Ascospore Ontogeny Research Articles

Evolutionary trends in the Physciaceae

Abstract:The current delimitation of the family Physciaceae has been generally accepted since detailed descriptions of ascus characters allowed for a more natural circumscription of lichenized ascomycetes. The generic relations within the family are, however, still controversial and depend on the importance different authors attribute to specific morphological or chemical characteristics. The aim of this paper is to describe ascospore ontogeny and to test the present taxonomic structure of the family against a parsimony-based cladistic analysis, which includes three different scenarios of a priori character weighting. A study of ascospore ontogeny revealed two distinct developmental lines. One line revealed a delayed septum formation, which clearly showed transitions from spores with apical and median thickenings to spores without apical, but still well developed median thickenings, and to spores without any thickenings. In the second developmental line with an early septum formation, again taxa with no thickenings, median thickenings, and both median and apical thickenings were found. Although these characters were constant at a species level, median wall thickenings especially varied among otherwise closely related taxa. In the cladistic analyses the current taxonomic structure of the Physciaceae was only obtained after the five character groups, namely morphology and anatomy of the vegetative thallus, conidiomata and conidia, morphology and anatomy of the apothecia, ontogeny of the ascospores, and secondary metabolites of the thallus, were given equal importance, and after a subjective a priori weighting further increased the weight of the three characters ‘conidial shape’, ‘presence of apical thickenings’, and ‘spore septation delayed’. This structure was not supported by a cladistic analysis with equally weighted characters but reflected the biased character weighting of the present day Physciaceae taxonomy. The taxonomic importance of conidial characters and of anatomical and ontogenetical spore characteristics need, therefore, a careful reconsideration in future.

Notes on British Species of Thecotheus (Ascobolaceae, Pezizales), with Reference to Other Species of the Genus

Summary. Notes on the taxonomy and nomenclature of British species of Thecotheus are presented. British records of T apiculatus should be referred to T keithii, and of T cinereus to T crustaceus. The occurrence of T holmskjoldii in the British Isles is confirmed, and a detailed description of that species based on a recent British collection is given. A key to the five known British species and discussion on other species of the genus are provided. The genus Thecotheus Boud. was proposed more than a century ago (Boudier 1869), but it was not widely accepted until its history was reviewed by Kimbrough (1966) and its generic status firmly accepted by Kimbrough & Korf (1967). The genus was placed in the family Pezizaceae Dumort., based on amyloid asci, colourless ascospores, and the presence in some species of callose-pectate spore ornamentation and spore sheaths or apiculi (Kimbrough & Korf 1967). Eckblad (1968) included Thecotheus in the family Thelebolaceae (Brumm.) Eckblad, as the tribe Theleboloideae Brumm. was elevated to family level. Aas (1992) considered Thecotheus to belong to Ascobolaceae Boud. ex Sacc. based on the amyloid asci, the ascus protrusion beyond the hymenium at maturity and the thick-walled ascospores when young. Kimbrough (in Dissing & Schumacher 1994) provided further evidence to support the placement of Thecotheus in Ascobolaceae. The pore-plugging mechanism of septum structure is considered very consistent in Ascobolaceae and the ascospore ontogeny and ascus apex are different from that of Pezizaceae. Thecotheus lacks the accumulation of epiplasmal pigment but the deposition of secondary wall material is like that of certain species of Ascobolus Pers. (see Dissing & Schumacher 1994). Most species of Thecotheus are coprophilous. They can be found on dung of herbivorous animals, especially horse, deer and sheep in British collections. However, other species grow on bare soil or on water-soaked twigs and wood. The genus is characterised by apothecia which are sessile, globose to subglobose or cylindrical, rarely pulvinate to lenticular at first, becoming cupulate to discoid, white, becoming grey, yellow, orange to brownish; ectal excipulum of textura angularis and medullary excipulum of textura intricata and textura globulosa; asci operculate, cylindric, 4- to 32-spored (rarely 64-spored), wall amyloid, protruding from the hymenium at maturity; ascospores ellipsoid, rarely subglobose, eguttulate, hyaline to pale yellow, smooth or ornamented with granules or warts, having polar

Spore ontogeny of Galiella rufa (Pezizales)

Galiella is one of the genera of the dark-colored apothecial Sarcosomataceae, tribe Galielleae, with cyanophilous spore markings. Ultrastructural studies show that spore wall development of Galiella rufa is similar to the subgenus Discina of Gyromitra in Helvellaceae and to the other Sarcosomataceae, especially Plectania nannfeldtii, which both have fine secondary wall spore ornaments. The multinucleate ascospores found in G. rufa may show relationship to the Morchellaceae and the Helvellaceae. Keywords: ascospore ontogeny, Galiella, Sarcosomataceae, ultrastructure.

