Mycologia, 93(6), 2001, pp. 1164-1173.
© 2001 by The Mycological Society of America, Lawrence, KS 66044-8897
Conidiomatal morphogenesis and pleomorphic conidiogenesis in
Scleroconidioma sphagnicola
INTRODUCTION
A. Tsuneda1
Department of Biological Sciences, University of
Alberta, Edmonton, AB T6G 2E9, and Northern
Forestry Centre, Canadian Forest Service, Edmonton,
AB T6H 3S5, Canada
Scleroconidioma Tsuneda et al is a genus of Hypho­
mycetes with minute, dematiaceous, sclerotic coni­
diomata that extrude successive hyaline, bacilliform
conidia from papillate conidiogenous cells (Tsuneda
et al 2000). Scleroconidioma sphagnicola Tsuneda et al
is the type species of the genus, and was isolated from
diseased leaves of the hummock- and peat-forming
moss, Sphagnum fuscum (Schimp.) Klinggr. Our pre­
liminar y inoculation tests and ultrastructural investi­
gations confirmed that S. sphagnicola is the causal
agent of a necrotic disease of Sp. fuscum. Conidia are
most likely the major inoculum source for new infec­
tions. Information on conidiogenesis is thus impor­
tant for understanding not only taxonomy but also
pathology of the fungus. However, details of conidia­
tion, especially that occurring on conidiomata, could
not be clarified by light microscopy because of the
darkly pigmented conidiogenous cells and the mi­
nute conidiogenous loci. Therefore, an ultrastructur­
al study was conducted to elucidate conidium ontog­
eny as well as microsclerotial and conidiomatal mor­
phogenesis in S. sphagnicola.
M. H. Chen
Medicine/Dentistry Electron Microscopy Unit, l074B
Dentistry Pharmacy Building, University of Alberta,
Edmonton, AB T6G 2N8, Canada
R. S. Currah
Department of Biological Sciences, University of
Alberta, Edmonton, AB T6G 2E9, Canada
Morphogenesis of microsclerotia and con­
idiomata, and con idiogenesis in Scleroconidioma
sphagnicola were studied primarily by transmission
and scanning electron microscopy. Microsclerotia
were initiated as bulges from hyphae that later
swelled and became multicellular. They increased in
size by forming protrusions that subsequently were
delimited by multilayered, simple septa. The struc­
ture of septa indicated an ascomycetous affinity. Cells
of mature microsclerotia contained large lipid bodies
and poorly defined organelles. In culture, micro­
sclerotia often became conidiomata by conversion of
the surface cell layer to conidiogenous cells. These
conidiogenous cells were either percurrently prolif­
erating or were phialides with a collarette and peri­
clinal wall thickening. Conidia were also produced
from vegetative hyphae. Conidiogenous cells arising
from juvenile, hyaline hyphae proliferated percur­
rently or occasionally sympodially with the produc­
tion of successive conidia. As the colony aged and
hyphae became darkly pigmented, variously shaped,
solitary hologenous conidia became more dominant.
Secondary conidiation from these conidia was fre­
quent. Relative juvenility of the cell wall at the coni­
diogenous locus and the age of the colony appear to
be important factors in determining the mode of co­
nidium development in S. sphagnicola.
Key Words:
conidium ontogeny, dematiaceous
hyphomycete, microsclerotium
Abstract:
MATERIALS AND METHODS
Scleroconidioma sphagnicola (UAMH 9731) was grown on
corn meal agar with dextrose (CMAD; Difco, Detroit, Mich­
igan) at 20 C for 2 wk in the dark. Agar disks, 7 mm diam,
were cut from the colony periphery and placed singly in
the center of CMAD plates or 2% malt extract agar (MEA;
Difco) plates and kept at 20 C in the dark. These cultures
were used to examine morphogenesis of microsclerotia and
conidiomata as well as conidium development from the
conidiomata. Instead of mycelium, conidiomatal conidia
were used as the inoculum to prepare cultures for observ­
ing conidiogenesis on vegetative hyphae. Ten conidiomata
actively sporulating on CMAD were transferred into 0.5 mL
sterile distilled water in a glass vial. The vial was shaken
vigorously and then allowed to stand until the conidiomata
settled at the bottom. Two to three drops of the supernatant
(conidial suspension) were then spread evenly on CMAD,
incubated at 20 C in the dark, and examined periodically
from 12 h up to 30 d of incubation.
