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Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9 Canada
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Electron microscopy shows that ascomata of Myxotricum arcticum bear a striking resemblance to discocarps in morphogenesis and in previously overlooked aspects of gross morphology. Although mature ascomata of M. arcticum superficially resemble reticuloperidial cleistothecia common in the Onygenales, the bramble-like aggregation of thick-walled hyphae, previously considered to represent a closed peridium, forms a basket-like apothecium that overarches a distinct hymenium of stipitate, protunicate asci interspersed with paraphyses. There is no evidence of asci developing in chains and at different levels as is characteristic of the centrum of many Eurotiomycetes. Instead, more or less globose, stipitate and evanescent asci arise individually from penultimate cells of croziers and develop almost synchronously across a distinct hymenial layer derived from a richly branched network of crozier-bearing hyphae. After dissolution of the ascus wall, ascospores adhere to a membranous sheath that underlies the hymenium. These observations provide strong support for prior suggestions based on molecular phylogenetic comparisons that the Myxotrichaceae recently are derived from a helotialean ancestor. Observations of conidiogenesis show that the typical Oidiodendron anamorph is accompanied by a second conidiogenous form with ampullae and botryose clusters of blastic conidia.
Key words: conidiogenesis, discocarps, hymenium, Onygenales, ultrastructure
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) traditionally has encompassed four cleistothecial families, Arthrodermataceae, Gymnoascaceae, Myxotrichaceae and Onygenaceae (Currah 1985). Many taxa are keratinophilic, produce morphologically simple arthroconidial states, and all four families host some genera in which the ascocarp possesses a mesh-like reticuloperidium (Currah 1985, Greif and Currah 2003) that encloses at maturity a mass of single-celled, hyaline, ascospores. The Myxotrichaceae was considered an atypical family in the Onygenales because of their cellulolytic abilities, more elaborate dendritic arthroconidial states in the genera Oidiodendron and Geomyces, and ascospores that are fusoid to navicular and usually markedly striate, rather than oblate, globose or allantoid and smooth or pitted. In addition to Myxotrichum, the Myxotrichaceae includes Pseudogymnoascus and Gymnostellatospora (Sigler et al 2000).
Based on some ecological and morphological similarities between the Myxotrichaceae and Hymenoscyphus ericae (Helotiales), Currah (1994, 1995) hypothesized that the family, in spite of having cleistothecial ascomata (and to some extent, arthroconidial anamorphs), might have stronger affinities to the in-operculate discomycetes than to the Eurotiomycetes. Some support for this hypothesis subsequently came from comparisons of 18S DNA sequence data from these and other ascomycetous taxa (Hambleton et al 1998a, Sugiyama et al 1999, Sugiyama and Mikawa 2001), but strong morphological and morphogenetic evidences were missing. Light microscopy (LM) shows that, in species of Myxotrichum at least, asci are subclavate and borne on a stipe thus differing from the more or less globose asci of Eurotiomycetes. Studies of the ascomata of the Myxotrichaceae using electron microscopy have been done with Myxotrichum deflexum (Rosing 1985) and Pseudogymnoascus roseus (Tsuneda 1982), but these investigations neither looked for nor revealed potential similarities to apothecial forms. There are no published developmental studies of the species of Gymnostellatospora.
During a search for additional morphological clues to the ancestry of the Myxotrichaceae, we re-investigated the ultrastructural aspects of ascomatal morphogenesis using a strain of Myxotrichum arcticum (Udagawa et al 1994). This species was selected because its ascomata and conidial state are easily produced in culture, are generally typical of other members of the genus and its affiliation with other species in Myxotrichum had been confirmed on the basis of DNA sequence analysis (Hambleton et al 1998b).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) and accumulated in more or less spherical masses that developed singly or in clusters (FIG. 2). These elements were branched, thick-walled, septate, sometimes swollen at the nodes, and often bore minute, spine-like projections (FIG. 3, arrows). Short appendages, which also were asperulate, extended slightly beyond the periphery of the individual hyphal masses (FIG. 4). Each spherical mass was associated with one ascogonium (more when spherical masses were in clusters) (FIG. 5, arrow), although they often were absent at very early stages of accumulation (FIG. 1). The gametangial apparatus consisted of paired hyphae that originated from different vegetative hyphae. One hypha of each pair wrapped around a club-shaped partner to form a coil (FIG. 6) that enlarged at its distal end (FIGS. 7, 8, arrows) to become an ascogonium. TEM of the gametangial apparatus shows what appears to be the antheridial cell (FIG. 