[ 221 ]
Trans. Br. mycol. Soc. 91 (2), 221-226 (1988)
Printed in Great Britain
THE ZYGOSPORE OF MORTIERELLA INDOHII
By PENELOPE J. ANSELL AND T. W. K. YOUNG
Department of Human Environmental Science, King's College London (KQC), University of London,
Chelsea Campus, Hortensia Road, London SWlO oQR, U.K.
The mature zygosporangial apparatus of M. indohii, determined by means of SEM and
TEM, consists of a basal, supporting suspensor from which investing hyphae arise and
partially surround the essentially smooth-walled zygosporangium. The zygosporangial wall,
derived from the gametangial wall, is about 0'2 pm thick. The wall of the zygospore has a fine
electron-dense outer, primary layer and a zoned, inner, secondary layer about 4'0 pm thick.
Lipid droplets occupy most of the volume of the cytoplasm of the zygospore and the basal
suspensor.
Twenty of the seventy-three described species of
Mortierella (Gams, 1977) have been reported to
exhibit the sexual stage. Of the zygosporic species,
twelve produce naked zygosporangia (M. alpina
Peyronel (Kuhlman, 1975); M. bainieri Costantin,
M. candelabrum van Tieghem & Le Monnier
(Kuhlman, 1972); M. chlamydospora (Chesters)
Plaats-Niterink (Chesters, 1933); M. elongata
Linnemann, M. epigama Gams & Domsch (Gams,
Chien & Domsch, 1972); M. humilis Linnemann
ex W. Gams (Chien, Kuhlman & Gams, 1974);
M. marburgensis Linnemann (Williams, Gray &
Hitchen, 1965; Kuhlman, 1972); M. minutissima
van Tieghem, M. pulchella Linnemann (Kuhlman,
1972); M. spinosa Linnemann (Kuhlman, 1975);
M. zychae Linnemann (Kuhlman, 1972)). Nine
species produce zygosporangia surrounded to various extents by investing hyphae (M. ericetorum
Linnemann (Kuhlman, 1972); M. gemmifera M.
Ellis (Ellis, 1940); M. indohii Chien (Chien et al.,
1974); M. nigrescens van Tieghem (van Tieghem,
1876); M. polycephala Coemans (Dauphin, 1908);
M. renispora Dixon-Stewart (Dixon-Stewart,
1932); M. rostafinskii Brefeld (Brefeld, 1876;
Kuhlman & Hodges, 1972)). M. paroispora Linnemann is said to produce either naked (Gams &
Williams, 1963) or invested (Kuhlman, 1972)
zygosporangia. Twelve species of Mortierella are
known to be heterothallic, seven are homothallic
and M. elongata is reported as having both
homothallic and heterothallic isolates (Gams et al.,
1972). With the exception of M. ericetorum and
M. indohii, the heterothallic species produce naked
zygosporangia whereas among the homothallic
species only M. chlamydospora and M. epigama
produce naked zygosporangia.
.
Mortierella indohii, a heterothallic species, produces zygosporangia invested either wholly or
partially with hyphae arising from an enlarged
basal suspensor. The zygosporangial apparatus in
M. indohii has been described largely on the basis
of light microscopy and scanning electron microscopy (Chien et al., 1974; Ansell & Young, 1983).
In this communication, the structure of the
zygosporangial apparatus, determined largely by
means of ultrathin sections, is described.
MATERIALS AND METHODS
Compatible strains of M. indohii Chien, CBS
460.75 and CBS 720.71 (Centraalbureau Voor
Schimmelcultures, Baarn, Netherlands) were
mated on Czapek-Dox agar (Oxoid). Inocula of the
mating strains, placed 1-2 em apart on 8'5 em Petri
dishes, were incubated at room temperature (ca
21°) for up to 21 d. The preparative techniques
used for light microscopy, SEM and TEM of the
zygosporangia were those of Ansell & Young
(1983)·
RESULTS
Zygosporangia arise towards the periphery of the
zone of contact of the compatible mycelia after 4-5
days of growth at room temperature. In summary,
two morphologically differentiated progametangia
contact. A septum which develops in one of the
progametangia delimits the gametangium from the
basal suspensor. The gametangium enlarges and
develops into the zygosporangium. The other
progametangium does not enlarge, and constitutes
the lateral suspensor (Ansell & Young, 1983).
