Professor A. H . Reginald Buller (1874- 1944), President of the British Mycological Society, 1914·

Vol. 82, Part 1

Trans. Br. mycol. Soc. 82 (1), 1-11 (1984)

[ 1 ]

Printed in Great Britain

February 1984

THE FIRST BENEFACTOR'S LECTURETHE FUNGAL MYCELIUM: AN HISTORICAL PERSPECTIVE

By P. H. GREGORYRothamsted Experimental Station, Harpenden, England ALS 2JQ

I will start by expressing my grateful thanks to theCouncil of The British Mycological Society for theinvitation to give the first Benefactor's Lecture. Ifeel this both an honour and a responsibility.

The beginner in oil painting is advised to avoidgetting involved with detail, to use a large brush andhold it by the end of the handle at arm's lengthfrom the canvas. I intend to use this technique forsketching the history of ideas about the fungalmycelium. Frankly, I have left out almost everybodyand everything!

Biologically the vegetative mycelium is the mostimportant part of the fungus: it does all the hardwork of exploring and exploiting the substratumand feeding off it. Yet most textbooks dismiss themycelium in two or three pages. We write aboutfungi like those old-fashioned historians who treathistory in terms of the Napoleons and Cleopatras,but ignore Tom, Dick and Sally. We seem blind tothe vegetative mycelium.

This blindness is illustrated by the otherwiseexcellent and stimulating report of the SecondKananaskis Conference. In spite of its title: TheWhole Fungus, it ignores the vegetative mycelium(which it takes for granted) except that around page463 it gives 60 lines to the taxonomic relationshipsof the Mycelia Sterilia. Our neglect of themycelium is curious because sale of mushroomspawn has been a thriving industry for generations,and today much capital is invested in processes forgrowing mycelial pellets in submerged culture toproduce antibiotics such as penicillin.

DISCOVERY OF THE MAIN FEATURES

OF THE MYCELIUM

There is no need to define the term 'mycelium':without a shrewd idea already you would not be atthis Symposium. Basic facts about the myceliumwere summarized by de Bary (1887) in hisComparative Morphology. But the mycelium is adynamic entity, and to understand its activity wemust take account of three important phenomena:(1) hyphal fusions or anastomoses; (2) perforatesepta; and (3) cytoplasmic and nuclear migration.

Credit for discovering hyphal fusions probably

Vol. 81, Part 3, was issued 9 December 1983

goes to the brothers Tulasne, who depict hyphalfusions in five species in their great 'Carpologia'(1861-5): in Grove's translation of their Latin thephenomenon is twice referred to as 'wonderful'.

Marshall Ward (1888) in his paper on the lilydisease, may have been the first actually to watchvegetative fusion in progress under the microscope.In his three or four day old cultures of Botrytiselliptica fusions were numerous and of constantoccurrence.

The fusion process started with action at adistance between two neighbouring hyphae whichevidently signalled across a gap equivalent toseveral hyphal diameters (Fig. 1). Forty minutesfrom the start, a lateral hypha was emerging fromone of the two hyphae; in another 50 min this hadelicited growth of a branch from the neighbouringhypha and had changed course to meet it; 25 minlater they had almost met; in another 15 min theyhad met and fused so that protoplasmic continuity(the final test of true fusion) had been establishedbetween them. Like the Tulasnes, Marshall Wardwas impressed and called this attraction pheno-menon 'remarkable'. He also recognized that themycelium must be in some special but undefinedstate for fusions to occur, because young hyphaecould grow across one another without fusing.

We now know that vegetative hyphal fusions arevery common in the higher fungi (Meyer, 1902),but in contrast mycelium of the zygomycetesgenerally lacks vegetative hyphal fusions (Synce-phalis seems to be exceptional in this respect).However, in zygomycetes obvious sexual fusionsoccur between gametangia: the glamour of thissexual process in the lower fungi has eclipsed thework-a-day vegetative fusions of the higher fungi.

Septation

The living zygomycete mycelium is generallynon-septate and can be described as coenocytic-filamentous (although some Trichomycetes can bestrongly septate).

In contrast the outstanding characteristic of thehigher fungi (i.e. the ascomycetes, basidiomycetesand their imperfect states) is that their mycelium is

Myc82

2 The First Benefactor's Lecture

I r:I !

If

b

~-r----

~ '

(l

x

Fig. 27

g :=

~....

: '

~/ XJ:Y...........

