Mycologia, 87(5), 1995, pp. 597-603.
© 1995, by The New York Botanical Garden, Bronx, NY 10458-5126
The mating system of Phellinus tremulae
1988; Dahlberg and Stenlid, 1990). There are few re­
ports about the mating system and population struc­
ture of P. tremulae. This may in part be due to the
fungus not producing clamp connections; consequent­
ly, it has been difficult to distinguish compatible from
incompatible matings.
Verrall (1936) showed that P. tremulae was hetero­
thallic but was unable to demonstrate conclusively
whether the fungus had a bipolar or tetrapolar mating
system. Hennon and Hansen (1987) studied nuclear
behavior in several Phellinus species, including P. tre­
mulae, but were unable to differentiate between homo­
karyotic and heterokaryotic cultures and could not
determine P. tremulae's mating system. Fischer (1987),
in a study of 20 species of Phellinus, concluded from
matings of homokaryotic isolates from one basidiocarp
that P. tremulae was heterothallic and had a bipolar
mating system.
We have been studying the population structure of
P. tremulae to gain an understanding of the epidemi­
ology of white rot in trembling aspen. In this study
we report new observations on the mating system of
P. tremulae and describe the interactions between ge­
netically different heterokaryotic individuals.
K.I. Mallett
C.L. Myrholm
Department of Natural Resources, Canadian Forest
Service, Northern Forestry Centre, 5320- 122 St. ,
Edmonton, Alberta, T6H 355, Canada
Abstract: Basidiocarps of Phellinus tremulae were col­
lected from trembling aspen (Populus tremuloides) in
Alberta, Saskatchewan, and Ontario. A mating study
based on single spore isolates of 12 of these basido­
carps was performed using carrot agar, malt agar, and
malt-yeast agar media. Results from carrot agar showed
that the fungus is heterothallic and has a tetrapolar
mating system. Mating studies on malt agar and malt­
yeast agar were inconclusive. The mating factor genes
were multiallelic. Compatible and incompatible mat­
ings could be recognized microscopically. Radial
growth rate of heterokaryon colonies did not differ
significantly from that of homokaryon colonies. When
genetically different heterokaryotic isolates were paired
on agar media a zone of antagonism formed where
the two colonies met. No zone of antagonism occurred
between genetically identical heterokaryon colonies.
Key Words: genetics, incompatibility, population
structure, Populus tremuloides
MATERIALS AND METHODS
Collections of basidiocmjJs.-Basidiocarps were collect­
ed from trembling aspen trees (P. tremuloides) at var­
ious locations in Alberta, Saskatchewan and Ontario
during the summers of 1990,1991,and 1992 (fABLE
I). Individual basidiocarps were pried from aspen
stems, placed into paper bags and brought back to the
laboratory. Taxonomic identifications were made fol­
lowing the description in Lindsey and Gilbertson (1978).
Heterokaryotic isolates were grown from pieces of
context taken aseptically from the basidiocarps. Iso­
lations were made on acidified 3% malt extract agar
(AMA) [3% (w:v) Difco malt extract, 1.5% agar, 1 ml
25% lactic acid, 1 L distilled water]. Homokaryotic
(single spore) isolates were obtained by removing a
small piece (0.5 x 0.5 cm) of hymenial layer from the
basidiocarp, and placing it in a 1.5-ml microcentrifuge
tube containing sterile water. The microcentrifuge tube
was shaken on a vortex mixer for about 10 sec and
then 0.5 ml of the suspension was spread over the
surface of a petri plate containing 1.5% malt agar
INTRODUCTION
Trembling aspen (Populus tremuloides Michx.) is one
of the most widely distributed tree species in North
America (Peterson and Peterson, 1992) and comprises
a substantial portion of the wood volume in the boreal
forest. The principal decay fungus of trembling aspen
in Canada and the United States is the white rot fungus
Phellinus tremulae (Bond.) Bond. & Borisov (Basham,
1958; Thomas et al., 1960; Hinds, 1985). This fungus
is found throughout North America and Europe (Roll­
Hansen, 1967), but despite its ubiquitousness, rela­
tively little is known about its biology or the epide­
miology of white rot.
