THIS
VIEW OF LIFE
A
HUMONGOUS FUNGUS AMONG US
By: Gould, S.J., Natural History, Jul92, Vol. 101, Issue 7
The
discovery of an enormous underground organism raises some basic questions about
individuality
When
an animal achieves disembodied immortality by becoming a verb, human speakers
usually honor its behavior: we hawk our wares, gull or buffalo our naive
competitors, hound our adversaries, and clam up in the face of adversity; we
have also been known to man the barricades and kid around with our companions.
But plants and other rooted creatures do not feature so great a range of overt
actions, and our botanically based verbs therefore tout growth and appearance
as sources of metaphor.
Consider
the two most prominent examples, citing comparable phenomena but with such
different meanings-for one usually expresses our joy and the other our fear.
Art and prosperity ÒflowerÓ; taxes and urban violence Òmushroom.Ó The burden of
difference reflects an obvious source in our culture and legends. We love the bright
flowers of ÒhigherÓ plants, either radiant in the sunlight or jewellike in the
quiet darkness of a forest. We loathe the spongy, fruiting bodies of ÒlowlyÓ
fungi, growing in dank dampness, sprouting in cancerous formlessness from
rotting logs. (Even a colorful mushroom usually strikes us as sinister rather
than lovely.) I well remember a common schoolyard taunt, often cruelly directed
at unloved classmates: ÒThere's a fungus among usÓ--a cry that always inspired
the ritualistic retort: ÒKill it before it multiplies.Ó
Combine
this image of uncontrolled and noxious fungal growth with our general
fascination for superlatives-the biggest and most persistent in this case and
we can easily understand the flurry of press commentary that greeted a
technical article by M. L. Smith, J. N. Bruhn, and J. P. Anderson in the April
2, 1992, issue of Nature: ÒThe fungus Armillaria bulbosa is among the largest
and oldest living organisms.Ó On the same day, the New York Times reported the
discovery in a front page story with a more sprightly headline: ÒThirty-Acre
Fungus Called World's Largest Organism.Ó [Editor's note: As this column was
being prepared for press, the Associated Press reported the find of a larger
fungus, an Armillaria ostoyae, covering about two and a half square miles in
the state of Washington. Larger organisms probably will be found, but the
discoveries will not change the point of this piece.]
We
divide muticellular life into three great kingdoms: animals, plants, and fungi.
How does this new Armillaria stack up against the champions of the other
domains. Blue whales, by far the largest animals that ever lived (edging out
both Ultrasaurus and Supersaurus from the world of dinosaurs), may reach 100
feet and as many tons. (Incidentally, since female whales exceed males in size,
the largest individual animal of all time was undoubtedly female.) Sequoia
trees are bigger by far, sometimes exceeding 1,000 tons (although mostly in
nonliving wood), not to mention heights measured in hundreds of feet and years
in thousands.
Armillaria
bulbosa lives in and around tree roots in European and eastern North American
mixed hardwood forests. Clones begin as a single fertilized spore and then
spread out by vegetative growth. The basic unit of spread is a hypha, or thread-like
filament that forms the structural unit of growth in many fungi. In Armillaria,
the hyphae are then bundled into cordlike accumulations called rhizomorphs.
Since these rhizomorphs grow and extend underground (in and around tree roots),
a human observer sees nothing of this interwoven subterranean mat except for
the occasional and spatially discontinuous mushrooms that poke through the
forest floor.
Since
we mistake an individual mushroom body for a discrete organism, we might look
at the area of this Armillaria clone and, seeing nothing of the underground
continuity, view the species as consisting of a few widely scattered, entirely
separate items-a population of several insignificant individuals. But Smith and
his colleagues, working in a forest near Crystal Falls on the Upper Peninsula
of Michigan, found a region of some thirty acres underlain by the
interconnected rhizomorphs of a single Armillaria clone. Extrapolating from
best assessments of rhizomorph growth rates, they infer a minimal age of 1,500
years-probably a substantial underestimate since neighboring clones have
inhibited further spread for some time, while the calculation assumes
continuous expansion. They estimate the weight of rhizomorphs in the soil at
about ten tons, but this figure excludes much of the clone's bulk growing in
places harder to assess-for example, within tree roots and deep in the soil.
Moreover, this figure excludes the mass of fine hyphae that extend from the
rhizomorphs into surrounding soil and wood. Smith and his colleagues estimate
that the weight of the entire clone may exceed their figure by ten times or
more, yielding a total closer to 100 tons.