Ultrastructural observations on ascosporogenesis in Ciboria betulae (Helotiales : Sclerotiniaceae)

The ultrastructure of ascospore ontogeny including spore delimitation and spore wall formation is examined in Ciboria betulae. A peripheral delimiting cisterna is derived from the ascus plasmalemma apparently by the self-assembly of subunits in the cytosol. The cisterna is discontinuous and delimits each of eight incipient ascospores in the ascus. The particular, verrucose ascospore ornamentation in C. betulae appears early during primary wall formation. The electron-lucent primary wall differentiates into an endospore of finely fibrillar texture and a two-layered, fibrillar epispore. The ultrastructural changes of ascus epiplasm, sporoplasm, and spore wall are described and compared with previous studies on Ascomycetes. Key words: ascosporogenesis, ascospore ontogeny, Ciboria betulae, ascospore wall differentiation.

Ultrastructure of Ascospore Ontogeny in Aleuria, Octospora, and Pulvinula (Otideaceae, Pezizales)

Ultrastructural studies were used to help resolve questions as to the recognition and limits of the heterogeneous family Otideaceae (Pezizales). Ascospore ontogeny was examined in species of Aleuria, Octospora, and Pulvinula and compared with that of other members of the order. Stages of ascosporogenesis, including primary wall formation, were similar to those found in other pezizalean genera. An accumulation of electron-dense globules appeared early in the perisporic sac of species of Aleuria and Octospora and contributes directly to formation of the epispore layer. In Pulvinula, there was a late accumulation of small electron-dense granules that was deposited onto the primary wall to form the epispore. A pronounced development of dense granules occurred in the perispore of Aleuria that condensed onto the epispore to form prominent ornaments. In Octospora, the granules degenerated and formed a loose secondary wall separated from the primary wall by an electron-translucent zone. A secondary wall did not develop in Pulvinula, and the epispore layer became the outer spore wall. We conclude that there was not a close correlation in spore ontogeny among the tribes of Otideaceae currently recognized on the basis of light-microscope data.

Ultrastructural observations onNimbospora bipolaris(Halosphaeriaceae, Ascomycetes)

The ultrastructure of the ascus, ascospores and appendages ofNimbospora bipolariswere examined at the transmission electron microscope level. The mature ascus wall comprises a single narrow electron-opaque layer which deliquesces and lacks an apical elaboration. Ascospore walls comprise a bipartite mesosporium, a thinner electron-opaque episporium and an outer mucilaginous sheath. Before release from the ascus the mucilaginous sheath is compact and folded. The fibrillar, trailing ascospore appendages are arranged in eight equidistant tufts around the circumference of the septum, originate from the mesosporium and emerge through pores in the episporium. In early stages of development the appendages are retained within the mucilaginous sheath but upon release from the ascus the sheath expands and the appendages pierce the sheath and radiate out to form their characteristic appearance. Ascospore ontogeny inN. bipolarisis compared with other appendaged members of the Halosphaeriaceae.

Two new species ofSphaerophorus(Caliciales) from New Zealand

Abstract The new species Sphaerophorus whakapapaensis Wedin and S. ohlssonii Wedin (Sphaerophoraceae, Caliciales, lichenised ascomycetes) are described from New Zealand. The ascospore ontogeny is described. The spore ornamentation of both species consists of mazaedial material added to the spore wall after the spores have teen released from the asci. Concentric bodies occur in young ascospores. The conidiophores are branched and produce conidia both acrogenously and pleurogenoesly. The conidiophores thus correspond to Vobis' type V. The conidia are bacilliform in S. whakapapaensis and ellipsoidal in S. ohlssonii. The distribution of the species and their taxonomic position in the genus are commented upon.

