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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    6 FUNGAL GENETICS resulting groups of seven chromosomes form two da ughter nuclei, one in each half of the ascus. The separation of the two nuclei is due principally to the elongation of the spindle, which appears to push the two groups of chromosomes apart. During telophase the chromosomes lose their contraction and during the succeeding interphase appear as slender threads which are sometimes visibly double (Fig. 1 j). The second division of meiosis is initiated by a prophase, during which the chromosomes become contracted once again. This is followed by metaphase II, with the formation of a longitudinally orientated spindle within each nucleus and with the chromosomes coming to lie on the spindle equators. At metaphase II, and also at anaphase I, the chromo¬ somes can sometimes be seen to be divided but with the two halves still held together at least at one point. It is generally accepted, on the basis of analogy with meiosis as seen in higher organisms, that the undivided point on each chromosome is the centromere, though this conclusion could hardly have been readhed from a study of the Neurospora ascus alone. Genetic evidence, to be considered below, confirms that the chro¬ mosomes must already be divided along most of their length before ana¬ phase I. At anaphase II each chromosome completes its longitudinal division, and a movement of chromosomes essentially similar to that of anaphase I (with sister centromeres passing to opposite poles) results in the forma¬ tion of four nuclei arranged in a row (Fig. \k). Since the second division spindles seldom or never overlap, the two nuclei in one half of the ascus derive from the same interphase nucleus. Each nucleus now contains seven undivided chromosomes, one of each kind. At this point the process of meiosis has been completed. It is most simply regarded as two divisions of the nucleus accompanied by only one division of the chromosomes, the chromosomal division occurring partly during the first nuclear division (division of the chromosome arms) and partly during the second (division of the centromere). During telophase II the chromosomes become elongated once more, but soon contract again for a third nuclear division. This is a mitotic divi¬ sion, essentially similar in effect to the nuclear divisions which occur in vegetative hyphae. By analogy with higher organisms, although this de¬ tail can hardly be seen in Neurospora, the chromosomes are believed al¬ ready to be divided, except at the centromere, before the onset of pro¬ phase, and they certainly separate completely into identical daughter chromosomes at the end of metaphase. Anaphase and telophase follow in the usual way. The third division spindles are orientated somewhat