Abstract
By introgressing Neurospora crassa translocations into N. tetrasperma, we constructed heterokaryons bearing haploid nuclei of opposite mating types, and either the translocation and normal sequence chromosomes (i.e., [T + N]) or a duplication and its complementary deficiency (i.e., [Dp + Df]). The [T + N] heterokaryons result from alternate segregation of homologous centromeres, whereas adjacent-1 segregation generates [Dp + Df]. Self-cross of either heterokaryon produces [T + N] and [Dp + Df] progeny. Occasionally during N. tetrasperma ascus development, a pair of smaller homokaryotic ascospores replaces a heterokaryotic ascospore. Crosses with the Eight-spore mutant increase such replacement, and can generate asci with eight homokaryotic ascospores, either 4T + 4N from alternate segregation, or 4Dp + 4Df from adjacent-1 segregation. Crosses of some of the introgressed translocation strains with normal sequence N. tetrasperma produced more Dp than T or N homokaryotic progeny. We suggest this is due to an insufficiency for a presumptive ascospore maturation factor, which increases the chance that, in asci with > 4 viable ascospores, none properly mature. Since only four viable ascospores (Dp or [Dp + Df]) share the limiting factor following adjacent-1 segregation, whereas four to eight ascospores compete for it following alternate segregation, this would explain why Dp homokaryons outnumber T and N types, whereas the heterokaryons are not as affected. We believe that this novel form of transmission ratio distortion is caused by a Bateson-Dobzhansky-Muller Incompatibility (BDMI) triggered by an N. crassa gene in the N. tetrasperma background. Heterokaryons tend not to out-cross, and crosses of Dp strains are barren, thus the BDMI impedes interspecies gene flow.