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BEN # 178
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No. 178 November 25, 1997
aceska@freenet.victoria.bc.ca Victoria, B.C.
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Dr. A. Ceska, P.O.Box 8546, Victoria, B.C. Canada V8W 3S2
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EVOLUTION IN ACTION: FROM MUSHROOMS TO TRUFFLES? PART 2
From: Dr. Bryce Kendrick <mycolog@pacificcoast.net>
[Kendrick, B. 1994. Evolution in action: from mushrooms to
truffles. II. McIlvainea 11 (2): 39-47.]
In the first article, I described how various members of the
mushroom genus Lactarius (family Russulaceae, order Agaricales)
had evolved into rather strange forms. They had kept their
distinctive microscopic characters: latex-producing cells which
exude a unique milky fluid when broken; thin- walled, swollen
sphaerocysts which make the tissues of the mushroom characteris-
tically brittle; and a distinctive spore ornamentation of spines
and ridges which often form a network, and which stain dark blue
or almost black in iodine (what we call the amyloid, I+, or
starch-like reaction). But the fruit bodies had taken on a
distinctive appearance and also appeared to function rather
differently.
In these evolutionary offshoots, three things have changed: (1)
the peridium remains attached to the stipe at maturity, so the
gills are not exposed to the outside atmosphere; (2) the gills
are no longer plate- like, and are not oriented in a precise
vertical plane; and (3) the spores are not forcibly discharged
from the sterigmata. So despite having the characters listed
earlier as being diagnostic of Lactarius, these forms are put in
a separate genus, Arcangeliella, because the differences, espe-
cially the loss of the spore-shooting mechanism so characteris-
tic of most basidiomycetes, are regarded as being of some basic
biological importance. They affect the reproductive strategy of
the organisms and therefore need to be taken account of when the
taxonomy of the group is being established.
There are also even more reduced forms, in which the fruit body
develops underground, the stipe is lost, and the gill tissues
have become so folded and convoluted as to assume a spongy,
chambered appearance: they are no longer gills, though they
still bear basidia and produce basidiospores. So although these
forms still have latex, sphaerocysts and amyloid spore ornamen-
tation, they have been segregated in a third genus, Zel-
leromyces.
I concluded by saying that the Lactarius - Arcangeliella -
Zelleromyces evolutionary pathway is not unique. In this second
article, I will describe other similar developmental phenomena
that have come to light, and the way in which they are now being
interpreted.
The family Russulaceae, as understood by many mycologists,
contains only two genera. We have already looked at one of them,
Lactarius. Now let's consider the other one, Russula. This genus
is very easy to recognize in the field, and (along with Lac-
tarius) is one of the first genera the beginning amateur
mycologist learns to identify. Russula has substantial fruit
bodies, often with brightly coloured caps, stout stipes, and
beautifully regular, white or cream-coloured gills. The caps,
stipes and gills are brittle because their tissues contain
clusters of round, thin-walled, turgid sphaerocysts. And the
basidiospores have spiny, ridged and often net-like ornamenta-
tion that stains blue in iodine. Russula shares these two
characters with Lactarius (which is why they are in the same
family: these features are not found in any other agarics). But
Russula has no laticiferous cells, and so does not produce latex
(milk). This immediately distinguishes it from Lactarius, the
milky cap, at least in most young, fresh collections.
Specimens are sometimes found which match the genus Russula in
most ways, yet the peridium remains intact, attached to the
stipe, and the gills are not exposed, even at maturity. In such
specimens it will be seen that the hymenium has become highly
convoluted or lacunose. Microscopic examination shows that
sphaerocysts are present in the tissues, and the basidiospores
do have blue-staining ornamentation; but although the attachment
of the spores to the sterigmata is still somewhat asymmetrical
or offset, those spores are not forcibly discharged. That is
enough to exclude these specimens from Russula, and they have
been placed in a separate genus, Macowanites.
Other atypical russuloid fungi have been found which resemble
Macowanites in many ways: they still have sphaerocysts
throughout the tissues, and spores with amyloid ornamentation.
But they develop underground, and do not emerge, even at
maturity. The external stipe has been lost, although a stipe
remnant, in the form of a vertical column of sterile tissue, may
still run through the fruit body. The spores, which are not
forcibly liberated, are now symmetrically attached to their
sterigmata. And the hymenium is no longer on recognizable gills,
but lines convoluted or labyrinthine chambers. These specimens
are segregated in the truffle-like genus Gymnomyces.
But this is not all. A second line of reduced forms appears to
have originated from Russula. Some of these resemble Russula in
many ways, having a stalk and a cap, sphaerocysts in the outer
tissues and spores with amyloid ornamentation. But the gills
have entirely lost their vertical orientation and perhaps even
their integrity. The fruit body is now filled with a spongy mass
in which the hymenium lines finely convoluted chambers whose
walls lack sphaerocysts. And although the spores are asymmetri-
cally mounted on the sterigmata, they are not discharged. This
is the genus Elasmomyces.
Other specimens, while retaining sphaerocysts in their outer
tissues and amyloid spore ornamentation, have retreated (or
rather, remained) underground, have lost their stalk, and have
become essentially truffle-like. Their internal arrangements are
rather like those of Gymnomyces, but although they have
sphaerocysts in their outer tissues, they have none in the walls
of the hymenial chambers. These fungi are placed in the genus
Martellia.
