The Sexuality and Arrangement of the Spores in the Ascus of Neurospora Sitophila

Abstract

In Neurospora sitophila, the ascus is long and narrow and the spindle of the first mitosis is longitudinally placed. The resulting daughter nuclei come to rest some distance apart, one above the other in the ascus. The second division spindles are also longitudinal. The four resting nuclei are bottle-shaped and have two long horn-like appendages protruding from the neck. The spindles of the third division are transverse and are widely separated. Spores of known position were isolated from six asci of Neurospora sitophila and every possible combination made in cultures. It was found that the spore haplonts may be arranged in the ascus in either one of the four following combinations, AA, BB, AA, BB; BB, AA, BB, AA; AA, BB, BB, AA; or BB, AA, AA, BB, where A and B are reciprocal as to their sexuality. Since the spores carrying like factors for sexuality arrange themselves in pairs along the length of the ascus, the segregation of the sex factors must take place in the second mitosis. Had this factor been segregated in the first division, the four spores at one end of the ascus would be of one sex and the four spores at the other end of the opposite sex. Should this segregation take place in the third mitosis, however, the spores of opposite sex would alternate throughout the length of the ascus, which would be contrary to the experimental results.