Meiotic and Mitotic Recombination in Aspergillus and Its Chromosomal Aberrations

Abstract

This chapter analyzes the effects of one, or two, overlapping, reciprocal translocations on meiotic crossing-over and nondisjunction and to identify the processes of mitotic recombination in diploids with and without translocations, in triploids, and also in disomics from single and double translocation crosses, as well as the effects of inducing agents on these. It also assesses the various methods of genetic mapping, the uses of translocations for mapping, and the various problems arising from chromosomal aberrations for mapping by the parasexual cycle. The effects of chromosomal aberrations on meiotic recombination are of two general types. Based on an extensive analysis of effects by recombination-reducing aberrations on the disjunction of other chromosomes, two phases of pairing have been postulated for the meiosis of Drosophila: one early one, leading to and reinforced by chiasma formation, and a second one, “distributive pairing,” which involves all noncrossover chromosomes and may result in pairing of heterologous types. Several types of mitotic segregation which lead to spotting or variegation have been demonstrated in various organisms. In Aspergillus, two main types occur spontaneously: (1) Mitotic crossing-over, (2) Mitotic nondisjunction. A large variety of methods combining genetic, cytological, and biochemical techniques have been used in various organisms for the mapping of genes to specific chromosome segments. The chapter emphasizes the methods based on mitotic recombination that are new and especially useful in Aspergillus nidulans. In addition, techniques making use of translocations for genetic mapping in conjunction with meiotic and mitotic recombination are considered in detail.