28 



THE SIMPLE HOLOCARPIC BIFLAGELLATE PHYCOMYCETES 



APHANOMYCOPSIS 



Scherffel, 1925. Arch. Protistk. 52: 11. 



(plate 8) 



Thallus intramatrical. holocarpic, filamentous 

 and thread-like or irregularly tubular, simple or 

 branched, continuous or septate; walls showing a 

 marked cellulose reaction when tested with chloro- 

 iodide of zinc; whole thallus or segments trans- 

 formed directly into sporangia or oogonia (?). Zoo- 

 sporangia elongate, filamentous and cylindrical or 

 short and tubular with one to several straight, curved 

 or irregular exit tubes which project considerably 

 beyond the surface of the host. Zoospores diplanetic, 

 emerging in succession without flagella and encyst- 

 ing in a cluster at the mouth of the exit tube ; sec- 

 ondary zoospores grape seed-shaped with a ventral 

 furrow and conspicuous vacuole, isocont ( ?) or 

 heterocont (?), active shorter flagellum directed 

 forward in swimming with the longer one dragging 

 behind; swimming movement relatively slow and 

 even, not darting. Oogonia questionable. Resting 

 spores or oospores (?) single or several in thallus 

 or segments thereof; parthenogenetic (?), germina- 

 tion unknown. 



Scherffel regarded this genus as closely related 

 to Ectrogella monostoma and included it in the 

 Saprolegniaceae near Aphanomyces because of its 

 long thread-like zoosporangium and the formation 

 of zoospores in single linear rows. He furthermore 

 believed that the thalli of Aphanomycopsis with 

 their incipient resting spores should be interpreted 

 as rudimentary oogonia containing one or several 

 egg cells which develop asexually or partheno- 

 genetically, a viewpoint which was subsequently ac- 

 cepted by Sparrow ('33) and Coker and Matthews 

 ('37). The latter workers, however, included 

 Aphanomycopsis in the family Ectrogellaceae, and 

 later on Sparrow ('-12) followed this classification. 

 Tokunaga ('33) on the other hand, placed it in the 

 Lagenidiaceae. Scherffel pointed out that this genus 

 differs from Aphanomyces by the lack of well differ- 

 entiated zoosporangia and oogonia, but Tokunaga 

 found the former structures to be strikingly like 

 those of Lagenidium, as is shown in figures 15, 19, 

 and 25. The segments which contain the resting 

 spores are also similar in shape to the oogonia of 

 some species of Lagenidium. The fact that the zoo- 

 spores are diplanetic, encyst in clusters at the orifice 

 of the exit tubes, and are heterocont after emerging 

 from the cysts does not exclude this genus from the 

 Lagenidiaceae since such characters are also to be 

 found in L. Oedogonii and Lagenidium sp., Couch. 

 Likewise, several species of the latter genus are 

 parthenogenetic, but each so-called oogonium forms 

 but one spore. On the bases of Tokunaga's studies, 

 Aphanomycopsis is herewith included only tempo- 

 rarily in the Ectrogellaceae. It is not altogether im- 

 probable, however, that Tokunaga's fungus may be 

 different from Scherffel's and Sparrow's species. 



Further studies are therefore essential to a better 

 understanding of the identity and relationships of 

 A phano m ycops is . 



Scherffel reported that the zoospores which 

 emerge from the cysts are heterocont (fig. 2), but 

 Tokunaga described and figured them as isocont 

 (fig. 16) without indicating any specific flagellum 

 orientation during motility. After coming to rest on 

 the host the zoospores form a broad germ tube which 

 makes a small round hole in the silicified wall and 

 penetrates the diatom cell (figs. 4, 5). The penetra- 

 tion tube continues to elongate, broadens, branches, 

 and eventually develops into a full grown thallus, 

 while the zoospore case and extramatrical portion of 

 the tube remains attached on the outside (fig. 6). 

 Scherffel and Sparrow reported only elongate, sim- 

 ple (fig. 13) or branched (fig. 17) non-septate thalli, 

 but West and West and Tokunaga found that the 

 thallus may be divided into segments by one to sev- 

 eral septa (figs. 7, 15, 19, 25) at maturity. 



One or more exit tubes are formed as the thallus 

 develops, but they do not perforate the diatom cell 

 like the penetration tube, according to Scherffel. In- 

 stead, they pass out between the valves of the host 

 when young and then develop thickened walls at the 

 region of exit, whereby the valves of the diatom are 

 pushed apart (figs. 13, 18). This so-called "Spreiz- 

 apparat" was observed by Scherffel and Sparrow, 

 but Tokunaga did not illustrate it in his figures of 



plate 8 



Aphanomycopsis bacillariacearum 



(Figs. 1-6, 8-12, 14, 17, 18, 20-24 after Scherffel, '25; 

 fig-. 7 after West, '09; fig. 13 after Sparrow, '33; figs. 

 15, 16, 19, 25 after Tokunaga, '33.) 



Fig. 1. Zoospore emerging from cyst. 



Fig. 2. Laterally biflagellate heterocont zoospore. 



Fig. 3. Optical cross section of same showing ventral 

 groove and vacuole. 



Figs. 4, 5. Infection of host. 



Fig. 6. Empty zoospore case and infection tube on dia- 

 tom cell. 



Fig. 7. Branched septate thallus with zoospores clus- 

 tered at tip of exit tube. 



Figs. 8-11. Stages in cleavage and zoospore formation 

 in exit canal. 



Fig. 12. Branched exit tube with zoospores which failed 

 to emerge from main branch. 



Fig. 13. Two thalli with clusters of discharged zoo- 

 spores, greatly enlarged. 



Fig. 14. A thallus with three exit tubes. 



Fig. 15. Branched septate thallus with several exit 

 tubes. 



Fig. 16. Bean-shaped isocont zoospores. 



Figs. 17-19. Empty thalli. Note thickened basal portion 

 of exit tube, the so-called "spreading apparatus." 



Fig. 20. Thallus with three young oospores (?), two of 

 which occur in pairs. 



Figs. 21-23. Developmental phases of the latter two 

 oospores or resting spores. 



Fig. 24. Large oval resting spore. 



Fig. 25. Two branched septate thalli with oospores in 

 two expanded cells. 



