Karyosystematics

karyotaxonomy, a branch of systematics that studies the structure of the cell nucleus in the various groups of organisms (taxa) in order to determine the extent of their phylogenetic kinship and to use the results to construct the natural system of a given group of organisms.

Karyosystematics developed at the point of junction between the sciences of systematics, cytology (of which karyology is a branch), and genetics. The concern of karyosystematics is usually only the structure and evolution of the karyotype; however, all characteristics of the nuclear apparatus (alternation of diploid and haploid phases, comparison of types of nuclei) are used in the classification of a number of groups of protozoans. Not only the number but also the morphology of chromosomes, the amount of DNA in the nucleus and the nucleotide composition of the DNA have taxonomic significance. In karyosystematics the chromosomes are usually studied in the metaphase of mitosis (less commonly, of meiosis, this being important in determining the causes of sterility in interspecific first-generation hybrids). Russian and Soviet scientists, such as S. G. and M. S. Navashin, G. A. Levitskii, L. N. Delone, I. I. Sokolov, B. L. Astaurov, and A. A. Prokof’eva-Bel’govskaia, have made a substantial contribution to the field.

Plant karyosystematics has been developing rapidly since the beginning of the 20th century. E. Strasburger and L. Guignard were the first to determine chromosome numbers in plants (1882). The German cytologist H. Tischler described chromosome sets in 400 plant species (1915). The concern of plant karyosystematics is usually limited to the determination of chromosome numbers because of the exceptional part played by polyploidy in plant evolution. Flowering plants have been studied in the greatest detail: by 1967, the chromosome numbers of more than 35, 000 species (about 15 percent of all species of flowering plants) had been described.

Animal karyosystematics has developed more slowly, but the use of modern research techniques (tissue culture, autoradiography) has resulted in considerable progress in the 1960’s and 1970’s. Precise data have been obtained on the morphology of individual chromosomes, and the heterochromatin and euchromatin portions of externally similar chromosomes have been distinguished. Polyploidy occurs less widely in bisexual animals, but the karyotype is characterized by greater variety than in plants. In animals, less specialized species and genera (evolutionary earlier) have a larger number of chromosomes, with a predominance of single-armed chromosomes in the karyotype; specialized species and genera (evolutionarily later) have fewer chromosomes, and of these, the two-armed type is predominant. A diploid karyotype is considered the original form for plants, and the polyploid karyotype is derivative. The trends of evolution of the karyotype make it possible to assess the probability of karyotype transformation in a given direction and to determine the routes of dispersal of species.

Karyosystematics is of value in detecting twin species. For example, in the black rat (Rattus rattus), it has been found that there are in fact twin species within what was previously considered a single species: a 38-chromosome species from Europe and Southwest Asia, brought by Europeans into America and Australia, and a 42-chromosome species from Southeast Asia. Karyosystematics has shown that all breeds of domestic sheep are descended from the mouflon and that domestic horses do come from the tarpan, although not, as once was thought, from Przhevalski’s horse. The methods of karyosystematics are particularly useful in the case of taxa lying between the species level and the subfamily-family level. However, karyosystematics is of little help in differentiating intraspecific and higher taxa.

Karyosystematics has practical applications in breeding. Study of the karyotypes of the species to be bred should precede attempts at remote hybridization.