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.

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    24 FUNGAL GENETICS rales are heterothallic with two mating types called + and —. The great¬ est obstacle to genetical work is the difficulty of germinating zygospores. Another difficulty appeared to be the apparent lack of hyphal anasto¬ mosis between strains of the same mating type preventing the formation of heterocaryons. However, recently the synthesis of heterocaryons from morphological or nutritional mutants has been claimed in Mucor ram- manianus(E. H. Evans, personal communication) and Rhizopus javanicus [277]. Among the species which have been investigated there are varia¬ tions in the events following the fusion of gametangia and leading to the release of spores from the germ sporangium. The inheritance of mating type is a good example of this variation. Blakeslee [33], who discovered heterothalhsm, defined four types of zygospore germination. These were : (1) pure germinations in which all germ sporangiospores were of the same mating type either + or —; (2) mixed germinations in which both + and — spores were present; (3) mixed germinations in which +, — and homothallic (i.e. self-fertile) spores were present ; (4) pure germina¬ tions in which all the spores were homothallic, this last type being found in the homothallic species. It is generally agreed that during its initial de¬ velopment the zygospore contains many nuclei from the gametangia and that these appear to fuse in pairs. What is not clear, however, is how many of the diploid nuclei thus formed undergo meiosis and at what stage. In those species which give mixed germinations it is not known whether all the products of each meiotic division survive and can be re¬ covered. Since Blakeslee defined the four types of germination many workers have studied the cytology of zygospore development. In the heterothallic species Cutter [76] defined three basic types of nuclear behaviour accord¬ ing to whether nuclear fusion and meiosis occurred before the resting period, at germination or in the germ sporangium. Sjowall [387] however recognized only two types of behaviour. In the first group, which in¬ cluded Mucor hiemalis, M. mucedo and Absidia glauca, nuclear fusion and meiosis occurred before the resting period. In the second group nu¬ clear fusion was delayed until zygospore germination. Meiosis took place in the germ sporangium and involved many diploid nuclei in Phy corny ces blakesìeeanus but only one nucleus in Rhizopus nigricans. The cytological findings do not completely account for the genetic evidence. Thus Kohler [221] found that in M. mucedo not only are all the spores from a germ sporangium of the same sex but in crosses involving two pairs of alleles only one of the four possible genotypes was found in any one germ sporangium. However Sjowall claimed to have