Mating system and vegetative incompatibility studies
have been used to investigate the epidemiology of dis­
ease and the population dynamics of mycorrhizal and
decay fungi (Coates et al., 1981; Mallett and Harrison,
Accepted for publication May
I, 1995.
597
59 8
MYCOLOGIA
TABLE 1.
Isolate
Collection
number
date
List of Phellinus tremulae basidiocarp collections
Host
Location
NoF-1479
2-VIII-90
Populus tremuloides
Hanmore Lake, Alta.
NoF-1484
VII-90
P. tremuloides
Hinton, Alta.
NoF-1487
IX-90
P. tremuloides
Hawk Hills, Alta.
NoF-1492
VII-90
P. tremuloides
Fort Assiniboine, Alta.
Ont-L4
21-V-91
P. tremuloides
Sault St. Marie, Ont.
Ont-L5
21-V-91
P. tremuloides
Sault St. Marie, Onto
NoF-2009
13-V-92
P. tremuloides
Slave Lake, Alta.
NoF-2007
I-V-92
P. tremuloides
Elk Island National Park, Alta.
Christopher Lake, Sask.
NoF-2012
3-VI-92
P. tremuloides
NoF-2145
23-IX-92
P. tremuloides
Drayton Valley, Alta.
NoF-2146
23-IX-92
P. tremuloides
Drayton Valley, Alta.
NoF-2147
23-IX-92
P. tremuloides
Drayton Valley, Alta.
NoF-2148
10-XI-92
P. tremuloides
Chip Lake, Alta.
NoF-2149
ll-X-92
P. tremuloides
Edson, Alta.
NoF-2150
ll-X-92
P. tremuloides
Chip Lake, Alta.
NoF-2151
ll-X-92
P. tremuloides
Obed Lake, Alta.
NoF-2152
ll-X-92
P. tremuloides
Hinton, Alta.
(1.5% Difco malt extract, 1.5% agar). The plates were
incubated at 21 C for 2 days. Germinated single spore
isolates (homokaryotic isolates) were cut out of the
agar and placed onto carrot agar (CA). Carrot agar
was made by adding 300 g of washed carrots to 400
ml of distilled water and grinding in a blender for 45
sec. The carrot pulp was separately autoclaved from
the agar; 15 g of agar were dissolved in 400 ml of
distilled water. The carrot pulp and agar were mixed
after autoclaving the carrot pulp for 60 min and the
agar for 20 min. About 15 homokaryotic isolates per
basidiocarp were isolated.
Heterokaryon cultures and representative homo­
karyotic cultures from each basidiocarp were stored
in the Northern Forestry Centre culture collection.
Determination of mating system. -Mating types were de­
termined by pairing 3 x 3 x 3-mm plugs of mycelia
and agar, taken from the margin of 2-wk-old colonies,
about 1 cm apart on the surface of carrot agar, one
pairing per plate. The petri plates were then sealed
with Parafilm® and incubated at 25 C in the dark for
3 wk. At least 10 homokaryotic isolates from each
basidiocarp were paired with each other in every pos­
sible combination. Sexual compatibility was scored af­
ter 3 wk based on the morphology of the resulting
colony and by the reaction created between the two
colonies. After 21 days incubation, a 1 x 2-cm agar
strip was removed from the zone where the two col­
onies met and placed on CA. The resultant colony or
colonies were observed 21 days later to confirm wheth­
er a pairing was successful or incompatible.
Growth rate.-Radial growth of heterokaryotic and
homokaryotic cultures from the same basidiocarp were
compared. Inoculum, cut with a no. 3 cork borer from
the margin of a colony, was placed onto carrot agar
in petri plates, which were then sealed and grown in
the dark at 25 C. Diameter measurements were made
2 wk after inoculation.
