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summary The extension rates of Clitocybe nebularis (Batsch ex Fr.) Kummer strains on 2% malt agar were only 30–40%, of those, up to 3.4 mm d−1, observed in woodland at equivalent exponential mean temperatures. Extension of mature field systems was accomplished by mycelial annuli or arcs 30-40 cm wide, differentiated into a leading edge of mycelial cords followed by a zone of dense, diffuse mycelium which bleached litter components, and a trailing edge of greyish, Used mycelium. Disruption of mature annuli by natural obstacles or experimental re-orientation within the mycelial band resulted in regression of the affected segment of mycelium. Localized lysis following encounter with an obstacle by immature patches of mycelium with a diameter of 30–50 cm, led to polarized development of the residual mycelium. Strains from different fruit bodies were somatically compatible when paired on 2% malt agar if sampled from the same ring, but incompatible if from different rings, resulting in mutual antagonism and formation of a persistent demarcation zone. By contrast, collision between adjacent systems in woodland culminated in mutual obliteration of the interaction fronts. C nebularis was non-combative when paired against other decomposer basidiomycetes on 2% malt agar, being either replaced or deadlocked but not replacing mycelia of these fungi. The implications of these observations are discussed in terms of emerging concepts of ecological strategies, foraging theory and polarity in mycelial collectives.

summary Ratios of nuclear genotypes observed in conidia from heterokaryotic strains of Heterobasidion annosum (Fr.) Bref., obtained from pairings between sympatrically derived, sib-related and non-sib-related homokaryons, commonly deviated from 1:1. Ratios were temporally stable, and the genotypes examined could be ranked in a strict dominance hierarchy, linked both to the relatedness of the association partners and to the growth rates of the parental homokaryons. Parental homokaryons and sibrelated heterokaryons produced conidia with a mean number of nuclei of about two, whereas non-sib-related-d heterokaryons produced conidia that were predominantly uninucleate. Moreover, whereas conidia containing more than one nucleus germinated most rapidly when derived from homokaryons or sib-related heterokaryons, uninucleate conidia germinated more readily if derived from non-sib-related heterokaryons. In a study of naturally occurring heterokaryons, distribution patterns of the number of nuclei in conidia were found to be similar to those of the homokaryons. The possible interpretation of these findings in terms of inter-nuclear genomic conflict is discussed.

This contribution to the Common Knowledge symposium “Fuzzy Studies” argues that the inclusion of space in form brings varying degrees of fuzziness and fluidity to all natural identities. Such inclusion is vital to evolutionary creativity, from subatomic to cosmic scales of natural energy flow. Examples abound throughout the natural world of indeterminate forms and processes unclassifiable under any of the discrete categories that are preferred and imposed by definitive theoretical models. Here, the much-neglected kingdom of the fungi is used to illustrate the workings of a logic of fluid boundaries.

Psychology is not alone in its struggle with conceptualizing the dynamic relationship between space and individual or collective identity. This general epistemological issue haunts biology where it has a specific focus in evolutionary arguments. It arises because of the incompatibility between definitive logical systems of 'contradiction or unity', which can only apply to inert material systems, and natural evolutionary processes of cumulative energetic transformation. This incompatibility makes any attempt to apply definitive logic to evolutionary change unrealistic and paradoxical. It is important to recognise, because discrete perceptions of self and group, based on the supposition that any distinguishable identity can be completely cut free, as an 'independent singleness', from the space it inescapably includes and is included in, are a profound but unnecessary source of psychological, social and environmental conflict. These perceptions underlie Darwin's definition of 'natural selection' as 'the preservation of favoured races in the struggle for life'. They result in precedence being given to striving for homogeneous supremacy, through the competitive suppression of others, instead of seeking sustainable, co-creative evolutionary relationship in spatially and temporally heterogeneous communities. Here, I show how 'natural inclusion', a new, post-dialectic understanding of evolutionary process, becomes possible through recognising space as a limitless, indivisible, receptive (non-resistive) 'intangible presence' vital for movement and communication, not as empty distance between one tangible thing and another. The fluid boundary logic of natural inclusion as the co-creative, fluid dynamic transformation of all through all in receptive spatial context, allows all form to be understood as flow-form, distinctive but dynamically continuous, not singularly discrete. This simple move from regarding space and boundaries as sources of discontinuity and discrete definition to sources of continuity and dynamic distinction correspondingly enables self-identity to be understood as a dynamic inclusion of neighbourhood, through the inclusion of space throughout and beyond all natural figural forms as configurations of energy. Fully to appreciate and communicate the significance of this move, it is necessary to widen the linguistic, mathematical and imaginative remit of conventional scientific argument and explication so as to include more poetic, fluid and artistic forms of expression.

