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BEN # 177
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No. 177 November 21, 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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SEQUESTRATE BEN - IT STARTED WITH MACOWANITES
On October 11 and 12, 1997, the South Vancouver Island Mycologi-
cal Society organized a mushroom foray to the Pacific Rim Na-
tional Park near Tofino, British Columbia. With the help of
mushroom experts (Bryce Kendrick, Paul Kroeger, Ian Gibbson,
Oluna Ceska, Tony Trofimow, et al.) we compiled a list of about
140 mushroom species. One of the most interesting finds, at
least for me, was Macowanites chlorinosmus.
Macowanites looks like Russula that has never made it: an un-
opened ball with contorted gills inside. Dr. Bryce Kendrick
explained to us that Macowanites is a member of the so-called
sequestrate fungi (also called secotioid or gastroid fungi),
mushrooms that follow the example of truffles and remain buried
underground, or grow close to the soil surface. They don't
release their spores, but rely on animals for spreading their
spores around.
I asked Dr. Bryce Kendrick to write me a short note on these
mushrooms for BEN, and he sent me two articles that he published
in McIlvainea. I read these articles and realized that a short
note would not do justice to those interesting fungi. I have
decided to post them in full and the next three issues of BEN
(177, 178, and 179) will be fully devoted to this topic.
I apologize to all of you who believe that mushrooms really do
not belong to the botanical realm. I will post these BEN issues
in short intervals of two or three days. This will ensure some
degree of continuity, and at the same time, it won't fill your
mail boxes entirely. I hope you will enjoy this sequestrate
diversion. - Adolf Ceska
EVOLUTION IN ACTION: FROM MUSHROOMS TO TRUFFLES? PART 1
From: Dr. Bryce Kendrick <mycolog@pacificcoast.net>
[Kendrick, B. 1994. Evolution in action: from mushrooms to
truffles. I. McIlvainea 11 (2): 34-38.]
The fungi are very old. Their history extends over hundreds of
millions of years. Yet their origins, and the major evolutionary
pathways they have followed, are still cloaked in mystery. This
is largely because the fossil record of the fungi is fragmentary
and disconnected. Organisms that live on land, and particularly
such ephemera as most fungal fructifications, are much less
likely to be fossilized than are marine organisms with hard
parts. The paucity of fossil evidence has not deterred the
cognoscenti among mycologists from a little judicious specula-
tion, inevitably based largely on what we know about fungi that
are alive today. This speculation is probably wrong in many
respects and is usually heavily laced with the prejudices of its
authors, but it is not necessarily a bad thing: students of the
fungi need a conceptual framework on which to cut their teeth,
and at which to aim their more mature criticisms. But we are
still on shaky ground when we try to look into the evolutionary
history of most modern fungi.
It is, then, all the more exciting to encounter an area of
mycology in which not only the results of evolution, but also
the starting points, and the steps in the process, can still be
seen in living organisms. And all this has happened, not in
obscure microscopic fungi, but in conspicuous and fairly common
mushrooms that can be held in the hand and compared. We now know
that some mushrooms have given rise to radically changed but
still viable descendants: relatives which, although often look-
ing very different from their forebears, clearly betray their
ancestry at the microscopic and molecular level. Let us see how
this has happened in the well-known and easily recognized mush-
room genus Lactarius (the "milky-caps": family Russulaceae,
order Agaricales).
But before I describe the exciting changes we have seen, I must
establish a base-line or starting point by describing how mush-
rooms usually develop, and what they do: how their form and
function are interrelated. First, the thread-like mycelium,
which permeates the soil and is often involved in an intimate
and mutually beneficial mycorrhizal association with tree roots,
must accumulate considerable reserves of food energy. Then
conditions of temperature and moisture must be favourable.
Finally, the mushroom begins its development underground, form-
ing a clump of mycelium which differentiates into a "button,"
then rapidly expands upward and emerges from the earth as a
characteristic structure with a central stalk or stipe, bearing
an expanding circular cap or pileus. The top of the cap is
covered by a skin or cutis. The thin, plate-like gills of Lac-
tarius normally develop in a neat radial pattern (like the
spokes of a wheel) on the under side of the expanding cap. The
basidiospores will form on these gills. As the cap opens out
like an umbrella, the gills assume a precise vertical orienta-
tion and are now ready to make and liberate spores.