For scanning electron microscopy (SEM), 5-mm agar
discs were cut from cultures, washed in phosphate buffer
(pH 7.0), and fixed in 2% glutaraldehyde in buffer for 2 h.
After rinsing with buffer, these discs were immersed in 2%
tannic acid-2% guanidine hydrochloride solution for 4-5 h,
Accepted for publication April 25, 2001.
I
Corresponding author, Email: [email protected]
1164
TSUNEDA ET AL: CONIDIOGENESIS IN
rinsed thoroughly in distilled water, and postfixed overnight
in 2% OS04 at 5 C. The fixed material was dehydrated in
an ethanol series, taken to amyl acetate, and critical-point
dried in a Polaron E-3000 unit using carbon dioxide. The
dried samples were coated with gold before examination by
SEM. Cryo-fracturing of fresh fungal material was done in
an Emitech K 1250 Sputter-Cryo cr yogenic preparation sys­
tem using the method of Beckett and Read (1986). All the
dried as well as cryo-fractured samples were examined with
a JEOL JSM-6301 FXV field-emission scanning electron mi­
croscope at 5, 10 or 15 kV. Cultures were also examined
with a Zeiss D-7082 Axiophot photomicroscope.
For transmission electron microscopy (TEM), specimens
were fixed in 2% glutaraldehyde and 2 % OS04 in Millon­
ig's buffer at pH 7.3 and dehydrated in an ethanol series.
Samples were then embedded in Spurr's resin. Ultrathin
sections were stained with uranyl acetate and lead citrate.
Photomicrographs of samples were taken at 75 kV with a
Hitachi H-7000 electron microscope.
RESULTS
Morphogenesis of microsclerotia and conidiomata.­
Abundant microsclerotia, conidiomata and tufts of
aerial hyphae occurred on both MEA and CMAD
(FIG. 1). The typical development of microsclerotia
on MEA is shown in FIGS. 2-6. Bulges from the sides
of hyphae (FIG. 2) swelled (FIG. 3, arrows) and be­
came multicellular. These bodies increased in size by
forming protrusions (FIG. 4, arrowhead) that were
later delimited by septa. Under TEM, all the resultant
cells (daughter cells) possessed newly developed,
electron-light cell walls, surrounded by those of
mother cells that were impregnated with numerous
melanin-like granules (FIG. 7: also see 15 and 16).
Subglobose bodies, mostly 80-200 fLm diam and con­
sisting of several to many, darkly pigmented sclerotic
cells, were thus produced (FIGS. 4-8). These struc­
tures are referred to here as microsclerotia. Some­
times determinate aerial hyphae, often moniliform at
their basal parts, arose from the component cells
(FIG. 3, arrowheads; FIG. 5). After cell division ceased
in microsclerotia, a transparent, gelatinous secretion
formed a sheath or cuticle over the surface; thus the
mature microsclerotia appeared smooth under SEM
(FIG. 6, arrow). Adjacent microsclerotia frequently
merged (FIG. 6).
Some mature microsclerotia were converted to
conidiomata; others remained unchanged, forming
no conidiogenous cells even after 2 mo of incubation
on MEA. All cells of nonsporulating microsclerotia
appeared dormant and contained large lipid bodies
and poorly defined organelles (FIGS. 7, 8). Conver­
sion of microsclerotia to conidiomata occurred by
the direct conversion of existing surface cells to con­
idiogenous cells and/or by the development of new
SCLEROCONIDIOMA
1165
conidiogenous cells on the outer surface. The major­
ity of microsclerotia formed on CMAD became con­
idiomata.
Conidium development from conidiomata.-Typical de­
velopmental stages on CMAD are shown in FIGS. 914 (SEM) and 15-22 (TEM). Conidia were extruded
in succession from conidiogenous cells on the coni­
diomatal surface and were hyaline, one-celled, and
bacilliform. In 5-7-d-old colonies, juvenile conidi­
ogenous cells occurring in small groups occasionally
gave rise to conidia (FIG. 9). Juvenile conidiogenous
cells in very early stages of conidiation were readily
identified by the incomplete development of cuticle
and by the lack of a papilla (FIGS. 9, 10). After a
conidium (probably the first one) seceded from its
conidiogenous cell, a minute, round projection,
about 0.4-0.8 fLm diam became discernible by SEM
at the conidiogenous locus inside the cell apex (FIG.
