WHY ARE ORGANELLES UNIPARENTALLY INHERITED

Uniparental inheritance of organelles is a very common trait but one that has received few attempts at evolutionary explanation. The most commonly adopted theory emphasizes the potentially deleterious behaviour of selfish cytoplasmic replicators and considers uniparental inheritance as a character whereby nuclear genes may avoid the spread of these cytoplasmic replicators. Here, the autonomous evolution of cytoplasmic genes is considered and an alternative evolutionary pathway to uniparental inheritance is described. The process involves specialization of organelles to cell lines, with invasion of the cytoplasm by cytoplasmic replicators specialized to replicate in the female germ-line. As a result, the nature of selection is modified and other cytoplasmic genes (especially selfish replicators) cannot spread: uniparental inheritance appears, then, to be a stable mode of inheritance. The conditions that lead to this evolutionary process are studied; it appears that the constraints caused by the evolution of anisogamy are of great importance and that the distribution of mutations can cause major differences in the evolutionary outcome.