Speciation and distribution in - Springer Link

P1. Syst. Evol. 177: 9 3 - 110 (1991)

,Plant Systematics

and Evolution

© Springer-Verlag 1991 Printed in Austria

Speciation and distribution in

Corticiaceae (Basidiomy-

cetes) NILS HALLENBERG Received February 7, 1991 Key words" Basidiomycetes, Corticiaceae. - Speciation, distribution, biohistory, mating system, evolution, population structures. Abstract" Evolutionary processes in Corticiaceae (wood-inhabiting basidiomycetes) are dis-

cussed on the basis of sibling species analysis, colonization strategies, and the known present distribution. It is proposed that the phylogenetical differentiation of basidiocarp structures may be very old and many species have remained unchanged. Subsequent evolution has to a great part been the effect of biological interaction with the environment, like colonization of new substrata, and the formation of species complexes may be a consequence of this. The amount of spores liberated from a single basidiocarp is regarded as an adaptive character for dispersal mainly in the immediate environment. The actual, wide distribution of many species is supposed to be associated with expansion of pertaining forest types in the past. When rapid expansion of a species occurs it is likely to be connected with occupation of a new ecological niche. Finally, the consequences for fungal communities in modern forestry are discussed. For a long time, scientists working with C o r t i c i a c e a e ( B a s i d i o m y c e t e s ) have mostly been engaged in taxonomic problems, how to keep species and genera distinct from each other. In pace with increasing knowledge on the occurrence of species in different parts of the world and more exact delimitations of species by the use of compatibility tests and other experimental methods, attention has been paid to factors involved in speciation (BOIDIN & LANQUETIN1984, HALLENBERG 1987). Such factors have been revealed by analyses of species aggregates, which are consequences of relatively recent events in the evolution. However, it is also necessary to consider the historical perspective with the early evolution of basidiomycetous, wood-decaying fungi. An idea about what has happened to these fungi in the past will help understanding of the dynamic processes which may affect the future. Floristic reports from various parts of the world show that many species are widely distributed, at least within the boreal and nemoral zones over the northern hemisphere, but many of these species have also been found in the South (HJoRTSTAM & RYVAP.OEN 1985). The wide distribution of many species has been ascribed to high efficiency of wind dispersal by spores. This should further be facilitated by a high spore production in many species. A number of new species is described every year especially from little investigated areas, such as the tropics. Still, tax-

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onomy on genus level and higher ranks in Corticiaceae is very vague. Events in the history of the group where adaptive radiation may have occurred have been very little discussed (PARMASTO1986). Confusion in making good generic arrangements could depend on the restricted number of characters available for traditional taxonomy (mainly basidiocarp morphology), but also a too strong focusing on basidiocarp characters. The application of new methods, such as DNA-techniques, could help us to construct more phylogenetic systematics. To understand the phylogenetic scenario in this group of fungi, both the known historical events and the complete biology of living species should be considered. In the light of present knowledge, it is possible to give an hypothesis concerning systematics and distribution in Cortieiaceae and related wood-inhabiting basidiomycetes. This is based on ideas that many species or species aggregates are very old, in comparison with most angiosperms, and that high capacity in wind dispersal by spores does not necessarily imply genetic exchange between allopatric populations. These ideas have been mentioned or discussed by BOIDIN & LANQUETIN (1984), PARMASTO (1986), and HALLENBERG(1987). An important aspect concerns the future and the possible adaptations which may take place among wood-fungi in an environment which is so strongly influenced by human activities. The potential for adaptations to new conditions could as well be extracted from the study of evolutionary events in the past. Most examples here are taken from the temperate zone of the northern hemisphere, as this area is most thoroughly investigated. For the analysis, the following items will be considered: Comparisons with higher plants in a historical perspective. Biohistory of wood-inhabiting basidiomycetes. Present distribution of wood-inhabiting Corticiaceae. Microevolutionary tendencies observed. Selection factors of importance for speciation. Mating behaviour and evolution. Population structures.

Comparisons with higher plants in the northern hemisphere Within temperate, forested areas on the northern hemisphere there are two fundamentally different vegetational zones, boreal and nemoral (WALTER 1968). Within each zone there are great similarities in flora of higher plants between Europe, Asia, and N. America (KORNAS 1972). The coniferous taiga forest, which is the principal forest type in the boreal zone, was formed during Miocene or earlier, as a mountain taiga. At that time lowland plains were dominated by a vegetation of the nemoral type. Much later when the climate became cooler due to glaciations, the coniferous taiga extended towards lowland areas. Also, the nemoral forests formed a continuous vegetational zone in Miocene. Climatic changes with a general decrease in temperature, due to glaciations, caused this zone to move southwards. The nemoral zone became divided mainly into three isolated areas, E. N. America, E. Asia, and parts of Europe. The forest tree-species in these two zones may have wide distributional areas but they are not circumpolar. Closely related corresponding taxa occur in all three

