Uniparental mitochondrial inheritance in Cryptococcus is controlled by the pheromone and pheromone receptor mating-type locus

Abstract

Mitochondria are inherited uniparentally during sexual reproduction in the majority of eukaryotic species studied, including humans, mice, nematodes, as well as many fungal species. Mitochondrial uniparental inheritance (mito-UPI) could be beneficial in that it avoids possible genetic conflicts between organelles with different genetic backgrounds, as recently shown in mice; and it could prevent the spread of selfish genetic elements in the mitochondrial genome. Despite the prevalence of observed mito-UPI, the underlying mechanisms and the genes involved in controlling this non-mendelian inheritance are poorly understood in many species. In Cryptococcus neoformans, a human pathogenic basidiomyceteous fungus, mating types (MAT and MATa) are defined by alternate alleles at the single MAT locus that evolved from fusion of the two MAT loci (P/R encoding pheromones and pheromone receptors, HD encoding homeodomain transcription factors) that are the ancestral state in the basidiomycota. Mitochondria are inherited uniparentally from the MATa parent in C. neoformans and this requires the SXI1 and SXI2a HD factors encoded by MAT. However, there is evidence additional genes contribute to control of mito-UPI in Cryptococcus. Here we show that in Cryptococcus amylolentus, a sibling species of C. neoformans with unlinked P/R and HD MAT loci, mitochondrial uniparental inheritance is controlled by the P/R locus, and is independent of the HD locus. Consistently, by replacing the MAT alleles of the pheromones (MF) and pheromone receptor (STE3) with the MATa alleles, we show that these P/R locus defining genes indeed affect mito-UPI in C. neoformans during sexual reproduction. Additionally, we show that during early stages of C. neoformans sexual reproduction, conjugation tubes are always produced by the MAT cells, resulting in unidirectional migration of the MAT nucleus into the MATa cell during zygote formation. This process is controlled by the P/R locus and could serve to physically restrict movement of MAT mitochondria in the zygotes, and thereby contribute to mito-UPI. We propose a model in which both physical and genetic mechanisms function in concert to prevent the coexistence of mitochondria from the two parents in the zygote and subsequently in the meiotic progeny, thus ensuring mito-UPI in pathogenic Cryptococcus, as well as in closely related non-pathogenic species. The implications of these findings are discussed in the context of the evolution of mito-UPI in fungi and other more diverse eukaryotes.