The ciid beetles Octotemnus glabriculus and Cis boleti exploit different developmental stages of fruit bodies of their preferred host fungus Coriolus versicolor. Larvae of the smaller beetle, O. glabriculus, mainly use young, expanding, fruit bodies; adults of O. glabriculus are predominantly found in young fruit bodies. By contrast, adults and larvae of the larger beetle, C. boleti, are prevalent in fully developed fruit bodies of C. versicolor. Because fruit bodies of most genets emerge during spring and early summer and mature by autumn, O. glabriculus and C. boleti breed in separated seasons. Adults and larvae of O. glabriculus are abundant in spring and early summer. By contrast, the number of adults and larvae of C. boleti increases gradually from late spring to summer and peaks in autumn. We conducted a field experiment that suggests that the phenological dynamics of C. versicolor fruit bodies drive the separation of breeding seasons between O. glabriculus and C. boleti. Additionally, laboratory experiments revealed that O. glabriculus and C. boleti have differential behavioural responses to odour compounds from young and mature fruit bodies of C. versicolor. We conclude that age-related changes in the chemical composition of fruit bodies may allow O. glabriculus and C. boleti to discriminate among C. versicolor, thus providing a mechanism for the partitioning of the resource.

Most ciids (Ciidae) are strict fungivores specialized on fruit bodies of wood-rotting fungi. The Ciidae includes both specialist and generalist species. Recent evidence suggests that ciids locate and discriminate their potential hosts based mainly on fungal odours. In this study, we investigated the field distribution of ciids in a local woodland near Bath, U.K. We also evaluated experimentally the behavioural responses of ciids to host and non-host fungi in an olfactometer, and explored potential differences in putative aroma compounds in host fungi. Our field data showed that Octotemnus glabriculus, Cis boleti and Cis nitidus have preference for one host species, whereas C. bilamellatus has no preference. The experimental evidence was in accordance with the field observations. The specialists O. glabriculus, C. boleti and C. nitidus were specifically attracted to odour compounds of their preferred host fungi: Coriolus versicolor for the first two beetles and Ganoderma adspersum for the latter one. By contrast, the generalist C. bilamellatus was attracted to odours from C. versicolor, G. adspersum and Piptoporus betulinus. The evidence from this and other published studies suggest that ciids depend mainly on fruit body odours for location and discrimination of their host fungi. In accordance with this, we observed diffierential behavioural responses of ciids to aroma compounds of host and non-host species.

On the basis of the evidence that insect fungivory has the potential to affect fungal reproductive fitness, we investigated the effects of two specialist ciid beetles (Octotemnus glabriculus and Cis boleti) on the reproductive potential of their host fungus, Coriolus versicolor. We found, from field data, a negative correlation between the number of individuals of O. glabriculus inhabiting C. versicolor fruit bodies and the percentage of the fungal spore-producing surface (hymenium) that was functional. By contrast, the number of C. boleti inhabiting C. versicolor fruit bodies did not correlate with the percentage of functional hymenium. Experimentally, O. glabriculus and C. boleti reduced the reproductive potential of C. versicolor by 58% and 30%, respectively, whereas the combined trophic activity of both beetles caused a reduction of 64%. This latter effect was not significantly different from that caused by O. glabriculus alone. These findings disagree with previous assertions that insect fungivory on fruit bodies has only neutral effects on fungal fitness. We conclude that in the short-term, fungivory by ciids significantly decreases the area of functional hymenium of C. versicolor and is likely to reduce fungal reproductive fitness. Within this perspective the evolution of certain fungal characteristics (i.e. chemical composition, consistency and phenology) can be interpreted as being driven by fungivory.

Perhaps like many other mycologists, my initial encounters with fungi filled me with a desire to engage with mysteries: mysteries embedded in the diversity of form and function in fungi and how this diversity might be related to the life of the forest in which so many of these organisms appear to find their homes. In all good mysteries, the resolution of underlying causes at one level of comprehension only serves to reveal deeper levels for exploration.This can open the way into a cascade that nourishes a growing empathy with fellow beings and so assuages that most fundamental need to have a sense of belonging – to be as one; to be at peace; secure in the knowledge of idiosyncrasies embraced in the fondness of others. I dreamt that fungi would somehow eventually lead me and my fellow travellers down that cascade. I still do.

