Fungal genetics / J.R.S. Finchman, P.R. Day.

  • Fincham, J. R. S.
Date:
1971
Catalogue details

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

Credit: Fungal genetics / J.R.S. Finchman, P.R. Day. Source: Wellcome Collection.

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    30 FUNGAL GENETICS growing cultures at any time. None of these technical advantages is readily available in fungi with a mycelial organization. Matings in Saccharomyces are carried out by pairing single cells [136], by isolating mating pairs from mass matings, or by selecting prototrophic (i.e. nutritionally wild type) diploids from mass matings of complementary auxotrophic parental strains spread on minimal medium [599]. Fused haploids of S.cerevisiae usually bud off diploid cells only, but occasionally form haploid buds which are not recombinant for parental markers. Thus the pro¬ duct of fusion of haploid cells appears to be a transient heterokaryon [264, 842]. The diploid cells are larger than haploids, but are otherwise very similar to them. Meiosis and ascus formation take place when diploid cells are placed on a starvation medium which is usually a gypsum block [456] or sodium acetate agar [374a]. The cytology of the yeast cell has long been a subject of dispute. Indeed, the evidence for haploid and diploid states in S. cerevisiae is largely genetical except for the demonstration by Ogur et al. [467] that diploid cells possess twice as much deoxyribonucleic acid as haploids (cf. p. 196). There was dis¬ agreement not only on chromosomes but even on which of the visible struc¬ tures in the cell was the nucleus [183, 452, 458, 848]. More recent studies with light and electron microscopy (501a and Moens and Rapport (520a) have clearly identified the nucleus and have shown that mitosis and meiosis take place within an intact nuclear membrane. Countable chromosomes cannot be seen and although synaptinemal complexes (see p. 156) were observed at meiosis attempts to estimate the number of bivalents were unsuccessful. The linkage map of Hawthorne and Mortimer [340, 342] has sixteen groups of markers linked to different centromeres, eight of which have markers in both chromsome arms. The fission yeast Schizosaccharomyces pombe multiplies by a process of equal cell division rather than by budding. The diploid cell formed by the fusion of two haploids usually undergoes meiosis without any intervening growth or cell division. The ascus is dumb-bell shaped and has four haploid spores arranged in a row (see Fig. 6C). The experimental selection of diploids may be carried out by a method devised by Leupold [439]. Cells from a mating mixture are spread on nutrient agar. Diploid colonies, identified by their production of ascospores which stain brown when brieñy 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 mycelial organization, and a few have a well-defined diploid phase like 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