Botany online 1996-2004. No further update, only historical document of botanical science!
Analyses of karyotypes helped decisively to understand the processes of evolution. Their results answered many questions, but did simultaneously also ring up new questions. The product of chromosomal number multiplied with the size of the chromosome, for example, is not at all linked to an increased performance of the genetic material. In many polyploid species, the chromosomes are far smaller than in the respective diploid species pointing at a loss of genetic material during polyploidization.
The progress of molecular biology led to new methods for the characterization of genomes and allowed to document and sometimes also to understand its changes. The three following methods proved to be especially promising:
Cytophotometric analysis of the DNA-content of cell nuclei,
Chromosome Banding chromosomal dyes helping to visualize the structural subdivisions of chromosomes, the distribution of euchromatin and heterochromatin, and the differences between homologous chromosomes of related species. The identification of translocations and inversions is especially impressive.
Fragmentation and analysis of isolated DNA helps understanding the amount of repetitive DNA and its distribution in the genome. This again answers how large the amount of actually transcribed DNA is and how many active genes exist. The importance of genome enlargement and reduction had during evolution can thus be estimated.
The last years illustrated quite clearly that DNA is no unchangeable molecule, but is constantly under construction. Jumping (mobile) genes or controlling elements, also called transposons, contribute decisively to these changes, and their existence seems to be the cause of translocations, inversions and other chromosomal changes. The frequency of their appearance seems to have an important influence on the velocity of the organisms’ evolution and they decide which taxa are more successful than others.
Mobile genetic elements were first in maize, than also in Drosophila and finally found everywhere. Up till now, not enough results for the clarification of the problems of evolutionary research exist. But the discovery of such elements and their activity alone shows that it is a powerful factor enhancing variations and not the simple mutation known to us from classic genetics (SCHWARZ –SOMMER et al., 1985, KUBITZKI et al., 1991).