Affiliation of the parasite Herpodiscus durvillaeae (Phaeophyceae) with the Sphacelariales based on DNA sequence comparisons and morphological observations

Figures & data

Fig. 1. Phylogenetic tree of the Phaeophyceae based on the maximum likelihood analysis of combined partial rbcL and 26S data (GTR+I+G model, –ln likelihood=14013.8779). Numbers above lines indicate bootstrap values (left: ML; right: MP), numbers below Bayesian posterior probabilities. Dashes indicate that branches received a bootstrap support of 50% or less, or less than 70% Bayesian probability. The scale bar indicates substitutions per site.

Fig. 2. Phylogenetic tree (ML) for basal Phaeophyceae inferred from partial rbcL sequences (GTR+I+G model, –ln likelihood=−8376.330601). Numbers above lines indicate bootstrap values (left: ML; right: MP), numbers below Bayesian posterior probabilities. Dashes indicate that branches received a bootstrap support of 50% or less, or less than 70% Bayesian probability. The scale bar indicates substitutions per site.

Figs. 3–9. Morphology of the parasitic of Herpodiscus durvillaeae. Fig. 3. Population of the host, Durvillaea antarctica, showing healthy fronds without signs of infection with H. durvillaeae. St. Kilda, 20km north of Brighton, Otago, New Zealand, March 1997. Fig. 4. Phylloids of the host (D. antarctica) with extensive red-brown patches of external filaments of H. durvillaeae. Brighton, August 1997. Fig. 5. Transverse section through external filaments of fresh H. durvillaeae Brighton, August 1997. Filaments above the host surface (arrowhead) show a row of unilocular sporangia (arrow), the tips of the filaments covered with pennate diatoms. Figs 6, 7. Transmission electron microscopy of sections of host and parasite. Fig. 6. Longitudinal section through the meristoderm at the end of the host's growth season showing two apical cells of H. durvillaeae (p) among host cells at the surface. Parasite cell contents appear concentrated at their apices. Shrinkage of the host cells and the loss of most of their physodes (holes indicated by arrows) are preparation artefacts. Fig. 7. Longitudinal section through the base of a parasite patch. Filaments of H. durvillaeae showing acroblastic branching (arrow). Figs 8, 9. Reaction with Eau de Javel. Fig. 8. Transitory blackening of cell walls of H. durvillaeae immediately after treatment (arrow). Gametangia-turned gametophytes have coloured internal walls (arrowhead). Dried material, scraped from the surface of a parasite patch. (WELT A027896). Fig. 9. Reaction of external terminal cells of fresh H. durvillaeae with Eau de Javel as front moves through the tips of the external filaments. Some terminal cells are completely bleached (double arrowhead), while others display blue bands of colouring just below their apical hemisphere (arrows). Terminal cells have swollen tips and contain large nuclei (arrowheads).

Table 1. Comparison of the morphology of Herpodiscus durvillaeae and of members of families of the Sphacelariales s.s. currently recognized (Prud'homme van Reine, 1982, 1993; Womersley, 1987; Peters, 1990; D.G. Müller, personal communication; present study).

	Stypocaulaceae	Cladostephaceae	Sphacelariaceae
Taxon	Alethocladus corymbosus, Halopteris sp., Phloiocaulon sp., Ptilopogon botrycladus, Stypocaulon sp.	Cladostephus spongiosus	Sphacelaria sp.	Sphacella subtilissima	Herpodiscus durvillaeae
Growth form	Leptocaulous or auxocaulous	Auxocaulous	Leptocaulous	Leptocaulous	Possibly leptocaulous
Formation of laterals	Mostly acroblastic, irregularly, radial or distichous	Main axis: hypacroblastic, laterals: acroblastic	Hypacroblastic, irregularly, radial or distichous	Hypacroblastic, irregularly	Irregular, rarely acroblastic (branching only present in the internal phase)
Subdivision of subapical cells	Present	Present	Present	Not regularly present	Present (possibly not regularly)
Secondary growth of segments	Present	Present	Absent	Absent	ND (Possibly only enlargement in cell length)
Longitudinal walls	Present	Present	Present	Absent (rarely present)	Absent
Thallus structure	Polystichous	Polystichous	Polystichous	Haplostichous	Haplostichous
Cortication	Present (heavily corticated)	Present (heavily corticated)	Only basal rhizoidal	Absent	Absent
Propagules	Absent	Absent	Present, with 2–4 lobes, or absent	Absent	Absent
Phaeophycean hairs	In axillary clusters or absent	Present	Present	Absent	Absent
Life history	Isomorphic	Isomorphic	Isomorphic or slightly heteromorphic	ND	Heteromorphic with very reduced gametophyte
Reproduction	Aniso- or oogamous	Isogamous	Iso- or anisogamous	ND	Isogamous
Position of unilocular sporangia	Axillary	Lateral	Lateral	Terminal or lateral	Terminal
Position of plurilocular sporangia	Axillary or N/A	Lateral	Lateral	Lateral	N/A (gametophyte completely turns into a four-celled gametangium)
Natural habitat	Usually epilithic	Epilithic (rarely epiphytic)	Epilithic, epiphytic, epi-endophytic	Obligate epi-endophytic	Obligate epi-endophytic
N/A: not applicable; ND: no data.

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