Heterokaryon-homokaryon pairings. -Heterokaryon­
homokaryon pairings were performed using the pro­
cedure developed by Angwin and Hansen (1993). Iso­
lates from the interaction zones of putative compatible
and incompatible pairings from five basidiocarps were
paired on CA with two homokaryotic isolates from two
unrelated and geographically distant basidiocarps. The
pairings were incubated at 25 C in the dark and ob­
served after 4 wk.
Cytology.-Homokaryotic and heterokaryotic isolates
were placed on 1 x l -cm pieces of sterile cellophane
over CA and grown for 10 days in the dark at 25 C.
The cellophane with fungal colonies was removed and
dried on a slide warmer at 50 C for 10 min. Samples
were then fixed in Singleton'S fixative (Singleton, 1953)
before staining with Giemsa (Colotelo and Grinchen­
ko, 1962).
Heterokaryotic pairings. -Mycelial interactions be­
tween heterokaryotic individuals that were genetically
different (from different, geographically distant basi­
diocarps) were investigated by pairing inoculum plugs,
approximately 3 x 3 x 3 mm, 5 mm apart in petri
plates containing 3% malt agar (MA). The cultures
were grown for 3 wk in the dark at 25 C.
Fruiting in culture.-Four aspen poplar branch seg­
ments, approximately 1 cm in diam and 4 cm long,
were placed in a I-L erlenmeyer flask with 100 ml of
599
MALLETI AND MYRHOLM: PHELLINUS TREMULAE MATING SYSTEM
distilled water before autoclaving for 1 h. Flasks were
cooled over night before adding 10
, 0 ml of malt-pep­
tone-dextrose broth (30 g malt extract, 1 g Bacto­
peptone, 20 g dextrose, and 1 L distilled water) and
then autoclaving for an additional 40 min. The branch­
broth mixture was inoculated with mycelia taken from
the margin of a colony that originated from a basid­
iocarp. The flasks' mouths were stuffed with sterile
cotton-batten. Two replicate flasks were made for each
of the three isolates (NoF-1456, NoF-1459, and NoF1464) used. The inoculated flasks were placed in growth
chambers and grown at 25 C until mycelia covered the
segments. The cultures were then subjected, every day
for 6 months, to 12 h of light and a temperature of
15 C.
RESULTS
Growth of P. tremulae was initially compared on 3%
MA and CA. The resultant colonies were substantially
different in radial growth and appearance after 3 wk
incubation. On CA, heterokaryotic and homokaryotic
isolates of P. tremulae grew faster and had thicker more
luxuriant aerial mycelia. The mycelial mat was similar
to the "bleaching type" of colony that Hopp (1936)
and Niemela (1977) described for P. tremulae. On 3%
MA the colonies resembled those described as the
"staining type" (Hopp, 1936; Niemela, 1977). They
were slow growing with little aerial mycelia and pro­
duced a deep red-brown stain in the agar. Initial mat­
ing studies utilizing 3% MA were inconclusive because
of the "staining type" of colonies. No reactions were
observed in the interactions of the mycelia, possibly
because so little aerial mycelia is produced by the
"staining type" of colony. As a result, CA was chosen
for all further mating studies.
Mating studies utilizing CA showed that P. tremulae
has a tetrapolar mating system (TABLE II). Matings of
homokaryotic cultures from the other basidiocarps also
had a tetrapolar pattern. The five types of reactions ob­
served with homokaryotic pairings are shown in FIG. 1.
Compatible reactions could be recognized by the
growth of a secondary mycelium over the two paired
colonies after 3-4 wk of incubation. The secondary
mycelia was mottled white, appressed, and not as lux­
uriant as compared to the homokaryotic mycelia.
Three types of incompatible reactions were ob­
served. The first type of incompatible reaction resulted
in a "line of demarcation" between the two colonies.
The mycelia along this line were raised and entangled.
