Diverse interactions mediate asymmetric incompatibility by the het-6 supergene complex in Neurospora crassa

Abstract

Heterokaryon incompatibility (HI) in filamentous fungi is a form of nonself recognition that operates during the vegetative phase of the life cycle. One HI gene complex in Neurospora crassa, the het-6 locus, comprises two incompatibility genes, het-6 and un-24, each having two allelic variants, Oak Ridge (OR) and Panama (PA). The un-24 gene also encodes the large subunit of ribonucleotide reductase while het-6 appears to be a member of a repetitive gene family with no other known function aside from HI. These two genes are in severe linkage disequilibrium such that only un-24ORhet-6OR and un-24PAhet-6PA haplotypes occur in nature. In this study we unravel several genetic interactions that govern the HI functions of this gene complex. We use novel un-24PAhet-6OR strains and het-6 deletion strains to demonstrate that nonallelic interactions occur between un-24 and het-6 and reveal an allelic incompatibility interaction between the OR and PA forms of un-24 that is asymmetrically enhanced by the presence of het-6OR or het-6PA. We also show how two allelic forms of vib-1, a suppressor of het-c- and mat-associated incompatibility, differentially act as recessive suppressors of HI associated with nonallelic interactions between un-24PA and het-6OR. In contrast, vib-1 is a dominant suppressor of HI associated with allelic differences at un-24 and a dominant partial suppressor of the un-24OR and het-6PA nonallelic interaction. The range of suppressor activities is largely explained by an interesting differential effect on het-6OR and het-6PA transcript levels by VIB-1.