Speciation and distribution in Corticiaceae

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continents. It is noteworthy that corresponding taxa also are ecologically corresponding. Despite their morphological differentiation they occupy the same ecological niches, because these remained unchanged since the time prior to taxonomical divergence. Shrubs and herbs in the conifer forests are circumpolar to a high degree while local species predominate in the isolated nemoral areas, but even here the generic affinities are obvious (KORNAS 1972). Biohistory of wood-inhabiting Basidiomycetes Filamentous fungi, as saprophytes and as partners in mutualistic relationships with higher plants, are very old and probably contemporary with the first terrestrial plants (TAYLOR 1990). Fossil records indicate that white rot fungi were present already in the Permian. Vesicular-arbuscular mycorrhizae (Zygomycetes) have been indicated in fossils from the Triassic, while there are no signs of ectomycorrhiza (Basidiomycetes) from Paleozoic or Mesozoic. PARMASTO (1986) assumes that the origin and early development of Hymenomycetes was synchronous with the early evolution of Pinidae and Angiospermae, and connected with the formation of forests, similar to the present ones. There are good reasons to believe that primitive Basidiomycetes, which were the origin of present Hymenomycetes, were present and widespread in the fungus litter flora at that time. PARMASTO(1986) further emphasizes that the appearance of these forests with accumulation of dead wood and a new kind of forest litter, created two new ecological mega-niches which caused adaptive radiation and rapid evolution. 1. Fungi growing on wood primarily formed effused basidiocarps. Later evolution resulted in the development of different kinds of corticioid and stereoid basidiocarps as well as polypores. 2. Fungi which were growing on litter on the ground produced vertical basidiocarps. They were subsequently differentiated into clavarioid, cantharelloid and agaricoid basidiocarps. Within this group of fungi, the mutualistic relation of ectomycorrhiza most probably evolved. Representatives of these two groups of fungi also exhibit general and consistent physiological differences (HINTIKKA1982). The rapid evolution implied adaptations of the species in different respects: Adaptations for different climatic conditions. The humidity factor must have been an especially important delimiting factor for species in both the two groups mentioned above. In species, whose mycelia endured growth in a relatively dry environment, the attached basidiocarps also attained corresponding adaptations for enduring dry periods or for rapid fructification in short periods of high humidity. Adaptations for effective spore dispersal. The extent of spore production is correlated with the possibilities for establishment and the supply of suitable niches in the environment. In the beginning of the rapid evolution of hymenomycetes, a high spore production was certainly a factor of a certain competitive value. Adaptations for survival in an micro-environment where compounds which are hostile for fungal growth are present. When our present boreal and nemoral forest types were formed on the northern hemisphere, a great deal of now existing corticioid species were most probably already existing and widespread (Table 1).

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Table 1. Survey of macroevolution in Corticaceae Period:

Carbon

Triassic - Jurassic

Tertiary

Vegetation:

tree ferns, primitive gymnosperms

origin of present forests

coherent nemoral vegetation over the N. hemisphere; mountain coniferous forests

Basidiomycetes:

widespread as litter decomposers

adaptive radiation in wood-decay and ectomycorrhiza fungi

present corticiaceous fungi widespread to a high extent

Strong indications that now living fungal species are old in the above mentioned concept are given by DEMOULIN (1973) with examples from the genus Lycoperdon, and by RAJCHENBERG (1989) for south hemispherical polypores.

Present distribution of wood-inhabiting Corticiaceae In a survey of wood-rotting fungi of N. America, GmBERTSON (1980) concluded that a high percentage of species were common in Europe and N. America. The similarities were highest when only the boreal zone was considered. Any precise value for this similarity is unimportant at the moment, as our knowledge is still uncomplete. Corticioid species look relatively undifferentiated when observed with the naked eye, and some of them may seem to be of erratic occurrence, explaining why a number of species escape attention in the field. In a number of cases, such species which were considered as endemic in one continent, subsequently were found to be widespread outside this area. HALLENBER~ (1981) investigated the corticiaceous flora in northern Iran, a phytogeographically isolated area with nemoral vegetation, and found many similarities in comparisons with N. Europe and N. America. HJORTSTAM & RYVAP,DEN (1985) investigated the Aphyllophorales flora on No thofagus in Tierra del Fuego (Argentine). The corticiaceous species found were similar to a high extent to those found in N. Europe. Mating tests between representatives from N. America and Europe, for many species in Corticiaceae, showed that they were compatible, despite the geographic isolation (HALLENBERG 1984a, 1985). The list of floristic reports showing conformity in the flora of corticiaceous fungi, could be extended. A condition for this high similarity is, however, that the same kind of vegetational zones are compared. The frequency of some aphyllophoroid species in central parts of the Russian taiga was analyzed along a humiditytemperature gradient, from forest tundra in the North to forest step in the South (MUKmN 1987). The analysis showed a continuity within this gradient over a vast geographical range but also particular preferences for the individual species. An imporant conclusion is that there seems to be no correlation between number of spores produced by each basidiocarp and the distributional area. Among the widespread ones are several species with small and little differentiated fructifications. Further, species may appear rare or frequent irrespective of their capacity in spore production.

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Table 2. Outlines of present distribution of higher plants and corticaceous fungi over the N. hemisphere Trees

Shrubs and herbs

Corticiaceae

Boreal zone:

widespread but vicareous species

circumpolar and vicareous species

circumpolar to a high extent

Nemoral zone:

vicareous species

vicareous species

circumpolar to a high extent

There seems to be no doubt that the majority of species in Corticiaceae is well distributed, at least within the boreal and nemoral zones of the northern hemisphere. Reports from comparative areas in the southern hemisphere are still too scanty, to allow adequate comparisons but as indicated above, a similar Corticiaceae flora exists even there. The comparison of higher plants in the northern hemisphere showed high similarities on species level for the field layer flora in the boreal zone (KORNAS1972). Between the disjunct nemoral areas, similarities were restricted to genus level. Woodinhabiting species in Corticiaceae are obviously widespread and fairly evenly distributed in both boreal, coniferous forest and broad-leaved, nemoral ones (Table 2).

