Evolution of hierarchical cytoplasmic inheritance in Plasmodial slime mould, Physarum Polycephalum

A striking linear dominance relationship for uniparental mitochondrial transmission is known between many mating types of plasmodial slime mold Physarum polycephalum. We herein examine how such hierarchical cytoplasmic inheritance evolves in isogamous organisms with many self-incompatible mating types. We assume that a nuclear locus determines the mating type of gametes and that another nuclear locus controls the digestion of mitochondria DNAs of the recipient gamete after fusion. We then examine the coupled genetic dynamics for the evolution of self-incompatible mating types and biased mitochondrial transmission between them. In Physarum, a multiallelic nuclear locus matA controls both the determination of the mating type of the gametes and the selective elimination of the mitochondrial DNA of one gamete type.

We herein theoretically study two potential mechanisms that might be responsible for the preferential digestion of mitochondria in the zygote. In the first model, the preferential digestion of mitochondria is assumed to be the outcome of differential expression levels of a suppressor gene carried by each gamete (Suppression-Power Model).

In the second model (Site-Specific-Nuclease Model), the digestion of mitochondria DNAs is assumed to be due to their cleavage by a site-specific nuclease which cuts the mtDNA at unmethylated recognition sites. Also assumed is that the mitochondria DNAs are methylated at the same recognition site prior to the fusion, thereby being protected against the nuclease of the same gamete, and that the suppressor alleles convey information for the recognition sequences of nuclease and methylase. In both models, we found that a linear dominance hierarchy evolves as a consequence of the build up of a strong linkage disequilibrium between the mating-type locus and the suppressor locus, though it fails to evolve if the recombination rate between the two loci is larger than threshold. This threshold recombination rate depends on the number of mating types and the degree of fitness reduction in the heteroplasmic zygote. If the recombination rate is above the threshold, suppressor alleles are equally distributed in each mating type at evolutionary equilibrium.

Based on the theoretical results of the site-specific nuclease model, we propose that a nested subsequence structure in the recognition sequence should underlie the linear dominance hierarchy of mitochondrial transmission.

Epidemiology and disease-control under gene-for-gene plant-pathogen interaction

An introduction of disease-resistant variety of a crop plant often leads to the development of a virulent race in pathogen species that restores the pathogenicity to the resistant crop. This often makes disease control of crop plants extremely difficult. In this paper, we theoretically explore the optimal ‘multiline’ control, which makes use of several different resistant varieties, that minimizes the expected degree of crop damages caused by epidemic outbreaks of the pathogen. We examine both single-locus and two-locus gene- for-gene (GFG) systems for the compatibility relationship between host genotypes and pathogen genotypes, in which host haplotype has either susceptible or resistant allele in each resistance locus, and the pathogen haplotype has either avirulent or virulent allele in the corresponding virulence locus. We then study the optimal planting strategy of host resistant genotypes based on standard epidemiological dynamics with pathogen spore stages. The most striking result of our single-locus GFG model is that there exists an intermediate optimum mixing ratio for the susceptible and resistant crops that maximizes the final yield, in spite of the fact that the susceptible crop has no use to fight against either avirulent or virulent race of the pathogen. The intermediate mixture is optimum except when the initial pathogen spore population in the season consists exclusively of the virulent race. The optimal proportion of resistant crops is approximately 1/R0 , where R0 is the basic reproductive ratio of pathogen―the rest (the vast majority if R0 is large) of crops should be the susceptible genotype. By mixing susceptible and resistant crops, we can force the pathogen races to compete with each other for their available hosts. This competition between avirulent and virulent races prevents the fatal outbreak of the virulent race (the super-race) that can infect all the host genotypes. In the two-locus GFG control, there again exists the optimal mixing ratio for the fraction of universally susceptible genotype and the total fraction of various resistant genotypes, with the ratio close to 1/R0.