Thus,
this champion Armillaria clone may weigh as much as a blue whale (although
substantially less than a large sequoia) and probably lies in the ballpark of
the oldest tree for age. Its claim to Òfirstest with the mostestÓ must rest
upon the added criterion of lateral spread-in other words, of mushrooming.
Thirty acres, however thinly distributed in spots, makes quite a creature. Even
the Blob, of B-movie fame, will have to take a back seat.
As
so often happens in the first flurry of commentary on scientific findings, the
wrong aspects get emphasized and the truly interesting conceptual issues fall
into the background. I can get as het up as the next guy about claims for
maximal age, size, and weight. I know that Robert Wadlow nearly reached nine
feet, that Robert Earl Hughes exceeded 1,000 pounds, and that Mr. Izumi of
Japan lived past his 120th birthday. The Michigan clone of Armillaria, while
failing short in weight, and perhaps in age, does enter my mental list as the
biggest organic spread. But the deeper fascination of this tale lies
elsewhere-in the striking way that this underground fungal mat forces us to
wrestle with the vital biological (and philosophical) question of proper
definitions for individuality.
First
of all, the novelty of Smith, Bruhn, and Anderson's study lies not in the
discovery of something so big but in the establishment of criteria to test the
clone's status as an individual. Large and continuous underground mats of
fungal rhizomorphs have long been recognized. Heretofore, however, we had no
way of ascertaining whether or not they formed from a single source. Fungal
spores are everywhere (as we all know from our experiences with bread mold). A single mushroom can
produce up to a million fertile spores per hour over several days. With so many spores,
representing so many different clones, falling all over the forest floor, why
suspect that a large mat of rhizomorphs should represent growth from one
initiating spore, and therefore count as a single individual by genealogy? Why
not propose that such gigantic mats of rhizomorphs are actually congeries, or
aggregations, made of products grown from several founding spores (representing
many different parents), all twisted and matted together-in other words, a heap
rather than a person?
Spores
are certainly ubiquitous, but a constant rain does not guarantee a cooperative
matting together to form massive aggregations, for organisms have immunological
mechanisms for distinguishing self from nonself and rejecting amalgamation
(abutting coral colonies do not generally fuse on growing reefs), while
Darwinian theory views competition among different genetic units within a
species as a major thrust of life's game. Thus, single versus multiple sources
for large fungal mats has been viewed as an open and interesting question for
some time.
Genetic
tests are now available for resolving such an issue. Smith and colleagues first
sampled several portions of the Armillaria mat both for genes that determine
compatibility in breeding (called mating-type alleles) and for several
fragments of mitochondrial DNA. Both the mating-type and the mitochondrial
genes are highly variable within the species Armillaria bulbosa, but all
samples of the Michigan mat yielded the same array. This demonstrates close
genetic relationship but does not prove individuality, for how can we know
whether the mat is made from many highly inbred siblings or from a single
founder'? An additional genetic test then indicated a single founder. Repeated
close inbreeding leads to a marked reduction of genetic variability among
offspring-a presumed Òdeep reasonÓ behind our various laws and taboos against
incestuous marriage. Genes that are variable within an individual (called
heterozygous because the material and paternal copies differ) will tend to
become uniform in offspring if that individual mates with very close relatives.
Smith and colleagues traced markers of several DNA sequences from heterozygous
genes throughout the mat and found no reduction in their variability-a sign
that the entire mat has grown from a single source and does not represent an
incestuous amalgamation of numerous sibling subclones.
But
this elegant demonstration that the Michigan mat formed from a single source
only opens the more interesting and portentous issue of defining individuality
the central question, as we shall see, for applying Darwinian theory to nature.
The Michigan Armillaria mat grew vegetatively from a single source, but does it
qualify as an individual under our usual vernacular definitions? Clive Brasier
addressed this question in a commentary that accompanied Smith, Bruhn, and
Anderson's original article:
The
suggestion of Smith et al. that [the Michigan mat] deserves recognition as one
of the largest living organisms, rivaling the blue whale or the giant redwood,
invites closer scrutiny. The blue whale and redwood exhibit relatively
determinate growth within a defined boundary, whereas fungal mycelia do not.
In
other words, a whale is a whale, with flippers and tail, but the Michigan mat
just spreads. Moreover, we cannot be sure that the Michigan mat is truly
continuous throughout. Pieces may break off and become physically separated
from the main mass; but a whale's flipper, if broken off, is dead meat, not a
miniwhale. Brasier concludes, with justice:
So
although [the Michigan mat's] reputation as a champion genotype [discrete
genetic system I may yet be secure, its status as a champion organism depends
upon one's interpretation of the rules.