Mieroevolutionary tendencies observed Although the high similarity in the wood-fungus flora is the most striking feature when comparing two similar areas, the observed differences may help us to understand the microevolution. A few species seem to have a rather limited, geographical distribution (more or less endemic) and are taxonomically well distinguished. Such species are found in the genus Echinodontium ELL. & EVERH. Historical factors affecting the environmental forest type seem to be responsible for that kind of limited distribution. Still, a few examples on allopatric speciation exist, while the number of inter-incompatible sibling species complexes, probably originating from the same geographical area, is larger and continuously increasing. Also between populations which still are completely intercompatible, some ecological divergences have been noticed. Allopatric speciation certainly occurs within Corticiaceae but a long lasting geographic isolation does not always imply differentiation in basidiocarp morphology or may not even affect the mating behaviour. Too little is known about the ecology of individual species to allow designations of "corresponding taxa", as in its case for higher plants. Discussions on allopatry must therefore be restricted to species-pairs which undoubtedly are very closely related, even in a taxonomical sense. Such species-pairs often show slight morphological differentiations and may still have a certain degree of mating ability (partially compatible). Thus, Bomry LANQUETIN (1977, 1983) found that Dichostereum durum (BovRD. & GALZ.) PILAT (France) and D. sordulentum (COOKE& MASSEE)BOID. & LANG. (N. America) constitute such a pair, Peniophora malenconii subsp, malenconii BolD. & LANQ.and subsp, americana C~IA~URIS are another. In a world-wide study they also found

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several incompatible sibling species in the Scytinostroma galactinum (FR.) DONK complex, where each component seems to be geographically limited (BOIDIN & LANQUETIN 1987). Peniophora pini (FR.) BOLD. from Europe was found to be partially compatible with P. duplex BURT from N. America (WERESU~ & GIBSON 1960). Other cases of allopatric speciation in Hymenochaetaceae have been discussed by PARMASTO (1985). Partial compatibility between geographically isolated populations has also been reported in connection with so-called ABC-matings. In these cases two closely related species (taxonomically distinct or not) with sympatric occurrence may be inter-incompatible, while both are completely or partially compatible with representatives from another continent. In such cases, the incompatible relation must depend most probably on sympatric or parapatric speciation, as sterility barriers did not develop between truly allopatric populations. Within Corticiaceae, ABCmatings have been well documented in Peniophora cinerea, Hyphodontia subalutacea (HALLENBERG 1987), and among three, closely related species in Amylostereum, Amylostereumferreum (BERK. & CURT.) BOLD. & LANG.,A. chailletii (FR.) BOLD., A. laevigatum (FR.) BOLD.(BoIDIN & LANQUETIN1984). Several examples also exist within the polypores. Sympatric or parapatric speciation seems to occur in a number of species and is also distributed within many different genera. In some cases, like Sistotrema brinkmannii, it has been possible to link several of the supposed siblings to distinct, taxonomical species, using traditional, morphological criteria although in a strict sense (HALLENBERG 1984 b). New siblings or "compatibility groups", however, are continuously added, to S. brinkmannii as well as to other species complexes. Even if a compatibility group is represented by only a single specimen, later additions of new material may include compatible specimens to such groups. This is a frequently made observation. Table 3 lists species where such siblings have been detected at our mycology laboratory. The precautions for interpretation of results from compatibility tests mentioned by BOIDIN (1986) have been considered but in accordance with the discussion above, also intercompatibility groups represented by a single specimen are enumerated. The number of specimens within each compatibility group is given. Occurrence of presumably homothallic siblings is marked with a "H". Where special preference to a certain substrate for any of the siblings has been noticed, this is especially marked, and also for supposed geographically isolated siblings. However, most examples of geographically isolated siblings mentioned below, originate from the same continent. Further examples are given by MCKEEN (1952) who found that specimens of Hyphoderma mutatum (PECK) DONK growing on Populus were mainly incompatible with specimens growing on wood from other hardwood trees. BOIDIN (1950) reported that Hyphoderma praetermissum (KARST.) ERIKSS. & STRID consisted of two bipolar siblings as well as homothallic ones. Since then two additional siblings have been found (HALLENBERG,unpubl.). BOIDIN (1977) further noted the existence of four heterothallic and one homothallic forms of Laxitextum bicolor (FR.) LENTZ, which at least partly seem to be geographically delimited. SALIBA & DAVID (1988) found that Steccherinum ochraeeum (FR.) S. F. GRAY could be divided into several sibling species, without finding any differences in niche selectivity between them.

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Speciation and distribution in Corticiaceae Table 3. Corticiaceous species where the occurrence of sibling species has been detected Species

No. of specimens per sibling

Substrate

Ceraceomerulius serpens (Fro) EmKss. & RYV. Crustomyces subabruptus (BOuRD. & GALZ.) JUL. Dacryobolus sudans (FR.) FR. Fibricium rude (KARsT.) JOL. Galzinia incrustans (H6~N. LITSCFI.) PARM.

