Ascus Morphology Research Articles

First Report of Peduncle Dieback of Rose Caused by Colletotrichum gloeosporioides in Korea

Roses are an ornamental crop and grown commercially as cut flowers. They are also used in the perfume, jam, and jelly industry. Dieback of rose is a major disease caused by different types of fungi including Lasiodiplodia pseudotheobromae, Acremonium sclerotigenum, Coniothyrium fuckelii, and Botrytis cinerea (Mirtalebi et al. 2016; Wee at al. 2017). Severe peduncle dieback of rose was observed in a garden Sngju, South Korea, in August 2018. Typical symptoms are characterized by progressive death of peduncle from the flower bud. The presumptive causal agent was isolated from infected tissue. Small sections of peel (2 × 2mm) were cut from the margin of infected tissue and disinfected with 0.5% NaOCl for 1 min, followed by rinsing in sterile water. A disinfected section of infected tissue was blotted dry, placed on potato dextrose agar (PDA), and incubated at 25°C in the dark. Two pure isolates (ICKR1 and ICKR2) were obtained by transferring newly emerging hyphal tips to the new fresh PDA plates. Colonies on PDA were creamy white with small black perithecia produced across the colony after 3 weeks. Asci were eight-spored, clavate to cylindrical, and 57.6 to 83.5 × 9.2 to 14.6 μm (mean ± SD = 70.1 ± 9.2 × 12.1 ± 1.35 μm) (n = 35). Ascospores were hyaline, unicellular, curved or straight with round ends, and 15.2 to 30.8 × 5.2 to 8 μm (mean ± SD = 21.4 ± 3.6 × 6.40 ± 0.8 μm) (n = 30). The morphology of asci and ascospores overlapped with the previous morphological description of Glomerella cingulata (teleomorph of C. gloeosporioides) (Ireland et al. 2008; Weir et al. 2012). Only a few conidia were produced on PDA, which were hyaline, cylindrical, straight with one or both ends rounded, and 18.8 to 24.6 × 5.9 to 6.8 μm (mean ± SD = 21.45 ± 1.9 × 6.3 ± 0.45 μm) (n = 20). Appressoria were brown and unlobed. Molecular analysis was reformed using the internal transcribed spacer (ITS) region of rDNA and partial GAPDH, TUB2, CHS-1, ACT, and CAL genes sequence data. The complete ITS region and partial GAPDH, TUB2, CHS-1, ACT, and CAL genes were amplified using ITS1F/ITS4, BT2a/BT2b, ACT-512F/ACT-783R, CHS-79F/CHS-345R, ACT-512F/ACT-783R, and CL1C/CL2C primer sets, respectively (Weir et al. 2012), and sequenced. The resulting sequences were deposited in GenBank (accession nos. LC469311 to LC469322). Phylogenetic analysis (maximum likelihood and neighbor joining) using MEGA 6 software delineated the present isolates from rose as Colletotrichum gloeosporioides. Pathogenicity tests were conducted on small twigs of rose containing at least one bud. Ten twigs were washed in tap water, disinfected with 90% ethanol, rinsed with distilled water, and prepared for inoculation. Each prepared twig was put in a plastic tube containing distilled water. The conidial suspension (1 × 10⁶ conidia /ml) of ICKR1 isolate was sprayed over six twigs. Another four twigs received distilled water and served as control. Both treated and control twigs containing tubes were place in a tube holder rack and incubated at 25°C and 16 h/6 h light/dark. After 12 days, typical dieback symptoms appeared on inoculated twigs, whereas control twigs remained asymptotic. The causal agent was reisolated and identified as C. gloeosporioides and confirmed Koch’s postulates. C. gloeosporioides is a ubiquitous pathogen on a wide range of crops and is responsible for anthracnose. C. gloeosporioides has been reported as the causal agent of Eucalyptus dieback in South Africa and tip dieback of Lygodium microphyllum and L. japonicum in Australia (Ireland et al. 2008; Smith et al. 1998). To our knowledge, this is the first report of dieback of rose caused by C. gloeosporioides in Korea.

