Linkage group establishment in Neurospora tetrasperma by interspecific hybridization with N. crassa.

HE purpose of this paper is to provide evidence for the establishment of one Tor more genetic markers on seven linkage groups in Neurospora tetrasperma so as to enhance the usefulness of this species in genetic research. The linkage groups were determined by hybridization with the genetically well known species, N . crmsa. Markers were first transferred from one Neurospora species to another by DODGE (1931) and FINCHAM (1951), using N . crassa and N . sitophila. Previous genetic studies with N . tetrasperma have been conducted with little knowledge of linkage relationships involved, and relatively few genetic markers have been reported. These markers include mating type (DODGE 195%; LINDEGREN 1932); I (DODGE 1934); o and m (DODGE 1935; TAI 1936); p and w (MALMCH 1942); d and U (DODGE, SCHMITT and APPEL 1945); E (DODGE, SINGLETON and ROLNICK 1950) ; col 1 , col 2 and mus (BRAVER 1952); ten biochemical and morphological mutants ( HOWE 1963,1963a) and 12 other biochemical and morphological loci (JOHNSON 1963). Linkage studies with these markers have been limited mostly to demonstrating whether linkage to the mating type locus was apparent. The haploid chromosome number is 7 (DODGE, SINGLETON and ROLNICK 1950). Gene-centromere map intervals can be determined by using the percentage of asci showing homokaryotic ascospores, as pointed out by SANSOME (1946) and CATCHESIDE (1951), who used the data of DODGE, SCHMITT and APPEL (1945). This method was subsequently employed by BRAVER (1952), HOWE (1963) and JOHNSON ( 1963). Determination of gene-gene map intervals is complicated by the normally dicaryotic condition of the ascospores. This difficulty was circumvented by DODGE, SINGLETON and ROLNICK (1950) and by BRAVER (1952), who included E in crosses. Crosses heterozygous for E produced asci containing eight uninucleate ascospores, but the ascospores containing the E allele survived poorly. Better survival of uninucleate ascospores was obtained with the rather infrequently occurring dwarf spore types that all strains produce (HOWE 1964). JOHNSON and SRB (1962) increased the frequency of production of uninucleate ascospores by use of the N . crassa mutant peak-2 (an allele of biscuit) transferred into N . tetrasperma. JOHNSON (1963) also used the transferred pk-2 marker to assign a lysine mutant to linkage group V.