7 + 2 + 2+ H 2+ 1 5+ 2+ 1 2+ 1 2+ 1 13+6+5+2+H 21 + 1 + 1 + 1 4+ 2+ H 3+ 1 16 + 1 + 1 4 + 3 + 2 + 1+ 1+ 1 7 + 1+ 1+ 1 11 + 1 + 1 10+ 1 + 1 + 1 3 + 2 + 1+ 1+ 1+ 1+ 1+ 1 7 + 5 + 2 + 2 + 1+ 1

x

Hyphodermapraetermissum (KARST.) ERIKSS. &

STRID

H. puberum (FR.) WALLR. H. setigerum (FR.) DONK Hyphodontia alutacea (FR.) ERIKSS. H. aspera (FR.) ER~KSS. H. breviseta (KARST.) ERIKSS. H. crustosa (FR.) EmKSS. H. nespori (BREs.) ERIKSS. & HJORTST. H. rimosissima (PECK) GILl3. H. sarnbuci (PERs.) EmKSS. H. subalutacea (KARST.) ERIKSS. Hypochnicium bombycinum (FR.) ERIKSS. -H. karstenii (BREs.) HALLENB. H. eichIeri (BREs.) ERIKSS. & RYv. H. polonense (BREs.) STR~D H. sphaerosporum (HOHN. & LIrSCH.) ERIKSS. -H. subrigescens BOID. Peniophora cinerea (Fro) CKE. P. nuda (Fro) BRES. Phlebia lilascens (BouRD.) ERIKSS. & HJORTST. P. livida (Fro) BRES. P. rufa (FR.) M. P. CHRIST. P. subochracea (BREs.) ERIKSS. & RYv. P. subserialis (BouRD. & GALZ.) DONK Radulomyces confluens (Fro) M. P. Cr~RIST. Resinicium bicolor (FR.) PARM. R. furfuraceum (BREs.) PARM. Sistotrema brinkmannii (BREs.) ER~KSS. S. oblongisporum M. P. C~RIST. & HAUERSL Sistotremastrum niveocremeum (HGHN. & LITSCH.) ERIKSS.

S. suecicum LITSCH. ex ERiKss. Tubulicrinis gracillimus (ELL. & EWRH.) CUNN. 7". strangulatus LARSS. & HJORTST. T. subulatus (BouRD. & GALZ.) DONK

7+3+1 8+2+1+1 3+2 2+1+1+1+1+1 26+7+4+3+1 11+4+1 3+2+2+1+1+H 10+4+1+H I0+2+1+1+1+H 3+2 3+1 7+1+1 14+1 9+2 20+2+1+1+H 7+1+1+1

Geography

x x X x

x

x

X x

x

X

×

x x

x X ×

x

X X X

x 2+l+H 3+1 4+3+2+2+1 7+1 12+3

x x x

x

Several siblings have also been reported form Stereum spp. and from Coniophora puteana (Fp..) KARST. (AINSWORTH1987). The majority of investigated species seem, however, to mate perfectly in intercontinental matings and a taxonomical delimitation of these species corresponds well with results from compatibility tests. A further example of microevolution of allopatric origin is the ecological differentiation observed in some species of Corticiaceae despite that the isolated pop-

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ulations are still fully compatible. Phlebia centrifuga KARST. occurs in Europe exclusively on conifers (Picea, Abies). In N. America, however, it is found on both conifers and hardwood. The situation is similar in Cystostereum murraii (BERg:. & CURT.) Pouz. which strongly prefers deciduous trees in N. America, whilst almost restricted to conifers in Europe, at least in the North (KOTLABA 1987). Hyphoderma roseocremeum (BRES.) DONK is normally found on deciduous trees in Europe, but on conifers in Canda. In all three examples the noted ecological differences is linked with minor but distinct morphological differences.

Selection factors of importance for speciation As species characters in a taxonomical sense mainly are based on basidiocarp morphology (including micromorphology) it is first necessary to make the following distinctions. Evolutionary processes resulting in the differentiation of various basidiocarp types most probably originated a long time ago. This kind of evolution was connected with the occupation of the new adaptive zone which appeared when coniferous and angiosperm forests, similar to the present ones, were formed. The very wide distribution of many (taxonomical) species within comparable forests strongly supports the assumption that such species have an origin which goes back to the origin of present forest types. Much of the subsequent evolution, at least within temperate areas of the northern hemisphere, seems to be related to the occupation of new substrata. Comparisons between siblings within above mentioned species complexes indicates, that biological interactions with the immediate environment (substratum) is of primary importance for the establishment and survival of individual mycelia. These interactions may be divided into several components, related to different parts of the life-cycle. Some parts of these interactions have been studied more closely and are reported below.