First report of Taphrina sp. causing leaf distortion and reddening in glue berry, Cordia dichotoma in India

Hitherto unknown fungal infection of Cordia dichotoma with leaf chlorosis distortion-reddening was observed in naturally growing plants at various places near Panchgaon Manesar, Gurgaon in March 2014 onwards covering 70% area of both surfaces. The pathogen was primarily identified as Taphrina sp. on the basis of morphology of asci and ascopsores. Aqueous suspension of whitish woolly growth (blooming) of the fungus sprayed on leaves produced typical blooming symptoms in about 15 days followed by reddening, distortion and cupping of C. dichotoma leaves.

Sphaceloma coryli: A Reemerging Pathogen Causing Heavy Losses on Hazelnut in Southern Italy.

In 2013 to 2014, a disease causing severe crop losses was observed in several growing areas of the Campania region in southern Italy. Disease symptoms consisted of necrotic spots on leaves, bracts, and shells along with vein necrosis. Nuts were necrotic or aborted and symptomatic fruit dropped prematurely. A fungus was consistently isolated from symptomatic tissues and morphologically identified as Sphaceloma coryli. The fungus was first reported in France and Italy (Campania) over 30 years ago and was not subsequently documented until 2006 in the Latium region of Italy. When artificially inoculated on healthy hazelnut plants, disease symptoms were reproduced. The teleomorph of S. coryli was recorded for the first time on overwintering tissues of hazelnut trees. It was also induced in vitro. The morphology of asci and ascospores together with phylogenetic analysis based on internal transcribed spacer sequences indicated that the teleomorph is an undescribed species within the genus Elsinoë for which the name of Elsinoë coryli is proposed. Optimum temperatures for ascospore and conidium germination and mycelium growth were determined in vitro. The inhibition effect of some commercial fungicides on S. coryli was also evaluated in vitro.

Placement of Medeolaria farlowii in the Leotiomycetes, and comments on sampling within the class

Medeolaria farlowii, an ascomycetous parasite of Medeola virginiana, has been included as the only member of the family Medeolariaceae and order Medeolariales. Its assignment within the Ascomycota has been problematic due to the lack of distinctive ascomatal form and ascus morphology. Asci are formed in a loosely organized hymenium on hypertrophic portions of Medeola virginiana stems. Phylogenetic analyses of nuclear 5.8S, large subunit, and small subunit rDNA gene sequences place M. farlowii in the Leotiomycetes with parsimony, Bayesian, and maximum likelihood analyses, but its position within this class remains unresolved. In general, limited taxon and gene sampling in the Leotiomycetes hampers placement of taxa within this class. A survey of available sequence data in the Leotiomycetes is given, and the implication for phylogenetic studies within the class is discussed. Medeolaria farlowii should be treated as a monotypic genus in the monotypic order Medeolariales, class Leotiomycetes.

New fungal genera, Tectonidula gen. nov. for Calosphaeria-like fungi with holoblastic-denticulate conidiogenesis and Natantiella gen. nov. for three species segregated from Ceratostomella

Two morphologically similar groups of ascomycetes with globose to subglobose perithecia, elongate necks, unitunicate asci floating freely at maturity, and hyaline ascospores currently placed in Calosphaeria s. lat. and Ceratostomella s. lat., respectively, are studied. The Calosphaeria-like fungi have groups of perithecia growing between cortex and wood, arranged in circular groups with converging necks and piercing the cortex in a common point; the asci with a shallow apical ring and U- to horseshoe-shaped hyaline ascospores are compared with Calosphaeria pulchella, the type species of the genus. Conidiogenesis of the investigated Calosphaeria-like fungi is holoblastic-denticulate; ramichloridium-like and sporothrix-like conidiophores and conidia were formed in vitro. Ascospore and ascus morphology, structure of the ascal apex, ascogenous system, mode of conidiogenesis and the large subunit rRNA sequences of this group differ considerably from C. pulchella and both groups are unrelated. Thus a new genus, Tectonidula, is described with two accepted species, T. hippocrepida and T. fagi; they are separated by ascospore and ascus morphology and holoblastic-denticulate conidiogenesis from the core species of Calosphaeria. The placement of Tectonidula among perithecial ascomycetes is discussed. The relationship of Tectonidula with Barbatosphaeria and two ramichloridium-like hyphomycetous genera Rhodoveronaea and Myrmecridium is investigated. Three species formerly attributed to Ceratostomella are studied. The revision of the herbarium type specimen and fresh material of Ceratostomella ligneola revealed that it is conspecific with Ceratostomella ampullasca and Ceratostomella similis. The LSU phylogeny clearly separated C. ligneola from Ceratostomella s. str. and morphologically similar Lentomitella. On the basis of molecular sequence data and detailed comparison of morphology of asci, ascospores and ascogenous system the genus Natantiella is described for C. ligneola with C. ampullasca and C. similis as its synonyms. Natantiella produced sterile mycelium in vitro.

