Somatic mutations hypothesis
- The hypothesis of somatic mutations (Gorshkov & Makarieva 1999) is based on the suggestion that one of the important functions of diploidy could consist in protection against somatic mutations. Somatic mutations occur during the ontogenesis of every multicellular organism. In large organisms, the number of cell divisions between the zygote and the adult organism is rather large. Consequently, during ontogenesis, a great many loss and thus frequently recessive lethal mutations accumulate in their cells. Their presence does not matter in diploidal organisms, as they do not enter the germinal line and are generally not manifested in the somatic cells, because there is only a small probability that similar mutations would occur in both copies of a single gene. However, sex chromosomes are present in the cells of members of the heterogametic sex in the haploid state which is quite fundamental for the viability of the organism in the case of the X-chromosome, which generally contains a large number of genes. While, in homogametic females, the X-chromosomes act as autosomes, so that loss somatic mutations of their genes are recessive and are not externally manifested, in members of the heterogametic sex with a single copy of the X-chromosome, the presence of these mutations is manifested in reduced functioning of the cells and tissues and thus reduced viability of the organism. Similar to the recessive gene hypothesis, the hypothesis of somatic mutations also explains the strong manifestations of this effect in interspecific hybrids by an exponential dependence between the number of mutations and the decrease in the fitness of the individual. In contrast to the recessive gene hypothesis, the reduced average frequency of recessive negative mutations on X-chromosomes, which is necessary consequence of the greater effectiveness of selection acting on the representatives of the heterogametic sex, in which the harmful effect of mutations is not masked by the presence of a functional copy on the second X-chromosome, does not represent a complication here.. The frequency of these mutations in the gene pool is totally irrelevant; the reduced viability of hybrids is a result of somatic mutations occurring during the life of the individual.
The hypothesis of somatic mutations also explains the results of experiments with hybrid drosophila with an “unbalanced” genome (see XXI.4.3.1). It is apparent that, in males, which bear two X-chromosomes from the same species, the effect of recessive somatic mutations on the X-chromosome must be less than in hybrid males with a single X-chromosome, i.e. just that demonstrated by experiments with drosophila, i.e. contrary to expectations following from the dominance hypothesis.
The hypothesis of somatic mutations also offers a simple explanation of why the Haldane rule for viability is basically not valid for mammals. In this taxon, compensation of the genetic dose, i.e. inactivation of one of the X-chromosomes, occurs in the somatic cells. Because of this inactivation, each somatic cell of a female, similar to males, contains only one active copy of the X-chromosome, and the viability of hybrid females is thus reduced here similarly to the viability of hybrid males. However, this effect can analogously also be explained by the dominance hypothesis.
The hypothesis of somatic mutations also explains other phenomena encountered in nature and not directly related to the Haldane rule. It explains why all large fauna are diploid, while small fauna, whose somatic cells undergo only a small number of divisions during ontogenesis and accumulate only a small number of mutations, are sometimes haploid. It also offers an explanation of why male haplodiploid insects are much more sensitive to irradiation in the early stages of ontogenesis, when their cells are haploid, where the differences between the sexes disappear in adulthood, when diploidization or tetraploidization occurs in most of the cells in the somatic tissues of males. This explains why large species have small X-chromosomes, while small species, for example drosophila, can have X-chromosomes that bear up to 40% of all the genes. This also provides an explanation for the fact that, in mammals (with heterogametic males), the males mostly exhibit higher mortality and a shorter average lifetime, while the opposite is true of birds (with heterogametic females) (Promislow 2003).
The hypothesis of somatic mutations, in itself, cannot explain the existence of all the phenomena described by the Haldane rule. The sterility of the members of the heterogametic sex is a result, to a major degree, of defects in the germinal cells themselves, where the number of cell divisions that occur in the germinal line is low for large organisms. It is thus apparent that a number of mechanisms are responsible for the phenomena described by the Haldane rule; however, they most probably also include the accumulation of somatic mutations in the sex chromosomes.
- more or less2443
- not at all1393