Complementation and recombination between Methionine-2 alleles in Neurospora crassa

Abstract

AMONGST the many new methionine mutant strains of Jt[eurospora crassa which have been isolated (Murray, 1960) the sulphur requirement of forty-two is satisfied by homocysteine or methionine, but not by cystathionine. Heterocaryon tests show that the mutant genes in these strains are physiologically allelic with H98 and 48004 (me-2) and that some pairs of them exhibit the phenomenon of interallelic complementation. Methionine-2 is located in• the right arm of linkage group IV, in a region well supplied with genetic markers, and as interallelic crosses proved to be relatively fertile, it appeared that this gene would be suitable for the investigation of the relationship between genetic and complementation maps. The data from ordered tetrads (Murray, 1960) indicated that me-2 is 87 units distal to typ-4 and 37 units proximal to pan-I. Therefore, trp-4 and pan-I were chosen as suitable markers for the analysis of recombination between me-2 alleles. The discovery of recombination between the two physiologically allelic mutants "Star" and "asteroid" in Drosophila (Lewis, 1945), and more recently between alleles at loci in Aspergillus (Pritchard, 1955), JVeurospora (Giles, 1951, 1956; Mitchell, 1955a, I.955b) and yeasts (Roman, 1956; Leupold, 1957), suggests that the gene may be interpreted as comprising a number of linearly arranged mutable sites which are separable by crossing-over. Pritchard (1955) was able to arrange a number of adenine alleles of Aspergillus in linear order by an analysis controlled by markers on each side of the locus. In general, recombination between the adenine alleles was accompanied by recombination of the associated markers. Prototroph formation was therefore explicable as the result of a single cross-over between the defective sites of the locus. Such a cross-over would produce one normal and one doubly mutant chromatid strand, and Pritchard was able to recover the doubly mutant recombinant. Other adenine prototrophs, of parental phenotype with respect to the marker genes, could not be interpreted as the products of single intragenic cross-overs, but were, perhaps, comparable in origin with the prototrophs from crosses of allelic pyridoxine mutants of .J'Teurospora (Mitchell, 1955a, i955b). By studies of ordered tetrads Mitchell showed that * Work done while holder of a postgraduate grant from the

Interallelic recombination at the his-5 locus in Neurospora crassa

“…Three criteria (Catcheside, Jessop and Smith, 1964) based on the distribution of the markers amongst the prototrophs of each cross when applied to the data systematically indicate, with remarkable consistency, an order for the sites of difference that agrees with their order in the genetic map based on the frequency of prototroph formation. These same criteria when applied to the me-2 (Murray, 1960(Murray, , 1963, pan-2 (Case and Giles, 1958) and his-s data (Jessop and Catcheside, 1965) all indicate with reasonable consistency, orders for the sites of difference that are the same as those based on the frequency of prototroph formation. In other data the asymmetries of marker distribution are not always so S sharp but are usually shown to some extent by one of the flanking markers.…”

“…In the his-5 gene the non-complementing mutants appear to be scattered throughout the length of the genetic map. This is also the case in pan-2 (Case and Giles, 1958) and me-2 (Murray, 1960(Murray, , 1963 and seems to be the more usual arrangement, although in his-i (Jcssop and Catcheside, unpub.) the non-complementing mutants arc all concentrated at the proximal end of the map.…”

“…The ascospores were screened for prototrophs by a modification of the layer plating technique (Newmeyer, 1954;Murray, 1960). Germination of the ascospores suspended in o'i per cent.…”

Meiotic Recombination: The Essence of Heredity

“…These elegant studies culminated in four key concepts that formed the foundation of molecular models of recombination: gene conversion, an exception to Mendel's principle of segregation, signaled a local nonreciprocal transfer of genetic information (Winkler 1930;Lindergren 1953;Mitchell 1955); postmeiotic segregation (PMS) indicated the presence of heteroduplex DNA (Olive 1959;Kitani et al 1962) (Lissouba and Rizet 1960;Murray 1960); and the strong correlation between gene conversion/ PMS events and crossing-over led to the proposal that these processes were mechanistically linked (Kitani et al 1962;Perkins 1962;Whitehouse 1963).…”

“…gene conversion lead to the idea that recombination initiated from pseudofixed sites (Lissouba and Rizet 1960;Murray 1960); and the strong correlation between gene conversion/ PMS events and crossing-over led to the proposal that these processes were mechanistically linked (Kitani et al 1962;Perkins 1962;Whitehouse 1963).…”