Spore germination and initial mycelial growth. It is a well known experience that the success in getting basidiospores to grow on artificial media varies. In certain species it is impossible to get mycelia from spores, in others only a very small percentage will grow out and then very slowly. In the now classical works by NILs FR~ES (summarized in FRIES 1987) a number of biological factors controlling spore germination in ectomycorrhiza fungi, have been demonstrated. Wood-inhabiting basidiomycetes, like the species in Corticiaceae, have generally been looked upon as species which are easy to culture. Even in these species, however, the number of obtained germlings is just a small portion of the total number of spores added to the medium. When the initial process of germination is studied directly under the microscope it seems that almost all spores will germinate, but after a short period of further growth most of the initial germlings have obviously been suppressed by the developing ones, which still are regularly haploid in heterothallic species. This process has been observed many times and in many species. In Hyphoderma praetermissum spore growth is initiated by the formation of a special structure, a stephanocyst (HALLENBERG 1990). In the very young germlings additional stephanocysts are formed regularly on developing hyphae but later in the developing mycelium, they are more scattered. One function of this structure is probably related to the establishment of the young mycelium on a new substrate. In Hyphodontia aspera (FR.) JOHN ERIKSS. the spores grow out in a very low percentage and some germlings are visible at first 2 - 3 weeks after dispersal. By

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accident a spore suspension was infected by bacteria isolated from the wood where the fungus was growing. Already after one week almost all spores had grown out with a hypha, 2 0 - 4 0 pm long, but later development was inhibited for some unknown reason. Spore growth was also studied in Phlebia subcretacea (LITSCH.) M. P. CHRIST. Within a week a number of germlings, 1 mm in diameter, were observed. Direct observation in the microscope showed that initial germination took place in almost all spores. The small germlings were carefully transferred to individual dishes with the same kind of medium. There, they remained for almost three months without further growth but after that period some of these haploid cultures started a more rapid growth and could be handled normally. Meanwhile, one dish with many spores dispersed, was left in order to obtain a polyspore culture. This developed normally without any resting period. The subsequent change in growth behaviour by the spore germlings may depend on physiological changes caused by the substrate, which initially was unsuitable for this particular species. These examples show that biologically interacting processes are of great importance and that the germinating mycelia themselves take an active part in these interactions.

Establishments of mycelia. In several reports it is shown that the environment of a corticioid species may be shared with a great number of other basidiomycete, ascomycete and mucoraceous species. Together with the hostile microenvironment offered by the substrate itself, especially when still living, the complete flora of decaying organisms interacts and strictly delimits the niches available for any particular species. Also, pure physical parameters appear to be very important as selection factors. There is a drastic change in the gaseous regime inside wood compared with the surface. The CO2 content rises while the amount of available oxygen is diminishing. Some species are adapted to a more extreme situation (HINTIKKA 1982), while the access of most basidiomycetes is faciliated by a more aerated wooden tissue, which is a result of initial decaying processes (RAYNER & BoooY 1988). In living tissues there is a flora of endophytic fungi (mainly ascomycetes) among which several are host specific (PETRINI & FISHER 1990, BOODY & GRIFFITH 1989). Some fungal species may be present as small mycelia, which rapidly will develop as soon as substrate tissues die off (CHAPELA& BODDY1988 a, b; GRIFFITH & BODDY 1988). Frequently, there is a general succession in the decay, starting mainly with nonbasidiomycetes. During this first phase of colonization, toxic phenolic substances produced by the tree are eliminated. Subsequently, basidiomycetes take an increasing part in the colonization (K;~XRIK 1972), and a characteristic succession of invading species is established. Depending on their ecological strategies (ruderal, stress-tolerant, combative) different species have distinct positions in this succession (COOKE & RAYNER 1984). Different ecological strategies are further manifested in modes of arrival to the wood substrate (spores born by wind or insects, penetration by mycelia from species living in the soil), size and age of individuals. It is worth to note that a few wood-inhabiting basidiomycetes have a short life-span of less than a year, while the life-time of individual clones (genotypes) of some root pathogens, like Phellinus weirii (MURR.) GILB., may persist for more than l 000 years (DICKMAN& COOK 1989).

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From the discussion above follows, that selecting factors of utmost importance exist in the relation between species in Corticiaceae and their substrates, including the sum of biological interactions taking place there. There are characteristic fungal communities for any particular host species. Accordingly, it is not surprising that a number of siblings within the species complexes listed above differ from each other in their host preferences (for details, see HALLENBERG 1987). The noted differences between hosts for such sPecies complexes, are mainIy of three characters: A. The host species itself. B. One of the siblings may grow on a host which naturally belongs to a forest type, different from the habitat of the other siblings host. Thus, the two siblings will primarily have their distributions in different forest or vegetation types. Examples of this are Peniophora cinerea in Europe, with one sibling restricted to Fagus forests while another occurs on a variety of deciduous trees, other than Fagus. In Peniophora nuda, there is one sibling on Ulmus carpinifolia GRED. on the islands Gotland and Oland in the Baltic Sea, while the main populations are found on various deciduous trees elsewhere (HALLENBERG1986 a). In Ceraceomerulius serpens and Hyphodontia subalutacea there are siblings which are restricted to pine forests in Europe while other siblings have not been collected on this substrate or in this kind of forest (HALLENBERG 1987). In Hyphoderma mutatum there is one sibling restricted to Populus in N. America, while another one occurs on deciduous trees, other than Populus (MCKEEN 1952). In this area Populus may form pure forests and also here speciation can be presumed to be of parapatric nature. C. Differences in type or degree of decay. In Tubulicrinis strangulatus two siblings occur, the only noted difference being degree of decay (both associated with brown-rotted, coniferous wood; see HALLENBERG(1986 b).The sibling in Peniophora cinerea which is restricted to Fagus as substrate, grows on decorticated branches which are affected by primary decay, while the other sibling is mainly found on the bark of newly fallen branches. "Species complexes" or "aggregates of siblings" is a human conception based on our inability to keep the respective basidiocarps distinct from each other on morphological grounds. From a biological point of view, other species pairs Could just as well be looked upon as a result of the same kind of speciation. In Peniophora there are a number of host specific species with only minute differences in basidiocarp morphology but as these differences are large enough to keep them distinct, they are not usually looked upon as siblings. The same situation exists in Amylostereum, where A. laevigatum differs only slightly in basidiocarp morphology from A. chailletii, .while the difference is clear in host selectivity (Juniperus - Taxus and Abies - Picea, respectively). The close relationship between the two is demonstrated by the ability of both to be partially compatible with A. ferreum (Podocarpus, in S. America; see BOIDIN & LANQUETIN1984). Further, host selectivity is not uncommon among polypores. Some siblings within the Phellinus igniarius (FR.) QUEL. -- complex are separated only on host characters (F~SCHER 1987). It seems that host specificity is more pronounced for species adapted for an early stage in fungal colonization which is logically, as the resistance from host tissues is of high importance compared with resistance of degraded wood. In the long run, a parapatric or sympatric speciation resulting from a change of substrate may involve morphological changes in the basidiocarps as well. Such

