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MICROMYCOPSIS OEDOGONII SP.NOV.

By MARJORIE ROBERTS

Birkbeck College, University ofLondon

(With Pla te 17 and 12 T ext-figures)

Micromycopsis oedogonii sp.nov. parasitizing Oedogonium sp. is descr ibed andillustra ted, and compa red with previously recorded species of Micromycopsis,all of which attack members of Conjugales.

Micromycopsis oedogonii sp.nov,Thallus in Oedogonii cellulis invenitur tumidis et in speciem oogonii globatis. Pro­

sorus plerumque globosus 14- 30,.,. dia. , nonnunquam autem pr aelongus (cum in adultacellula insedit), tunica glabra, hyalina. Sporangiosorus epibioticus, plerumque sessilis,glob osus, in 7-15 sporangia pyramidata divisus, Sori tu nica bru nea, aculeis minutisobtusis ornata. Zoospora subglobosa 2,.,. diam. , guttula oleosa un ica , uni co post icoflagello muni ta .

Hab. in Oedogonio, Bolde r Mere, Wisley, Surrey, Angliae.T ype mater ial dep osited a t Commonwealth Mycological In stitute, Kew, (Herb.

I.M.I. 50464.)

During the autumn of 1950 and the summer and autumn of 1951 Mi cro­mycopsts oedogonii was found in considerable abundanc e in collections ofOedogonium sp. from Bolder Mere, Wisley, Surrey, En gland. It seemed tobe strictly limited to a single host species. This grew intimately mixed withother members of Oedogoniales (two species of Bulbochaete and two ofOedogonium, including O. undulatum) but these were never attacked , norwere species of .?,Ygnema and M ougeotia which were some times in thecollections. As winter approached the host became scarce r and the riseof th e water level mad e collection difficult. The fate of the parasite duringlate winter and until its reappearance in the summer is unknown.

The fungus normally attacks the upper daughter cell formed by thepeculiar cell division in Oedogonium. This cell, which initially has a verythin wall, is caused to swell until it resembles an oogonium (T ext-fig. 2).Thalli of the parasite sometimes occur in unmodified cells, suggestingthat these have been attacked at a stage when the walls have becomerelatively rigid. The thalli in such cells are elongated having obviouslybeen forced into that shape by the narrow form of the host cell (T ext­fig. 3). Thus cells containing elongated thalli are of normal form but alwayshave thi ck walls, whilst in the hypertrophied host cells the wall is oftenquite thin, especially if the parasite is a t an early stage of development(Text-fig. 3) .

Before the parasite is seen, its presence can be inferred by the swellingof th e host cell and by a change in the optical density of the chloroplast.When it first becomes visible the th allus is naked , somewhat irregular andappears to consist almost entirely of oil drops (T ext-fig. 2). It is frequently

a

Text -figs. 1- E!. Micromycopsis oedogonii

Text-figs. 1- 4. Early stages. tl , very young thalli at ' ir regular ' stage ; b, thallus rounded offjust before the form at ion of the prosorus wa ll; c, young pro sorus. H ost chloroplast shownby coarser and more regu lar stippling .

T ext-figs. 5-8. Lat er stage s. ll, mature prosorus ; b, empty prosorus with young sorus ; c, soruswit h mature sporan gia.

Text-figs. 9 - 12. Text-fig. 9 , empty prosorus and remains of sorus wall. Text-fig . 10 , emptyprosorus and sorus sho wing polygonal pla tes of soru s wal l. Text-figs. II and 12, onespecimen with sorus in sur fa ce view and optical section .

M S 2 1

322 Transactions British Mycological Societyobscured by the remains of the chloroplast which at this stage is still green.The young thallus enlarges, becomes spherical and invested by a smooth,colourless wall whilst at the same time the oil drops cease to dominate thestructure of the protoplasm which becomes granular and greyish. Theparasite is now in the prosorous stage (Text-fig. 5), and the host chloro­plast has disintegrated to a greater or lesser extent, having lost most of itscolour. The mature prosorus contains protoplasm of a greyish browncolour, is surrounded by a smooth colourless cell wall and is normallyspherical. The size distribution of 60 spherical prosori encountered atrandom is shown in Table I.

Table I. Prosorus size rif Micromycopsis oedogonii

Size range in 1.£

> 14-16> 16-18> 18-20>20-22>22-24>24-26>26-28>28-30

Number

41010167841

Sometimes a double infection occurs and when this happens the twothalli may be at the same (Text-fig. 7) or different stages of development,within the same cell.