Twelve heterokaryotic strains of Heterobasidion annosum (Fr.) Bref. containing nuclei and mitochondria derived from British and North European populations were prepared by pairing homokaryotic strains and isolating hyphal tips from either side of interaction interfaces. Conidia derived from the heterokaryons had high germinability and were predominantly uninucleate. The nuclear genotypes recovered from these conidia were predominantly nonresident in origin, reflecting the marked asymmetry m nuclear ratios (as high as 9:1) in favour of invasive nuclei that developed in the mycelium. In spite of this asymmetry, the heterokaryons had similar phenotypes to the resident homokaryons that they were derived from. Whereas the relative ability of sib-related nuclei to become established in a homokaryon was about equal in sympatric protoplasm, it could be very different in allopatric protoplasm. Post-germination mortality due to the cessation of development following transfer of germlings to fresh medium was on average similar for allopatric and sympatric conidia, but there was a marked trend for reduction in mortality as nuclear ratio asymmetry increased. Homokaryons derived via conidia from a common heterokaryon exhibited less somatic incompatibility when paired together than did the original, basidiospore-derived strains. These findings indicate that phenotype-genotype relationships resulting from allopatric matings, such as could occur following geographical transposition, are liable to differ radically from those in sympatric matings and so have potentially profound effects on resident population structure.

Fungal mycelia can alter their organizational pattern in such ways as to produce alternative phenotypes. The latter allow mycelia to explore for, assimilate, conserve, and redistribute resources in spatially and temporally heterogeneous niches. It is suggested that mycelia produce alternative phenotypes by operating as nonlinear (feedback regulated), hydrodynamic systems with indefinitely expandable (indeterminate) boundaries. As such, mycelia can vary the resistances of hyphal envelopes to deformation and passage of molecules, and of hyphal interiors to displacement of contents, in accord with fortuitous local circumstances. Within the mycelial protoplasm are populations of nuclei and mitochondria. If disparate in genetic content or expression, these organelles can form diverse and unstable relationships that both influence and are influenced by metabolic processes affecting the hydraulic resistances of hyphae. Some of these processes may be autocatalytic, involving the generation, association and dissociation of free radicals and reactive oxygen species. Once initiated, such processes are beyond immediate genetic control. Fungal mycelia therefore epitomize the complex interplays between adaptive (genetic) and nonadaptive (organizational) processes that regulate the short term versatility and long term evolutionary divergence of indeterminate systems. Key words: epigenetics, mycelial development, networks, niche, nonlinearity, speciation.

Despite their different fundamental organization, ant colonies and mycelia of fungi exhibit striking similarities in their social organization. Both are collectives of genetically related or identical semi-autonomous units, consisting respectively of discrete multicellular individuals and hyphae. There is a variety of parallels in their foraging and resource capture strategies, developmental versatility and division of labour, and in the interactions between non-self recognition phenomena resulting in sexual or parasitic invasion of foreign genes into the collective, or in rejection of non-self. These parallels lend a new perspective to the nutritional relationships and modes of communication between developmentally different phases in collective systems of interacting individual units. They also have important evolutionary implications for the origin and regulation of mechanisms of gene flow between different colonies or collectives, including a link between mechanisms underlying sex, parasitism and sympatric speciation.

Summary The interrelationships between trees, and the fungi that inhabit and decay their xylem, are dynamic, complex and difficult to analyse into simple sequences of cause and effect. This is because both participants function, and interact, as highly versatile, open-ended or “indeterminate” hydrodynamic (“self-plumbing”) systems. As such they are capable of varying their pattern of development to accord with environmental circumstances by means of local feedback processes that lead to the opening up or sealing off of internal and external boundaries. Boundary-sealing, or “insulating” processes involve metabolic pathways that in leading to the production of water-resistant compounds and cell death would be detrimental within organisms with determinate development (e.g. many animals). However, they enable trees and fungi to switch functional modes, to conserve and redistribute resources, and to protect and repair their communication channels. The primary circumstance in trees that creates the opportunitie...

Three separate collections were made in southern England of Phanerochaete magnoliae forming hydnoid hymenial surfaces through the tubes of Datronia mollis basidiomes. Isolates of P. magnoliae exhibited unusual patterns of development and interaction with other fungi. Germinating basidiospores produced an extensive, sparsely branched, rapidly extending and fully coenocytic mycelial phase with a low fractal dimension. This phase was followed by events of septation, lateral branching, anastomosis, hymenium formation and autolysis in a strongly ordered ontogenetic sequence. These events were correlated with changes in constitutive and inducible laccase and tyrosinase activity. P. magnoliae also had a non-outcrossing breeding strategy and was able specifically to replace colonies of D. mollis without producing dense invasive mycelium.

Hyphal and mycelial interactions within progeny sets of single basidiospore isolates derived from Heterobasidion annosum fruit bodies from various parts of the northern hemisphere were studied microscopically using microculture chambers and with pairing tests, respectively. The behaviour within different sets was highly variable with respect to mating, nuclear exchange, morphogenetic patterns and somatic rejection responses. Evidence was obtained that rapid nuclear migration can occur between certain strains. An unusually large proportion of pairings (84 %) produced stable heterokaryons, as determined from back pairings with original homokaryons, of which 54 % possessed clamp connections and 46 % did not. Studies of isozyme patterns of pectinases of one selected pairing confirmed the presence of clampless heterokaryons. The genetic control of heterokaryosis is complex, with subsidiary systems being superimposed on those which govern formation of clamp-connections and mycelial morphogenesis. The implications of these findings in understanding gene flow within and between natural populations of H. annosum are discussed.