F ig. 1. Stages in formation of a hypha-to-peg fusion in Botrytis elliptica. Drawn by H. Marshall Ward (1888).

P. H. Gregory 3septate-reticulate . So, whereas the zygomycetemycelium maps a tree, the higher fungi map a net .

IncidentalIy, the oomycetes stand apart from thisscheme. In many respects they seem anomalous, forinstance in their predominantly diploid existence ,and in persistent claims of hyphal fusions (e.g.Stephenson, Erwin & Leary, 1974).

The higher fungi then are septate: but function-ally they are also coenocytic, because each septumhas a central perforation during most of the activelife of the hypha. In addition, electron microscopyshows that there are also plasmodesmata in thesepta. But the septal pores we are now discuss-ing are much larger than plasmodesmata - ten ortwenty times the diameter. Septal pores are evi-dently completely different from plasmodesmata.

Wahrlich (1893), using the chlor-zinc iodinereagent, discovered abundant cytoplasmic contin-uity between celIs via septal pores in nearly 50species of higher fungi.

Wahrlich also actualIy watched granular proto-plasm moving from one ceII to another in Eurotiumherbariorum, and he realized that the septal poresserve as channels for migration of protoplasm.

Migration of protoplasm was substantiated byRothert (1892 ) with a Sclerotium species; byReinhardt (1892) with Sclerotium sclerotiorum; andby Charlotte Ternetz (1900) with Ascophanuscarneus. In the zygomycete coenocyte two-waycirculation was described by Arthur (1897).

So, at the start of the zoth Century the fungi,especialIy the higher fungi, had been shown topossess features quite unlike vascular plants.However, these features remained as curiosities andno significance seems to have been accorded them.These scattered and unorganized facts were in needof a synthesizer who arrived in the person ofA. H. Reginald Buller .

THE BULLER ERA

A sketch of BulIer is relevant here in view of thesubsequent neglect of the vegetative mycelium.

In 1904 BulIer went to Canada from BirminghamUniversity to the Chair of Botany in the Universityof Manitoba. Arriving in Winnipeg at the CanadianPacific Railway Station, he walked a few yardsdown Main Street, saw the McLaren Hotel, turnedin for the night and made it his headquarters for thebest part of 40 years until his death in 1944 (thehotel had a good bi1liards table - spore ballistics I).All this is relevant because, when not teaching, thebachelor BulIer, unencumbered by domestic ties,could devote his time to research like some modernmonk, watching a fungus right through the nightif occasion required - as it often did. In thefavourable environment of his laboratory at the

University the functioning of the agaric hymeniumwas unravelIed, as recorded in the volumes of hisResearches on Fungi.

In 1931 I went to Winnipeg to work underBulIer's direction, not in his own laboratorythough, but at the Medical ColIege, and on arrivingat the CPR Station my first port of calI was also theMcLaren Hotel to dine with Buller and Bisby.

BulIer was excited over the phenomenon ofhyphal fusions which in the 1930S was in themycological air . He was just taking up the line ofstudy which, over the next few years, opened up anew understanding of the vegetative mycelium. Mywife and I got to know him quite well : he was atour wedding, and he once dropped in on usunexpectedly for breakfast after an alI-night vigil inhis laboratory. He was excelIent company, had acritical mind with a boyish enthusiasm. His aim wasto understand fungi. I was tremendously impressedby Buller: I sti1lam.

I will try briefly to summarize his maincontributions on the mycelium as expounded in hisResearches (BulIer, 1931; 1933) .

Hyphal fusions

Three types of true fusion were distinguishedaccording to their function : (1) vegetative fusions;(2) sexual fusions; and (3) the parasitic fusions seenwhen Chaetocladium and Parasitella attack certainMucoraceae.

True vegetative hyphal fusions (of the kindwatched by MarshaII Ward) lead to protoplasmiccontinuity being established across the bridge, andmust be distinguished from mere hyphal contact orhyphal adhesion in which continuity of ceII lumenis not established. (T his distinction is fundamentalin Mycology .)

Vegetat ive fusions develop in an older part ofthemycelium, converting it into a three-dimensionaltransport system . The process is self-regulated sothat the final number of fusions is limited. (FromBulIer's illustrations of his Pleurage curvicola Iestimate that the number of fusions is of the orderof 1 per 200 pm length of hypha.)

