Fungal genetics / J.R.S. Fincham and P.R. Day.

  • Fincham, J. R. S.
Date:
[1963]
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Fungal genetics / J.R.S. Fincham and P.R. Day. Source: Wellcome Collection.

    43/334 (page 27)
    OUTLINE OF THE BIOLOGY OF FUNGI 29 identified by their production of ascospores which stain brown when briefly exposed to iodine vapour, are then selected. Non-sporulating cells are isolated from the diploid colonies and a proportion of these cells are stable diploids. The diploids can be distinguished from haploids by their larger cells. Some other members of the Endomycetales have a myceUal organiza¬ tion, and few have a well-defined diploid phase Uke that of the yeasts. In Ascocybe grovesii the haploid mycelium produces an erect diploid branch, called an ascophore, with clusters of four-spored asci at its tip. When dissected off the parental haploid mycelium the diploid ascophore will grow indefinitely in liquid culture, retaining its unbranched morpho¬ logy [94]. Mutants with specific growth requirements have been isolated by Dixon (unpubUshed), but attempts to produce ascophores of hybrid origin have been unsuccessful so far. The Euascomycetes may be divided into three sub-groups : Plectomy- cetes, Pyrenomycetes and Discomycetes, which are separated by differ¬ ences in the structure of their fruit bodies. Plectomycetes The fruit body of the Plectomycetes is called a cleistothecium, and the enclosed ascospores are released by rupture of the cleistothecium wall. The most important genera are Aspergillus and PenicilHum. Some of the species usually included in these genera, on the basis of vegetative morphology, have no known sexual stage. The species which have been studied genetically include Aspergillus nidulans [332], A. niger [331], A. oryzae and A. sojae [191], and Pénicillium chrysogenum [333]. All of these have been studied by exploiting parasexual recombination (see Chapter 5), but only A. nidulans has also been subjected to more ortho¬ dox genetic analysis utilizing sexual reproduction. The life cycle of A. nidulans is shown in Fig. 7. The following brief account of A. nidulans is largely based on the authoritative paper of Pontecorvo et al [332]. The colourless mycelium is made up of multinucleate cells and bears conidiophores, each of which arises from a foot cell and ends in a globose multinucleate vesicle (cf Fig. 7). From the surface of the vesicle uninucleate primary finger-like sterig- mata protrude, and each of these gives rise to two or more secondary sterigmata. Each secondary sterigma remains uninucleate, but buds off a chain of uninucleate conidia by repeated division of its nucleus ; the youngest conidia are at the base of the chain. The conidia are dark green to dull-grey -green in colour in wild strains.