9, arrows: also see FIG. 17, arrow). These juvenile con­
idiogenous cells (FIG. 10, arrows) proliferated per­
currently with the production of successive conidia
and had distinct annellations.
In older colonies, percurrent proliferation of con­
idiogenous cells was not detected either by SEM (FIG.
11, arrowheads) or TEM, as explained later. Conidi­
ogenous cells extruded transparent gelatinous mate­
rial (FIG. 20, arrow) that accumulated as the conidia­
tion proceeded, and solidified to form a papilla
above the conidiogenous locus. Therefore, the size
of the papilla more or less reflected the number of
conidia produced by the cell. Once a papilla had
formed, subsequent conidia emerged through it
(FIGS. 11, 12; arrows). Each conidiogenous cell usu­
ally had one conidiogenous locus, though occasion­
ally two were present (FIG. 12, arrowhead). On
CMAD, each conidioma produced a large number of
conidia into a gelatinous matrix covering the coni­
dioma. Exhausted conidiogenous cells collapsed.
However, fresh clumps of conidiogenous cells often
emerged through the collapsed surface layer (FIG.
13, arrows) and soon produced conidia (FIG. 14, ar­
rowheads).
Under TEM, actively sporulating conidiogenous
cells were readily distinguishable by the absence of
large, well-defined lipid bodies (FIGS. 15, 16) and,
where the section was close to median, by the pres­
ence of a collarette (FIG. 15, arrows). In many cases,
lipid bodies in the cell below the conidiogenous cell
also became mobilized and cytoplasmic continuity
between the two cells was maintained through a sep­
tal pore (FIG. 16, arrowhead). Inside the apex of each
conidiogenous cell was a single conidiogenous locus
(FIGS. 16, 17, arrows). FIGURES 18 AND 19 show a co­
nidial initial and an elongating conidium, respective-
1166
FJG�. I-x.
:\!orphogencsis of microsclcrotia in
.\df'1"O(()lIiriiollia sjJhal-!;niw!a
on :\!EA. 1. �!icrosclcro!ia and t.ufts of aerial
hyphae. I mo. Bar =2:)() ILm. 2. Bulges (arrows) developing h-om Il\-phal cells. 3 d. Bar
and aerial hyphae arising Ii-OIl] thell! (arrm....heads). 3d. Har
:{ ILm. ,\. Inilated bulges (arrows)
10 p_m. ·4. Immature microsckrotiu1ll (arrow) den'loped
terminally on all aerial hypha. Arrowhead indicates a swollen protrusion. I () d. Bar
20 1L1ll. f). Ylaturing microsckrotiuIll
with aerial hyphae. 20 d. G. Microscl('!"otia near maturation. :\;ote merged adjacent microsclcr{)tia and devdoping cuticle on
Olle of them (arrow). Bar
=,
40 1L1ll. 7. X. TE:\1 and Crnl-SE:\l micrographs, respectively. of component cells of a mature
microsclerotiulll. Cells contain large lipid bodies
Hal' =] 1L1ll ill 7. � f.Ull in tl.
(1.)
and poorh-dctincd organelles. Arrows ill tl illdicat(' septal pore sites.
II ()7
TSl:\Ell.\ El ,\1.: CO:\lJ)IO(;F:\FSIS J:\ SUIJIOCO.\ll)j()\f.l
F!(;s, 9-14,
SF"! micrographs showillg conidium developlllent from cOlliciiomata of Sr/PlDconidiolllfl
�J. .!mcnile cOllidiogc)loUS cdIs that gave rise
to
OIl C\IAD,
conidia. Formation of cuticle is incomplete, ,\ITOWS indicate cOllidiogenoHs
loci, :1 d, Bar "" :2 j.lm, J 0, Annellated conidiogcl1ouS cell apexes (arrows) of a juvenile conidioma, 7 (L Bar "" I j.lnL J J,
Conidia
through papillae (arrows), Annellidic conidia!ioll is not evident (arrowhe;1(ls), I:J d, Bar
:2 p.m. 1:2.
\Yell-dcveloped papillae through which conidia are arising (arrows), SOlllctimes two cOllidingcllou, loci occur in a cOllidiogcnolls cdl (arrowhead). :20 d. Bar
exhausted surl�lC(, cOllidingcHolls
rise to conidia. :\0 d, Bar =:20
lun
:2 j.lln. J:l, ]·1. Fresh conidiogellolls cells (arrowheads) groWll out of lht' collapsed,
(arro\,') of the c()!lidiomat;t, The llewIY f(1rmcd
in ];), 10 j.l11l ill [1.
