, and continuously maintained in its

Acknowledgements

We are grateful to Dr. Kath White, Vivian Thompson, and Tracy Davey (Electron Microscopy Research Services at University of Newcastle upon Tyne, UK) for their support in maintaining the electron microscope facilities. SS gratefully acknowledges a post-graduate award from British Mycological Society to enable him to visit and carry out the part of this work in the UK. GWB gratefully acknowledges funding from The Daiwa Anglo-Japanese Foundation (Daiwa Foundation Small Grant: 6490/6788). FCK

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