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Resting Spore Germination in Synchytrium.*)
By John S. K a r 1 i n g.
Department of Biological Sciences Purdue University, Lafayette,
Indiana, USA.
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With 1 Textfig.
The restin,g spores of Synchytrium, which may or may not be
of zygotic origin, are reported to function as sporangia, or sori or
prosori in germination, and such differences are recognized as basic
distinctions of the subgenera into which the genus is divided. Discovery of the method by which the resting spores germinate is, therefore, essential to the identification and proper classification of species.
Up to the present time it has been observed in less than a score of the
175 or more species which comprise the genus.
In recent studies on Synchytrium the author has collected resting
spores of several species with the objective of determining their
manner of germination. Report has already been made (1954, 1955 G)
of this process for «S. brownii and S. austräte, and the present contribution concerns germination in S. stellariae fS. chiltonii), S. gerann, S. edgertonii, S. cerastii and S. aureum. The resting spores
of these species were collected on their hosts during March to
June, 1954 in Texas and Louisiana and stored in envelopes
at room temperatures until February 1955. On February 5 pieces
of the host with the spores were crumbled as finely as
possible between the thumb and forefinger and allowed to fall
on a petri dish of piain agar. By this method numerous spores are
dislodged from the galls and lie free on the agar where their subsequent germination may be readily observed under the stereoscopic
microscope. The surface of the agar plate was allowed to dry until
the spores and host debris were firmly stuck to it, after which it
was flooded with a film of animal charcoal water. The dishes were
covered and left on the laboratory table at room temperatures.
On February 15, ten days after sowing, numerous spores had
germinated, and by February 18 sori, sporangia and zoospores were
present. In all species the resting spore functioned as a prosorus and
gave rise to a superficial, attached sorus (fig. 1). Inasmuch as the
process was basically similar in all species, it is illustrated in
only one species, S. gerann (fig. 1—5). The process of germination
*) This study has been supported by a grant from the National Science
Foundation.
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is fundamentally the same as described by the author (1955 C) for
S. australe and need not be described again in detail.
In all species many of the spores germinated within the galls,
and in some instances the sorus was pushed out through the apical
pore or opening of the gall. In the species on Solidago sp., which the
author is calling S. aureum because it forms only resting spores and
has previously been reported on this host, the sorus was occasionally
formed beneath the resting spore in the base of the gall. As it ex-
Fig. 1—5. Synchytrium geranii. Fig. 1. Empty resting spore with attached
sorus. Fig. 2. Variations in size of sporangia. Fig. 3. Discharge of zoospores. Fig. 4. Variations in uniflagellate zoospores. Fig. 5. Gracked wall
of sorus.
panded the empty spore was pushed out of the apical gall opening.
In all species the sorus was rather firmly attached to the empty
spore at the pore through which the protoplasm had emerged.
Gloser examination of this region showed that the pore had
been filled with a plug of tough, resistant material (fig. 1) like
that shown by K u s a n o (1930) for S. fulgens. The sori and sporangia
usually remains viable on the agar plate for a long time. Sporangia
which were three weeks old produced abundant zoospores when
placed in fresh animal charcoal water. This prolonged viability is
obviously advantageous to the fungus in the spring as the young
host plants are emerging from the soil and insures periodic infection.
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In the case of S. aureum the sporangia produced and emitted its
zoospores while still in the sorus, and eventually such partially open
sori had the appearance of a sphere filled with empty hyaline polyhedra. In this species mounted sporangia "germinated" and produced
zoospores periodically over a period of three days.
The wall of the empty sorus of 5. geranii was usually scored
or cracked in a definite pattern so that it appeared to be made up
of irregulär pentagons or hexagons in cross section. The borders of
these areas meet in 3s at angles of approximately 120°, as shown in
fig. 5. These areas may separate along the cracks and lie free and
isolated. The cause and significance of this Cracking is unknown,
but it is a frequent and striking occurrence.
Gareful attention and study has been given to the zoospores produced by the germinated zoospores to determine if they function as
gametes and fuse. Fusion was observed in only a few cases in
S. aureum after the zoospores had passed through the actively motile
stage. Both ,gametes were only sluggishly motile, and no difference
in motility was noted between the so-called male and f emale gametes.