New insights into classification and evolution of the Lecanoromycetes (Pezizomycotina, Ascomycota) from phylogenetic analyses of three ribosomal RNA- and two protein-coding genes

The Lecanoromycetes includes most of the lichen-forming fungal species (>13 500) and is therefore one of the most diverse class of all Fungi in terms of phenotypic complexity. We report phylogenetic relationships within the Lecanoromycetes resulting from Bayesian and maximum likelihood analyses with complementary posterior probabilities and bootstrap support values based on three combined multilocus datasets using a supermatrix approach. Nine of 10 orders and 43 of 64 families currently recognized in Eriksson’s classification of the Lecanoromycetes (Outline of Ascomycota—2006 Myconet 12:1–82) were represented in this sampling. Our analyses strongly support the Acarosporomycetidae and Ostropomycetidae as monophyletic, whereas the delimitation of the largest subclass, the Lecanoromycetidae, remains uncertain. Independent of future delimitation of the Lecanoromycetidae, the Rhizocarpaceae and Umbilicariaceae should be elevated to the ordinal level. This study shows that recent classifications include several nonmonophyletic taxa at different ranks that need to be recircumscribed. Our phylogenies confirm that ascus morphology cannot be applied consistently to shape the classification of lichen-forming fungi. The increasing amount of missing data associated with the progressive addition of taxa resulted in some cases in the expected loss of support, but we also observed an improvement in statistical support for many internodes. We conclude that a phylogenetic synthesis for a chosen taxonomic group should include a comprehensive assessment of phylogenetic confidence based on multiple estimates using different methods and on a progressive taxon sampling with an increasing number of taxa, even if it involves an increasing amount of missing data.

New insights into classification and evolution of the Lecanoromycetes (Pezizomycotina, Ascomycota) from phylogenetic analyses of three ribosomal RNA- and two protein-coding genes

The Lecanoromycetes includes most of the lichen-forming fungal species (> 13500) and is therefore one of the most diverse class of all Fungi in terms of phenotypic complexity. We report phylogenetic relationships within the Lecanoromycetes resulting from Bayesian and maximum likelihood analyses with complementary posterior probabilities and bootstrap support values based on three combined multilocus datasets using a supermatrix approach. Nine of 10 orders and 43 of 64 families currently recognized in Eriksson's classification of the Lecanoromycetes (Outline of Ascomycota--2006 Myconet 12:1-82) were represented in this sampling. Our analyses strongly support the Acarosporomycetidae and Ostropomycetidae as monophyletic, whereas the delimitation of the largest subclass, the Lecanoromycetidae, remains uncertain. Independent of future delimitation of the Lecanoromycetidae, the Rhizocarpaceae and Umbilicariaceae should be elevated to the ordinal level. This study shows that recent classifications include several nonmonophyletic taxa at different ranks that need to be recircumscribed. Our phylogenies confirm that ascus morphology cannot be applied consistently to shape the classification of lichen-forming fungi. The increasing amount of missing data associated with the progressive addition of taxa resulted in some cases in the expected loss of support, but we also observed an improvement in statistical support for many internodes. We conclude that a phylogenetic synthesis for a chosen taxonomic group should include a comprehensive assessment of phylogenetic confidence based on multiple estimates using different methods and on a progressive taxon sampling with an increasing number of taxa, even if it involves an increasing amount of missing data.