The second was the formation of a "bow-tie" similar
to that described by Coates et al. (1981). In some re­
actions the "bow-tie" did not occur but a white line
formed in the mycelia between the two colonies. The
third, was a "stop" reaction. The two colonies met but
no further interaction occurred. When a homokary-
TABLE II.
Mating interactions between homokaryotic iso­
lates of Phellinus tremulae isolate NoF-1484
Isolate no.
40
20
27
38
45
c
c
c
42
c
c
c
66
c
c
c
40
69
-'
69
59
c
c
60
c
c
21
c
c
44
c
c
59
60
21
44
cb
C
C
c
c
c
c
c
45
42
66
20
c
c
c
27
c
c
c
38
c
c
c
• A "-" indicates an incompatible interaction, no sec­
ondary mycelium formed.
b A "c" indicates a compatible interaction, secondary my­
celium formed.
otic isolate was paired with itself, the colonies grew
into each other forming one colony.
To confirm compatible and incompatible reactions,
pieces of agar from the interaction zones were cut out
and plated onto fresh CA. FIGURE 2 shows the various
types of compatible and incompatible reactions after
the agar strips were plated on carrot agar. Compatible
reactions resulted in one colony; incompatible reac­
tions resulted in two or more colonies.
A comparison of colony radial growth was made
between heterokaryons and homokaryons using the
Mann-Whitney rank sum test (Zar, 1974). The median
colony diameter for the heterokaryon isolates was 1.9
cm compared to 2.0 cm for the homokaryotic isolates.
The difference between heterokaryon and homokar­
yon cultures in median colony diameter was not sta­
tistically significant (P
0.125). Growth rate could
not be used to distinguish between homokaryon iso­
lates and putative heterokaryon isolates (those derived
from a compatible pairing).
Heterokaryon-homokaryon pairing results are shown
in FIG. 3 and confirm a tetrapolar mating system. Lines
of demarcation formed between a homokaryotic tester
and a putative heterokaryotic isolate. No lines of de­
marcation formed between an isolate taken from a
putative incompatible mating and a homokaryotic tes­
ter. In both cases the homokaryotic colonies formed
a secondary mycelia.
Homokaryotic cells were multinucleate but the nu­
clei did not appear paired; however, paired nuclei were
seen in many heterokaryotic cells. Numbers of paired
nuclei per cell varied; however, detailed counts of nu­
clei were not done.
=
600
FIGS, I, 2,
MYCOLOGIA
Mating reactions of IJhellimLI treml/lal! on carrot agar. 1, Compatible and incompatible reactions betwccn
homokaryotic isolates taken from the basidiocarp of P. tremulae (NoF-1487); a, "stop" incompatible reaction; b, compatible
reaction with secondary mycelium; c, homokaryotic isolate paired with itself; d, "bOlv-tie" incompatible reaction; e, "line of
demarcation" incompatible reanion, 2, Colonies of I', tremulae grown from inoculum taken from the interaction zones of
incolllpatible and compatible re:1(tiol1s, a, "bow-tie" incolnpatible reaction; h, "stop" reactioIl incompatible reaction; (, "linc
of demarcation" incompatible reaction,; ct, compatible reaction,
Compatible and incompatible reactions were stud­
ied using the light microscope, Microscopically, the
colony resuit.ing from a compatible reaction was sim­
ilar to that taken from a heterokaryotic culture derived
a basidiocarp, Mycelia from the "bow-tic" and
"line of demarcation" reaction exhibited hyphal knot­
ting. Mycelia from the "stop reaction" did not inter­
mingle,
from
MALLETT AND MYRHOLM: PHELLINUS TREMULAE MATING SYSTEM
601
Mating experiments were repeated using mall. yeast
agar (MYA) [3% (w:v) malt extract, 0.1 % yeast extract,
and 2% agar]. Fischer (1987) used this media for mat­
ing studies of Phellinus. A comparison of the reactions
on MYA to those OJI CA are shown in FJ(;. 4. The
"bow-tie" and the compatible reaction could not be
distinguished on MYA.
