|Akiko IWANAGA's web page|
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.