Molecular systematics of Ceratostomella sensu lato and morphologically similar fungi

The systematic position and phylogenetic relationships of Ceratostomella sensu lato and phenotypically similar fungi using comparative morphological and culture studies and phylogenetic analyses of the nuclear large- and small-subunit ribosomal DNA were explored. In the light of inferred phylogenies and morphological data the genus Ceratostomella is redescribed, the generic concept is emended and four species are accepted (viz. C. cuspidata, C. pyrenaica, C. rhynchophora and C. rostrata). A new genus Xylomelasma is introduced and delimited from Ceratostomella, with two new species described (viz. X. novaezelandiae and X. sordida). In culture species of both Ceratostomella and Xylomelasma produced sterile mycelium. The genus Lentomitella with a phaeoisaria-like anamorph formed in vitro is reinstated to encompass taxa formerly attributed to the broadly perceived Ceratostomella with three accepted species (viz. L. cirrhosa, L. crinigera and L. tomentosa). Lentomitella and Ceratostomella are clearly distinguishable by the morphology of asci, ascospores and centrum. Lentomitella is compared to phenotypically similar Ceratosphaeria, which formed a harpophora-like anamorph in vitro. In the present phylogenies Ceratostomella, Ceratosphaeria, Lentomitella and Xylomelasma are shown as clearly separate genera belonging to three different groups of perithecial ascomycetes. Ceratostomella, Lentomitella and Xylomelasma reside within a large unsupported clade consisting of members the Ophiostomatales, the freshwater Annulatascaceae and a group of nonstromatic, terrestrial taxa. Ceratosphaeria is well supported within the Magnaporthaceae. The systematic value of morphological characters of ascospores, paraphyses, asci, centrum and conidiogenesis in segregating taxa from Ceratostomella sensu lato and their relatives is discussed.

Characterization of rec15, an early meiotic recombination gene in Schizosaccharomyces pombe

In S. pombe strains mutant for rec15 aberrant ascus morphology, reduced spore viability and severe reduction of meiotic recombination was detected. Genetic and cytological analysis identified frequent interruption of meiosis after the first division, and nondisjunction I, as the main segregation errors in the mutant. Chromosome segregation at meiosis I was not random in rec15, suggesting the presence of a backup system for correct segregation of achiasmate chromosomes. The analysis of meiotic progression in time-course experiments revealed that the major meiotic events, such as the onset of premeiotic DNA synthesis, of horse-tail nuclear movement, and of the first meiotic division occurred earlier in rec15 than in wild-type. The early onset of meiotic events is a novel observation for an early recombination mutant and implies a function of rec15 protein already at or before DNA synthesis.

Halosarpheia kandeliae sp. nov. on intertidal bark of the mangrove tree Kandelia candel in Hong Kong

Halosarpheia kandeliae sp. nov. collected on intertidal bark of Kandelia candel is described and its unique ascus morphology illustrated. It is compared with other Halosarpheia spp. and Aniptodera salsuginosa.

A RE-EXAMINATION OF GENERIC CONCEPTS OF BAEOMYCETOID LICHENS BASED ON PHYLOGENETIC ANALYSES OF NUCLEAR SSU AND LSU RIBOSOMAL DNA

Abstract: The lichen symbiosis has evolved several times within the fungal kingdom, although the total number of lichenization events leading to extant taxa is still unclear. Two lichenized families, theIcmadophilaceae and Baeomycetaceae have been classified in the Helotiales. Because the Helotiales are predominantly non-lichenized, this suggests that these families represent independent evolutionary episodes of lichenization from the Lecanorales. As a first step towards understanding the evolution of the lichen symbiosis within this order, we tested recent hypotheses concerning the segregation of lichen genera between the two lichen families. Specifically, we used phylogenetic analyses of nucleotide sequence data from nuclear small-subunit and large-subunit ribosomal DNA to test the morphology-based hypotheses thatDibaeis is a distinct genus from Baeomyces and that Dibaeis is a member of the Icmadophilaceae rather than the Baeomycetaceae. Phylogenetic analyses of nuclear SSU rDNA and combined SSU and LSU rDNA data support the hypothesis that Dibaeis is more closely related to Icmadophila than it is to Baeomyces. Therefore, these data support the resurrection of Dibaeis from its previous synonymy with Baeomyces based on the characters of ascocarp colour and ascus morphology. The recognition of two distinct genera is also consistent with character state distribution of unique lichen acids.