Homokaryotic isolates with representative mating
types from the 12 basidiocarps were paired in every
possible combination. All of the homokaryotic isolates
from the I 2 basidiocarps were compatible when paired
with hOI1lokaryotic isolates from different basidi­
ocarps. This indicates that the two genes responsible
for mating in P. tremulae arc multiallelic.
Heterokalyotic isolates that differed genetically were
paired on :{% MA. Zones of antagonism formed where
the two colonies met. The hyphae in this zone were
melanized.
No basidiocarps were observed to form in the 6
months of culture 011 aspen poplar branch sq,'1l1ents
and malt-peptone-dextrose broth.
DISCUSSION
The tetrapolar mating pattern for P. trernulae de­
scribed in this study differs from Fischer's (1987) find­
ings. Fischer paired homokaryotic isolates from a sin­
gle basidiocarp on MYA and described rwo distinct
reactions: an incompatible reaction in which the my­
celia from the two hOlllokaryotic isolates met and
formed a demarcation line, and a compatible react.ion
where, what he called, a "crossing-mycelium" was
formed in the reaction zone. These results led him to
conclude that P. {remulae had a bipolar mating system.
The same reactions were observed in this study when
hOIllokaryotic isolates were paired on MYA; however,
mating reactions on CA allowed the differentiation of
the compatible reaction from the "bow-tic" reaction
which supports a tetrapolar mating system hypothesis.
This result was further substantiated by the colony
Flcs. ;),4.
Phellinus IrelllnlaF colonies resulting from hel­
erokaryol1-ho!llokaryoll pairing and hOlllokaryon (Tosses. ,�.
P. Itemulal' isolates from putative incompatible and com­
patible reactions with two unrelated P. 11'l'lIIl1/ac hOIllokary­
otic isolates; a, inoculum taken from the interaction zone of
a putativc incompatible crossing paired with two unrelated
hornokaryolic isolates; b, a putative heterokaryon P. Iremulae
isolate paired with two unrelated hOlllokaryotic isolates. Ar­
rows indicate zones of antagonism. 4. IloJl]okaryoIl crosses
on carrot agar (petri plates 011 right) and malt yeast agar
(petri plates on left);
<I,
compatible reaction; b, ·'line of de­
marcation" incompatible reaction; c, ·'bow-l.ie" incompati­
ble feact ion.
602
MYCOLOGIA
development when strips of agar from the interaction
zones were plated on fresh agar. Only single colonies
developed from putative compatible reactions, where­
as two to three morphologically distinct colonies de­
veloped from incompatible reactions.
The "bow-tie" reaction has been noted in Stereum
hirsutum (Willd.:Fr.) S.F. Gray (Coates et aI., 1981),
Stereum gausapatum Fr. (Boddy and Rayner, 1982), and
Mycena galopus (Pers.:Fr.) Kummer (Frankland, 1984).
Coates and Rayner (1985) provided evidence that the
"bow-tie" reaction was due to heterozygosity at a sin­
gle, mutiallelic locus they termed the B-factor. Al­
though Stereum hirsutum has a bipolar mating system,
Coates and Rayner (1985) suggested that this factor
may be similar in function to the B-mating factor in
tetrapolar species. No further genetic analysis has been
done on the "bow-tie" phenomenon in P. tremulae.
Results from heterokaryon-homokaryon pairing ex­
periments are also indicative of a tetrapolar mating
system. When synthesized putative heterokaryons were
paired with unrelated homokaryons, a secondary my­
celium developed on the homokaryon colony and a
zone of antagonism formed between the two colonies.
This reaction indicates that there were two nuclear
types in the original putative heterokaryon colony and
that a "di-mon" type mating occurred. The zone of
antagonism formed because of somatic incompatibil­
ity. When an inoculum plug from the reaction zone
of an incompatible mating ("bow-tie", "line of de­
marcation", or "stop" reaction) was paired with an
unrelated homokaryon, a secondary mycelium formed
over both colonies and no zone of antagonism was
observed. This indicated that only a single nuclear type
was present in the original incompatible plug. Coates
et ai. (1981) and Angwin and Hansen (1989, 1993) have
reported similar observations in S. hirsutum and Phel­
linus weirii (Murr.) Gilb.
