Mitochondria, hydrogenosomes and mitosomes: products of evolutionary tinkering!

One hundred years ago, C. Mereschkowsky, “Privatdozent an der Kaiserlichen Universitat in Kasan (Russia)” published a notoriously ignored landmark paper: “Uber Natur und Ursprung der Chromatophoren im PXanzenreiche.” (“On the nature and origin of the chromatophores in the plant kingdom”; Mereschkowsky 1905). In spite of the fact that this paper was written in German (the lingua franca in biology at the time), its fate was similar to Mendel’s publication, which signiWed, in retrospect, the birth of genetics (Mendel 1865). Both before and after the publication of Mereschkowsky’s article there were many publications dealing with plant “chimera’s” and cytoplasmic inheritance in plants, which should have favoured the interpretation of plastids as “semi-autonomous” symbiotic entities in the cytoplasm of the eukaryotic plant cell (e.g. Braun 1873; Hildebrand 1908; Baur 1909; Renner 1922, 1924, 1934, 1936a, b; Darlington 1929; Stubbe 1959; Tilney-Basset 1963). In addition, millions of people had variegated Pelargonium and other green-and-white spotted plants in their homes, or variegated plants such as Euonymus, Hedera or Ilex aquifolius in their gardens, which could have provided comprehensive evidence for plastid inheritance to the naked eye. Therefore, it is one of the mysteries of the 20th century that an endosymbiotic origin of plastids had not been generally accepted before the 1970s and 1980s, especially after the courageous paper of Lynn Margulis (Sagan 1967) and the unequivocal demonstration of DNA in plastids (Gibor and Izawa 1963). Twenty years after Mereschkowsky’s plea for an endosymbiotic origin of plastids, Wallin (1925, 1927) postulated the “bacterial nature of mitochondria”. The reasons for this postulate were less obvious, since—in contrast to chloroplast mutations—you cannot experience the consequences of mutations in the mitochondrial genome by naked eye—you need at least a good microscope and basic experience in cytochemical staining techniques (reviewed by Ernster and Schatz 1981). Moreover, Wallin’s claim of having cultivated mitochondria in vitro turned out (of course) to be wrong, just as Portier’s fancy speculation that food-associated bacteria could fuse with mitochondria in order to rejuvenate the latter (Portier 1918). Most importantly, however, the genetic evidence for the presence of hereditary factors in mitochondria was diYcult to interpret. The reasons for these peculiarities of mitochondrial genetics are well understood today, but still notoriously ignored in many textbooks. First of all, mitochondrial DNA is usually present in multiple copies in one and the same mitochondrion and, notably, the hundreds to thousands of mitochondria in a single Communicated by R. Bock