The mature zygosporangial apparatus comprises
a lateral, sub-cylindric suspensor, and a basal,
larger suspensor from which hyphae of limited
growth arise which invest the subglobose zygosporangium (Fig. 1). The zygosporangial wall,
derived from the gametangial wall and present
throughout the development of the zygospore,
222
The zygospore of Mortierella indohii
Figs 1-7. For legends see page 224.
Penelope J. Ansell and T. W. K. Young
Figs
~.
For legends see page
224.
223
The zygospore of Mortierella indohii
224
consists of a single layer of intermediate electron
density, ca 0·2 p,m thick (Fig. 2). During formation
of the wall of the zygospore, lipid droplets tend to
accumulate in the peripheral cytoplasm near the
developing wall, although they are dispersed
throughout the cytoplasm of the mature zygospore
(Figs 8, 9). Centripetal formation of the wall of the
zygospore proper occurs, commencing with deposition of the primary (outer) layer adjacent and
internal to the wall of the zygosporangium. In
material from 5-day-old cultures fixed in potassium
permanganate solution, the primary layer of the
wall of the zygospore is similar in electron density
to the zygosporangial wall. Electron-dense areas
are present at the interface of the primary layer and
the plasmalemma (Fig. 3). A secondary wall layer,
deposited internal to the primary layer, is less
electron-dense than the primary layer (Fig. 5). The
secondary wall layer thickens and increases in
electron density.
Comparable study of the development of the
wall of the zygosporangiumjzygospore using material from cultures 5-20 days old, fixed in glutaraldehyde, shows an electron-grey zygosporangial
wall and an electron-dense primary zygospore wall
layer (Fig. 4). The secondary wall layer which
appears electron-lucent initially is less electrondense than the primary layer. Three zones, which
vary in electron density, are usually discernible in
the fully developed secondary wall layer, which is
up to 4 p,m thick (Fig. 6). Sections of developing
zygospores are illustrated (Figs 2-5). Cytoplasm of
the basal suspensor stains more densely than that
of the zygospore, and numerous lipid bodies and
nuclei are present throughout development. In the
cytoplasm of the zygospore, prior to extensive wall
thickening, conspicuous strands of smooth endoplasmic reticulum, with occasional cisternae and
lipid vacuoles located mainly towards the wall,
several nuclei and mitochondria are present. Lipid
accumulates in the developing zygospore and
constitutes the bulk of the volume of the cytoplasm
of the mature zygospore. Most of the lipid bodies
in the cytoplasm of the zygospore are markedly
larger than those in the basal suspensor (Figs 8, 9).
The septum at the junction of the basal suspensor
and the zygospore comprises the wall of the
zygospore proper and the inner layer of the wall of
the suspensor, with an electron-grey interstitial
middle layer between them which is expanded at
Figs 1-7. Mortierella indohii.
Fig.
1.
Surface view of a zygosporangium partially surrounded by investing hyphae. SEM.
Fig. 2. A developing zygosporangium in vertical section; basal suspensor (bs). Two investing hyphae are
shown in transverse section. Potassium permanganate fixation.
Fig. 3. Pan of the wall of the developing zygosporangial apparatus showing the laminate zygosporangial wall
(z) and the primary wall layer of the zygospore with electron-dense areas (arrows). Potassium permanganate
fixation.
Fig. 4. A developing zygosporangium in vertical section showing the primary, electron-dense layer of the
zygospore wall, lipid droplets associated with the peripheral cytoplasm, the septum at the junction of the
zygosporangium and basal suspensor (bs) and investing hyphae (h). Glutaraldehyde/osmic acid fixation.