Early stages in the germination of the prosorus have not been observed,which is remarkable as so much living material has been examined. Itappears, however, that an exit tube pierces the host wall and swells outsideto form a spherical body which is sessile on the host (Text-fig. 5). Exittubes vary in length from 2 to 13fL. The external swelling becomes dividedinto 7- 15 sporangia to form the sporangiosorus. Each sporangium appearstriangular in optical section of the sorus (Text-fig. 12), but is reallypyramidal with a curved external wall polygonal in surface view (Text­fig. I I). Thus the wall of the sporangiosorus appears to consist of a numberof polygonal plates which are somewhat thickened, brownish andcovered with short blunt spines. The internal walls of the sorus, separatingthe sporangia are thin and smooth. At maturity the sporangia swellslightly causing the plates of the sorus wall to separate to some extent(Text-fig. 8). At this stage the sporangiosorus looks rather like a black­berry. Only in a few examples were fully delimited zoospores seen in thesporangia, but each seemed to contain about 50.

The liberation of zoospores was seen only once and it was not clear ifthe escape was through a pore in the thin part of the sporangium wall, asreported for other species. The zoospores were sub-spherical, about 2 fL indiameter, each with a conspicuous oil drop and a single posterior flagellum.They were zoids of the characteristic chytridian type. In this speciesthere appears to be only one kind of zoospore in the life cycle, not two asreported by Canter (1949) in Micromycopsis fischeri and as suggested byScherffel (1926) in M. cristata. Infection of host filaments with zoosporeswas not observed.

Trans.

Brit.

Myc.

Soc.V

ol.36.

Plate17

Micromycopsis oedogonii. M. Roberts 323

Since M. oedogonii can apparently be found only for a few months eachyear, there is the problem of its survival during the rest of the year. Nospecialized resting spores were seen, but it is likely that certain prosorifunction as such. This is in conformity with what is known of other aquaticmembers of Synchytriaceae.

Micromycopsis oedogonii is distinguished sharply from other known speciesof aquatic Synchytriaceae in parasitizing Oedogonium sp. Previous records(Table 2) name desmids or filamentous Conjugales as hosts. Morpho­logically Micromycopsis oedogonii resembles M. intermedia most closely, butdiffers in having a larger number of sporangia in the sorus, and in havinga prosorus wall that is colourless and never spiny. It differs from M.fischeri in having only one type of zoospore, the typical chytrid zoidliberated directly from the sporangia of the sorus, and from M. cristatain its pyramidal sporangia.

Table 2. List of aquatic Synchytriaceae with recorded hosts based oninformation in Sparrow (1943) and Canter (1949)

Species

Endodesmidium formosumMicromycesqgogonii

AI. peterseniiM. longispinus1\1. laecisMicromycopsis crtstata

M, fischeriM, zygnemicolaM, interrnediaM. rnirabilis

Host

Netriurn oblongum, Cylindrocystis spp.<ygogoniurn sp., Spirogyra quadrata,

Mesacarpus scalaris, Netrium sp.,Mougeoiia spp.

Mougeotia sp.Spirogyra sp.Mougeotia sp.Hyalotheca spp., Spirogyra sp.,

<ygogoniurn sp.<ygogonium sp., Tetrnernorus brebissonii<ygnema stellina<ygnema spp.Closterium spp.

My thanks are due to Prot: D. Jones for help with the Latin diagnosisand especially to Professor C. T. Ingold under whose direction thisinvestigation was carried out.

REFERENCES

CANTER, H. (1949). Studies on British Chytrids. VI. Aquatic Synchytriaceae. Trans.Brit. mycol. Soc. 32, 69-94.

SCHERFFEL, A. (1926). Einiges uber neue oder ungenligend bekannte Chytridineen.Arch. Protistenk, 54, 167-260.

SPARROW, F. K. (1943). Aquatic Phycomycetes, Ann Arbor, C.S.A.

EXPLA:\,ATlON OF PLATE 17iUicromycopsis oedogonu

Fig. J. Naked thallus inside hypertrophied Oedogonium cell. Chloroplast still large and green.x 860.

Fig. 2. Mature prosorus. Chloroplast of swollen host cell largely disintegrated. x 970.Fig. 3. Two infected cells of an Oedogonium filament. Upper cell not hypertrophied with an

elongated prosorus in its upper end. Lower cell hypertrophied with an oospore-likeprosorus. x 720.

Fig. 4. Swollen host cell containing empty prosorus partially collapsed. Outside is the sporan­giosorus seen in optical section showing the individual triangular sporangia. x 690.

iAccepted for publication 2 October 1952)21-2