Mutations at the level of the entire chromosome set
Other categories of mutations, genome mutations, affect entire chromosomes or entire chromosome sets. In contrast to the previous types of mutations, these mutations are not formed as a consequence of irregularity in DNA replication or repairing, but as a consequence of irregularities and errors in the progress of cellular division. As a consequence of these disorders, organisms can be formed in which a certain chromosome is multiplied or, on the other hand, is lacking (aneuploidy); in other cases, the entire chromosome series is multiplied (polyploidy). Depending on the number of specimens of a given chromosome occurring in the cell, this can correspond to nulisomics, disomics, trisomics, etc. On the basis of the number of chromosome sets, these are then haploid, triploid, tetraploid, etc. organisms. It was found in a study of human sperm that the frequency of diploid sperm varies around 0.2% and the frequency of haploid sperm with multiplication of one of the four monitored chromosomes varied from 0.1 to 0.17% (Miharu, Best, & Young 1994).
In most animals, the sex of the organism is determined by the gene dose and individuals with aberrant autosome ratios and sex chromosomes have transitory traits between males and females, i.e. are intersexes, and are mostly not capable of reproduction. Analogous disorders can occur for uneven gene doses at various chromosomes (cf. the Down syndrome in human beings, caused by trisomy of chromosome 21). Thus, these mutations are of only limited evolutionary importance for animals. However, the situation is very different, for example, for plants, in which the gene dose does not play a role in determining the sex and in which polyploidy thus generally does not have a detrimental effect on the viability or fertility of the mutant (Muller 1925).
A large proportion of these extensive genome restructurings cause partial or complete sterility in their bearers or at least form very effective interspecies barriers.However, polyploidization is simultaneously a mechanism that enables hybrid speciation.If two species cross, the zygotes cannot normally develop, because the gametes of the two species contain different sets of chromosomes.Thus, during cell division, they cannot form regular pairs of homologous chromosomes, so that the two chromosome sets divide unevenly into the daughter cells.As a consequence, most cells are not viable.However, if polyploidization occurs prior to hybridization, either autopolyploidization as a consequence of mitosis, which is not followed by cellular division, or alopolyploidization as a consequence of fusion, e.g. of two diploid cells derived from two different species in a single tetraploid cell or, more frequently, through a triploid intermediate stage (see XXI.5.3), the situation is substantially more favourable.If, for example, two tetraploid organisms cross together, their gametes already contain pairs of homologous chromosomes, so that a quite regular dividing spindles are formed during cell division and the chromosomes can be divided completely evenly amongst the daughter cells.Examples of hybrid speciation are encountered very frequently in some families of plants and also in animals with parthenogenetic reproduction, for example in some daphnia (Daphnia) (Dufresne & Hebert 1994).
Mutations at the level of the entire chromosomes – Translocationsof major extent can even be manifested in the structure of entire chromosomes.Simultaneously, classical cytogenetic methods can be employed to determine the occurrence of fusion or fission of chromosomes, i.e. processes entailing a change in the number of chromosomes in a chromosome set without a simultaneous change in the amount of DNA in the genome.A change in the number of chromosomes is not usually manifested in the phenotype of the organism, but often acts as a strong interspecies barrier.
Robertson transloctionis an interesting type of chromosome mutation in which one chromosome with a centromere in the vicinity of the centre (metacentric chromosome) is formed from two chromosomes with centromeres in the vicinity of the ends of the arms (metacentric chromosomes) (Fig. III.4).However, the altered karyotype contains one less chromosome, where the number of chromosomal arms does not change.There are a number of species, the best known of which are, e.g., the house mouse, in which the occurrence of local populations with one or more Robertson translocations is very common (Zima et al. 1990).It is very probable that meiotic drive is responsible for spreading of these chromosomal mutations (see VI.3.5) (Zimmering, Sandler, & Nicoletti 1970; Prout, Bundgaard, & Bryant 1973; Thomson & Feldman 1974), i.e. a phenomenon in which meiosis does not occur completely evenly and one of the pair of homologous chromosomes enters the sex cells preferentially.In case of polymorph populations, in which a certain chromosome with a Robertson translocation occurs and simultaneously two original unfused chromosomes are present in other individuals, it can readily happen that, in a heterozygote with two acrocentric and one metacentric chromosome, the metacentric chromosome will enter the sex cells with greater (in other species with lower) probability during meiosis (Gropp, Winking, & Redi 1982; Everett, Searle, & Wallace 1996).In this case, the particular type of chromosomes will spread in the population even under conditions when the heterozygotes will have reduced fertility compared to the original parent form because of the formation of part of the gametes with an incomplete (aneuploidal) chromosomal set – cf. the blue beard model (IV.9.1).