A Re-Examination of Generic Concepts of Baeomycetoid Lichens Based on Phylogenetic Analyses of Nuclear ssu and Lsu Ribosomal Dna

The lichen symbiosis has evolved several times within the fungal kingdom, although the total number of lichenization events leading to extant taxa is still unclear. Two lichenized families, the Icmadophilaceae and Baeomycetaceae have been classified in the Helotiales. Because the Helotiales are predominantly nonlichenized, this suggests that these families represent independent evolutionary episodes of lichenization from the Lecanorales. As a first step towards understanding the evolution of the lichen symbiosis within this order, we tested recent hypotheses concerning the segregation of lichen genera between the two lichen families. Specifically, we used phylogenetic analyses of nucleotide sequence data from nuclear small-subunit and large-subunit ribosomal DNA to test the morphology-based hypotheses that Dibaeis is a distinct genus from Baeomyces and that Dibaeis is a member of the Icmadophilaceae rather than the Baeomycetaceae. Phylogenetic analyses of nuclear SSU rDNA and combined SSU and LSU rDNA data support the hypothesis that Dibaeis is more closely related to IcmadophUa than it is to Baeomyces. Therefore, these data support the resurrection of Dibaeis from its previous synonymy with Baeomyces based on the characters of ascocarp colour and ascus morphology. The recognition of two distinct genera is also consistent with character state distribution of unique lichen acids.

Molecular Systematics of Clavicipitaceae Supporting Monophyly of Genus Epichloe and Form Genus Ephelis

The family Clavicipitaceae (Ascomycota) is comprised of fungi with perithecia borne on stro- mata, unitunicate asci, and filamentous, multiseptate ascospores. All are biotrophic symbionts, either mu- tualistic with plant hosts or pathogenic to plants, in- vertebrate animals or other fungi. Genera of plant- associated Clavicipitaceae (tribes Balansieae and Clavicipieae) are distinguished, in part, by stromal and ascus morphology, ascospore germination pat- terns, whether sclerotia are formed, and host inter- actions. Their anamorphs include enteroblastic mi- croconidial states, classified in anamorphic genera Neotyphodium and Sphacelia (for teleomorphs Atkin- sonella, Claviceps, Echinodothis, and Epichloe), and ho- loblastic macroconidia, classified in the anamorphic genus Ephelis (teleomorphs Atkinsonella, Balansia, and Myriogenospora). Epichloe' species often are mu- tualistic with grass hosts, and are ancestral to asexual, seed transmitted endophytes symbiotic with many cool-season grasses. Partial 28S nuclear rDNA se- quences were determined from isolates of five species and two undescribed mating populations of Epichloe, one asexual Epichloe hybrid (Neotyphodium coeno- phialum), and representatives of six other genera in the family. Results from phylogenetic analysis of the sequences supported monophyly of plant-associated Clavicipitaceae, with insect-pathogenic Cordyceps spe- cies more deeply rooted. Four clades were distin-

Molecular systematics of Clavicipitaceae supporting monophyly of genus Epichloë and form genus Ephelis