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morphological changes may be related to the character of the host surface (mode of attachement by basidiocarps) or the arrival by spores (spore-sizes, -shapes). Mating behaviour and evolution It has been postulated by many authors that the multi-allelic, bi- or uni-factorial mating system, combined with effective wind dispersal by spores, is a strongly stabilizing factor in basidiomycete populations. Apart from these heterothallic species also homothallic forms exists, in which the life-cycle is completed directly from one spore without any preceding mating. Nuclear behaviour during the lifecycle varies (BoIo~N 1971) and in one mode, holocoenocytic behaviour, there seem to be no differences in individual hyphae between mycelia resulting from single spores and those isolated from basidiocarp tissues. In both kinds of mycelia there are multinucleate cells and clamp connexions may occur, both single and in whorls. While homothallic forms of non-holocoenocytic species are known to exist in parallel with heterothallic forms in a number of species without any closer phylogenetical relationship, holocoenocytic behaviour has been found to be characteristic for complete genera (Phanerochaete, Coniophora, Stereum, Phellinus, Inonotus). BOIDIN (1971) supposed that the species in these genera could be homothallic owing to similarities in nuclear behaviour between single spore and polyspore isolates. Later, COATES& al. (1981) showed that Stereum hirsutum (FR) S. F. GRAY must be regarded as a heterothallic, bipolar species by analyzing the differentiation in mycelial forms obtained in mating tests. This was a real breakthrough in the theory of matings in basidiomycetes as earlier conceptions took for granted that the differences between single spore and polyspore isolates should be visible under the microscope (absence/presence of clamps, dikaryons). A continuation of these new ideas was the detection of heterothallic behaviour in a number of Stereum spp. (RAYNER& TURTON 1982), in Phlebiopsis gigantea (FR.) J~I~. (KoRHONZN & KAUPPILA 1987), Phellinus and Inonotus (FlSCI-IER 1987), Phanerochaete and Coniophora (AINsWORTH 1987). An important discovery was made by L£GER & LANQUZTIN (1989), who found that a species in Hymenochaete, a genus related to Phellinus, was heterothallic and bipolar. This is the first record of a species in Hymenochaetaceae, where a single spore mycelium has uninucleate cells and a polyspore mycelium is dikaryotic. In a number of species without any closer relationships homothallic forms are known to exist in parallel with heterothallic ones. Nevertheless, it seems to be more common in certain genera. Probably, it is more common than generally believed, as single spore cultures may sometimes be consistently clamped when isolated in many specimens. Frequently, this is explained as a consequence of inaccurate isolation, i.e. isolated germlings have been supposed to be already mated at a very early time upon germination. This explanation is questioned, mainly from experience in the practical work with polarity tests. Moreover, isolated germlings have several times been found to lack clamps on their hyphae, polarity tests have been carried out successfully, but later all originally isolated single spore cultures appeared to be clamped. The short step between heterothallism and homothallism was excellently shown in Coprinus cinereus (FR.) S. F. GRAY (VERRINDERGIBBINS & Lu 1984), which normally is heterothallic and tetrapolar. A monocaryotic culture was subjected to nutritional stress which caused the development of a secondary mycelium with

