Plastids, Mitochondria, Intracellular Symbionts, and Plasmids

Abstract

A plasmagene (Darlington, 1939b) can be defined as an extranuclear hereditary determinant that shows non-Mendelian inheritance. Goldschmidt (1945) proposed that the term should be used only in such instances where a self-replicating unit in the cytoplasm produces definite genetic effects similar to those produced by genes in the chromosomes. The sum total of all plasmagenes constitutes the plasmon or the plasmotype (Imai, 1936). The plasmotype and the genotype are referred to as the idiotype (Siemens, 1921) or the entire genetic system of the cell. At the time the concept of the plasmon was established, the details of extra- chromosomal inheritance were not too well known. Now it seems to be clear that most of the cases of cytoplasmic inheritance could be included in one of the following three groups:

• 1. plastids and mitochondria

• 2. intracellular symbionts

• 3. plasmids