What,
then, shall we accept as definitive of individuality, and why is this issue
important to biological theory, not merely a verbal game? A single genetic
origin followed by growth in physical continuity might represent a good first
stab, but the Michigan mat raises a cardinal issue also illustrated by many
other creatures, including grass blades and bamboo stalks (although the problem
may only represent an unfortunate parochialism based upon our unfair
extrapolation of what we know about the character of our own bodies to a false
criterion for all life).
Our
canonical individuals are bounded entities with definable form-a whale, a tree,
a cockroach, a human being. By analogy, we wish to label a grass blade, a
bamboo stalk, or a mushroom as an individual. But consider the proposed
criterion of contiguity and unique genetic origin. A bamboo stalk looks like
what we call an entire plant in other circumstances, but each stalk in a field
may arise from a common system of underground runners, all united and all
formed by vegetative growth from a single seed. Isn't the bamboo stalk then
just an organ of this larger individual, as the mushrooms of Armillaria are
only the visible fruiting bodies of an underground totality? (Bamboo stalks are
giant grass blades, so the same argument applies to our lawns.)
Botanists,
more often than zoologists, must deal with this problem of apparent organs that
look like individuals (although a colony of coral animals raises exactly the
same issue). Botanists have therefore devised a special terminology to treat
these ambiguous cases of parts that are entire organisms in vernacular usage
but organs of a larger totality by genetic definition. They speak of the
morphologically well-defined part (the grass blade, bamboo stalk, or mushroom)
as a ramet and the entire interconnected system (the underground runners and
stalks, the mat of rhizomorphs with occasional mushroom buds) as a genet. In
other words, the vernacular individual is a ramet; the genetic individual, a
genet. This terminology does not solve the conceptual problem of defining
individuality but merely devises names to acknowledge the classical case of
inherent ambiguity.
The
vernacular and genetic definitions can be driven even further apart by
recognizing that even the chief character of connectedness can fail when we
advocate the genetic criterion. In 1977, zoologist Dan Janzen wrote a
provocative article on this subject in the American Naturalist, with the
subheading: ÒWhat Is an Aphid?Ó Many aphid species breed sexually just once a
year. Females born from this sexual union produce subsequent all-female
generations by parthenogenesis-that is, without fertilization. The parthenogenetic
offspring are all identical, both to their sisters and their mothers (except
for rare new mutations). In other words, they form a clone of genetically
identical bodies-and billions of aphids may ultimately arise from one
stem-mother. (Eventually, the females begin to produce some male offspring and
fertilization can then ensue but this is a different story.)
Now
what are all these aphid bodies in a single parthenogenetic clone? By any
vernacular criterion, they are individuals. They look like any other
unambiguously individual insect-like a cockroach, a ladybug, or a cricket. They
have mouths and six legs, just like any ordinary insect. Yet they are part of
one genetic system, all formed by an analogue of vegetative growth from a
single starting point (the stem-mother). The entire clone of separated aphid
bodies is just like the stalks of a bamboo field or the mushrooms of an
Armillaria mat, but without the underground connections. The aphid bodies are
disconnected ramets of a single genet. Why, Janzen argues, should we not label
the aphid bodies as pans and the entire clone as a single EI, or Òevolutionary
individualÓ? This redefinition has some startling consequences, as Janzen notes
(you may need to read this quotation twice to put yourself in its
unconventional framework, but the implications are surely arresting):
Each
EI grows rapidly by parthenogenesis, with occasional pieces (aphids) being
bitten out of it by parasites (in conventional discussions these would be
called ... predators). Only very rarely is an EI preyed upon (i.e., all of it
eaten), since part of its growth pattern is to spread itself very thinly over
the surface of the plants in its habitat, so thinly that a potential predator
is very unlikely to find all of it at once.
Before
this sequence of increasing ambiguity becomes any more maddening, let me
propose a different approach, and a potential solution. Terms are best defined
within the context of explanatory theories. Gravity may have a variety of
vernacular meanings, but its changing technical definition in the successive
formulations of Newton and Einstein define its character as a scientific
concept. Similarly, individual has a central role and meaning within Darwinian
theory, our ruling paradigm for understanding nature. Shouldn't we accept this
theoretical definition as our primary biological meaning? (The vernacular may
stray, but science cannot and should not control all ordinary usage.)