BulIer lists the main functions of hyphal fusionsas follows : (1) conduction of food materials; (2)mating between + and - mycelia in a heterothallicspecies; (3) passage of nuclei during' diploidiza-tion ' ; (4) reducing the effect of mechanical damageto a hypha by providing an alternative route formovement of protoplasm; and (5) enabling severalmycelia of the same species to form a singlecompound mycelium which acts as a social unit inthe production of fruit-bodies and spores from alimited food base.

The last point is i1lustrated by BulIer's diagram1-2

4 The First Benefactor's Lectureof a horse-dung ball colonized by 24 basidiosporesof the homothallic Coprinus sterquilinus. In compe-tition not one of the 24 mycelia would have enoughfood to fructify, but their pooled resources afterfusion would give a large sporophore - more of thislater.

Observations by earlier workers on 'telemor-phosis' and 'zygotropism' were confirmed, thoughthe phenomena remain largely unexplained to thisday. Vegetative fusions occur between hyphae ofone and the same species, but not between differentspecies. This specificity offusions was stressed, andthe possibility of using the ability of two mycelia toanastomose as a test of conspecificity was exploredin early experiments with Panellus stipticus '(Buller,1924; and see Davidson, Dowding & Buller,1932 ) .

By continuous observation with the microscopeduring all stages of numerous fusions, Bullerconcluded that all vegetative fusions take placebetween growing hyphal tips albeit the lateral hyphaelicited by an approaching hypha may be merely ashort 'peg'. Four different kinds of hyphaI fusionswere distinguished: hypha to hypha; hypha to peg;peg to peg; and hook to peg in clamp connexions.

Protoplasmic streaming

Buller confirmed in general the presence ofan openpore in the septum of living hyphae in the higherfungi by observing protoplasm flowing through theseptum: in Sordaria fimicola he recorded amaximum speed of 6 cm/h". In Pyronema stream-ing was watched through a series of 161 succes-sive septa, away from hyphae which were beingemptied and towards a rapidly growing area ofmycelium.

As to the function of septal pores - Bullerconcluded that perforate septa offer little resistanceto cytoplasmic flow, but the pore can be pluggedrapidly if a cell is injured: and at the same time flowis not interrupted because alternative routes areavailable through the hyphal network.

Among basidiomycetes, flow was observed inRhizoctonia solani, both through main hyphae andacross bridges formed by hyphal fusions. Howevermost hymenomycetes have very clear transparentcytoplasm, so flow could not easily be observed.

But Buller discovered a new phenomenon-migration of nuclei through septal pores. Toanticipate, he saw the structure which Moore &McAlear (1962) 30 years later using the electronmicroscope described as the dolipore septum ofhymenomycetes, but Buller concluded that what hesaw was an optical illusion because it did not inhibitnuclear migration (more of this later).

Nuclear migration

Mating tests with Coprinus lagopus (sensu Buller) invitro showed that a fragment of haploid (mono-karyotic) mycelium placed at the edge of a largehaploid mycelium would rapidly lead to dikaryoti-zation of the large mycelium. And so, surprisingly,would a fragment of a dikaryotic mycelium used asinoculum. The appropriate nuclei travelled at1'4 mm/h? into the larger mycelium (or rathertheir progeny did in a kind of relay race).

The dikaryotization of a monokaryon by adikaryon has become known as 'the Buller pheno-menon'. Buller concluded that the persistenceof conjugate nuclei (instead offusing to true diploidnuclei) aids dikaryotization of naturally occurringhaploid mycelia in nature. The function of clampconnexions he saw as preserving the correct balancebetween nuclei and cytoplasm in the terminal cell(never less than two nuclei).

The conclusion was that nuclei can travel throughlong series of cells, not merely to the next cell as inthe sexual processes of the lower fungi and otherorganisms.

REACTIONS TO BULLER'S WORK ON THE

VEGETATIVE MYCELIUM

Inevitably, after nearly 50 intervening years ofresearch, Buller's conclusions need some editing.Surprisingly however, while most of his findingshave been confirmed, they have never beenassimilated into mycological philosophy.

I believe that the main reason is that the facts aredisquieting to anyone accustomed to classifyingfungi as plants. One can almost hear the traditionalbotanist despairingly adapt Henry Higgins's com-ment on Eliza Doolittle in 'My Fair Lady': 'Whycan't a fungus be more like a plant?'.

Are Fungi plants?

