UNIPARENTAL INHERITANCE AND REPLACEMENT OF MITOCHONDRIAL-DNA IN NEUROSPORA-TETRASPERMA

This study tested mechanisms proposed for maternal uniparental mitocho ndrial inheritance in Neurospora: (1) exclusion of conidial mitochondr ia by the specialized female reproductive structure, trichogyne, due t o mating locus heterokaryon incompatibility and (2) mitochondrial inpu t bias favoring the larger trichogyne over the smaller conidium. These mechanisms were tested by determining the modes of mitochondrial DNA (mtDNA) inheritance and transmission in the absence of mating locus he terokaryon incompatibility following crosses of uninucleate strains of Neurospora tetrasperma with trichogyne (trichogyne inoculated by coni dia) and without trichogyne (hyphal fusion). Maternal uniparental mito chondrial inheritance was observed in 136 single ascospore progeny fol lowing both mating with and without trichogyne using mtDNA restriction fragment length polymorphisms to distinguish parental types. This sug gests that maternal mitochondrial inheritance following hyphal fusions is due to some mechanism other than those that implicate the trichogy ne. Following hyphal fusion, mututally exclusive nuclear migration per mitted investigation of reciprocal interactions. Regardless of which s train accepted nuclei following seven replicate hyphal fusion matings, acceptor mtDNA was the only type detected in 34 hyphal plug and tip s amples taken from the contact and acceptor zones. No intracellular mtD NA mixtures were detected. Surprisingly, 3 days following hyphal fusio n, acceptor mtDNA replaced donor mtDNA throughout the entire colony. T o our knowledge, this is the first report of complete mitochondrial re placement during mating in a filamentous fungus.