Fig. 5. Pan of the wall of the zygosporangial apparatus showing the zygosporangial wall (z) adpressed to the
primary layer of the wall of the zygospore in which the electron-dense areas have become continuous. The
secondary layer of the zygospore wall appears electron-lucent (arrow). Glutaraldehyde/osmic acid fixation.
Fig. 6. Pan of the wall of the mature zygospore showing the fine electron-grey zygosporangial wall (z),
the electron-dense, primary layer (arrow) and zoned, thick secondary layer of the wall of the zygospore.
Glutaraldehyde/osmic acid fixation.
Fig. 7. The junction of the basal suspensor (bs) and the zygospore. The zygosporangial wall is arrowed.
Glutaraldehyde/osmic acid fixation. Fig. 1, bar = 10 usn, Figs 2-7, bars = 1·0 pm.
Figs
~.
Mortierella indohii.
Fig. 8. The zygosporangial apparatus in vertical section. Cytoplasm of the basal suspensor (bs) and zygospore
is packed with lipid droplets. Glutaraldehyde/osmic acid fixation.
Fig. 9. A mature zygosporangium in transverse section surrounded by investing hyphae. Glutaraldehyde/
osmic acid fixation. Bars = 1·0 pm.
Penelope J. Ansell and T. W. K. Young
the periphery of the septum. In potassium permanganate-fixed samples, initially, vesicles conraining electron-grey/dense material are present in
the middle layer of the septum. At maturity,
however, the vesicles are absent. Such vesicles
have not been observed in glutaraldehyde/
osmium-fixed samples. The zygosporangial wall,
continuous with the outer layer of the wall of the
basal suspensor, is closely applied to interstitial
material of the septum.
DISCUSSION
It is generally assumed that mucoralean zygospores
are adapted to survival in adverse environmental
conditions, and the relatively thick-walled, lipidfilled zygospore of M. indohii would appear to fulfil
the structural requirements for such a role in
nature. The entire zygosporangial unit constitutes
a stalked, sheathed propagule with a substantial
energy reserve. Whether the sheathing hyphae are
likely also to contribute to the survival apparatus in
this species is not known. As both naked and
invested zygosporangia are produced in species of
the Mortierellaceae, and there is no detailed
account of the process of germination of mortierellaceous zygospores, the function of these units in
relation to structure is an area for investigation.
Initiation of the zygosporangial apparatus
through contact of apposed progametangia occurs
in the other zygosporic species of Mortierella.
Another heterothallic species which produces invested zygosporangia is M. ericetorum, The heterothallic M . paroispora may (Gams & Williams, 1963)
or may not (K uhlman , 1972) produce invested
zygosporangia. Development of the investing
hyphae at an early stage in the production of the
zygosporangiurn in these species and in homothallic
species with invested zygosporangia has precluded
clear observation of the details associated with
copulation. With the exception of M . ericetorum,
M . nigrescens, M. polycephala, M. renispora and M.
rostafinskii, where the information currently available is inadequate, the zygosporangial apparatus of
Mortierella species shows an enlarged basal suspensor and a lateral suspensor that remains small
and may not be visible under the light microscope
at maturity. Whether a septum which delimits the
gametangium develops in each of the progametangia, or in one of them only, seems to depend on
the species. Gametangial septa have not been
described in M. alpina, M . spinosa (Kuhlman,
1975), M . humilis (Chien et al., 1974), M . bainieri,
M. candelabrum, M. elongata, M . minutissima and
M. pulchella (Kuhlman, 1972). In M . marburgensis,
and M . paroispora, both gametangia are delimited
by septa (Williams et al ., 1965) whereas in M.