So, with a little imagination, we can visualize three lines of
evolution, beginning with "normal" members of the family Rus-
sulaceae, mushrooms like Russula and Lactarius, and ending in
truffle-like fungi which fruit underground.
Lactarius -> Arcangeliella -> Zelleromyces
Russula -> Macowanites -> Gymnomyces
Russula -> Elasmomyces -> Martellia.
Notice that the Russulaceae really contains not just two, but no
fewer than eight genera, and that six of them, while microscopi-
cally "correct," do not give spore prints.
By now, you may suspect that there must be other such strange
evolutionary pathways hiding among the rest of the agarics, and
even in other groups of fungi. And your suspicion would be
correct.
In fact, no fewer than 14 _ yes fourteen _ mushroom families
have given rise to closed or underground forms which are treated
as separate taxa. Let me sketch for you these lines of evolution
as they are understood at present:
(1) Russulaceae - see above
(2) Cortinariaceae: the genus Cortinarius gets its name from
the presence on the expanding basidioma of a special filamentous
or cobwebby partial veil called a cortina (from the Italian for
curtain). Many species also have brightly coloured caps. The
basidiospores are rusty-brown in mass, and characteristically
ornamented. Cortinarius has some species in which the partial
veil does not open. But since the basidia still shoot their
spores (they end up sitting on the inside of the veil), these
species are retained in Cortinarius. In other Cortinarius-like
specimens, the cap also remains closed, but careful examination
shows that these have lost both the spore-shooting mechanism and
the vertical plate-like organization of the gills: a section
shows that the hymenium-bearing tissue has become convoluted and
labyrinthine or spongy. These "aberrant" forms have been placed
in the genus Thaxterogaster.
Some species of Thaxterogaster seem to have lost their external
stipe, but there is still a central column of white sterile
tissue running up the middle of the fruit body. Other offshoots
of Cortinarius have become entirely hypogeous, never emerging
above the surface of the soil. These have lost all semblance of
stipe and gills, look just like a truffle, and have been put in
the genus Hymenogaster, although their basidiospores still
closely resemble those of Cortinarius.
(3) Agaricaceae: the genus Agaricus has given rise to se-
questrate forms placed in the genera Endoptychum and Longula.
(4) Lepiotaceae: Notholepiota is a sequestrate member of this
family.
(5) Amanitaceae: Torrendia is a sequestrate segregate of
Amanita.
(6) Bolbitiaceae: this family has given rise to a common and
widespread sequestrate form called Gastrocybe. This is a strange
fungus which appears in the grass during hot, humid weather. A
narrowly conical, wet-looking brown cap arises on a long, nar-
row, delicate white stipe, which soon flops over. The spores sit
squarely and persistently on the sterigmata. The whole cap soon
dissolves into a slimy mass, which sticks to the grass. The
spores never become airborne. We tend to assume that these
spores are dispersed by grazing arthropods, although there is as
yet no hard data to support that hypothesis.
(7) Coprinaceae: Coprinus has given rise to a sequestrate
form which is known as the desert shaggy mane. This fungus,
which is put into the genus Podaxis, looks externally very like
Coprinus comatus. Yet when a mature cap is cut open, the inside
is seen to be filled, not with closely-packed, upwardly deli-
quescing gills, but with a dry mass of black spores, which will
eventually blow away like dust when the outer skin of the fruit
body erodes away or breaks. I have an excellent videotape se-
quence of this happening to a large specimen growing out of a
termite mound in Africa (the Podaxis, unlike Termitomyces,
apparently does not enjoy a mutualistically symbiotic relation-
ship with the termites). The relationship of Podaxis with
Coprinus is confirmed by the fact that under wet conditions,
Podaxis, too, can undergo some deliquescence or self-digestion.
(8) Strophariaceae: Stropharia is the presumed ancestor of
the sequestrate genera Nivatogastrium and Weraroa.
(9) Entolomataceae: Entoloma has spawned the sequestrate
Richonia, the relationship being established by the pink colour
and the distinctive angular shape of Richonia spores, which are
almost identical to the spores of Entoloma itself. Nolanea may
have given rise to Rhodogaster.
(10) Tricholomataceae: Hydnangium appears to be a sequestrate
derivative of Laccaria.
(11) Gomphidiaceae: Gomphidius has hived off the sequestrate
genus Gomphigaster, and Chroogomphus has produced Brauniellula.
(12) Paxillaceae: Austrogaster and Gymnopaxillus are se-
questrate derivatives.
(13) Boletaceae: Boletus, Suillus and Leccinum have spawned
above-ground sequestrate forms in Gastroboletus, Gastrosuillus
and Gastroleccinum. Alpova, Truncocolumella and the extremely
common Rhizopogon are below-ground, sequestrate derivatives of
Suillus. The techniques of molecular biology have recently shown
that, at least for certain parts of its genome, Rhizopogon is
very closely related to the epigeous, spore-shooting Suillus
(more closely, in fact, than Suillus is related to other genera
of boletes).
(14) Strobilomycetaceae: Gautieria is a fairly common
hypogeous derivative, probably of Boletellus.
[Continuation in BEN # 179]
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