Plasmids in natural populations ofNeurospora

Abstract

Eukaryotic plasmids are mainly fungal. Global Neurospora populations have been surveyed and show that approximately half the isolates have one or more plasmids. There are distinct homology groups of plasmids and most groups are distributed across species. One example is also in a related genus, Gelasinospora. Most plasmids are apparently benign but several examples are virulent, killing by insertion into mtDNA. In Hawaii, one of the virulent plasmids, kalilo, might be increasing in frequency. Plasmids can be transmitted horizontally between laboratory strains by forced heterokaryosis or by transient fusion. Simulations of population transmission have shown that the linear kalilo and circular Hanalei-2 plasmids are aggressively transmitted from one compatible prototroph to another. This transmission is slowed but not necessarily eliminated by het gene incompatibility. Paternal transmission of plasmids has been demonstrated and this seems to be affected by the incompatibility genotypes of the parents. However, paternal transmission might not be through the normal trichogyne route, but by a "back-door" route possibly through maternal mycelium. There are several documented ways in which plasmids can change their structure. One novel way is by recombination between two different heterologous plasmids. These processes might be relevant to plasmid evolution. Two suppression processes have been discovered that can be used by the host to eliminate specific plasmids: a cell autonomous type, and a nonautonomous type that acts during the sexual cycle. In conclusion, plasmids are diverse and widely distributed in fungi, undoubtedly reflecting their multiple modes of transmission and structural modification. Key words: Neurospora, plasmids, mitochondria, populations, kalilo.