Darwin's
central postulate states that natural selection works upon individuals engaged
in a struggle
(metaphorical and without conscious intent to be sure) for reproductive
success. Individuals that leave more surviving offspring obtain a Darwinian
edge, and populations change thereby. Fine, but who is the ÒindividualÓ engaged
in such a struggle? Darwin gives us a clear answer: individuals are
organisms-that is, conventional bodies (with some nuancing for ambiguous cases
like mushrooms and aphids). Natural selection works on creatures-on individual
lions fighting for a limited supply of zebras; on individual trees struggling
with others for light.
This
emphasis on ordinary vernacular organisms was central to Darwin's radical
reformulation of nature, for he was consciously striving to overthrow the
comforting and conventional idea of nature's intrinsic benevolence, with a
creator fashioning good organic design and harmonious ecosystems directly. How
delicious to contemplate that these ÒbenevolentÓ results arise only as side
consequences of a mechanism operating ÒbelowÓ divine superintendence, and
having no ÒgoalÓ but the selfish propagation of individuals that is, organisms
struggling for personal reproductive success and nothing else.
We
continue to accept Darwin's abstract formulation today-individuals struggling
for personal reproductive success-but a substantial rethinking has enlarged
Darwin's concept of individuality. For Darwin, only organisms are individuals,
or Òunits of selection.Ó But what properties does an entity need to operate as
a Darwinian individual-and are organisms the only such entities? We can specify
five such properties: an individual must have a clear beginning (or birth)
point, a clear ending (or death) point, and sufficient stability between to be
recognized as an entity. These first three properties suffice to define an
individual in the most abstract sense. But an entity requires two more
properties to enter a Darwinian process of reproductive competition: it must
bear children, and these children must be produced by a principle of
inheritance that makes offspring resemble parents with the possibility of some
differences.
Darwin
was surely right that ordinary organisms (also Armillaria mats and aphid
clones) possess these five properties: they are born and die at definable
points; they are stable enough during their lifetimes; and they have children
that resemble them with the possibility of difference. Organisms can therefore
be units of selection.
But
what about entities either more or less inclusive than organisms? What about
genes ÒbelowÓ organisms, and species ÒaboveÓ? We usually think of genes as
parts and species as collectivities, but maybe this conventional view only
represents the bias of restricted focus on our personal lives. Maybe genes and
species are just as good Darwinian individuals as bodies. After all, species
are born when a population becomes isolated and branches off from its parental
stock. Species die unambiguously at their extinction (unless of course the
species slowly evolves into one or more other species!jdDec1202). Most species are quite
stable throughout their geological duration. Genes also possess the five key
properties of birth, death, stability, reproduction, and inheritance with the
possibility of difference.
Thus,
individuality extends beyond Armillaria mats and aphid clones to encompass
different levels of biological organization-so different that we have usually
called them parts or collectivities under the parochial assumption that only
organisms can be individuals. Genes and species are also Darwinian individuals,
and selection can operate upon them as well. Selection does not only sort
organisms, as Darwin thought; it operates simultaneously at several levels of a
genealogical hierarchy-on genes and cell lineages ÒbelowÓ organisms, and on
populations and species ÒaboveÓ organisms. All these levels of the hierarchy
produce legitimate Darwinian individuals-and this is the large, inclusive, and
proper biological sense of the term individual.
Evolutionary
theory does not operate as Darwin imagined when selection acts simultaneously
upon several kinds and levels of individuals. Balances and feedback, rather
than adaptive perfection, become the source of temporary stabilities. Fancy
feathers are wonderful for an individual peacock but harmful (in geological
time) for the individual species Pavo cristatus. Roger Clemens's salary is
terrific for him and his family line but of dubious value for long-term
persistence of the individual called Major League Baseball.
Nature
is not an intrinsic harmony of clearly defined units. Nature is built by
multiple levels, interacting fuzzily at their borders. We cannot even formulate
an unambiguous definition of individual at the single level of organic
bodies-as Armillaria mats and aphid clones demonstrate. Moreover, in Darwinian
terms, legitimate individuals exist and operate at several levels of a
genealogical hierarchy-genes and species, as well as organisms. But what a
fascination when this maelstrom of differing individuals forms its meshwork of
interaction to produce life's history by Darwinian evolution. Does nature
herself then sing Walt Whitman's Song of Myself?
Do I contradict myself?
Very
well then I contradict myself,
(I am large, I contain multitudes.)