The present article is an essay rather than a review. The discovery in 1952 by Pontecorvo & Roper (1) of parasexual recombination in Aspergillus nidulans, i.e., recombination of hereditary determinants outside the sexual cycle, has been followed by research on two questions. First, which processes underlie what we now know to be a cyclical series of events, the parasexual cycle? Second, how widespread is the parasexual cycle among filamentous fungi, and what part does it play in their genetic systems, particularly in the case of species in which the sexual cycle is not known to occur? There is a corollary to the second question, relevant to the applied fields of phytopath ology and industrial fermentations: what is the bearing of the parasexual cycle on variation in nature and in the factory? In nature because it might, and indeed Buxton's work (2) suggests that it does play a part in the varia tion of host-pathogen relationships; and in the factory because it might, and the work of Caglioti & Sermonti (3) shows that it can be harnessed to the breeding of more productive strains. With the exception of work on heterokaryosis, which we now know to be one of the steps in the cycle, practically all the published work on the parasexual cycle has been done either in our Laboratory or by workers pre viously associated with it, at the Istituto Superiore di Sanita, Rome, and at the Plant Pathology Department, Rothamsted Experimental Station. Research on the first question has led to an understanding of the sequence of steps in the cycle, and to a working knowledge of some of these steps (4, 5). This working knowledge suggests new ways of investigation of the phenom enon observed by Buller (6), Quintanilha (7), Papazian (8), and Raper (9) in the Basidiomycetes, i.e., the formation of nuclei of new types in dikaryons. Furthermore, this knowledge is already sound enough for use in the mapping of chromosomes. In A. nidulans, for instance (5), certain operations for map ping are more conclusive and less laborious if the analysis is through the para sexual cycle than if it is carried out in the usual ways through the sexual cycle. Research on the second question has ascertained that the parasexual cycle occurs in the asexual species Aspergillus niger (10, 11), Penicillium chryso genum (3, 12) and Fusarium oxysporum (2), besides the homothallic A. nidulans in which it was first found. Recombination also occurs in Strepto myces coelicolor (13), but here it is not yet known whether or not the underly ing process is the same as in the four species of fungi mentioned above. In short, a search for the parasexual cycle in four species of filamentous fungi
Read full abstract
Jan 1, 1958
P H Leslie + 1
Open Access