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In N. crassaand N. intermedia, the kalilo plasmid triggers senescence by insertion into mitochondrial DNA. To investigate the cell death pathway induced by this plasmid, juvenile and senescent subcultures of several senescent strains were examined by light and transmission electron microscopy, and at the DNA level. There were no signs of apoptotic events, such as shrinkage of the cytoplasm away from the cell wall, apoptotic bodies, internucleosomal DNA fragmentation or condensation of the cytoplasm while retaining mitochondria and endomembrane structure. Instead, swollen mitochondria lacking cristae and containing amorphous inclusions, and disruption of nuclear and mitochondrial membranes indicated a necrotic mode of cell death.

The widespread occurrence of vegetative incompatibility in fungi and the high level of incompatibility gene polymorphisms in fungal populations means that generally parents in a cross will be vegetatively incompatible. In Neurospora crassa the mating type locus even functions as a vegetative incompatibility locus assuring vegetative incompatibility in a cross. In this paper we have tested the effect of vegetative incompatibility on regulating transmission of mitochondrial plasmids and nuclear genes in sexual interactions of N. crassa. In the absence of mating type vegetative incompatibility between the parents, transmission of plasmids from the conidial paternal parent was found to be approximately ten times higher than in normal crosses. Control experiments in which conidia of a contaminating plasmid bearing strain of similar mating type as (and fully vegetatively compatible with) the established maternal culture were added together with the presumed paternal conidia (opposite mating type) showed plasmid transmission to ascospores. Since trichogynes do not fuse with conidia of the same mating type, it may be concluded that plasmids from the contaminating strain entered the maternal tissue by somatic fusion. The fate of conidial nuclei in such sexual interactions has been investigated as well, both under conditions of vegetative compatibility and incompatibility between the strains. These experiments demonstrated that conidia that are vegetatively compatible to the established protoperithecial strain manage to get access to the resources of the maternal culture and initiate new fruiting bodies. This type of nuclear parasitism was never observed when vegetatively incompatible conidia were used. We propose that vegetative incompatibility may function in sexual crosses to protect unfertilised cultures from looting of maternal resources.