| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Department of Forest Mycology and Pathology, Swedish University of Agricultural Sciences, Box 7026, S-750 07 Uppsala, Sweden
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
The aim of the present work was to investigate the potential for territorial and dispersive clonality in natural populations of the postfire root rot ascomycete Rhizina undulata. Population studies based on vegetative compatibility tests were done with strains isolated from individual sporocarps at five burned sites in three different localities (separated by 20–40 km) in the Curronian Spit of western Lithuania. Among a total of 103 strains, the tests identified 14 distinct vegetative compatibility groups (VCGs) of R. undulata, 13 of which were represented by 2–48 strains and three were encountered at 2–4 different sites. Occurrence on spatially separated sites of the same VCG of the fungus indicated a presence of dispersive clonality in R. undulata populations. On a local scale clusters of vegetative compatible sporocarps usually occupied discrete territories, implying territorial clonality. The two largest VCGs covered areas up to 7 and 3 m across. The results show that both dispersive and territorial clones are characteristics of natural populations of the fungus.
Key words: ascomycetes, clone, population structure, root pathogens, soil-borne fungi, vegetative compatibility group
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
, Brooks 1910, Weir 1915) and 15–60 y old conifer trees (Gremmen 1961, Murray and Young 1961). Germination of dormant R. undulata ascospores in the soil is activated by the heat of fire ( Jalaluddin 1967a, b), and, during the subsequent growth, the mycelium attacks conifer roots (Brooks 1910, Weir 1915, Murray and Young 1961). This might result in groups of <100 killed trees within 0.04–0.1 ha (Murray and Young 1961). Large numbers of R. undulata sporocarps commonly are distributed over burned sites (Zeller 1935). For example within the radius of 6–7 m a total of 319 R. undulata sporocarps were recorded around a single pine stump (Hagner 1962) and 89 around a single tree (Hanso and Hanso 1998). The disease centers often are reported to expand radially (Sato et al 1974, Butin and Kappich 1980, Lee and Kim 1990). Thus, in case of a single starting point, R. undulata advances through the soil in the manner of a fairy ring and sporocarps appear in a wider and wider, complete or discontinuous circle in successive years (Phillips and Burdekin 1982). In other cases, however, expanding clusters of sporocarps show no geometrical center (Gremmen 1961, Murray and Young 1961).
Studies on local population structure in large soil-borne basidiomycetes (e.g. Suillus bovinus [L. : Fr.] Kuntze) demonstrate that within the clusters, sporocarps often are genetically identical, thus originating from the same territorial network formed by a single mycelium (Dahlberg and Stenlid 1990). In certain cases spread of basidiomycete mycelial structures in forest soil is so efficient that single fungal genets of Armillaria spp. may cover more 200–3800 m wide areas (Legrand et al 1996, Ferguson et al 2003) and sometimes even 15 ha or more (Smith et al 1992, Dettman and van der Kamp 2001, Ferguson et al 2003). An extensive territorial clonality consequently might become characteristic for local populations of soilborne basidiomycete fungi (Anderson and Kohn 1995).
Until recently population studies of ascomycetes have been directed mainly toward species that produce microscopic sporocarps and their population structures therefore have been viewed from an angle of "genetic diversity vs. dispersive clonality" (Anderson and Kohn 1995, Correll and Gordon 1999). Nevertheless more recent investigations on wood-inhabiting species have shown that individual mycelia of some ascomycetes (e.g. Daldinia loculata [Lev.] Sacc. and Sarea resinae Kuntze [despite microscopic sporocarps of the latter]), might be large and expand 2–3 m within living tree stems ( Johannesson et al 2001, Vasiliauskas et al 2001). In contrast to the above-mentioned fungi, individual mycelia of which are confined within a certain resource unit of wood (e.g. trunk, log, stump), R. undulata grows in soil, thus the space for its vegetative spread over forest areas potentially is unlimited. Moreover R. undulata is known to be a homothallic species, meaning that individual mycelium of a single genet produces sporocarps without mating (Vasiliauskas and Stenlid 2001).
