| Possible stages in the evolution of division of labour between nuclear and cytoplasmic (chloroplast and mitochondrial) genetic systems. It is assumed that any gene may be copied between the endosymbiont (to the right of the schematic cell) and the host cell nucleus (to the left). Following endosymbiosis, most endosymbiont genes are subject to free radical mutagenesis, which they evade upon transfer to the nucleus: the endosymbiont copy is lost. In contrast, genes for certain key redox components of bioenergetic organelles remain in situ, since their expression must be subject to redox regulation in order to respond directly to environmentally-induced changes in electron transport. Loss of the symbiont or organellar copy of such genes would uncouple redox control, and hence increase free radical production within the organelle and the cell as a whole. The nuclear copy of each of this subset of genes is therefore redundant, and is lost. Adapted from Allen (1993) and Allen & Raven (1996). |