Mitochondrial Four-Point Crosses in ASPERGILLUS NIDULANS : Mapping of a Suppressor of a Mitochondrially Inherited Cold-Sensitive Mutation.

Four-point mitochondrial crosses were conducted in heterokaryons of Aspergillus nidulans. The mutations used were (oliA1), conferring resistance to oligomycin, (camA112), conferring resistance to chloramphenicol; (cs-67), conferring cold-sensitivity, and ( sumD16), a suppressor of (cs-67). Initially, the crosses were conducted by observing the segregation of extranuclear markers in heterokaryotic sectors emerging from the original point of heterokaryosis. This showed that (camA112), (cs-67) and (sumD16) were linked but were probably all unlinked to (oliA1). Second, four-point crosses were conducted using a double marker selection technique, in which (camA112 ) and (oliA1) were always set in repulsion and the frequency of the phenotypes produced by the segregation of the mutant and wild-type alleles of (cs-67) and (sumD) were observed in (camA112 oliA1) recombinants. From these results we concluded that (camA112 ), (cs-67) and (sumD16) were linked and probably mapped in the order given. It was observed that the two nuclear types of conidia from a heterokaryon often had a dissimilar frequency distribution of the segregants of a mitochondrial cross.