Ultrastructural observations on Helvellaceae (Pezizales, Ascomycetes). IV. Ascospore ontogeny in selected species of Gyromitra subgenus Discina

The ultrastructure of ascospore ontogeny and spore wall microchemistry are described in three sessile, discoid species of Gyromitra previously placed in Discina. Silver proteinate and barium permanganate were used as poststains to enhance the definition of various wall layers and spore organelles. Early stages of ascosporogenesis and primary wall formation are similar to those described in other species of Pezizales. Secondary wall formation, which results in characteristic spore ornamentation, is similar in Gyromitra brunnea, Gyromitra leucoxantha, and Gyromitra perlata. Mature spores of these species differ in the size and shape of translucent lacunae within the secondary wall, and in the morphology of apiculi. The lacunae originate through blebbing of primary wall material through the epispore into the secondary wall, resulting in the isolation of electron-translucent primary wall clumps within the electron-dense secondary wall. These and other ultrastructural observations of apothecial tissues support the maintenance of the Helvellaceae (sensu lato) to include taxa of the tribes Helvelleae, Discineae, and Rhizineae. Phylogenetic linkages of these taxa to other families of Pezizales are suggested. Key words: ascosporogenesis, ascospore wall structure and microchemistry, discomycete systematics and phylogeny.

Evolutionary Aspects of Ascospore and Conidium Ontogeny in Scoleconectria cucurbitula

Morphological changes and nuclear behavior during ascospore ontogeny in a collection of Scoleconectria cucurbitula with two-spored asci were characterized, and asexual states observed in artificial culture. Following meiosis, two single-celled, uninucleate ascospores were formed within the developing ascus; the other two post-meiotic nuclei did not take part in ascospore development. Ascospores progressed through two-celled, four-celled, and multi-celled stages, finally becoming highly elongate and transversely septate, with numerous uninucleate cells. While within the ascus, ascospore cells differentiated into phialides and produced numerous conidia. Synanamorphs formed in culture included a phialidic hyphomycetous state, a pycnidial state, and a state producing spores within hyphal cells by a process resembling free cell formation. It is suggested that intra-ascal germination obviates the need for greater numbers of ascospores, and that the two-spored form of S. cucurbitula likely arose from the four-spored form. Intra-ascal germination appears correlated with loss of ascus rings and possibly with loss of forcible spore discharge in certain Nectria species. The possible role of heterochrony in the evolution of phialidic ascospore germination in S. cucurbitula and allied fungi is discussed.

Evolutionary Aspects of Ascospore and Conidium Ontogeny in Scoleconectria Cucurbitula

Morphological changes and nuclear behavior during ascospore ontogeny in a collection of Scoleconectria cucurbitula with two-spored asci were characterized, and asexual states observed in artificial culture. Following meiosis, two single-celled, uninucleate ascospores were formed within the developing ascus; the other two post-meiotic nuclei did not take part in ascospore development. Ascospores progressed through two-celled, four-celled, and multi-celled stages, finally becoming highly elongate and transversely septate, with numerous uninucleate cells. While within the ascus, ascospore cells differentiated into phialides and produced numerous conidia. Synanamorphs formed in culture included a phialidic hyphomycetous state, a pycnidial state, and a state producing spores within hyphal cells by a process resembling free cell formation. It is suggested that intra-ascal germination obviates the need for greater numbers of ascospores, and that the two-spored form of S. cucurbitula likely arose from the four-spored form. Intra-ascal germination appears correlated with loss of ascus rings and possibly with loss of forcible spore discharge in certain Nectria species. The possible role of heterochrony in the evolution of phialidic ascospore germination in S. cucurbitula and allied fungi is discussed.

Fine structure of ascospore formation in Poronia punctata

Ascospore ontogeny in Poronia punctata is described. At the time of complete delimitation the ascospore is enclosed within two membranes, the spore plasma membrane to the inside and the investing membrane to the outside. The initial ascospore wall is the primary wall, formed by deposition of electron-transparent material between these two membranes. Differentiation within the region occupied by the primary wall begins with the development of an electron-dense epispore adjacent to the highly convoluted spore plasma membrane. Immediately after the epispore is differentiated the mesospore begins to form between the epispore and the spore plasma membrane. The mesospore becomes about twice as thick as the epispore. With the formation of the mesospore, differentiation and development within the region formerly occupied by the electron-transparent primary wall is complete. The outer portion of the primary wall, that between the epispore and the investing membrane, is the penspore. After formation of mesospore, epispore, and perispore, electron-transparent material is deposited centripetally between the spore plasma membrane and the mesospore. This innermost wall is the endospore.