The flat surfaces of the gills are covered by a fertile layer
called a hymenium. This contains huge numbers of special cells
called basidia, which produce and liberate astronomical numbers
of spores. Each basidium bears four spores (sometimes more,
occasionally fewer). These develop asymmetrically, in an offset
manner, at the tips of four sterigmata, which are tiny projec-
tions from the mother cell. When ripe, the spores are delicately
but deliberately launched into the air between the gills. They
float slowly and gently downward until they emerge from the
gills, and are then carried away like dust by air movement. In
this way the fungus broadcasts its spores far and wide. The
different genera and families of agarics often follow sig-
nificantly different developmental pathways, some with gills
exposed from the beginning, others with gills enclosed almost
until maturity, but they all eventually arrive at the same end-
point, with vertical, exposed gills dropping spores into the
air.
One of the ways in which we identify agarics is by placing a cap
on a piece of white paper in a draft- free place and letting it
drop millions of spores onto the paper overnight. The deposit
will form a visible radiating pattern, which reflects the ar-
rangement of the gills from which the spores came. This spore
print may be white, cream, pink, brown or black, according to
the mushroom genus which produces it. The spore print of Lac-
tarius is white or cream coloured.
Everything I have said so far applies not just to Lactarius, but
also to many other genera of mushrooms. So how does Lactarius
differ from the rest? That's easy: it has a unique combination
of three features which are not found together in any other
genus of agarics.
[1] The cap and gills of Lactarius contain special cells
filled with a milky juice or latex (white, yellow, orange or
red) that oozes out in visible drops when the tissues are cut
(and sometimes change colour after exposure to air).
[2] The flesh of Lactarius contains large numbers of
swollen, thin-walled cells called sphaerocysts: these make the
flesh extremely and characteristically brittle and granular.
[3] The spores of Lactarius are ornamented with con-
spicuous warts and spines, lines and ridges, which often join up
to form a network. These ornamentations are chemically different
from the rest of the spore wall, because they stain darkly
(grey, blue, purple or black) in iodine, while the spore wall
itself remains unstained, or stains only slightly. Ornamentation
that gives this colour reaction is often described as iodine-
positive, or amyloid.
Even beginners can easily identify Lactarius by the milky juice
it exudes when the brittle flesh is broken: no other ordinary
mushroom has anything like it.
But in addition to specimens of Lactarius as I have just
described it, we occasionally find extraordinary specimens.
Specimens which have a few important differences from the milky
caps we are used to seeing. They are similar (and theoretically
could therefore be included in the genus) because they have all
three characters listed above: brittle flesh full of
sphaerocysts; latex exuded when the tissues are ruptured; and
spores with ornamentation that is iodine-positive. Yet they are
different because their cap develops in such a way as to enclose
their gills, and the gills are no longer vertical plates, but
have become crumpled or convoluted to form a spongy, chambered
mass. Since the cap remains closed, the spores obviously cannot
escape. This sounds as if it would be a serious problem for the
fungus: after all, have not mushrooms evolved to be spore-making
and spore-launching machines? And if the spores are not released
into the atmosphere, how will they be dispersed? Yet if we
remember that the lungs of land vertebrates evolved from the
swim-bladders of their fish ancestors, and the wings of birds
from the forearms of their earthbound reptilian ancestors, we
will appreciate that evolution, guided by environmental forces,
often drives organisms in unforeseen directions. Something of
this kind appears to be happening to the Lactarius, and we must
assume that some other way of dispersing the spores has been
evolved.
If we now cut away the edge of the unopened cap which is obscur-
ing the gills, and try to make a spore print, we will not
succeed. No spores will be deposited. This is not because the
mushroom is either unripe or overmature. If we examine some of
the basidia under a microscope, we will see that they have
produced mature basidiospores. But the basidia have subtly
changed. The four spores tend to develop symmetrically (not
offset) on the sterigmata, and they tend to remain attached to
the sterigmata: they are never forcibly discharged, as they were
in normal Lactarius fruit bodies.