The fact is that the life-styles of green vascularplants and of fungi are vastly different. Greenplants have an excellent plumbing system forconducting water and solutes through the tissues,but they do not permit living protoplasm with itsorganelles (sometimes including nuclei) to migratethrough up to 160 consecutive cells! Angiospermsresemble fungi most closely in the behaviour of thepollen tube, in other respects their life-styles arepoles apart.

In the 19th Century emphasis was laid ondifferences between species. Today it is customaryto emphasize the unity of living organisms: bothfacets are worth exploring.

Significantly this passage was deleted by the editorof the posthumous second edition of Langeron'sPrecis: part of the unconscious cover-up? The onlywork I know which gives Buller an impartialtreatment is Burnett's Fundamentals of Mycology,and I am pleased to note that the second edition(1976) has not been censored!

P.H.Gr~ory 5Higher plants draw water and inorganic nutrients empty and moribund. In this way a fungal thallus,

from the soil, while maintaining their green tops in whatever its shape, always comprises a living zonethe air and sunshine where they synthesize growingcontinuously, and a central skeletalzoneformedelaborate organic compounds. In contrast fungi of empty dead tubes. The growth of the thallus is onlyobtain all their food from the substratum. The limited by exhaustion of the cytoplasmic mass, which

finishes up bypassingintothe propagative or reproductivemycelium is organized to rootle it out and carry it spores (Langeron, 1945).towards the site where the resting or dispersalorgans are forming. All this supports the modernclaim for recognizing Kingdom Fungi (Whittaker,1969; etc.), My only reservation is that we still lackcomparable information about the algae, wherevegetative hyphal fusions undoubtedly occur, as forexample in the Laminariaceae (Fritsch, 1945).Perhaps the natural solution will be to recognizeKingdom Thallophyta.

An unconsciouscover-up?

So, while one would have expected the newknowledge to be welcomed, instead it was largelyignored, and there has been an unconsciouscover-up.

It must be admitted that Buller was somewhatvulnerable, not in his observations but because ofhis isolation during three quarters of the year inWinnipeg (where he had in fact built up an activecentre of mycological research). His Researches,published largely at his own expense, are oftenprolix. And once, when he wrote about a fungusgrowing on cow dung, he added a photograph ofcows in Kildonan Park! It seemed easy to disparagethe unwelcome new findings as the product of aboyish enthusiast from the Wild West, and to pushthe disturbing ideas under the carpet.

To most botanists the new ideas were disturbingindeed. The only parts of his work on the vegetativemycelium referred to in the Royal Society ofLondon's obituary notice are the Buller Pheno-menon and his explanation for clamp connexions.(The British Mycological Society did not publishan obituary.)

Only Maurice Langeron in his Precis deMycologie (1945) seems to have grasped theimplications of Buller's work, and he drewconclusions more far-reaching than Buller himselfever did. Some years ago I quoted Langeron's briefattempt at a synthesis (Gregory, 1966), which Itranslate freely as follows.

A fungus is a nucleatecytoplasmic masswhich movesina centrifugal direction,either freelywithout restraint, orinsidetubeswhichit buildsgraduallyas it movestowardsthe periphery. At the same time as these tubes areelongating under cytoplasmic pressure, the fungusgradually quits the central part of the thallus, whoseelements become vacuolate progressively, then becoming

Observations on living fungi

There was a further reason for neglect of the newideas, and that lay in technical progress. Buller wenton looking down his light microscope at moving,growing fungi, continuing obstinately (but fortun-ately) into the era when most research had movedover to study dead mycelia, killed and embalmedwith elaborate ritual before being stained or cut upwith steel, glass or diamond knives. Don'tmisunderstand me: I am not belittling the brilliantadvances gained by the microtome and electronmicroscope. I am merely stressing what everymodern microscopist realises, - that one disadvan-tage of modern techniques is that they not merelycut sections through tissues, but they also cutsections through time. Unfortunately the success ofthe modern lethal techniques is so great thatinformation gained from the older techniques inflowing time has been largely forgotten.

For the study of the mycelium, mycology nowneeds a synthesis between observations on livingmycelia and observations on dead sections. First,observations in flowing time must be revived: howelse can we interpret E.M. and S.E.M. picturesunless we have a clear picture of timing of eventsin the mycelium ? Use of a television microscopecould help share the tedium of watching. Andfortunately development of phase contrast opticshas revived interest in the living mycelium, leadingto studies of nuclear migration, for example byDowding (1958) on Gelasinospora, and recently byNiederpruem (1980) on Schizophyllum.