Phytopathologists have long been aware of the considerable intraspecific variation that can exist in natural populations of phytopathogenic mycelial fungi. However, the significance of this variation has largely been perceived in terms of fungal-plant rather than interfungal relationships. Corresponding­ ly, questions about variation in the pathogenicity and virulence of different fungal lineages have received more attention than questions about the way in which these lineages have the potential to interact antagonistically or syn­ ergistically with one another (55). The purpose of this article is to explore the many ways in which mycelial interactions may, nonetheless, be relevant to our understanding of the etiology and control of plant disease. An important step in understanding mycelial interactions is to have a clear perspective of the units of selection in natural fungal populations. Historical­ ly, this issue has been clouded both by the indeterminate nature of fungal mycelia, which have the potential to continue extension indefinitely, and by the concept that hyphal anastomosis could allow extensive cooperation be­ tween colonies of diverse genetic origin (22, 57, 58). The resultant view that members of the same species could pool their genetic resources in a versatile

Outcrossing populations of the Coniophora puteana complex had a multiallelic unifactorial (bipolar) homogenic incompatibility (mating) system regulating the emergence of a secondary mycelium (stable mating-type heterokaryon) between paired single basidiospore-derived strains (homokaryons). Emergence of the secondary mycelium involved a switch to fast-effuse development at the interaction interface, probably dependent on resources redistributed from progenitor hyphae whose extension was arrested. This switch was preceded by a short phase of nuclear migration associated with eccentric erosion of dolipore septa, accompanied by formation of lateral bulges superficially resembling clamp connexions. The secondary mycelium was sometimes delimited from progenitor mycelium by a brown zone. Pairings between genetically different secondary mycelia invariably resulted in such zones, due to somatic incompatibility. Pairings between homokaryons from Europe and the U.S.A. revealed four breeding units. There was no clear correlation between breeding ability and geographic origin, angiospermous or gymnospermous hosts, but one breeding unit was solely from Britain and another was from a single domestic collection.

SUMMARY Populations of the wood-decaying basidiomycete, Stereum hirsutum (Willd. ex Fr.) S. F. Gray, from different geographical regions, reproduce either by outcrossing or non-outcrossing. Outcrossing generates variable progeny whilst non-outcrossing results in clonal sub-populations. Pairings between single-basidiospore-derived strains from non-outcrossing and outcrossing populations commonly result in mutual rejection or mycelial invasion of the outcrosser by the non-outcrosser. However, on the basis of both morphological evidence and studies of DNA restriction fragment length polymorphisms, some pairings result in unilateral transfer of nuclear DNA from the non-outcrosser into the outcrosser. In some combinations the resultant putative heterokaryon may be stable and give rise to variable basidiospore progeny with unusual development and non-self recognition responses and at least one mating-type specificity evidently transferred from the non-outcrosser. In other, combinations, genomic takeover by the non-outcrosser has been demonstrated, leading respectively to the emergence of mycelial sectors or production of basidiospore progeny in which solely or predominantly non-outcrossing nuclear DNA is associated with outcrossing mitochondrial DNA. These observations add a new perspective to the selective pressures influencing sexual outcrossing and nonoutcrossing and the role and interplay of these breeding strategies in reproductive isolation and speciation.

Individual mycelial genotypes (genets) of Hymenochaete corrugata are able to traverse the canopies of coppice-grown hazel, Corylus avellana, by producing sclerotized mycelial pads which bridge between and bind together contiguous stems from the same and different stools. The pads may also allow attachment to other trees, e.g. elm (Ulmus sp.), climbers, e.g. Clematis vitalba and fallen leaves. Underlying this hitherto unsuspected means of aerial spread in temperate woodland is the ability of the fungus to oscillate between distinctive modes of mycelial development, concomitant with production of melanized pseudosclerotial plates.

As energy-using systems, all life forms must possess boundaries. However, these boundaries can neither be fully open nor fully closed to energy transfer if they are not to become infinitely dispersed or forever static. Instead, they define the reactive interfaces between the insides and outsides — i.e. the “dynamic contexts” — of living systems. The properties of these interfaces are fundamental to the origins of phenotypic patterns and the manner in which neighbouring life forms interact with one another. Moreover, these properties depend on the interaction of ingredients coming from inside, outside and within the boundaries themselves — not least the water which envelopes the genes of all life forms.