These differences are important enough for taxonomists to con-
clude that the fungus can no longer be called a Lactarius, and
it has been placed in a different genus, named Arcangeliella.
This genus has sometimes been excluded from the family Rus-
sulaceae and even from the order Agaricales, and has instead
been put in a separate order, the Hymenogastrales. But there is
no doubt that it has evolved from Lactarius in relatively recent
times, that it is still closely related to that genus, and that
it should be retained in the Agaricales, and even in the Rus-
sulaceae.
Arcangeliella still looks very like a mushroom, even if its
behaviour is a little strange. But we have found other specimens
which have evolved even further away from Lactarius. These
specimens develop, and remain, just below the surface of the
ground, looking rather like truffles. They are rounded or ir-
regular in shape. The skin that covered the Lactarius now com-
pletely surrounds the truffle-like specimens. They have no
stalk. There are no gills: the hymenium lines labyrinthine
chambers. And the basidiospores, now sitting straight on the
sterigmata of the basidia, are not actively shot away.
Note that the outer skin and often the walls of the labyrinthine
spore-bearing tissues contain sphaerocysts; latex oozes from the
cut surfaces of fresh specimens; and the spores have spiny or
ridged ornamentation that stains dark in iodine. Once again, the
three diagnostic characters of Lactarius. A vestige of a stalk
may even occur in the form of a pad of sterile tissue inside the
base of the fruit body; the walls of the labyrinthine chambers
could be derived from crumpled gills; and the presence of
sterigmata on the basidia is a reminder that these structures
were originally evolved as part of a mechanism to launch spores
into the air.
Yet it would be stretching the concept of Lactarius beyond the
breaking point to include these specimens in it: surely no-one
would call them agarics. It is also clear that they are con-
siderably more "reduced" even than those placed in Arcan-
geliella. So mycologists put them in another new genus, called
Zelleromyces.
Although Zelleromyces differs from both Arcangeliella and Lac-
tarius in important ways, the fact that it has latex,
sphaerocysts and iodine-positive (amyloid) spore ornamentation
is a compelling argument for keeping it in the family Rus-
sulaceae of the order Agaricales. After all, this disposition
seems to best reflect its true relationships. Arcangeliella and
Zelleromyces are what we now call sequestrate (see the note
below) derivatives of the original agaric. The word sequestrate
implies that they sequester or retain their spores, rather than
broadcasting them into the air. This retentive habit, diagnosed
by spores sitting symmetrically on the sterigmata of non-
shooting basidia, is clearly characteristic of both genera.
Before drawing the first part of this discussion to a close, I
must address one final issue. If these sequestrate genera share
all the essential diagnostic features of Lactarius, how are we
to distinguish the Lactarius we all know from its sequestrate
derivatives? It is apparent that the three diagnostic characters
I described earlier must be supplemented by three more, as
follows:
[4] the cap of a true Lactarius expands at maturity and
the gills are exposed.
[5] its gills are vertically oriented.
[6] its basidiospores are asymmetrically mounted on the
sterigmata and are forcibly discharged at maturity.
If the Lactarius -> Arcangeliella -> Zelleromyces sequence was
the only case in which this strange evolutionary sequence had
been observed, we might be able to dismiss it as a quirk of
evolution, a freak. But we have evidence that similar pathways
have evolved in other mushroom genera. These will be explored in
the second part of this article, in the next two issues of BEN.
The term "sequestrate" has recently been introduced (Kendrick
1992) to describe all these closed or hypogeous offshoots of
regular fungi. It means that the spores are sequestered or
hidden away, kept from contact with the outside world, at least
until the fruit body decays or is eaten. The term sequestrate
appears to be a more useful and more widely applicable term than
such frequently-used words as `gastroid' (which inappropriately
implies close relationship with gasteromycetes) and `secotioid,'
an arcane word suggesting similarity with the genus Secotium
(which is a sequestrate derivative of Agaricus). Most amateur
and many professional mycologists have never seen Secotium, so
the term derived from that name conveys little or no meaning.
[Continuation in BEN # 178]
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