The family Clavicipitaceae (Ascomycota) is comprised of fungi with perithecia borne on stromata, unitunicate asci, and filamentous, multiseptate ascospores. All are biotrophic symbionts, either mutualistic with plant hosts or pathogenic to plants, invertebrate animals or other fungi. Genera of plant- associated Clavicipitaceae (tribes Balansieae and Clavicipieae) are distinguished, in part, by stromal and ascus morphology, ascospore germination patterns, whether sclerotia are formed, and host interactions. Their anamorphs include enteroblastic microconidial states, classified in anamorphic genera Neotyphodium and Sphacelia (for teleomorphs Atkinsonella, Claviceps, Echinodothis, and Epichloë), and holoblastic macroconidia, classified in the anamorphic genus Ephelis (teleomorphs Atkinsonella, Balansia, and Myriogenospora). Epichloë species often are mutualistic with grass hosts, and are ancestral to asexual, seed transmitted endophytes symbiotic with many cool-season grasses. Partial 28S nuclear rDNA sequences were determined from isolates of five species and two undescribed mating populations of Epichloë, one asexual Epichloë hybrid (Neotyphodium coenophialum), and representatives of six other genera in the family. Results from phylogenetic analysis of the sequences supported monophyly of plant-associated Clavicipitaceae, with insect-pathogenic Cordyceps species more deeply rooted. Four clades were distinguished among plant-associated Clavicipitaceae: a monophyletic genus Epichloë (including N. coenophialum), a monophyletic genus Claviceps, a clade represented only by Echinodothis tuberiformis, and a clade including all sampled strains with Ephelis anamorphs. Morphological relationships of anamorphs and ascospore development better fit the molecular phylogenetic relationships than did host relationships or host tissue specificity.

Neotropical Ascomycetes 3. Reinstatement of the Genus Xenolophium and Two New Species from French Guiana

The genus Xenolophium is recognized for X. pachythele and X. applanatum, both new combinations, and their affinities to Ostropella albocincta are discussed. Two new species X. samuelsii and X. guianense are described from French Guiana. A key to the species of Xenolophium is given. Investigations of Neotropical fungi at the her? barium at New York uncovered two ascomycetes from French Guiana having large, superficial as? comata with an apical crest, slit-like ostiole and distinctive ascus morphology. Ascomatal characteristics suggested a relationship with Ostropella albocincta (Berk. & Curt.) von Hohnel and Schizostomapachythele(Berk. & Br.) Sacc, both known from tropical areas. This paper reexamines the relationships of O. albocincta, S. pachy? thele and the genus Xenolophium Sydow, with descriptions, illustrations and a key to species. Ostropella albocincta, S. pachythele, and Xenolophium are linked by similarities in ascom? atal, ascus and ascospore characteristics. Barr (1990) provides the history of Ostropella (Sacc.) von Hohnel and its relationship to these other taxa and to the genus Ostreionella Seaver. Holm and Yue (1988) present the most recent work on Schizostoma Ces. & de Not. ex Sacc. in which they tried to clarify some misconceptions that have surrounded the genus since its establishment by Saccardo in 1878 and to resolve the placement of some species. Sydow (in Stevens, 1925) established Xenolophium for two species from Hawaii, X. leve Sydow and X. verrucosum Sydow, but the genus has been in synonymy until now.

Metschnikowia gruessii sp. nov., the Teleomorph of Nectaromyces reukaufii but not of Candida reukaufii

Summary One strain identified by conventional phenetic methods as Metschnikowia reukaufii did not hybridize with a species-specific probe produced in our laboratory. Molar % G+C composition and a relative heteroduplex formation of 7% with the type strain of M. reukaufii revealed that they did not belong to the same taxon. Comparisons with the type strain of Candida reukaufii (CBS 1903) and the authentic strain of Nectaromyces reukaufii (CBS 611) showed no similarity with the former (22% reassociation) and conspecificity with the latter (97% reassociation). Low nuclear DNA reassociation values with all the type strains of other Metschnikowia species provided sufficient evidence for proposing a novel species, Metschnikowia gruessii that represents the teleomorph of Nectaromyces reukaufii but not of Candida reukaufii . It differs from the type strain of M. reukaufii in its cell and ascus morphology, assimilation of trehalose and maximum temperature for growth, and from the other described Metschnikowia species by additional characteristics. More strains of Metschnikowia gruessii were repeatedly isolated from flowers in the natural park of Arrabida, Portugal.