The way that organisms respond to one another and their environment in natural populations depends critically on the dynamic processes that produce and limit phenotypic diversity. However, the fundamental nature of these processes, and the extent to which they are genetically prescribed, is arguably the most challenging issue facing evolutionary biology. In this chapter we aim not only to show how the approach taken to this issue affects understanding of the structure and dynamics of fungal populations, but also to reveal how fungi may provide insights relevant to all kinds of life forms.

Cord systems of Hypholoma fasciculare, Phallus impudicus, Phanerochaete laevis, Phanerochaete velutina and Steccherinum fimbriatum developed progressively in non-sterile soil from advancing mycelial fronts within which linear aggregation of hyphae was followed by lysis of diffuse mycelium. Extension rates of the cord systems were generally linear and mostly increased with temperature from 5 to 20 or 27 °C. Extension rates varied between species and sometimes between strains of the same species. In S. fimbriatum an abrupt change occurred from a slowly extending, highly branched mycelium to a rapidly extending, sparsely branched cord system, which appeared to be triggered by an internal switch rather than by environmental factors. Extension rate was little affected over the range of initial soil matric potentials from − 0·007 MPa (field capacity) to − 0·9 MPa, but extension was hindered above field moisture capacity and only cord systems of H. fasciculare and P. impudicus persisted. Lateral branching decreased as soil matric potential decreased. All strains decayed wood block inocula after 70 d. The minimum temperature at which decay occurred varied between 5° to 15°; decay increased with temperature up to 30° or 27° for all species, but this increase was not always correlated with cord extension rate.

were similar to those found in natural populations of the fungus, with individual dikaryons present in discrete regions delimited by zones of mutual antagonism. Different dikaryons had similar growth rates and occupied approximately equal areas or volumes when inoculated simultaneously on to agar or into wood respectively. Simultaneous and sequential inoculation of sexually compatible monokaryons on to agar demonstrated that growth rate of the monokaryons and patterns of dikaryotization and nucleus migration between them are likely to be crucial determinants of the spatial distribution of dikaryons in a population. Monokaryons inoculated into logs in the field quickly became dikaryotized, and the resulting dikaryons contained diverse mating-type factors indicating the presence of a heterogeneous spore rain.

Mycelia of Hymenochaete corrugata exhibited two distinctive developmental patterns when cultivated on 2 % malt agar, resulting in either an appressed, yellow-brown pigmented colony or a white colony with extensive aerial mycelium. The switch from one colony type to the other sometimes occurred spontaneously, resulting in formation of sectors. On other occasions it followed subculture, damage or inter- or intra-specific interactions with other mycelia. Juxtaposition of the two colony types resulted in formation of a dark brown pseudosclerotial plate. These observations suggest that endogenous mechanisms regulate the formation of different colony types.

Cultural studies of the forest root pathogen Heterobasidion annosum (Fr.) Bref. have shown that the mycelium of the fungus possesses a high degree of developmental versatility, being able to grow in a variety of distinctive functional modes. Consequently the fungus can alternate between conidiogenous and meiotic reproductive pathways, rapid, sparsely branched and slow, densely branched mycelial extension, nutritive and non-nutritive modes, and'juvenile'and 'senescent'phases. This ability is probably of considerable importance in natural infection cycles and provides an interesting model for examining interrelationships between developmental regulation and host-pathogen interactions generally. It also raises important questions about the interface between environmental and endogenous controls of developmental pathways. CONTENTS Summary 245 I. Introduction 245 II. Spore germination and mycelial extension 246 III. Production of non-nutritive mycelium 249 IV. Sporophore production, senescence, somatic incompatibility and extracellular enzyme production 251 V. Discussion 254 Acknowledgements 255 References 255.

SUMMARY The characteristic shape of diamond-cankers in sycamore may be attributable to spread of fungal infection via the medullary rays. The rays become densely filled with fungal hyphae in infected bark, and when viewed tangentially are arranged in a lattice-like array with angles between the diagonal rows similar to the apical and side angles of diamond-cankers observed in the field. Computer simulation has shown how a range of types of random growth processes could result in diamond-shaped patterns if a fungus spread through such a lattice, with each ray serving as a centre from which its neighbours could become occupied and act in turn as centres.

Single basidiospore isolates from different fruit bodies of Stereum hirsutum always produced a morphologically distinct secondary mycelium when paired in culture. This type of mycelium was only formed in certain crosses between sibs, and the strains belonged to two incompatibility groups, indicating that the species is heterothallic. Secondary mycelia obtained from various compatible combinations of non-sibs were always strongly antagonistic when paired. Field isolates from fruit body tissue or wood were similarly antagonistic if obtained from different columns of decay in the same log or from separate localities, indicating that they represented separate heterokaryotic individuals. An unusual reaction occurred in about one-third of the incompatible pairings between sibling monospore mycelia. This took the form of a band of appressed, sparse mycelium, widest at the ends, and bounded by narrow regions of exuded droplets. These bands usually expanded unilaterally, with one of the isolates partially or totally replacing the other. In some cases this expansion stopped along a line where typical antagonism later developed. On the basis of these reactions, a simple procedure for detecting interfertility between Stereum isolates is described. This may facilitate an experimental taxonomic approach hitherto thought impossible for Stereum due to the widely held belief that members of the genus are homothallic.