DEVELOPMENTS SINCE 1950

Although Buller's ideas have not been adopted bygeneral mycology, the phenomenon of hyphalfusions has stimulated two specialized groups:geneticists and virologists. For convenience thevirus work will be considered first.

6 The First Benefactor's Lecture

Mycoviruses

Beginning with the identification of mushroomviruses by Hollings (1962), an important develop-ment of the last 20 years is the discovery that a largeproportion of mycelia of higher fungi (in vitro or inthe field) are infected with viruses (see reviews byHollings, 1976; Buck, 1980; Lemke, 1981). Usualmethods of experimental inoculation fail withmycoviruses but fusion with another conspecificmycelium may transfer virus infection. On mush-room farms, before spawned beds are coveredwith casing soil, the mycelium may becomeinfected by fusing with airborne basidiosporesacting as virus vectors, the ease of transferdepending on which strains of Agaricus bisporus areinvolved (Schisler, Sinden & Sigel, 1967).

Concept of the mycelium mosaic

The dawn of the antibiotic era in the 1940S withthe development of penicillin stimulated anenormous interest in fungal genetics.

Applied workers had been puzzled to account forthe variability oftheir fungi, - often in the apparentabsence of any sexual process. Kohler (1930), andBrierley (1931) independently suggested thathyphal fusions might facilitate heterokaryosis andso increase variability. Much laboratory experi-mentation on heterokaryosis followed, especiallyafter Pontecorvo and his colleagues described the'parasexual cycle' or mitotic recombination(Pontecorvo, 1956), and a new understanding oforigins of variability seemed to be opening up.

Perhaps it was Buller's concept of the social unitmycelium and his diagram of Coprinus sterquilinusgrowing on its multiple-inoculated dung ball thatsparked off a new concept that all mycelia of aspecies form a physiological unit.

On tree stumps, Burnett & Partington (1957)found adjacent groups of Coriolus versicolor sporo-phores carrying the same mating-type factors indifferent combinations. Mycelia derived from thesewould anastomose freely in culture. One possibleinterpretation was that in the wild the mycelium'exists as a single physiological and ecological unit,although genetically a mosaic', thus setting thefungi apart from almost all other organisms in whatBuller called their social organization, and makingan individual fungus hard to define (Burnett, 1976).

Raper (1976) and Raper & Flexner (1970)also envisaged mosaicism in Schizophyllum andCoprinus, - 'physiologically unified dikaryons con-stituted of many accretions of diverse origins. Theproduct is a genetic mosaic, all parts of which mayproduce fruiting bodies and liberate basidiospores... a mosaic population capable of harbouring

enormous variability'. It seemed that the conceptof an individual could not be applied in the higherfungi, - the individual was merged in a cooperativesociety co-extensive with the species.

However, doubts began to arise when Caten &Jinks (1966) pointed out that in the ascomycetes theinduction of heterokaryosis was mainly a laboratoryphenomenon, not paralleled in the wild. The roleof hyphal fusions began to appear in a new light.

Some complications

The consequences of hyphal fusion proved to bemore varied than Buller envisaged. Fusions withinone and the same mycelium indeed still appearentirely beneficial. Confrontations between myceliaof different species can lead to repulsion orindifference, only rarely to fusions. In some casesapproaching hyphae may attract one another, buton making contact their apices remain roundedinstead of mutually flattening as in the prelude totrue fusion (Kohler, 1930).

Confrontation between different mycelia of thesame species usually leads to consummation offusion, but this is often followed by a rejection re-action, with death of groups of cells, as for instancein Thanatephorus species (Flentje & Stretton,1964). In Britain, Aspergillus nidulans exists in manycompatibility groups, between which heterokaryonsare not formed (Jinks et al., 1966). The latitudinar-ianism of the 1950S began to wilt.

Concept of the individualistic mycelium

A new phase started a few years ago when Rayner& Todd (1977) and Todd & Rayner (1980) wentinto the woods with a saw and cut up decaying treestumps which had multiple natural basidiosporeinfections. They discovered that hitherto largelyunsuspected mycelial rejection phenomena couldoperate after successful hyphal fusion to limit thescope of the social unit mycelium concept.