)US (ells ill J ,1 arc giving
MYCOLOCIc\
1 16�
FJ(;s. 15-23.
IjJhagniro/a. 15.
TE.'vl micrographs showing different stages of conidium den'lopmellt from conidiomata of
Sdrmmnidioma
CO!lidiogCllous cell with a collarellc (arrows), �()lC cell ,,'ails of motlwr cclls are imprcgnatc'd with numerous
melanin-like granules. L
lipid hodies, Bar
2 fUlL Iti. Conidiogenolls cdl and immediatch' adjaccllt cell (backup cell).
These cdls arc cOl1l1ected through a septal pore (arrowhead). Lipid bodies (L) i ll the backup cell show signs of mobilization.
Arrow indicates the conidiogcnolls locus. :\
.�
llUCh'US .'vIC
.
mothl'l' cell walls containill g 1ll1111CrOllS melanin-like granules,
TSUNEDA ET AL: CON IDIOGENESIS
ly. After conidial secession, the space above the con­
idiogenous locus was filled with amorphous (gelati­
nous) material (FIG. 20, black arrow).
We could not obtain thin sections of juvenile, con­
idiogenous cells with annellations like those shown
in FIG. 10 because these cells were too few and scat­
tered to collect for TEM under a dissecting micro­
scope. All conidiogenous cells observed by TEM were
regarded as phialides because the conidiogenous lo­
cus was more or less fixed (FIGS. 17-19. 20, white
arrow) and periclinal wall thickening occurred at the
conidiogenous cell apex inside the collarette (FIGS.
18-21, arrowheads). Exhausted conidiogenous cells
were almost devoid of contents and had a well-devel­
oped, irregularly shaped papilla on top (FIG. 22,
black arrowhead). Cells below the conidiogenous
cells either showed concomitant loss of their cell con­
tents (FIG. 22, arrow) or appeared metabolically in­
active, containing large lipid bodies (FIG. 22, white
arrowheads). Septa were multilayered, with a simple
pore (FIG. 23).
Conidium development from vegetative hyphae. Conid­
ia spread on CMAD quickly germinated and hyphae
were forming hyaline, one-celled conidia within 12 h.
These conidia were almost indistinguishable in shape
and size from those on conidiomata, although some
were longer. Conidiogenous cells were peg-shaped,
proliferated percurrently with successive conidiation,
and became up to 3 fLm long, or rarely more (FIGS.
24-26). In 40 h, sympodially proliferating conidi­
ogenous cells as well as those forming solitary holo­
genous conidia were also evident. All the conidia
were hyaline at this stage of colony development.
Sympodially produced conidia occurred only occa­
sionally and were one-celled, hyaline to lightly pig­
mented, oblong to long cylindrical with truncate ba­
ses, many of the latter were slightly curved and up to
about 10 fLm in length (FIGS. 27-29). Solitar y, holo­
genous conidia arising directly from vegetative hy­
phae became abundant as the colony became darkly
pigmented. In 5 d-old or older cultures, the solitary
conidia were variable in shape, size, and degree of
pigmentation and some of them were two or more
celled (FIGS. 30-32). Secession was by schizolysis of
-
IN SCILROCONIDIOMA
1169
the basal septa, but in the case of two- or more-celled
conidia, cell separation occurred at any septum (FIG.
31, arrow). Free conidia frequently formed second­
ary conidia by yeast-like budding (FIG. 32, arrows),
or sometimes secondary conidia appeared to have de­
veloped successively, leaving a minute, annellated peg
on the mother conidium. Darkly pigmented sclerotic
cells and young conidiomata appeared in 5-6 d. Sol­
itary, hologenous conidia and a conidioma often de­
veloped from the same hypha in close proximity (FIG.
30). The results of this study are schematically sum­
marized in FIG. 33.
DISCUSSION
In our previous paper (Tsuneda et al 2000), we used
the term stromata to describe all the sclerotic, mul­
ticellular bodies of S. sphagnicola regardless of wheth­
er the body carried conidiogenous cells or not. How­
ever, we decided to use separate terms in the present
paper, i.e., microsclerotia (micro because of their mi­
nute size) for those without conidiogenous cells, and
conidiomata for those bearing conidiogenous cells
because: (1) the majority of sclerotic, multicellular
bodies of S. sphagnicola, collected in nature during
the growing season, did not bear conidiogenous cells
(Tsuneda et al 2000); (2) the primary function of
these bodies in nature appeared to be survival under
adverse conditions; and (3) definitions of the terms
stroma and sclerotium vary among authors and may
create confusion (e.g., Coley-Smith and Cook 1971,
Chet and Henis 1975, Hawksworth et al 1995, Alex­
opoulos et al 1996, Ulloa and Hanlin 2000). In the
present paper, we use the term microsclerotium to
represent a minute, hard, multicellular structure
whose primary function is survival.
