Sordaria, hybrid asci, pre and post-reduction

Objectives To study the distribution of light and dark spores in the asci of this hybrid mold to understand the role of the successive divisions of the egg cell, the first, reducing, restoring the haploid state. List the types of distribution encountered, their frequency allowing us to understand the results of crossing-over and the principle of establishing genetic maps. Principle The mold mycelium contains n chromosomes: from the formation of the egg with 2n chromosomes, the two divisions of meiosis occur successively followed by mitosis; this explains the presence of 8 spores with n chromosomes per ascus. The hybrid studied is the result of crossing a species with dark spores with a species with light spores; the colors of the spores are associated by 2 or multiples of 2 (last mitosis), but their distribution is random. The statistical study of associations leads to 2 conclusions: 1 - the existence of 6 types of storage is linked to crossing-over, 2 - the frequency of each type is a function of the gene-centromere distance (principle of establishing the genetic map).

Bouquets of asci open between blade and lamella. Resin mounting.