An ultrastructural analysis of organelle arrangement during gun (infection) cell differentiation in the nematode parasite Haptoglossa dickii

Infection of C. elegans by Haptoglossa Species Reveals Shared Features in the Host Response to Oomycete Detection

Oomycetes are a group of eukaryotic organisms that includes many important pathogens of animals and plants. Within this group, the Haptoglossa genus is characterised by the presence of specialised gun cells carrying a harpoon-like infection apparatus. While several Haptoglossa pathogens have been morphologically described, there are currently no host systems developed to study the infection process or host responses in the lab. In this study, we report that Haptoglossa species are potent natural pathogens of Caenorhabditis nematodes. Using electron microscopy, we characterise the infection process in C. elegans and demonstrate that the oomycete causes excessive tissue degradation upon entry in the body cavity, whilst leaving the host cuticle intact. We also report that the host transcriptional response to Haptoglossa infection shares similarities with the response against the oomycete Myzocytiopsis humicola, a key example of which is the induction of chitinase-like (chil) genes in the hypodermis. We demonstrate that this shared feature of the host response can be mounted by pathogen detection without any infection, as previously shown for M. humicola. These results highlight similarities in the nematode immune response to natural infection by phylogenetically distinct oomycetes.

Attachment, penetration and early host defense mechanisms during the infection of filamentous brown algae by Eurychasma dicksonii

Eurychasma dicksonii is one of the most common and widespread marine pathogens and attacks a broad spectrum of more than 45 brown algal species. The present study focuses on the mechanism used by the pathogen to attach on the host cell wall and force its way into algal cells. Ultrastructural examination revealed a needle-like structure which develops within the attached spore and extends along its main axis. Particular cell wall modifications are present at the basal part of the spore (adhesorium pad) and guide the needle-like tool to penetrate perpendicularly the host cell wall. The unique injection mechanism is shared with Haptoglossa species which suggests that this is an important characteristic of early diverging oomycetes. Furthermore, the encystment and adhesion mechanism of E. dicksonii shows significant similarities with other oomycetes, some of which are plant pathogens. Staining and immunolabelling techniques showed the deposition of β-1,3-glucans on the host cell wall at the pathogen penetration site, a strategy similar to physical responses previously described only in infected plant cells. It is assumed that the host defense in terms of callose-like deposition is an ancient response to infection.

A new oomycete species parasitic in nematodes, Chlamydomyzium dictyuchoides sp. nov.: developmental biology and phylogenetic studies

The genus Chlamydomyzium is a little studied holocarpic oomycete parasite of nematodes of uncertain phylogenetic and taxonomic position. A new holocarpic species, Chlamydomyzium dictyuchoides, is described which has usually refractile cytoplasm and a dictyuchoid pattern of spore release. This new species infects bacteriotrophic rhabditid nematodes and was isolated from diverse geographical locations. Infection was initiated by zoospore encystment on the host surface and direct penetration of the cuticle. A sparsely branched, constricted, refractile thallus was formed which eventually occupied almost the entire host body cavity, often accompanied by complete dissolution of the host cuticle. Walled primary cysts formed throughout the thallus and each cyst released a single zoospore via an individual exit papillum, leaving a characteristic dictyuchoid wall net behind. At later stages of infection some thalli formed thick-walled stellate resting spores in uniseriate rows. Resting spore formation appeared to be parthenogenetic and was not accompanied by the formation of antheridial compartments. These spores had ooplast-like vacuoles and thick multi-layered walls, both of which suggest they were oospores. The maximum likelihood tree of sequences of the small ribosomal subunit (SSU) gene placed this new isolate in a clade before the main saprolegnialean and peronosporalean lines diverge. A second undescribed Chlamydomyzium sp., which has direct spore release forms a paraphyletic clade, close to C. dictyuchoides and Sapromyces. The fine structure of other documented Chlamydomyzium species was compared, including an undescribed (but sequenced) isolate, SL02, from Japan, Chlamydomyzium anomalum and Chlamydomyzium oviparasiticum. Chlamydomyzium as currently constituted is a paraphyletic genus that is part of a group of phylogenetically problematic early diverging clades that lie close to both the Leptomitales and Rhipidiales.

The evolutionary phylogeny of the oomycete "fungi"

Molecular sequencing has helped resolve the phylogenetic relationships amongst the diverse groups of algal, fungal-like and protist organisms that constitute the Chromalveolate "superkingdom" clade. It is thought that the whole clade evolved from a photosynthetic ancestor and that there have been at least three independent plastid losses during their evolutionary history. The fungal-like oomycetes and hyphochytrids, together with the marine flagellates Pirsonia and Developayella, form part of the clade defined by Cavalier-Smith and Chao (2006) as the phylum "Pseudofungi", which is a sister to the photosynthetic chromistan algae (phylum Ochrophyta). Within the oomycetes, a number of predominantly marine holocarpic genera appear to diverge before the main "saprolegnian" and "peronosporalean" lines, into which all oomycetes had been traditionally placed. It is now clear that oomycetes have their evolutionary roots in the sea. The earliest diverging oomycete genera so far documented, Eurychasma and Haptoglossa, are both obligate parasites that show a high degree of complexity and sophistication in their host parasite interactions and infection structures. Key morphological and cytological features of the oomycetes will be reviewed in the context of our revised understanding of their likely phylogeny. Recent genomic studies have revealed a number of intriguing similarities in host-pathogen interactions between the oomycetes with their distant apicocomplexan cousins. Therefore, the earlier view that oomycetes evolved from the largely saprotrophic "saprolegnian line" is not supported and current evidence shows these organisms evolved from simple holocarpic marine parasites. Both the hyphal-like pattern of growth and the acquisition of oogamous sexual reproduction probably developed largely after the migration of these organisms from the sea to land.

Ultrastructural morphogenesis of dimorphic arcuate infection (gun) cells of Haptoglossa erumpens an obligate parasite of Bunonema nematodes

Haptoglossa is a genus of biflagellate organisms that has been placed in the oomycetes and is characterised by producing unique infective gun cells, which usually infect by physically rupturing the nematode cuticle. Haptoglossa erumpens is a parasite of Bunonema nematodes that produces arcuate infection cells and aplanospores that are discharged following the swelling and rupture of the thallus wall and distended host cuticle. Recent isolations of H. erumpens have revealed that the germinating aplanospores develop into two similar-sized but morphologically distinct infection cells. The uni-nucleate, convexly arcuate, gun cells were observed to fire in response to host nematodes, producing a cylindrical sporidium inside the host body. These gun cells had an apical missile chamber containing a needle with a unique arrangement of investing cones. Unlike previously described gun cells, the tube tail did not wind around the nucleus but continued into the basal vacuole where it terminated. The second type of infection cell was a concavely arcuate, bi-nucleate, cell that had an unusually large and elongate annulus component in the missile chamber. These modified bi-nucleate gun cells were never observed to fire in response to contact with Bunomena nematodes. The patterns of morphological and structural variations in these infection structures in this genus are reviewed in the light of these findings.