9, A) and a trychogyne-like terminal cell (FIG. 9, T) arising from the ascogonium, but their roles in fertilization could not be confirmed. Slender sterile hyphae often arose from the basal cells of ascogonia (FIG. 9, SH) or from the parent hypha below the ascogonium, but these were not involved in peridium formation. Ascogenous hyphae radiated from the ascogonium and gave rise to croziers that grew out and branched to form additional croziers until an elaborate and distinct hymenial network was in place (FIGS. 10 and 11). Ascus mother cells arose from the penultimate cells of the croziers in this network (FIGS. 12 and 13, arrowheads), elongated (FIG. 14) and developed into short-stipitate, subglobose to ovoid asci (FIGS. 15 and 16); this process proceeded more or less synchronously throughout the hymenium. Smooth, slender, paraphyses that sometimes had swollen apices (FIGS. 14, arrowhead; 16) were interspersed among the asci. Paraphyses were thin-walled, possessed typical simple septa associated with Woron-in bodies (FIG. 17), and were derived from the basal cells of the ascogonia (FIG. 9, SH) and their parental hyphae, as well as from surrounding peridial and smooth-walled vegetative hyphae. Under a dissecting microscope, hymenia at this stage were grayish white and the peridial hyphae above them frequently were sparse; thus ascomata looked like discocarps (FIG. 18). By SEM, numerous immature asci, interspersed with paraphyses, were visible throughout the hymenium (FIG. 19). Another striking feature was the presence of membranous sheath, revealed as an electron-dense line by TEM (FIG. 20, arrow), underlying the hymenium (FIG. 19, arrow). Immature ascospores were navicular and surrounded only by the primary wall, which was thick and smooth (FIG. 21, arrow). As the ascospores matured, a ridged secondary wall layer was deposited on the primary wall (FIG. 22). Epiplasm remained until late stages of ascosporogenesis (FIG. 22, E). As the asci evanesced, the released ascospores accumulated on the exposed hymenium (FIG. 23). Paraphyses also deliquesced. The released ascospores had longitudinal striations characteristic of the genus and were supported en masse on the underlying membranous sheath (FIG. 24). This sheath was clearly visible in ascomata in which ascospores had been washed free of the hymenium during preparation for critical-point drying (FIG. 25, arrow).
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). Arthroconidia were slightly verrucose and often had distinct connectives between them (FIG. 27). Intercalary cells of conidiophores occasionally swelled to form ampullae that gave rise to blastic, botryose clusters of conidia either singly or in short chains (FIG. 28, arrow). Secession scars remained on the conidiogenous cells where conidia had detached (FIG. 28, arrowheads).
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DISCUSSION |
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, the same subfamily, Gymnoasoideae (Benny and Kimbrough 1980) or even considered congeneric (Kuehn 1955a, b). Differences between these two genera in ascospore morphology and patterns in substrate degradation later were used as arguments to move the genera to separate families, i.e., the Myxotrichaceae and Onygenaceae, respectively, within the Onygenales (Currah 1985). A combination of morphological (Currah 1985, 1994) and molecular data (Hambleton et al 1998a, Sugiyama et al 1999, Sugiyama and Mikawa 2001) later indicated that the differences between these two families were substantial, with the Onygenaceae representing a clade among the true cleistothecial fungi and the Myxotrichaceae a group derived ostensibly from the inoperculate discomycetes through excipular modification and loss of forcible ascospore discharge. A rationalization of the similarity in peridial morphology based on developmental sequences between the representatives of these two lineages was sought, but few studies were available to allow the comparisons of ascocarps morphogenesis. The ascogonial apparatus of M. arcticum resembles that of Emericellopsis microspora (Hypocreales, incertae sedis, Eriksson et al 2003) in ultrastructure (see FIG. 2
in Wu and Kimbrough 1990) but differs in that vegetative hyphae arising from the base cells of ascogonia take part in the formation of peridium in E. microspora, whereas such hyphae become paraphyses of the hymenium in M. arcticum. In Gymnoascus reessii (Gymnoascaceae), a cleistothecial species with a reticuloperidium superficially similar to species of Myxotrichum, the peridial elements develop from the surrounding vegetative hyphae after the formation of gametangia, a sequence also observed in other cleistothecial fungi (Kuehn and Orr 1959, Wu and Kimbrough 1990). We found that the reticuloperidial hyphae of M. arcticum arise from vegetative hyphae and often are devoid of ascogonia in their initial stages of accumulation. This indicates that peridium formation in M. arcticum is induced by stimuli other than those associated with the presence of gametangial initials and may account in part for the ‘‘sterile’’ Myxotrichum-like cleistothecia formed by strains of Oidiodendron maius (Rice and Currah 2002).