The manner of forming groups of genetically uniform sporocarps by a single mycelium is similar to that exhibited by heterokaryotic mycelium of larger basidiomycetes. It consequently provides an opportunity for sporocarp-based studies on spatial distribution of fungal genets in local populations where defining fungal individuals is of primary importance. In natural populations of fungi often it is unclear whether a cluster of sporocarps (or several adjacent clusters) represent several genets or each is derived from a single clone (Burnett 2003). Regarding R. undulata, the investigation of the local population structure of the fungus could provide information on the number of individuals that colonize burned forest areas, as well as on their growth rates, physical boundaries and size. To our knowledge the presence of territorial clonality in populations of soilborne ascomycetes has not been reported. However, during earlier work three R. undulata sporocarps collected within a distance of 1.2 m proved to be genetically identical, showing clear potential for clonal spread over discrete territor y (Vasiliauskas and Stenlid 2001).
The potential for dispersive clonality also is obvious in R. undulata because in addition to mycelial growth in the soil from tree to tree, the fungus spreads by airborne ascospores (Hartig 1900, Laine 1968, Phillips and Young 1976). In homothallic fungi a given strain is self-fertile and gives rise to sexual spores that genetically are identical among themselves and to parental mycelium (Elliott 1994, Correll and Gordon 1999). In our earlier study evidence for this was reported also in R. undulata (Vasiliauskas and Stenlid 2001). Therefore the possibility can not be excluded that the clonal genotypes of R. undulata are propagated over spatially separated geographic areas. Population studies of another homothallic ascomycete Sclerotinia sclerotiorum (Lib.) de Bary have demonstrated that such mode of spread can be efficient having notable effect on genetic variation in fungal populations over large scales (Anderson and Kohn 1995). The aim of our work therefore was to investigate the potential for territorial and dispersive clonality in natural populations of R. undulata.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
) were investigated. The areas contained stands of more than 100 y old mountain pine (Pinus mugo Turra.) on sandy soils. The stands were partly clear-cut 2 years before the study, with concurrent slash burning. A total of 141 sporocarps were collected and mapped around five discrete fireplaces: three sites in Preila (128 sporocarps, FIG. 1B–E), one site in Smiltyne (eight sporocarps, map not shown) and one site in Juodkrante (five sporocarps, map not shown).
|
). We obtained 103 pure cultures in total. These consisted of 93 single-ascospore isolates and 10 hymenium isolates. Of those 103 cultures, 90 were from Preila, eight from Smiltyne and five from Juodkrante.
Identification of vegetative compatibility groups (VCGs).— –
Identification of VCGs of R. undulata isolates was based on mycelial interactions in dual cultures on Petri dishes containing vegetable juice agar (Vasiliauskas and Stenlid 2001). At first all isolates originating from the same site (fireplace) were paired in all possible combinations, including self-pairing controls of two pieces from the same mycelium. Then, intersite pairings were carried out confronting the representatives from different vegetative compatibility groups in a similar manner. Interactions between two mycelia were regarded as compatible and the strains were assigned to the same VCG when a continuous mycelial mat was formed between the isolates, corresponding to that of self-pairing controls. Antagonistic types of mycelial interactions (demarcation line) after contact were classed as incompatible, and the tested strains in such cases were assigned to the different VCGs. Examples of compatible and incompatible mycelial interactions between the isolates of R. undulata are shown in Vasiliauskas and Stenlid (2001).
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
). We consequently assumed that different VCGs represent different clonal lines of the fungus. As a result, in a whole study material of 103 strains, the tests outlined presence of 14 distinct VCGs of R. undulata, 13 of which were represented by 2–48 strains (sporocarps), and three were encountered at 2–4 different sites (TABLE 1, FIG. 1A–E
). Occurrence on spatially separated (<40 km) sites of the same VCG of the fungus indicated presence of dispersive clonality in R. undulata populations (TABLE I, FIG. 1A). However this does not necessarily imply the potential of airborne ascospores to travel such distance on a single occasion. On the contrary, stepwise multiply short distance dispersal is more likely explanation of observed population structure of the fungus. In fact throughout the decades pine stands of Curronian Spit were subjected repeatedly to local forest fires, slash burning and the attacks of R. undulata and the disease centers with abundant fruiting were observed commonly in many parts of the peninsula (Vasiliauskas 1998). Previous population studies in homothallic ascomycetes and basidiomycetes have revealed their dispersal over much larger geographic territories, at distances of hundreds of kilometers (Kohn 1995, Vasiliauskas and Stenlid 1998). Therefore studies of R. undulata populations on a larger scale might provide interesting results on spatial distribution of clones of the fungus.
