Mitochondrial plasmids are known from many angiosperm species. Some have been molecularly characterized, but their broader biological properties are generally unknown. From observations and circumstantial evidence it appears as if mitochondrial plasmids are not always strictly maternally transmitted but show a pattern of inheritance intermediate between the nuclear and the mitochondrial modes.
We describe the conditions under which a mitochondrial plasmid can spread in a large plant population. The plasmid is assumed to be associated with a, possibly weak, deleterious effect. Attention is paid to selfing and the effect the plasmid may have on the sexual development of its carrier plants. Horizontal transmission is also considered. The rate of transmission via pollen is important in all these cases.
The plasmids seen in plants today must be highly evolved. Using modifier theory we analyse how the key parameters characterizing plasmids are likely to change. The evolutionary process driven by plasmid incompatibility is also studied. Generally, the most likely direction for the long-term evolution of mitochondrial plasmids is towards weak phenotypic effects and a maternal mode of transmission.