Summary The spatial development, in beech logs exposed to air-borne and soil-borne inoculum, of mature fungal communities containing mutually or unilaterally exclusive mycelia of decay species is described and related to the ecological strategies and interaction of participant individuals. A combative heirarchy is recognized between (i) ruderal and/or stress-tolerant individuals which are spatially dominant early on and then decline, through (ii) air-borne combative individuals of decay fungi, such as Coriolus versicolor (L. ex Fr.) Quel. and Stereum hirsutum (Willd. ex Fr.) S. F. Gray, which establish mutually exclusive decay columns expanding away from the aerial cut surface, and culminating with (iii) highly combative individuals of mycelial cord-formers such as Phallus impudicus (L.) Pers. and Phanerochaete velutina (DC ex Pers.) Parmasto, which invade slowly from the base, ultimately occuping large volumes of wood, causing intense decay, and replacing many of the pioneers. Addition of basidiospore suspensions of Bjerkandera adusta (Willd. ex Fr.) Karst., C, versicolor, Hypholoma fasciculare (Huds. ex Fr.) Kummer or S. hirsutum to the aerial cut surface appeared to affect community dynamics and functioning markedly. Development of decay columns by air-borne fungi was inhibited and associated with enhanced vertical penetration by basally-colonizing fungi, and persistence of the relatively non-combative species Xylaria hypoxylon (L. ex Hooker) Greville and Armillaria bulbosa. (Barla) Kile & Watling. Re-cutting of the aerial cut surface resulted in community development patterns somewhat intermediate between those in naturally colonized and inoculated logs.

Heterozygosity at a single, multiallelic locus (‘B-factor’) leads to development of a ‘bow-tie’ reaction between homokaryotic mycelia of Stereum hirsutum paired against each other on malt agar. This reaction involves formation of a band of appressed mycelium with abnormal branching, widest at its edges, which is usually asymmetric and tends to advance preferentially into one homokaryon leading to partial or complete replacement by the other. Expression of the reaction is probably dependent on continuance of access of nuclei from a donor into an acceptor mycelium and is affected both by the particular combinaton and relatedness of the strains, and by their physiological condition. First generation sibs can be ranked according to their relative ability to accept or donate nuclei, from which the reaction pattern for particular combinations can usually be predicted. With such strains the reaction develops more rapidly than with second generation (inbred) sibs, and, unlike in the latter, often culminates in expression of somatic incompatibility. Repeated subculturing tends to advance expression of somatic incompatibility relative to the bow-tie reaction.

SUMMARY Isolates of the mycelial cord-forming agaric, Tricholomopsis platyphylla, were obtained from an oak stand and their mating and mycelial interactions studied. The fungus was found to have tetrapolar incompatibility and genetically different dikaryons were strongly mutually antagonistic in culture. On the basis of such antagonism, 22 mycelial types, each comprising from one to 15 isolates were identified amongst a random sample of 113 dikaryotic mycelial isolates from an area of 20·8 ha. Isolates belonging to the same mycelial type were frequently found in close proximity in areas from which other types were excluded. However, some were from samples as much as 150 m apart and separated by areas occupied by other mycelial types. The likely origin of this distribution is discussed in relation to the pattern of dispersal, establishment and spread of this saprophytic fungus in a woodland habitat.

Mycelial cord systems of Phallus impudicus, Phanerochaete velutina, Steccherinum fimbriatum, Stropharia aeruginosa and Tricholomopsis platyphylla, when excavated in the field, were found to be between 318 m (5. aeruginosa) and 317 m (P. velutina) in overall length. Cord systems of P. impudicus, P. velutina and T. platyphylla increased in length by 0.8, 0.95 and l.0 m respectively over 12–14 months. In all cases extension occurred via the peripheries of the systems, whilst certain more central parts disappeared. Mycelial development from colonized wood-blocks in tubes of non-sterile soil showed similar patterns to that observed in the field. However, in tubes of γ-irradiated soil mycelia were not aggregated into cords to such an extent as in non-sterile soil, and, except for T. platyphylla, had slower extension rates, although sometimes being more luxuriant. Wood-blocks colonized by Hypholoma fasciculate, S. fimbriatum, P. impudicus and P. velutina underwent significantly greater losses in dry weight in sterile soil than did blocks in non-sterile soil, except for those colonized by T. platyphylla. Cord systems of like and unlike mycelial types of P. velutina from wood blocks fused in soil, but whilst fusion of those of like mycelial type gave rise to cords of substantial diameter, fusions between different mycelial types were followed by death of the fine cords in the fused regions and avoidance of such regions thereafter. Cord formation by a coprophilous species occurred in non-sterile but not sterile conditions, and was associated with strong substratum directed growth responses.