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clamp-connexions and later production of basidiocarps. Normal meiosis took place in the basidia and a new generation of monokaryotic cultures were obtained from these basidiospores. These cultures were all of the same mating type, identical with the original monokaryotic culture. By the use of nutritionally forced matings LEMKE(1969) and ULLRtCH& RAPER (1975) found that homothallic and heterothallic, bipolar Sistotrema brinkamnnii were compatible. It seems quite possible to consider homothallism as a mode for adaptation to recurrent disturbances in the environment or to other environmental conditions where immediate genetic fitness is required (BRASIER 1987). Homothallism would then be a part of the ecological strategy of a particular species and not be looked upon as an event of further speciation. This view also fits a situation where the heterothallic parts of a population become extinct. An important event in the life-cycle of a basidiomycete is the transition from primary (homokaryotic) mycelia to secondary, where the latter is the result of a mating process. For heterothallic species with clamps on their secondary mycelia and which are not holocoenocytic, the mating type factors have been assigned a particular role in this morphogenesis (RAPER& FLEXER 1971). Compatible homokaryotic mycelia will recognize each other during matings and genetic expression typical for the growth of secondary mycelia will be released in the cells where heterokaryotic nuclei interact. A final result will be the basidiocarp with meiosis and basidiospore production. In homothallic species the secondary mycelium is obtained without matings being involved. In a variety of species also homokaryotic mycelia may produce basidiocarps (parthenogenesis), although clamps and dikaryons are lacking and there is no meiosis in the young basidium (PRILL~NGEP,1982). From this it follows, that genes responsible for secondary mycelium growth, may also be effective in a homokaryotic mycelium. In heterothallic, holocoenocytic species, compatible homokaryotic mycelia seem to have another kind of mating factors which also are multialMic (COATES & al. 1981). In some of these species even clamp-connexions occur on homokaryotic hyphae but this is just another example that the corresponding genes may not be linked to morphogenesis leading to secondary mycelia. Somatic incompatibility between secondary mycelia belonging to the same species but of different origin is widespread among heterothallic basidiomycetes (RAYNER • BODDY 1988). Between compatible homokaryotic mycelia, this rejection phenomenon is overridden by the mating system. In a few species, however, genetically controlled rejection between compatible homokaryons have been detected, which prevent dikaryotization and establishment of secondary mycelia (HAI.LE~qBERG 1988, CHASE & al. 1989). It has been suggested that this kind of restriction in outbreeding potential may be an expression for the first step in speciation processes (see Fig. 1).

host/niche specialized ~ sibling species

original heterothallic species - - - ~ h o m o t h a l l i c forms allopatric differentiation

Fig. 1. Modes of microevolution in Corticiaceae

Speciation and distribution in Corticiaceae

105

Population structures Populations are assemblages of individuals of the same species living in proximity in space and time (COOKE & RAYNER 1984). The living space for many woodinhabiting basidiomycetes is primarily delimited by the substrate, which to a great extent occurs as discrete units in nature. Also, the fungal individuals themselves occur as genetically and physically discrete units within a substrate (COOKE & RAYNER 1984). In undisturbed forests there is a continuous flow of new material made available for wood-fungi and thus, different successional stages are coexisting within a limited area. Individuals of the same species may occupy a number of equivalent niches without having any physical contact with each other. This means that the conception of a population has two different significations among wood fungi. Individuals occupying a limited wood resource constitute a population in a very strict sense which is limited also in time as the resource itself becomes extinct. On the other hand, within an area with uniform vegetation there will be several comparable resource units, occupied by other individuals of the same species. Most probably, individuals from different substrates within a limited area of uniform vegetation, have closely related ancestors and the close genetic similarities will be preserved within this area as long as the habitat persists. The size of a population of this kind is hard to define but it is certainly limited by the type of vegetation to which the particular species is confined. Indirect methods like the study of allele frequencies could be used for such delimitations (SLATKIN1987). Incompatibility factors are multi-alMic and in interspecific matings where the two partners have the same alleles, no secondary mycelium will be formed. It could be reasonable to assume that the number of such factor repeats would decrease with the distance between the mating partners. In a survey over the natural distribution of these factors in basidiomycetes, however, ULLRICH (1977) found that the number of factor repeats was about the same between samples from a small area (75 acres, in experiments with Schizophyllum commune FR.) compared with a large area or even world-wide. In this case only one specimen per resource unit was considered. Between samples from the same resource unit the number of factor repeats was significantly higher. The population structure of a species is highly dependent on its ecological strategy and niche specialization. The importance of an accurate "place" for a species in a community system within a limited resource unit has been discussed above. Many species are highly selective in their substrate preferences and this specialization is easily seen among primary colonizers and those adapted for growth and survival under physical stress. A further aspect of great importance here is related to succession. Different successional stages in the decay of a tree have been analyzed in several reports (JAHN 1966, KNNRIK 1972, LANCE 1986). The decay itself is limited in time and universal in its function. Decaying organisms take part in different periods of the process, depending on their ecological strategies. However, which species that are engaged depends on several factors. In southern Sweden, there are a number of planted, coniferous forests on former farmland. The flora of wood-decaying fungi is rather poor here and is composed of species, generally considered as common because they have a high propagation

