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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    THE CHROMOSOME THEORY 3 of different mating type, are brought together. Neurospora Ике other Asco- mycetes, has only two mating types which are usually symbolized by A and a. There is no morphological difference between A and a strains, and both can form abundant female reproductive structures, the protoperithecia, when grown on solid (agar) medium of suitable composition [803]. A protoperi- thecium consists of the ascogonium, which is a coiled multicellular hypha, enclosed in a knot-like aggregation of hyphae. The tip of the ascogonium is extended as a branching system of very slender hyphae, called the trichogyne, which projects beyond the sheathing hyphae into the air. Fertilization occurs when a cell of opposite mating type, which may be a macroconidium, a micro- conidium, or even a piece of ordinary mycelium, comes into contact with a part of the trichogyne. When such a contact is made, fusion may occur, and one or more nuclei from the fertilizing cell will then migrate down the tricho¬ gyne and into the ascogonium [23] (cf. Fig. 9). The details of the nuclear events within the ascogonium after fertilization are not fully understood, but from the work of Colson [143] on the closely related Neurospora tetrasperma, as well as from what is known about Asco- mycetes generally we can assume that what happens is as follows. No fusion of nuclei of different mating types occurs at this stage. Instead, a pair of nuclei, one from the ascogonium and one from the fertilizing cell, become associated and begin to divide synchronously. The products of these divisions pass, still in pairs of unlike mating type, into numerous ascogenous hyphae which now begin to grow out of the ascogonium. While this is happening, the mycelial sheath which enveloped the ascogonium begins to develop as the wall of the perithecium, and becomes heavily impregnated with melanin. The mature perithecium is a flask-shaped structure with a narrow beak-like neck. Oc¬ casional perithecia include ascogenous hyphae which have evidently arisen from more than one pair of parental nuclei; such perithecia may well be due to the inclusion of two adjacent ascogonia in a single perithecial wall [545]. The later stages of development of the ascogenous hyphae, and the whole course of development of the asci which arise from them, are well known, thanks to the work of McCHntock [468], Singleton [708] and Barry [39] who studied squash preparations of perithecial contents stained with acetic orcein. Each ascogenous hypha bends to form a hook (or crozier) at its tip and the two nuclei of opposite mating type within the crozier divide synchronously (Fig. la, b). Septa now form to divide the crozier into three cells, the central one, in the curve of the hook, containing two nuclei of unlike mating type. This binucleate cell is the ascus initial, and the two uninucleate cells on either side of it commonly fuse to reconstitute a binucleate cell which can grow on to form a further crozier. Almost immediately after the formation of an ascus initial the two nuclei within it fuse together (Fig. 1 c). The chromosomes at this time are in a relatively