Summary Pairings between strains of Stereum hirsutum (Willd. ex Fr.) S. F. Gray pp 2% malt agar were analysed using changes in mycelial morphology and non-self recognition properties as evidence of genetic transfer between the participants. In some combinations the morphological evidence suggested the- uniform emergence of a heterokaryon following mating and reciprocal nuclear migration between two compatible homokaryons. In other combinations there was evidence of strongly selective invasion by one of two nuclear types from a heterokaryon into a homokaryon, and in a third type of pairing there was evidence for acquisition by a homokaryon of a second mating type but not an entire nuclear genome. Combinations representing each of these three apparent classes of genetic transfer were selected, and mycelia grown from single hyphal tips and/or basidiospores taken either from pure cultures of the progenitor strains or from subcultures from interacting colonies. The DNA from each of these mycelia was extracted and analysed using restriction fragment length polymorphisms to detect genomic differences between strains. The results were consistent with the interpretation of events based on morphological criteria. The combination of morphological and molecular approaches holds considerable potential for rapid, yet precise investigation of genetic exchange between fungal strains isolated from natural populations which have not been the subject of extensive genetic analysis by classical means.

The induction of tumour-like mycelial growths and apically extending rhizomorphic organs in the non-rhizomorphic basidiomycete, Stereum hirsutum , is described and related to the activity of a transmissible factor with pleiotropic effects on sexual and somatic differentiation.

The ultrastructure of rhizomorph apices of Armillaria bulbosa was studied by means of thin sections. Samples of colonized wood obtained from the field were incubated in polythene bags to stimulate rhizomorph production. The apices, up to 1 mm diam, were surrounded by a zone of mucilage up to 200 μ m wide in which widely dispersed hyphae and individual cells with dense protoplasmic contents were embedded. The mucilage appeared to radiate from these cells. Mucilage production began in tightly packed cells at the interface between the mucilaginous and cellular regions of the rhizomorphs. In this region, mucilage-containing vesicles coalesced with the plasma membrane creating a mucilage-filled space between the membrane and all parts of the cell wall, including the septal plate, which was traversed by numerous membrane-bound protoplasmic protuberances. This space, together with the protuberances, disappeared following apparent partial or complete digestion of the wall which was concomitant with release, in a radiate pattern, of mucilage to the exterior of the cell. Mucilage released into septal regions filled locations previously occupied by septal swellings, and led to cleavage of compartments into separate cellular entities.

SUMMARY Almost 1000 wood blocks, 8 cm3, of beech (Fagus sylvatica L.), colonized by the mycelial cord-forming basidiomycetes Hypholoma fasciculare (Huds. ex Fr.) Kummer., Phallus impudius (L.) Pers., Phanerochaete (Ph.) laevis (Fr.) Erikss & Ryv., Ph. Velutina (DC ex Pers.) Parmasto, Steccherimum fimbriatum (Pers. ex Fr.) Erikss. and Tricholomopsis platyphylla (Pers. ex Fr.) Sing, were placed at the soil-litter interface in five different woodland sites. All species except T. platyphylla grew out radially from the blocks to form a network of mycelial cords ramifying amongst the litter and upper soil horizons. Initial outgrowth was more strongly related to microclimatic factors than to availability of nutrient resources, and after 3 months significantly greater (P

Various fully compatible and hemi-compatible pairings were made on 2 % malt agar between laboratory-synthesized heterokaryons and homokaryons of British collections of Stereum hirsutum . In all cases heterokaryons arose within the originally homokaryotic mycelia. However, the patterns of interaction between the colonies, and the number and genotypes of heterokaryons resulting from mating varied in different combinations. A zone of mutual antagonism developed in all cases along the initial junction between paired colonies. This was generally more intense, and strongly pigmented, between unrelated than related strains, and was eventually suppressed entirely in those hemi-compatible combinations where the homokaryon acceptor contained one of the component nuclear types in the heterokaryon. In certain combinations, passage of a ‘lytic zone’ of appressed mycelium preceded heterokaryotization, this being most common where two or more participant nuclear types were sib-related, and rare where all types were non-sib-related. In fully compatible combinations, i.e. where both heterokaryon component nuclei were sexually compatible with, and hence capable of migrating into, the homokaryon, the emergence within the homokaryon of heterokaryons with alternating genotypes and separated by narrow ‘tracks’ (somatic incompatibility zones) was often observed. However, selective exclusion of one of the heterokaryon component nuclei often occurred, and was associated either with the absence of tracks, or the replacement in alternating sectors of one of the expected heterokaryon types by the parental heterokaryon. Non-sib nuclei were generally selected in preference to sib nuclei.