Fusion occurred side by side with the flagella extending in opposite
directions. This arrangement of the flagella remained for some time
in the zygote, but eventually they become posterior. The zygote was
sluggish in movement for a short time and then came to rest completely. Fusion also occurred between uni- and biflagellate as well as
between two biflagellate cells.
In discussing the possible relationships and phylogeny of the
genus the author (1955 a) suggested that secondary sporangia and
zoospores might possibly be found in Synchytrium which would link
it more closely with Endodesmidium and Micromycopsis, the other
genera of the family Synchytriaceae in which such stages occur.
Therefore, careful observations of the zoospores were made to see
if they encysted, developed into sporangia, and produced minute
secondary zoospores, but no evidence of this was found.
A brief description of the sori, sporangia and zoospores produced
by the germinated resting spores of these species is presented as
follows:
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Synchytrium stellariae on Stellaria media.
Resting spores subspherical to spherical, 52—160 ^; sorus yellowish-orange, spherical 65—170 ^; sporangia 8—35 per sorus, highly
variable in size, predominantly polyhedral, 22—80 ^ in greatest diameter; zoospores oval to slightly elongate, 3.6—4^4.8^5.6 \i.
Synchytrium geranii on Geranium carolinianum.
Resting spores spherical 140—210 fj,, oval, 130—180^150—200 ji;
sorus yellowish-orange, spherical, 145—215 ^; sporangia up to more
than 200 per sorus, light-orange, predominantly polyhedral, 30—68 u
in greatest diameter: zoospores oval to slightly elongate, 3 #4—4.5 ^.
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Synchytrium aureum (?) on Solidago sp.
Resting spores spherical, 120—144 jx with an olive-drab colored
wall; sorus yellowish-orange, spherical, 132—150 u; sporangia up to
more than 100 per sorus, orange, small, predominantly polyhedral,
18—35 n in greatest diameter; zoospores oval to slightly elongate,
3—3.2^4—4.8 JA.
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Synchytrium edgertonii on Dichondra repens.
Resting spores oval to spherical, 120—192 ^, with a light amber
wall; sorus brilliantly yellow, spherical 140—210 ^; sporangia up to
245 per sorus, yellow, small, predominantly polyhedral, 16—34 jx in
greatest diameter; zoospores almost spherical, 3.2—4 ji.
Synchytrium cerastii on Cerastium viscosum.
Resting spores spherical, 30—80 \i, oval, 35—52^54—68 ^; sorus
yellow, spherical, 38—75 |x, oval, 40—60^65—80 ^ sporangia 10—50
per sorus, yellow, predominantly polyhedral, 20—58 ^ in greatest
diameter; zoospores oval to slightly elongate, 2.8—3.2^3.6—4.2 ^.
The discovery of resting spore germination in Synchytrium stellariae, S. geranii, S. edgertonii, S. cerastii and S. aureum make it
possible to classify these species with certainty. The author (1954,
1955 d, e) has already shown that the initial cell in the summer
sporangial cycle of the first four of these species functions as a
prosorus. The present discovery that their resting spores function
in the same manner shows that they belong in the subgenus Microsynchytrium, according to the author's (1953, 1955 b) interpretation of
this group. These four additional species brings the total to six terrestrial species which can now be definitely assigned to this subgenus. Also, the method of spore germination in the so-called 5.
aureum on goldenrod shows that it belongs to the subgenus Pycnochytrium.
Literature cited.
K a r l i n g , J. S., 1953. Mycromyces and Synchytrium. Mycologia 45: 276—
287.
— 1954 a. Synchytrium brownii, a new speoies with sexual reproduction.
Sydowia 8: 27—30.
— 1954 b. Possible relationships and phylogeny of Synchytrium. Bull.
Torrey Bot. Club 8 1 : 353—362.
— 1955 a. A key to the subgenera of Synchytrium. Proc. Indiana Acad.
Sei. 64: (In press).
— 1955 b. Resting spore germination in Synchytrium austräte in relation
to its classification. Mycologia 47: 185—189.
— 1955 c. Prosori in Synchytrium. Bull. Torrey Bot. Club, 82: 218—236.
— 1955 d. Additional species of Synchytrium with prosori. Ibid. 82
(In press).
K u s a n o , S. 1930. The life history and physiology of Synchytrium fulgens
Schroet., with Special reference to its sexuality. Jap. Jour. Bot.
5: 35—132.
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