|
). In Smiltyne and Juodkrante, among eight and five strains collected, two distinct VCGs were found on each respective site (TABLE I). Moreover in some cases relatively large territories were occupied by clusters of vegetative compatible sporocarps. For example 25 and 15 sporocarps of VCGs I and VII respectively covered areas up to 7 and 3 m across (FIG. 1C, D). Territorial distribution of R. undulata VCGs on our study sites indicates that each of the sporocarp clusters probably is produced by a single mycelium after its vegetative spread through the soil, thus implying territorial clonality in the local populations of the fungus. However the possibility cannot be excluded that several sib-related ascospores might have germinated in the vicinity and the mycelium of each produced vegetative compatible sporocarp on its own. This event for example looks likely for the VCG I on site 1 at Preila because two fronts of its sporocarps are advancing in different directions from opposite sides of the fireplace (FIG. 1C). Cases, when the territorial clonality is mixed with the dispersive in fungal populations, have been reported previously (Anderson and Kohn 1995). Nevertheless the remaining R. undulata sporocarp clusters probably represent territorial clones, each formed by an individual mycelium. Monitoring of R. undulata disease centers has shown that once germinated the mycelium is capable of extensive growth for several years. Thus Murray and Young (1961) reported a radial increase of R. undulata disease centers of about 2.7 m/y, emphasizing that in Britain no other root rot-causing fungus grows so quickly. In Japan and Korea mortality of pine trees caused by R. undulata spreads in an irregular ring at 3–6 m/y (Sato et al 1974, Lee and Kim 1990). In USA and Europe the radial progression of tree mortality was recorded to be 0.6–1 m/yr (van der Lek 1917, Phillips and Burdekin 1982, Tainter and Baker 1996). On our sites, within a year of fire, the distance between the edge of a burned area and the emerging R. undulata sporocarps was 0.1–3.0 m (FIG. 1). However in our work germination points of the ascospores is unknown and the sporocarps were not monitored, thus it does not provide any information on mycelial growth rates of R. undulata. We conclude that both dispersive and territorial clones are characteristics for natural populations of the fungus.
|
ACKNOWLEDGMENTS |
|---|
|
FOOTNOTES |
|---|
1 Corresponding author. Email: rimvydas.vasiliauskas{at}mykopat.slu.se
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONLITERATURE CITED |
|---|
Brooks FT. 1910. Rhizina undulata. Quart J For 4:308–309.
Burnett J. 2003. Fungal populations and species. Oxford: Oxford University Press. 348 p.
Butin H, Kappich I. 1980. Untersuchungen zur Neubesied-lung von verbrannten Waldböden durch Pilze und Moose. Forstw Cbl 99:283–296.
Correll JC, Gordon TR. 1999. Population structure of Ascomycetes and Deuteromycetes. In: Worrall JJ, ed. Structure and dynamics of fungal populations. Dordrecht: Kluwer Academic Publishers. p 225–250.
Dahlberg A, Stenlid J. 1990. Population structure and dynamics in Suillus bovinus as indicated by spatial distribution of fungal clones. New Phytol 115:487–493.[CrossRef]
Dettman JR, van der Kamp BJ. 2001. The population structure of Armillaria ostoyae and Armillaria sinapina in the central interior of British Columbia. Can J Bot 79:600–611.
Elliott CG. 1994. Reproduction in fungi: genetical and physiological aspects. London: Chapman and Hall. 309 p.