A striking characteristic of M. arcticum ascomata is the presence of a distinct hymenial layer of paraphyses and stipitate asci that mature synchronously. During ascus development the penultimate cell of the first-formed croziers extend and branch to produce more croziers, each of which extends and branches repeatedly in the same way until a layer of richly branched ascogenous hyphae has formed. The penultimate cells of the most distal branches give rise to asci (FIGS. 10–14). This pattern of maturation and disposition of mature hymenial elements is discomycetous (Henssen 1981). Prior reports of centrum development in Myxotrichum that diverge from this pattern were based on misidentified strains of Auxarthron umbrinum (as M. emmonsii and M. thaxteri), A. conjugatum (as M. conjugatum) and in Gymnoascus uncinatus (as M. uncinatus) (Kuehn 1955a, b) that had irregularly disposed asci as would be expected in true cleistothecial fungi. Other genera such as Onygena, Talaromyces, Monascus and Auxarthron (Tsuneda and Currah, unpubl data) form chains of asci that are randomly disposed in the centrum and which ripen consecutively rather than synchronously (von Arx 1981, Emmons 1935, Fennell 1973, Malloch and Cain 1971, 1972, Paden 1971, Spiltoir 1955, Wong and Chien 1986, Wu and Kimbrough 1990). Further studies of centrum morphogenesis in prototunicate taxa would refine and clarify these patterns and be useful to ascomycete systematics. Ascosporogenesis in M. arcticum is similar to that described in M. deflexum Berkeley (Rosing 1985) and in a number of Ascomycota (Read and Becket 1996, Wu and Kimbrough 1990).
It is unlikely that the striking similarity in the form of the mesh-like hyphae in the Myxotrichaceae and the Onygenaceae is an evolutionary coincidence, but its role in the reproductive fitness of these taxa is unclear. Currah (1985) suggested that the hooked or recurved appendages associated with many mesh-like peridial types in the Onygenales attach the ascocarps to the animal vector and the spaces between the peridial elements allow ascospores to sift out as the carrier travels from one habitat to the next. Summerbell (2000) suggested the mesh-like peridium, in species of Arthroderma, at least, could function as a deterrent to grazers. Greif and Currah (2003) proposed that the function of the lattice-like structure of the reticuloperidium in Myxotrichum and Auxarthron was to allow impalement of entire ascomata on arthropod setae and were able to demonstrate this mechanism in vitro. This model suggests that once impaled, and thus fastened to the surface of an arthropod, the adhering ascomata are carried away to new substrata and/or elicit a grooming response in the carrier that results in disruption of the peridium and more local dispersal of the meiospores. Our observations of as-coma development in M. arcticum suggest that water also might play a role in ascospore dispersal. Dehisced ascospores remain on the basal membranous sheath in mature undisturbed ascomata but are readily washed off with water. Ascospores in suspension or in water film might move or be moved very easily from one substrate to the next. The membranous sheath is reminiscent of pseudoepithecium occurring in discocarps of some Discomycetes (Tsuneda 1983).
Mitosporic states in Oidiodendron are the confirmed or putative anamorphs of species of Myxotrichum (Hambleton et al 1998b, Rice and Currah 2002). Morphologically similar taxa in the Onygenales, e.g., Auxarthron, are associated with Malbranchea states. If the reticuloperidium is a convergent feature in both onygenalean and helotialean lineages, why would both also have arthroconidial anamorphs? Have habitat and dispersal strategies also influenced the morphology of the mitosporic stages? Empirical evidence concerning the role of arthropods in the dispersal of arthroconidial states of the Onygenales is unavailable but some other unrelated arthroconidial taxa are known associates of insects (Tsuneda et al 1993). The role of airborne arthroconidia in the epidemiology of Coccidioides immitis (Onygenales) (Rippon 1988) is understood to be significant, and it is possible that air currents are effective means of dislodgement and carriage for these propagules. In Myxotrichum (and in species of Pseudogymnoascus and Gymnostellatospora, which also have arthroconidia) and in the Onygenales this conidial morphology simply may confer an advantage by letting the fungus exploit a variety of dispersal agents. In addition to the Oidiodendron anamorph, ampullae occurred as anomalous conidiogenous structures (referred to by Udagawa et al [1994] as a type of ‘‘geniculate’’ conidiogenesis). This type of pleomorphic conidiogenesis is distinct and is not found in other species of Myxotrichum.
In summary, ascomata of M. arcticum, in spite of their superficial resemblance to the cleistothecia of some onygenalean fungi, clearly are derived from a discomycetous ancestor in morphogenesis and in gross morphology. Unlike cleistothecia, fully expanded ascomata of M. arcticum are not entirely closed and possess a distinct, disk-like hymenium. Stipitate, protunicate asci and paraphyses constitute the maturing hymenium. Asci develop from croziers rather than in chains and mature almost synchronously throughout the hymenium. These results add to the argument that the Myxotrichaceae are derived from an inoperculate discomycetous ancestor (Currah 1994, Hambleton et al 1998b, Sugiyama et al 1999).
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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1 Corresponding author. E-mail: atsuneda{at}ualberta.ca
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