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capacity, irrespective of their ecological strategies:! A lot of wood of different dimensions is assembled on the ground there and the decay is just as complete as in virgin forests with the same: coniferous trees. In the latter kind of forest the wood-fungus flora is much more diverse and this is not only related to a higher diversification of available habitats for the fungi. During a long continuity of forest vegetation a number of species have subsequently invaded the area and meanwhile, they have become restricted to niches where growth conditions are optimal. Consequently, there is a succession of fungal communities which interact with the general sucession related to different decay phases of wood. EHRLICH & RAVEN (1969) presented the idea that gene flow in nature is much more restricted than commonly thought, and listed a number of examples from animals and higher plants. This idea is not in opposition to the world-wide distribution of many wood-inhabiting basidiomycetes, if supposed that these species are very old. Even if some polypores may produce enormous amounts of spores this is not proof that these spores are destined for long range dispersal. There are probably more world-wide species in Corticiaceae, producing smaller amounts of spores, than polypores. Then, a high spore production is more likely to be an important factor for effective distribution in the near environment (few and small "entrances" to appropriate niches for the spores). A number of isolated islands and other isolated areas have the same flora of Corticiaceae species as comparable areas with coherent distribution (N. Iran - HALLENBERG 1981; Canary Islands - BELTRAN TEJERA 1980). Obviously, gene flow is highly restricted between these isolated areas and the main coherent population but the stable microenvironment has opposed genetic drift leading to speciation. Nevertheless, the complex fungus flora in isolated islands are just evidences that effective colonization of a new area may take place. Here, there are two postulates which seem to be contradictory. A supposed restricted gene flow in natur~ and efficient capacity of dispersal over relatively long distances. This contradiction is, however, just apparent when the demographic strategy of wood-fungi is considered. Both the wooden resource units and local forests where these substrates occur, are relatively ephemeral structures. Constantly recurrent forest fires have obliterated almost all wood-fungi while later recolonization after reforestation has not been affected. Any particular species occupying a single resource unit constitutes a source of further propagation which will be dominant for equivalent niches in the near environment. If different species with obvious overlaps in their habitats have colonized the same area, the one with the highest fitness will be dominant in niches with overlap within a few generations, because of the ephemeral nature of the resource unit. A high fitness is thus ascertained also within a population and any long-dispersed immigrants of the same species will certainly be driven out by competition. Basidiospores from a species adapted for growth on a particular substrate and naturally occurring in a particular forest type will, of course, also infect a variety of other substrates in different forest types, which are adjacently located. Owing to genetic variation, some of these spores may be established in this new environ.ment. Such events probably occur rarely but when successful establishment takes place, gene flow from the main, original population will be strictly limited. The new population can be affected by genetic drift and subsequently sterility barriers towards the original population will arise. Selection for establishment in the new

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>,.~_ "~

tD

species of wood-fungi 03 09

iv

"planted forest"

"virgin forest"

succession of wood-fungus communities

Fig. 2. Succession of wood-decay basidiomycetes along two gradients in any particular resource unit. Different species occupy specific areas in the diagram

habitat will be reinforced by genetic linkage to such sterility genes (SLATKIN 1987). The species complexes mentioned above (under "Microevolutionary tendencies") where siblings differed from each other in their host preferences, are examples of this process (see Fig. 2). Conclusions

From above it follows that there are good reasons to believe that a great portion of the species in Corticiaceae are very old, their primary distribution being linked to the progressive expansion of coniferous and angiosperm forests. This is further manifested by the wide distribution noted for many wood-fungi, of which some are characterized by a very restricted spore production. In an early evolutionary phase different types of basidiocarps were created in response to climatic conditions at the appropriate niches and for effective spore production. During later evolution the selectivity in host specialization increases, thus optimizing the decay communities in their manifold varieties. The recent evolution among forest trees has certainly only to a small extent affected the chemical and structural composition of wood, especially when it is dead. This factor is of great significance for the conserving character of corticiaceous species. The ability of effective distribution is vital for wood-fungi. The wooden substrate is ephemeral and environments may change drastically (forest fires), and a good ability for dispersal must be looked upon as an important part of the dynamics in a population. The colonization of remote and isolated areas with practically the same wood-fungus flora as in comparable areas with coherent vegetation, however, can be explained as a side effect of this propagation strategy. On the other hand, the quantity in spore production seems to be an adaptation for establishment in the near environment rather than to real long-dispersal. Microevolutionary events observed today suggest that adaptations to new substrate species, living in adjacent and different forest types, may occur. Later appearing sterility barriers towards the original population may reinforce the isolated population which eventually will be widely distributed and recognized as a new (sibling) species. The relatively frequent occurrence of siblings in a number of species without any closer relationship indicates that this may be a common phenomenon and an expression of a propagation strategy (colonizing power outside the original habitat). However, the high constancy in basidiocarp morphology and in niche specialization of most species and the intercompatibility shown in artificial mating experiments between widely separated populations indicates, that most existing siblings (including homothallic forms) may be of a relatively short duration.

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A characteristic feature of modern land management is the extinction of virgin forests and the uniformity in planted, coniferous areas. Several wood-fungi, adapted to rather undisturbed forests with a long forest continuity, have become imprisoned in the few existing protected areas, obviously without possibilities for further distribution. In surrounding forests other wood-fungus communities have developed, poorer in species number and rapidly colonizing species are promoted. Because of the management, more rich and diversified fungal communities will never be established. The balancing power of the latter kind of communities is lost and the possibility for new siblings to arise with unanticipated properties, is obvious. N o doubt, this is a field of great importance for future research. I thank J. POELT, Graz, for discussions and valuable criticism of the manuscript. D. PEGLER, London, has kindly revised the English. References

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Address of the author: NILS HALLENBERG,Department of Systematic Botany, Gothenburg University, Carl Skottsbergs gata 22, S-413 19 G6teborg, Sweden. Accepted February 12, 1991 by J. POELT