Microsclerotia of S. sphagnicola (FIGS. 1, 4) resem­
ble multicellular sclerotic bodies (microsclerotia) of
Phaeotheca fissureUa Sigler, Tsuneda & Carmichael
formed in its natural habitat (Figs. a, b in Sigler et al
1981, Tsuneda and Murakami 1985). In both fungi,
microsclerotia are minute, darkly pigmented, and de­
velop by swelling of mother cells that become sub­
divided by septa. The structure of the septa suggests
Bar
1 fLm. 17. Conidiogenous locus (arrow). Compare with those shown in 9. Bar
0.5 fLm. 18. Conidium initial arising
from the conidiogenous cell. Arrowheads indicate periclinal wall thickening. Bar
0.5 fLm. 19. Emerging conidium. Arrow­
heads indicate periclinal wall thickening. Bar
0.5 fLm. 20. Amorphous (gelatinous) material (black arrow) accumulating
above the conidiogenous locus (white arrow). Arrowheads indicate periclinal wall thickening. Bar
0.5 fLm. 21. Conidi­
ogenous cell after producing many conidia with a well-developed papilla (arrow). Enlarged view of the one indicated by the
black arrowhead in 22. Arrowheads indicate periclinal wall thickening. Bar
0.5 fLm. 22. Part of an actively sporulating
conidioma. Conidiogenous cells and some of their backup cells (arrow) are nearly exhausted but other cells (arrowheads)
appear to be metabolically inactive. C
conidia. Bar
2 fLm. 23. Multilayered, simple septum (arrow). Bar
0.3 fLm.
=
=
=
=
=
=
=
=
=
1 170
FJ(;s. :2'1-:'>:2.
Vario\lS forms of conidia developed from \egetative ll\phac of Sr/pmrolliriiullt(J
:2b. Allllcllidic
the arrowhead in :21. Hal'
:29 arc
of the
on C\IAD. :21-
(arrows) OIl juvenile. hyalille hyphae. ] 7 h. :2e) shows the enlarged \'iew of the anllellidc (arrow)
,-)
fUll ill :21, J f1.l11 in :2,), :2 1.L1ll ill :21i. :27-:29. Sympodial conidiogellcsis. :2 d. :2S and
vicY,'s of the pans ill :27 indicated 1)\ all arrow and all arrowhead, ITslwni\ch. Arrow in :2S indicates schilol\sis
septum. Lcucrs (a. b. c) ill :2�J indicate the sequclltial order ill the (it-WjoPlllclll of the three cOIlidi,l. Bar
j() �m in :27. :2 �m in :2x, :l l.Llll ill :29. 30. Variothh shaped solitan, hologcnolls conidia (arrow) and a conidioma
(arrowhead) formcd
Oil
the samc ll\-pha. F) d. Bar
co
j() �Ill. 31. CeIls of
schil.ohsis (arrow). ,\rnmilcad indicates the hasal septum. ')0 d. Bar
cOllidiulll
a
scpt Hill
[wo-ccIled conidiulll
:2 �m. 3:2. Sccollcbn conidiatioll
(allows) ..\rlOwhcad indicates a basal septum. :)0 d. Bar =1 �Ill.
Oil
a two-celled
TSUNEDA ET
AL:
CONIDIOGENESIS IN SCU:'ROCONIDIOMA
1171
Conidiomatal Conidia
(Germination)
Juvenile Hyaline Hyphae
Annellidic
conidiogenesis
16H
Sympodial
conidiogenesis
Pigmented Hyphae
3D
Solitary hologenous
conidiogenesis
(Secondary conidiation)
6D
Annellidic
conidiogenesis
Phialidic
conidiogenesis
9D
12 D
I Darkly Pigmented Colonies
FIG. 33. Schematic representation of the pleomorphic con idiogenesis in Scleroconidioma sphagnicola on CMAD. H
hours of incubation, D
days of incubation.
