Plasmodial incompatibility in the myxomycetes: a review

Abstract

Two myxomycete phaneroplasmodia of the same species undergo somatic fusion only if they are phenotypically identical for a complex genetic incompatibility system. This system consists of a three tiered polygenic complex with dominant and recessive alleles. Thus, plasmodia must be phenotypically identical for approximately 16 loci in order to fuse (CC and Cc are phenotypically identical, but different from cc). The first level of the system (having a minimum of seven Fus loci) controls membrane fusion, and it apparently prevents fusion unless the two plasmodia have identical membrane or slime sheath components. The second level (having a minimum of six Cz loci) produces a rapid lysis of a small mixed region, of the two plasmodia, if membrane fusion has occurred. This lysis is directional in that it targets the recessive phenotype, and it is apparently triggered by some pre-formed substances when they come into contact with a different plasmodium. The third level (having a minimum of three Let loci) comes into play if membrane fusion occurs and there is no rapid lysis of the mixed plasmodium. It produces a slow lethal reaction, which targets and degrades the nuclei of the recessive phenotype. This reaction occurs over a period of five to twenty hours and requires the synthesis of new RNA and proteins. Since, this complex system produces a minimum of 65,536 different incompatibility phenotypes, it is highly unlikely that any two phaneroplasmodia will undergo a successful fusion unless they are very closely related. Species with aphaneroplasmoida apparently have a similar system, but species with small protoplasmodia do not appear to undergo any type of plasmodial fusion.