A stump with Coriolus versicolor for exampleshowed several decay columns, occupied bymutually antagonistic dikaryons, but whose mono-karyotic components were, significantly, fullyinterfertile. The columns were separated byrelatively undecayed dark zone lines. Rayner &Todd conclude that in nature antagonisms operateto prevent effective vegetative fusion betweengenetically different mycelia of the same species.They advance the hypothesis that such mechanismsare general in the higher fungi, operating asvegetative but not sexual mechanisms solely todelimit individuals within freely interbreedingpopulations. These mechanisms act only againstgenetically distinct mycelia: mycelia of the same

7

Mode 2 : divergent growth

During the phase of hyphal construction, in whichthe available environment is explored, new cell wallis formed at the growing apex of the hypha, andbranches are established in a pattern characteristicof the species by initiation of new lateral growingpoints behind the leader. In this mode younghyphae may approach and grow over each otherwithout initiating a hyphal fusion.

Mode 4 : migration of protoplasm(transmigration)

Hyphal fusions facilitate protoplasmic migration, -the mode by which the essential fungus begins to

It may be noted here that when starting a culturewith a dense spore suspension, the resulting germtubes may enter an anastomosing phase (Mode 3)temporarily, forming a mycelium which thenreverts to Mode 2.

In contrast with Modes 1 and 2, the remarkablephenomena of hyphal fusions have been very littlestudied during the last 50 years. How is the modetriggered? What signal is sent out by the hyphal tipinitiating the process, and how is it received so asto evoke the development of a growing apex fromone or more points on a neighbouring hypha? Whatspecial structures receive the signal? What stimuliorient the two growing points? What signals leadto recognition between mycelia (very complexsurely), and lead eventually to complete fusion ofboth cell wall and cytoplasm - the ultimate criterionof true vegetative hyphal fusions as distinct frommere contact and adhesion.

Time is now. ripe for a fresh enquiry into thesemysteries with biochemical and E. M. techniques.Some answers will follow later in this Symposium.

Mode 3: hyphalfusion (anastomosis)

In the higher fungi, but not in the phycomycetes,the mode of hyphal fusions establishes a three-dimensional network: this is a phase of middle age.The triggers for this mode are largely speculative:exhaustion of nutrients, accumulation of stalingsubstances perhaps? As already noted, the auto-nomy of Modes 2 and 3 was noted by MarshallWard (1888). It was more clearly stated by Buller(1933) who wrote:In a single mycelium the formation of hyphal fusions setsin the older parts where the culture medium is becomingexhausted, and in general the condition is promoted byconditions of starvation ...The actual formation of hyphaIfusions seems to alter the physiological conditions in sucha way that in the end the vegetative mycelium ceases toproduce hyphal fusions.

P. H. Gregory

clone are accepted. Membership of the co-op isrestricted to members of the clone.

If, as seems probable, this situation provesgeneral in the higher fungi, a number of puzzles fallinto place and others need re-thinking: the,barrage' phenomenon studied by Vandendries &Brodie (1933) and Esser (1966); the rejection inThanatephorus described by Flentje & Stretton(1964); zone lines (Campbell, 1933, etc.); and someof the strange features of fairy rings. A corollary ofthe theory is that in most species the prevention ofvegetative out-fusions must somehow be reconciledwith the usual requirement for out-breeding.

It seems that such reactions occur not onlybetween established mycelia, but also between amycelium and a germinating spore. Kemp (1975)studied homing and lethal reactions between oidiaand mycelium in Coprinus, and suggested that'oidia can function, not only as male gametes orspermatia, but also as a means of eliminating orreducing the growth ofhyphae belonging to closelyrelated competitors'. Niels Fries (1981) hints thatbasidiospores in Leccinum can function in the sameway. Biological warfare in the fungi!

When I speculated (Gregory, 1966) that 'besidestheir function as colonizers, spores may perhaps actas an unreliable airmail service transmitting genesbetween established mycelia' I did not foresee thatthe 'mail' delivered would consist largely ofletter-bombs and virus packets!

So much for the history of the mycelium. Nowfor the perspective.

MODES OF MYCELIAL FUNCTIONING

To understand the dynamics of the mycelium it ishelpful to use the analogy of a piece of electronicequipment, a 'music centre' for example, whichfunctions in different modes at the touch of aswitch. I can perceive nine possible modes, - theremay well be more. Note that these modes are notnecessarily part of a fixed cycle, but depend on theappropriate switch. And note that different parts ofa mycelium may be functioning in different modesat one and the same time.

Mode I : spore germination

Often we know little about what triggers a mode,but this is not the case with the first mode, whichhas been well studied.