Isolations of fungi were made from various categories of attached and fallen twigs of oak (Quercus robur). A similar range of species was isolated from non-living twigs of all types, the most characteristic being Tubakia dryina, Colpoma quercinum and Cryptosporiopsis quercina, but twigs abscissed at leaf fall were much more extensively colonized than those abscissed at other times of year. Basidiomycetes were obtained on several occasions from all categories of fallen twigs, but not from living attached twigs. Colpoma quercinum frequently produced numerous, somatically incompatible mycelia, separated by dark interaction zone lines in non-living twigs, both before and after abscission.

SUMMARY: A spontaneous change, possibly involving a mating-type switch, occurred during storage of a homokaryotic culture (F2.4) of the basidiomycete Stereum hirsutum and resulted in a dramatic transformation of its cultural and interactive properties. Subcultures grown on malt agar rapidly produced farinaceous hymenial surfaces if exposed to light, and were not receptive to nucleus migration when paired with compatible homokaryons. These properties were transmissible to other homokaryons by direct pairing, the recipients becoming transformed regardless of whether they were sexually or somatically compatible or incompatible with F2.4. With the incompatible sib F2.9, transformation apparently occurred without nuclear transfer. Progeny monobasidiospore cultures from transformed F2.4 and F2.9 strains consistently belonged to two interaction groups, standard and reversed, the former reacting similarly to the corresponding untransformed strains against a range of sib homokaryons, the latter interacting in exactly the opposite fashion. Reversed-standard pairings resulted in all cases in an unusual ‘quasi-compatible’ reaction, characterized by development of an asymmetrically migrating farinaceous region. Progeny from transformed F2.9 were dimorphic, being divisible into (a) slow-growing, dense and (b) fast-growing, effuse colony types, the former reverting spontaneously to the latter. In several cases mycelial ‘mounds’ developed on reverted slow, dense colonies. Basidiospore progeny and mycelial subcultures from mounds produced colonies with further mounds. These and further observations indicate pleiotropic effects initiated or mediated by a cytoplasmically transmissible, possibly insertional factor on recognition and developmental processes in S. hirsutum.

Attached or fallen branches of oak ( Quercus robur ) colonized by Phlebia radiata and P. rufa were collected from locations in South-West Britain, and the patterns of decay within the branches analysed. Isolates were cultured from decayed wood, fruit-body tissue and from single basidiospores. Isolates were paired on malt agar in various combinations and also against other species of decay fungi. Bipolar incompatibility was found in both species, although P. rufa behaved inconsistently, and intraspecific pairings between genetically different dikaryons were mutually antagonistic. Pairings between the two species always resulted in strong mutual antagonism, immediately after hyphal fusion, and no dikaryons with clamp connexions were formed between homokaryons. Both species tended to replace other decay fungi in culture, but P. radiata was generally the more successful. All of the above interactions appeared to depend on contact with septate mycelia. Colonies of both species on agar extended relatively rapidly from a wide peripheral zone of appresed, mostly coenocytic mycelium which generally lacked hyphal fusions. Behind this zone, which was wider in P. radiata than in P. rufa , hyphae with clamp connexions developed and aerial mycelium was formed. The aerial mycelium of P. rufa was generally rather localized and rapidly aggregated into small hymenial surfaces on homokaryotic and dikaryotic mycelia. P. radiata formed a much more diffuse, floccose aerial mycelium which formed hymenia only occasionally on old cultures. Homokaryotic and dikaryotic cultures of both species produced oidia, although P. rufa did so more abundantly and more consistently than P. radiata . The cultural differences between the two species were correlated with differences in their ecology: P. radiata is regarded as a secondary colonizer, but P. rufa is probably a pioneer colonizing weakened, but still-living wood.

The apical dome containing tightly packed cells in vigorously growing rhizomorphs of Armillaria bulbosa is underlain by a distinctive layer, up to several cells wide, adjacent to the central medullary region. Cells in this layer appear to be very active and are characterized by large numbers of mitochondria with distinctive outer membranes, and by axial bundles of microfilaments of which there may be several in an individual cell. The bundles appear often to extend for the entire length of a cell, and in some cases may traverse zones where lateral fusions have occurred. In comparison with similar bundles found in eukaryotes other than Eumycota, those presently described are unusual both in that associated microtubules are axially aligned, and in their thickness (normally 0.3–0.9 μm, and exceptionally up to 1.4 μm across). The likelihood is that this apparently specialized layer of cells, which extends at least 750 μm back from the apex, is responsible for short range supply of growth materials to the apical dome.