Compatible and incompatible reactions could also
be recognized on the basis of cytological characters.
Incompatible pairings had knotted or twisted hyphae
in the reaction zone. Paired nuclei were often seen in
heterokaryotic hyphae as well as in hyphae found in
the reaction zone of incompatible pairings. Numbers
of paired nuclei often varied. Paired nuclei were sel­
dom observed in homokaryons. These observations
are in contrast to those made by Hennon and Hansen
(1987) who found that paired nuclei occurred in homo­
karyotic mycelia nearly as frequent as in heterokaryotic
mycelia. They also found that there was a high fre­
quency of unpaired nuclei in heterokaryotic hyphae.
Heterokaryotic cultures are difficult to distinguish
from homokaryotic cultures except after a compatible
mating when a secondary mycelium is formed. Hen­
non and Hansen (1987) found that heterokaryotic cul­
tures grew faster than homokaryotic cultures. We
found growth rate to be an unreliable indicator of
whether a culture is homokaryotic or heterokaryotic,
because there was no statistically significant difference
in the growth rates of heterokaryotic and homokary­
otic cultures.
Vegetative incompatibility between genetically dif­
ferent heterokaryotic isolates was observed. No zones
of antagonism were formed when an isolate was paired
with itself. These observations confirm those made by
Verrall (1936). Vegetative incompatibility occurs in
many basidiomycete species (Adams and Roth, 1967;
Barrett and Uscuplic, 1971; Hansen, 1979; Mallett et
al., 1989) and has been used in population structure
studies (Adaskaveg and Gilbertson, 1987; Mallett and
Harrison, 1988; Dahlberg and Stenlid, 1990).
We were unable to fruit P. tremulae in culture and
therefore could not conclusively prove that the fungus
has a tetrapolar mating system. The reasons for the
failure to fruit are unknown as Badcock (1943) and
Montinaro et ai. (1993) were successful in fruiting Phel­
linus ignarius (L: Fr.) QueI. in culture. Fischer (1987)
was able to fruit some Phellinus species in culture but
was unsuccessful in his attempts to fruit P. tremulae.
Fischer (1987) studied the mating system of 20 dif­
ferent Phellinus species, 12 of which (P. ignarius, P.
cinereus (Niemela) Fischer, P. ossatus Fischer, P. con­
chatus (Fr.) QueI., P. laevigatus (Fr.) Bourd. & Galz.,
P. nigricans Niemela, P. pomaceaus (Pers.) Maire , P.
populicola, P. tremulae, P. torulosus (Pers.:Pers.) Bourd.
& Galz., P. pini (Brot.: Fr.) Ames, P. chrysoloma (Fr.)
Donk) were heterothallic and bipolar; the rest (P. fer­
ruginosus (Schrad.: Fr.) Pat., P. hartigii (Allesch. &
Schnabl) Bond., P. hippophaecolaJahn, P. nigrolimitatus
(Romell) Bourd. & Galz., P. punctatus (Fr.) Pilat, P. ribes
(Schum.: Fr.)P. Karst., P. robustus (P. Karst.) Bourd.
& Galz., P. viticola (Schw. ex Fr.) Donk) were homo­
thallic. The mating systems of these other Phellinus
species should be reexamined in light of the findings
presented here.
This study provides evidence that the mating system
of P. tremulae is tetrapolar rather than bipolar as pre­
viously described. The mating factor genes are mul­
tiallelic. Nuclei appear to be paired in heterokaryotic
isolates, although this requires further study. Vege­
tative incompatibility can be used to distinguish ge­
netically different heterokaryotic isolates.
ACKNOWLEDGMENTS
The authors thank Dr. A. Hopkin for providing the Ontario
collections.
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