=
I
an ascomycetous affinity for the fungus. Cells of ma­
ture microsclerotia in both fungi are packed with
large lipid bodies, and cell walls of mother cells are
impregnated with numerous melanin-like granules.
In culture, microsclerotia are often converted to con­
idiomata when the cells of the surface layers become
conidiogenous cells. Conidiomata of these fungi,
however, markedly differ in conidiogenesis. In S.
sphagnicola, they are primarily phialidic whereas P.
fissurella produces endoconidia by means of schizo­
lysis of septa delimiting the daughter cells (Tsuneda
and Murakami 1985).
All cells of the microsclerotia in S. sphagnicola con­
tain a large amount of storage material and their cell
organelles are poorly defined. This indicates that
these cells are dormant. It is striking that most cells
inside the actively sporulating conidiomata of S.
sphagnicola appeared metabolically inactive, showing
no signs of mobilizing storage material (FIGS. 15, 22).
Only one or two cells below the conidiogenous cells
appeared active, ostensibly supplying nutrients
through septal pores to sustain successive conidiation
(FIG. 16). Our results (FIGS. 13, 14) indicate that in­
ternal cells are capable of developing fresh conidi­
ogenous cells when conditions are favorable. We con­
sider this to be an adaptive feature in a fungus that
lives in peatlands where sudden changes of environ­
mental conditions commonly occur.
Conidiogenesis, when present, represents one of
the most important sets of characters when describ­
ing ascomycete and anamorphic taxa (Hennebert
and Sutton 1994). However, attempts to classifY ana­
morphic fungi based on conidium ontogeny
(Hughes 1953, Tubaki 1963, Barron 1968, Sutton
MYCOLOGIA
1172
1980) have encountered difficulties because conidi­
um ontogeny is much more plastic than was initially
realized (e.g., Wang 1979, Kendrick 1980, 1981, Tsu­
neda and Hiratsuka 1984, Okada et al 1993, Benade
et al 1995). We found that conidiogenesis in S. sphag­
nicola is also highly variable. First, two types of coni­
diogenous cells occurred on conidiomata. Conidi­
ogenous cells in very young colonies, 4-6 d on
CMAD, proliferated percurrently and bore distinct
annellations (FIG. 10). In older colonies, conidiogen­
ous cells exhibited a collarette and periclinal wall
thickening (FIGs. 17-21) and the conidiogenous loci
were stationary. These types of conidiogenous cells
were traditionally called annellides and phialides, re­
spectively, and have been suggested to be extremes
of a continuum of developmental types (e.g., Cole
and Samson 1979, Minter et a11982, 1983, Wingfield
et al 1989, Seifert and Okada 1993, Verkley 1998).
We consider that whether the conidiogenus loci in
conidiomatal conidiogenous cells of S. sphagnicola
are progressive (extending) or stationary (fixed) is
determined by the degree of maturity of the cell wall
at the conidiogenous locus as well as the age of the
colony. Likewise, Madelin (1979) suggested that
whether the conidium development is holoblastic
(hologenous) or enteroblastic (enterogenous) is sim­
ply a matter of juvenility or maturity of the conidi­
ogenous cell wall.
Second, conidium development from vegetative
hyphae in culture was also pleomorphic. Conidiogen­
ous cells were either percurrent or sympodial in pro­
liferation, or determinate, and conidia varied in
shape, septation, size, and degree of pigmentation.
Conidiogenous cells arising from hyaline hyphae in
juvenile colonies, after 12-24 h on CMAD, were al­
most exclusively percurrent in proliferation, but as
the hyphae became darkly pigmented, solitary, vari­
ously shaped, hologenous conidia became more
dominant (FIGS. 30-33). These results also indicate
that juvenility of the cell wall at the conidiogenous
locus, as well as the age of the colony, are important
factors determining the mode of conidium develop­
ment. A similar phenomenon has been observed in
some other fungi, such as Rhinocladiella atrovirens
(Tsuneda et al 1986) and Ophiostoma clavigera (Tsu­
neda and Hiratsuka 1984). In summary, S. sphagni­
cola is extremely plastic in conidiogenesis, exhibiting
both hologenous and enterogenous conidium devel­
opment, and the spatial sequence of successive locus
positions on the conidiogenous cell is either sympo­
dial, progressive, or stationary.
ACKNOWLEDGMENT
We are grateful to Dr. B. Kendrick for suggestions to im­
prove an earlier version of this manuscript.
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