The mycelium is typically initiated at sporegermination. Here we merely need to note that ingermination the protoplasm (by which term Iunderstand cytoplasm+nucleus) migrates into thegerm tube apex leaving a vacuole enclosed by thespore wall; it may even pop back again within thespore wall if conditions become adverse.

8 The First Benefactor's Lecture

Fig. 2. Stages in formation of a peg-to-peg fusion in Pyronema. Drawn by A. H. R. Buller (1933, p. 58).

quit the tubes of its mycelium and migrates towardsdeveloping sporulating or resting structures, nodoubt at times moving with extreme slowness.

Naturally this process can be observed only inliving material, and little has been added sinceBuller's era. But movement does depend on openseptal pores, and here electron microscopy hascontributed valuable information. Following dis-covery of the basidiomycete dolipore by Moore &McAlear (1962), it seemed that the parenthosomemight prove a barrier to migration of cytoplasm andnuclei. But this cap is now known to have openings,and, in some Coprini at least, the cap can disappearin the nuclear migration mode. Bracker & Butler(1964) and Giesy & Day (1965) illustrate nuclei inthe act of passing through dolipore septa. Possiblythe function of the dolipore is to imprison thenucleus in its cell until the nuclear migration Mode5 sets in.

Controversy has followed Buller's belief thatmigration of protoplasm through the mycelium isdue to vacuolar pressure from the rear. While somehave supported this view, others have produced

evidence of suction from in front (Plunkett, 1958);or active creeping could be involved (Isaac, 1964).

What is surprising is that argument about thecauses of protoplasmic migration can continuewithout explicitly acknowledging its function. Thisis evident from confusion, even 50 years afterBuller, between evacuation of a hypha andautolysis. For instance we read statements to theeffect that sporulation of some fungus or other isfavoured by 'autolysis'. This idea results evidentlyfrom ignorance of the phenomenon of migration,the devitalized appearance of the evacuated hyphabeing incorrectly attributed to autolysis. Trueautolysis in fungi has been known for a long time,most spectacularly in the deliquescing gills ofCoprinus.

Radioisotope techniques developed in the 1950Sled to much work on translocation in fungi,reviewed by Wilcoxon & Sudia (1968). It is clearto me that in the fungi two distinct phenomena havebeen confused under the term 'translocation'.

(1) True translocation, movement of water andsolutes through the hyphae, as studied for example

P. H. Gregory 9by Jennings (1982), is a process probably going oncontinuously through all modes, except in dry,concentrated structures like sclerotia and spores.

(2) Mass migration of living protoplasm along aroute in the mycelium (a phenomenon not met within higher plants) is the characteristic of this Mode4·

Transfer of terms from green plant physiologyhas sometimes proved inapt for mycology. A newterm needs to be added for fungi . I suggest that inaddition to 'translocation' we use the term ' trans-migration' to denote the distinctive processes bywhich protoplasm and organelles are carried forlong sequences of cells through the mycelium.

In mycological literature it is not always clearwhether translocation or transmigration is underdiscussion. For instance Schiitte (1956), when hereported that some species of Penicillium andAspergillus did not translocate, seemed mainly to beusing the term in the sense of transmigration.

It seems incredible that these moulds shouldshow neither translocation nor transmigration.How else are the aerial phialides supplied from thesubstratum? There may well be fungi which do nottransmigrate, but I interpret Schiitte's results asshowing that his moulds do not translocate ortransmigrate laterally. This interpretation wouldbe consistent with Burges (1960) who found thatsoil fungi belonging to his ' Penicillium Pattern 'sporulated densely on a small nutrient mass , but didnot grow out into the surrounding soil.

Transmigration is thus a process compatible withbut quite distinct from translocation. In transmi-gration movement can be in any direction throughthe mycelium, it is normally most vigorous towardsa reproductive or resting structure which is beingfurnished, and involves cytoplasm and organelles,at times including nuclei.

Mode 5 : nuclear migration

The phenomenon of nuclear migration, firstdiscovered by Buller (1931) and then doubted, hasbeen confirmed by later workers (see: Snider,1963). One trigger is clearly genetic, and themechanism that suggests itself is traction byprotein fibres. During transmigration it seems thatnuclei mayor may not migrate, but I wish we hadmore information on this little-explored aspect.

Nuclear migration may well be a common event .Although it is difficult to detect in the absence ofgenetic effects, there is no reason to suppose thatits only function is in diploidization.