Ferguson BA, Dreisbach TA, Parks CG, Filip GM, Schmitt CL. 2003. Coarse-scale population structure of pathogenic Armillaria species in a mixed-conifer forest in the Blue Mountains of northeast Oregon. Can J For Res 33:612–623.[CrossRef]
Gremmen J. 1961. A die-back of conifers caused by Rhizina undulata, particularly after slash burning. Ned Bosbouw Tijdschr 33:5–10.
Hagner M. 1962. Några faktorer av betydelse för rotmurklans skadegörelse. Norrl Skogsvårdsförb Tidskr 2:245–270.
Hanso S, Hanso M. 1998. Rhizina undulata Fr. as the cause of root rot in conifers. Forestry Stud (Tartu) 29:101–107.
Hartig R. 1900. Lehrbuch der Pflanzenkrankheiten. Berlin: Verlag von Julius Springer. 324 p.
Jalaluddin M. 1967a. Studies on Rhizina undulata. I. Mycelial growth and ascospore germination. Trans Br Mycol Soc 50:449–459.
———. 1967b. Studies on Rhizina undulata. II. Observations and experiments in East Anglian plantations. Trans Br Mycol Soc 50:461–472.
Johannesson H, Gustafsson M, Stenlid J. 2001. Local population structure of the wood decay ascomycete Daldinia loculata. Mycologia 93:440–446.[CrossRef]
Kohn LM. 1995. The clonal dynamics in wild and agricultural plant-pathogen populations. Can J Bot 73:1231–1240.[CrossRef]
Laine L. 1968. Rhizina undulata Fr., a new forest disease in Finland. Folia Forestalia 44:1–11.
Lee SY, Kim WK. 1990. Studies on Rhizina root rot disease of Pinus densiflora: physiological characteristics and pathogenicity of Rhizina undulata. J Korean For Soc 79:322–329.
Legrand P, Ghahari S, Guillaumin J-J. 1996. Occurrence of genets of Armillaria spp. in four mountain forests in Central France: the colonization strategy of Armillaria ostoyae. New Phytol 133:321–332.[CrossRef]
Murray JS, Young CWT. 1961. Group dying of conifers. London: Forestry Commission Forest Record No. 46. 19 p.
Phillips DH, Burdekin DA. 1982. Diseases of forest and ornamental trees. London: MacMillan Press. 435 p.
———, Young CWT. 1976. Group dying of conifers. London: Forestry Commission Leaflet No. 65. 7 p.
Sato K, Yokozawa Y, Shoji T. 1974. Studies on Rhizina root rot causing group dying of pine trees. Bull Gov For Exp Sta 268:13–48.
Smith ML, Bruhn JN, Anderson JB. 1992. The fungus Armillaria bulbosa is among the largest and oldest living organisms. Nature 356:428–431.[CrossRef]
Tainter FH, Baker FA. 1996. Principles of forest pathology. New York: John Wiley & Sons. 805 p.
van der Lek HAA. 1917. Rhizina inflata (Schäff) Sacc., een wortelparasiet van coniferen. Tijdschr Planteziekten 23:181–194.[CrossRef]
Vasiliauskas A. 1998. Main causes of dieback of Pinus mugo in the Curronian Spit and spread of Rhizina undulata disease centres. 
em
s Ûkio Mokslai 2:94–99.
———, Stenlid J. 1998. Influence of spatial scale on population structure of Stereum sanguinolentum in northern Europe. Mycol Res 102:93–98.[CrossRef]
Vasiliauskas R, Stenlid J. 2001. Homothallism in the postfire ascomycete Rhizina undulata. Mycologia 93:447–452.[CrossRef]
Vasiliauskas R, Ju
ka E, Stenlid J, Vasiliauskas A. 2001. Clonal differences and relations between diameter growth, stem cracks and fungi in 36-year-old clonal seed orchard of Norway spruce (Picea abies). Silvae Genet 50: 227–233.
Weir JR. 1915. Observations on Rhizina inflata. J Agr Res 4:93–97.
Zeller SM. 1935. Some miscellaneous fungi of the Pacific Northwest. Mycologia 27:449–466.[CrossRef]
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
