Sex Determination Directs Uniparental Mitochondrial Inheritance in Phycomyces

Uniparental inheritance (UPI) of mitochondria is common among eukaryotes. The underlying molecular basis by which the sexes of the parents control this non-Mendelian pattern of inheritance is yet to be fully understood. Two major factors have complicated the understanding of the role of sex-specific genes in the UPI phenomenon: in many cases (i) fusion occurs between cells of unequal size or (ii) mating requires a large region of the genome or chromosome that includes genes unrelated to sex determination. The fungus Phycomyces blakesleeanus is a member of the Mucoromycotina and has a simple mating type locus encoding only one high-mobility group (HMG) domain protein, and mating occurs by fusion of isogamous cells, thus providing a model system without the limitations mentioned above. Analysis of more than 250 progeny from a series of genetic crosses between wild-type strains of Phycomyces revealed a correlation between the individual genes in the mating type locus and UPI of mitochondria. Inheritance is from the plus (+) sex type and is associated with degradation of the mtDNA from the minus (-) parent. These findings suggest that UPI can be directly controlled by genes that determine sex identity, independent of cell size or the complexity of the genetic composition of a sex chromosome.