225
indohii (Ch ien et al., 1974; Ansell & Young, 1983)
and possibly M. elongata (Gams et al ., 1972) only
the gametangium associated with the basal suspensor is delimited by a septum. In those species of
Mortierella where each of the gametangia is
delimited by a septum, the process of zygosporangium formation parallels that found in other
Mucorales with apposed gametangia, for example
Phycomyces blakesleeanus Burgeff or Blakeslea
trispora Thaxter. No direct comparison can be
made, however, where garnetangial septum formation does not seem to occur, since the lateral
element appears to serve first as a progametangium,
then the gametangium and finally as a suspensor.
This triple role results from the apparent failure of
septum formation in the lateral copulation element.
Owing to the observed dissimilarity in structure of
the pairs of copulating elements in the various
species, the components of the sexual apparatus
may require interpretation. For example, the
, gametangia' in M. elongata (Gams et al., 1972)
appear to be equivalent both morphologically and
in function to the progametangia/gametangia of
M. indohii. A marked feature of zygosporangium
formation in several species is the apparent enlargement of one of the gametangia and its resultant
conversion into the zygosporangium. With this
observation in mind, perhaps the nuclear events
associated with zygosporangium formation in
Mortierella species should be studied in order to
determine whether the process may be considered
truly sexual.
REFERENCES
ANSELL, P .
J. &
YOUNG, T. W . K. (1983). Light and
electron microscopy of Morrierella indohii zygospores.
Mycologia 75, 64~9 ·
BREFELD, O. ( 1876) . Uber die Zygosporenbildung bei
Mortierella rostafinskii nebst Bemerkungen uber die
Systematik der Zygomyceten. Sitzungsberichte der
Gesellschaft naturforschender Freunde zu Berlin
9 1-97.
CHESTBRS, C . G. C. (1933). Azygozygum chlamydosporum
nov.gen. et sp , A phycomycete associated with a
diseased condition of Anurrhinum majus. Transactions
of the British Mycological Society 18, 199-214.
CHIEN, C-Y., KUHLMAN, E. G. & GAMS, W. (1974) .
Zygospores in two species of Mortierella with "stylospores '. Mycologia 66, 114-121.
DAUPHIN, M . J. (190 8). Contribution a l'etude des
Mortierelees. Annales des Sciences naturelles, Paris,
Bo tanique (9" serie) 8, 1-112.
DIXON-STEWART, D. (1932). Species of Mortierella
isolated from soil. Transactions of the British Mycological Society 17,208-220.
ELLIS, M . (1940). Fungi isolated from pinewood soil.
Transactions of the British Mycological Society 24,
87-7·
8-2
226
The zygospore of Mortierella indohii
GAMS, W. (1977). A key to the species of Mortierella.
Persoonia 9, 381-391.
GAMS, W., CHIEN, C.-Y. & DOMSCH, K. H, (1972).
Zygospore formation by the heterothallic Mortierella
elongata and a related species M. epigama. Transactions
of the British Mycological Society 58, 5-13.
GAMS, W. & WILLIAMS, S. T. (1963). Heterothallism in
Mortierella paroispora Linnemann. 1. Morphology and
development of zygospores and some factors influencing their formation. Nova Hedwigia 5, 347-357.
KUHLMAN, E. G. (1972). Variation in zygospore formation among species of Mortierella. Mycologia 64,
3 25-341.
KUHLMAN, E. G. (1975). Zygospore formation in Mortierella alpina and M. spinosa. Mycologia 67, 678-681.
KUHLMAN, E. G. & HODGES, C. S. (1972). Rediscovery of
Mortierella rostafinskii and Mortierella strangulata.
Mycologia 64, 92---98.
TIEGHEM, PH. VAN (1876). Troisierne memoire sur les
Mucorinees. Annales des Sciences naturelles, Paris,
Botanique (6· serie) 4, 312-398.
WILLIAMS, S. T., GRAY, T. R. G. & HITCHEN, P. (1965).
Heterothallic formation of zygospores in Mortierella
marburgensis. Transactions of the British Mycological
Society 48, 129-133·
(Received for publication
11
September 1987)