The transmission of cytoplasmic genes in Aspergillus nidulans

IntroductionThis manuscript concerns the spread of selfish cytoplasmic genes in the fungus Aspergillus nidulans. A.nidulans is a common soil fungus that grows vegetatively by forming a network (mycelium) of hyphae and reproduces via sexual ascospores and asexual conidiospores. Cytoplasmic genes are genes that are located in the cell cytoplasm and not in the cell nucleus where most genes are situated. The cytoplasmic genes investigated in this research are the genomes of mitochondria and viruses. Selfish genes are genes that are maintained in a population despite a negative effect on the fitness of their host.A possibility for the spread of selfish genes is created when they can compensate their negative effect on host fitness with an enhanced transmission rate. Because cytoplasmic genes can be transmitted independently from nuclear genes they can enhance their transmission rate in ways that nuclear genes cannot. Therefore there is evolutionary selection on mechanisms that regulate the transmission of cytoplasmic genes, thus preventing the spread of selfish cytoplasmic genes. Heterokaryon incompatibility may be such a mechanism. I have investigated the genetics of heterokaryon incompatibility and the transmission of mitochondria and viruses in A. nidulans. The implications of the results for the spread of selfish cytoplasmic genes are discussed.Genetics of heterokaryon incompatibilityContact between two fungal mycelia can result in their fusion and the formation of a heterokaryon (a mycelium containing nuclei from both strains). Such mycelial fusion creates possibilities for the horizontal transmission of cytoplasmic genes (transmission between individuals of the same generation). In most fungal species heterokaryon formation is restricted by a heterokaryon incompatibility mechanism. In A.nidulans heterokaryon incompatibility is regulated by heterokaryon incompatibility genes (het genes). Strains with different alleles for one or more het genes cannot form a heterokaryon, they belong to different heterokaryon compatibility groups (hcg's).Strains with the same alleles for all het genes can form a heterokaryon, they belong to the same hcg.A sample of 24 isolates collected in England and Wales in 1992 was found to contain 20 hcg's. Only 2 of these hcg's were present in the 20 previously described hcg's (Chapter 6). Due to this large amount of variation most pairwise strain combinations will be heterokaryon incompatible.Genetic analysis revealed the existence of partial heterokaryons, heterokaryons that grow less vigorously than 'normal' heterokaryons (Chapter 2). At first it was thought that partial heterokaryon incompatibility was caused by partial-het genes. Later it was discovered that partial heterokaryon incompatibility between parent and progeny could be induced by sexual reproduction (Chapter 3).I attempted to isolate strains in which heterokaryon incompatibility as a result of an allelic difference for het gene A or het gene B was suppressed (Chapter 3). 1 was successful for het gene B but not for het gene A. This may be because het gene A is an essential gene. All suppressor mutations were intragenic. Suppression of het gene B results in heterokaryon compatibility with both alleles of het gene B. Strains that had switched from compatibility with allele B' of het gene B to compatibility with allele B of het gene B were also isolated. This indicates that the two alleles are highly similar.Mitochondrial transmissionBiparental inheritance creates possibilities for the spread of selfish mitochondrial genes. If mitochondria are inherited biparentally the progeny contain the mitochondria from both parents. Selfish mitochondria can enhance their transmission rate by winning the ensuing competition between the maternal and the paternal mitochondria. In A. nidulans mitochondrial inheritance is strictly uniparental (Chapter 5). All of the +-* 10000 ascospores in a fruitbody are inherited from the maternal parent. An investigation of more than a hundred fruitbodies did not reveal the presence of a single paternal mitochondrion. Selfish mitochondria cannot spread through A.nidulans populations by biparental inheritance.The horizontal transmission of mitochondria is greatly restricted. Horizontal transmission within a kg was observed in a very low frequency and horizontal transmission between hcg's was never observed (Chapter 6). Selfish mitochondria cannot spread through A. nidulans populations by horizontal transmission.The combination of uniparental inheritance and horizontal transmission within a kg creates a possibility for the spread of selfish mitochondria. The recombination of het genes during sexual reproduction results in the presence of the maternal mitochondria in all the hcg's included in the progeny. Consequently a selfish mitochondrion can spread by transmission within a hcg. However due to the low transmission rate within a hcg and the presumed rarity of sexual reproduction under natural conditions the spread of selfish mitochondria will be severely restricted. It is doubtful whether such a small enhancement of the mitochondrial transmission rate will compensate for the negative effects of selfish mitochondria, on host fitness.Virus TransmissionThere are no reports of viruses being found in sexual aspergilli despite their ubiquity in asexual aspergilli. For this research we transferred viruses from the asexual A..niger to the sexual A..nidulans by protoplast fusion (Chapter 4). Virus infection was not observed to have any effect on the fitness of Infected fungi. This shows that the absence of viruses from A..nidulans isolates is not the result of resistance to virus infection.The horizontal transmission of viruses within a hcg is highly efficient. The transmission between hcg's is restricted but not prevented by heterokaryon incompatibility. Heterokaryon incompatibility in itself will not prevent the spread of viruses through A. nidulans populations.Viruses are excluded from ascospores (Chapter 4). This will not prevent the spread of viruses through A. nidulans populations because viruses are included in the conidiospores. However the exclusion of viruses from ascospores and the recombination of het genes during sexual reproduction does allow the formation of new virus-free hcg's. If virus-free hcg's are created faster than they are infected by horizontal transmission this will result in the exclusion of viruses from A..nidulans populations. This could explain the exclusion of viruses from populations of sexual aspergilli.