Mode 6 : my celium and yeas t transition

Some fungi can switch between mycelial and yeastmorphology. In Mucorthe yeast phase is determined

by anaerobiosis . Some systemic pathogens of manand an imals are mycelial in culture at roomtemperature but switch to yeast form in the animalbody (see : Nickerson, 1947). The industriallyimportant yeasts seem to be permanently fixed inyeast form . Factors controlling the transition arereviewed by Cole & Nazawa (1981).

Mode 7 : coherent growth

From spore germination onwards, early growth ofthe mycel ium is potentially three-dimensional.While the hyphae are exploiting the substratum,branches diverge in specifically characteristicpatterns. Sooner or later in most higher fungi (butnot in lower fungi ) the pattern switches fromdivergent to what I term 'coherent growth',perhaps forming sclerotia, strands or rhizomorphs,or forming sporocarps leading to teleomorph oranamorph reproduction (Modes 8 and 9). Needlessto add, the switch is reversible.

Nutritional factors are suggested for the tran-sition from divergent to coherent growth in Armil-laria mellea by Garrett (1953), in Serpula lacrimansby Butler (1958), and in the cultivated mushroomby Chanter & Thornley (1978). But it seems thatwe still do not understand what factors trigger thisextraordinary change from divergent to coherentgrowth, - a crisis in development which is probablyuni ver sal in the higher fungi . At this point we reachthe end of the mycelial modes. There remain twomore modes however.

Mode 8 : teleomorpb reproduction and Mode 9:anamorph reproduction

Factors switching Modes 8 and 9 are reviewed byMiiller (1979), but discussion of these Modes fallsoutside the scope of this survey.

ECOLOGY OF THE MYCELIUM

Viewed at its broadest, the mycelium is seen as anecological adaptation to a xerophytic environment.

My suggestion (Gregory, 1966) that a fungusresembles a myxomycete, except that its protoplastmoves about in a system of tubes, seemed at thetime no more than a helpful analogy. But currentpractice classifies the myxomycetes firmly in thefungi (Mart in & Alexopoulos, 1969; Whittaker,1969 ; Ainsworth & Bisby, 1971), and therefore therelevance of the myxomycetes may be one ofkinship rather than analogy. The unveiled activitiesof myxomycetes contrast with the more secludedlife style of eumycetes, We can equate themyxomycete plasmodium with the eumycete prot-oplast. Working our correspondences between the

10 The First Benefactor's Lecture

functional modes in the two groups is rewarding,but will not be pursued here.

Ecologically the fungi can be arranged in a seriesshowing increasing adaptation to a land environ-ment. (1) The myxomycetes (starting perhaps withsuch purely aquatic forms as Labyrinthula) haveplasmodium (or protoplast) more or less naked, aptfor ingestion of bacteria in a wet environment, butdangerously vulnerable to drought. (2) Thenon-lichenized mycelial fungi, with protoplastsmoving in tubes, are better adapted to xerophytism,but restricted to a liquid intake. (3) The seriesculminates in the lichen fungi, some of which mustbe among the most xerophytic organisms known.Here the mycelium does not explore the substratum,but is differentiated in one direction for anchoring,and in another direction for weaving the fabric ofthe alga-studded 'solar panels' which cover asubstantial part of the Earth's surface.

THE MYCELIAL THEME

A non-lichenized mycelial fungus is a heterotrophicnucleated cytoplasmic mass which constructstubular cell walls within which it moves as it feedsoff the substratum.

In the lower fungi the mycelium may becoenocytic, branching like a tree but not anasto-mosed; septa serve to repair wounds or to block offparts of the tubes from which the protoplast haswithdrawn.

In the higher fungi the mycelium is semi-cellular,branched, and becomes a three-dimensional trans-port network through hyphal fusions. Septa remainperforate during active exploration of the sub-stratum, and become closed at need.

When the mycelium switches in whole or in partto coherent growth or to a reproductive mode, thefungus inhabiting the tubes migrates towards thedeveloping sporocarp (or resting body). In sporu-lation the fungus leaves behind empty tubes,breaking up and launching out in the form of sporesto colonize new substrata, forming new mycelia,equipped like the parent mycelium for out-breedingand in-fighting.

On this theme are constructed a myriadvariations, but this is the basic life-style of themycelial fungi.

It is now clear that biologically the vegetativemycelium is the most important part of the fungus.

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