II.5.2 Genes in the genomes of cell organelles of endosymbiotic origin are mostly responsible for cytoplasmic heredity.
A eukaryotic cell contains some types of organelles that were formed in evolution through the transformation of endosymbiotic organisms. Some of these organelles, for example the hydrogenosomes of certain groups of protozoa, have already lost their own DNA or, to be more exact, have lost part of their genes and transferred part of them to the nuclear genome of their host cells. Other organelles, specifically mitochondria and chloroplasts, have retained part of their genome in the form of circular or linear DNA molecules. It is not very clear why some genes remained in the organelle DNA and others did not. According to some theories, primarily only the genes whose expression is regulated by the oxidation-reduction potential of their environment remain directly in the organelle (Allen & Raven 1996). Genes contained in the organelle DNA are responsible for cytoplasmatic heredity, i.e. genetic information that can be transferred experimentally from cell to cell by transplanting the cytoplasma and that is transferred from one generation to the next mostly along the maternal line under natural conditions. It is characteristic for cytoplasmatic heredity that Mendel’s laws do not apply to its transfer.
As, in most organisms, the number of genes in the organelle DNA is several orders of magnitude lower than the number of genes in the nuclear DNA, mutation of this DNA is frequently manifested very strongly in the phenotype of the individual. In humans and other organisms, including plants, a large number of various diseases are known whose occurrence is the result of mutation of genes in the organelle DNA. This phenomenon is caused by several special features that occur in transfer of organelle DNA from one generation to the next.
Genes contained in the organelle DNA are transferred to the next generations in a fundamentally different way than the genes of nuclear DNA. To begin with, they do not exist in the cell in only one or two copies, as is usually true for the genes of nuclear DNA, but frequently in many thousands of copies. As a consequence, a great many variants of an organelle genome can exist and regularly be formed in a cell. In addition, the individual DNA variants can compete together for the most effective transfer to the next generation. For example, a DNA molecule that is capable of replicating more rapidly is transferred to further generations in a greater number of copies regardless of whether it positively or negatively affects the viability of the cell. A second interesting characteristic of organelle DNA is that genetic recombination apparently occurs in it. This leads, for example, to the fact that all its genes (cistrons) are selected at once as a single evolutionary unit, one gene. Selection at the level of the individual genes, especially selection eliminating weakly detrimental mutations, is thus substantially limited. Another special feature is that organelle DNA mostly has only uniparental inheritance. In most known cases, it is inherited only along the maternal line; sometimes, however, inheritance is biparental and, for example, in a great many gymnospermous plants, chloroplast DNA is, to the contrary, transferred long the paternal line (Tilneybassett 1994). Of male sex cells, for example, mitochondria do not even enter the zygote or only get there after they are labelled on their surface with a certain protein whose presence signals that they are to be destroyed in the lysosomes. Consequently, from the standpoint of organelle DNA, males represent a dead end from which it is not possible do get to the next generation by normal pathways. As a consequence, their genes frequently contain alleles that “attempt” to change a male to a female, at the expense of the viability of the organism, or “attempt” to change the ratio of resources that a hermaphrodite individual invests in the development of both types of sex organs in favour of female organs. For example, a great many organelle genes for pollen sterility are known in plants (Budar & Pelletier 2001).