The bitunicate ascus paradigm

The bitunicate ascus paradigm recognized by E. S. Luttrell is reviewed. Species demonstrated to fit the paradigm are listed. Attempts to redefine the bitunicate ascus with functional as well as structural characters are outlined including Chadefaud’sTrois Categories d’Ascomycetes, Sherwood’s Unusual Ascus Types, Bellemere’sCouche Plan, and Eriksson’s Ascus Dehiscence Types. Discussion of related topics include: (1) Characteristics of the bitunicate ascus including sequential wall formation, separation of wall regions at ascospore ejaculation, thenasse apicale, and the “banded pattern” of the secondary wall layer; (2) Ascus morphology types based on characters observable with the light microscope; and (3) Ascostromatic species designated bitunicate that deviate from the bitunicate ascus paradigm and yet are not structurally similar to the unitunicate ascomycetes. The separation of two wall regions at spore ejaculation is recognized as the single criterion for the bitunicate ascus sensu stricto (=fissitunicate).

Nonlinear Asci without Apical Pores in the Peak Mutant of Neurospora

Previous workers have shown that asci of the Neurospora crassa morphological mutant peak are swollen, and that ascospores are not linearly ordered. In the swollen asci, spindle orientations are abnormal at all three divisions but other events of meiosis and ascospore formation are apparently normal. In the present study, cytological features of ascus development in crosses involving peak are reported in greater detail. The swollen peak asci fail to differentiate apical pores, and mature ascospores are not discharged from the perithecia. The rare linear peak asci form normal apical pores, however. Ascus morphology is described in crosses of peak and two other dominant ascus mutants.

Dominance modifiers in neurospora crassa: phenocopy selection and influence of certain ascus mutants.

When homozygous in zygotes, mutant alleles at the peak locus in linkage group V of Neurospora crassa initiate aberrant asci that are nonlinear, in contrast to the linear asci characteristic of wild type. Most mutant alleles are recessive, inasmuch as crosses of the mutant strains with wild type give linear asci. However, five different mutant alleles, when heterozygous with the wild-type allele, act in varying degrees as zygote dominants, initiating both linear and nonlinear asci, the relative proportions depending on the allele. Five modifiers that act on the dominance relationships of at least one of the five possible heterozygotes of a dominant peak and its wild-type allele have been characterized, four of them having been obtained by selection directed against a phenocopy of these mutants induced by treatment of wild type with l-sorbose. The pattern of modifier specificity observed among the various dominant peak heterozygotes indicates that the phenotypic effects are produced by a complex relationship between the modifiers and the dominant peak alleles in relation to their wild-type allele. In all but two cases the direction of modification, where present, is towards decreasing the dominance of the mutant allele in the heterozygote, evidenced by an increase in the percentage of linear asci when compared with control data. The modifiers exert their maximum modification when they themselves are heterozygous with their wild-type alleles and when the dominant peak allele is heterozygous with its wild-type allele. No modification occurs when heterozygous modifiers are included in zygotes homozygous for a dominant peak allele, reinforcing the notion that the modifiers act on the dominance relationship existent between a dominant peak allele and its wild-type allele, rather than influencing some activity of the mutant allele itself. The modifiers have no detectable effect of their own on ascus morphology, since homozygous modifier zygotes initiate entirely linear asci when only wild-type alleles of peak are present in the zygotes. Their only detectable effect, other than dominance modification, appears to be in conferring sorbose resistance to the mycelium. The modifiers are unlinked to the peak locus, and, except for two of them, they are nonallelic.

Cytology and genetics of two abnormal ascus mutants of Neurospora.

A new mutant of Neurospora, called scruffy, acts as a zygote recessive in determining asci with altered arrangements, and sometimes altered numbers, of ascospores. Cytological observations show that the morphological deviations seen in mature asci are preceded by differences in orientation of spindles for the nuclear divisions occurring within the developing asci, and that spindle disposition is influenced by the size and shape of the cells. The ultimate effects on spore shape and number appear to result from a disruption of the interaction between cell wall and elongating spindles that in N. crassa determines the uniseriate arrangement of eight spores. A nonallelic mutant, scumbo, also acts as a zygote recessive in effecting deviant ascus morphology. A demonstration of complementation between nonallelic mutants affecting ascus morphology provides additional evidence referring to zygotic control of the development of the ascus in Neurospora.