Each species can be identified by its karyotype. The karyotype is the full set of chromosomes of a species arranged with respect to number, size and shape of the chromosomes. The distribution of heterochromatin may also be taken into account.
Genome mutations result in a multiple set of chromosomes in a cell. Many organisms are either haploid, i.e. they contain a single set of chromosomes (n) or diploid (2n). Organisms, tissues or cells that contain a multiple of n are called polyploid.
There exist two types of polyploidy: auto- and allopolyploidy. Autopolyploidy is the multiple occurrence of the species own simple chromosomal set (n) while allopolyploidy means that the (partial) chromosomal sets of two or more species are present in a cell, tissue or a whole organism. Allopolyploidy may lead to the formation of new species. If multiple sets of the species own chromosomes occur as a result of its tissue differentiation, this is called somatic polyploidy or endopolyploidy. An organism that contains tissues with different states of ploidy is referred to as a cytochimera or a genetic mosaic. Polyploids are characterized by larger cells with higher water contents and smaller osmotic values, decreased growth rates, changes in the organs' proportions towards each other, a prolonged period until flower formation begins and by a prolonged flowering itself. The number of chloroplasts in the guard cells correlates to the degree of ploidy.
A compilation of chromosome numbers as well as further information about single plant species can be found on the server of the Missouri Botanical Garden - w3-tropicos-project:
http://mobot.mobot.org/Pick/Search/pick.html
After input of the species, it is possible to call up different properties, among them also the number of chromosomes.Every species is characterized by a given number of chromosomes that can be recognized by their specific shape. To describe the karyotype of a species, it is necessary to determine both number and shape of the chromosomes. Today the distribution of the heterochromatin is often also taken into account.
The analysis of many species' numbers of chromosomes showed that closely related species do often share a common basic number (x) of chromosomes. Their actual chromosome number is usually a multiple of x. The reason might be polyploidy (see next section)
Translocations and inversions lead to new combinations of the genetic information and thus to an increase in variability. Such chromosomal mutations may generate new species
The correlation of the cytological proof for translocations with the changes in coupling groups refuted the last criticism of the chromosome theory. In Drosophila, genetically detected deletions could be brought into line with the loss of a giant chromosome fragment. The size and the site of this fragment correlated with the respective distances in the gene map.
Several maize chromosomes, like chromosome 9, are characterized by a terminal knob. B. McCLINTOCK was able to show in a crossing experiment with easily detectable markers that the crossing-over and the cytological changes occur in parallel.
A Correlation Between Cytogenetic and Genetic Crossing-Over. The blue dot represents a cytologically visible knob. The hybridization of two individuals of the genotypes and karyotypes depicted in row P results after crossing-over in the F1 types below. The experiments shows that a crossing-over between the gene loci C and wx causes a new combination of both geno- and karyotypes. (H. B. CRAITON and B. McCLINTOCK, 1931)
Genome Mutations - Polyploidy The nuclei of an individual's somal cells do usually contain a diploid set of chromosomes (2n) while germ cells habour a haploid set (1n). Nuclei with a multiple of the simple set of chromosomes are deviations. They are said to be polyploid. Depending on how often a chromosomal set occurs, it is distinguished between triploid (3n), tetraploid (4n), pentaploid (5n), hexaploid, etc..sets.
Polyploidy is common in many plant species. Within an individual, single nuclei or the nuclei of certain tissues can be polyploid while the others are diploid. Such an organism is also refered to as a cytochimera. Cytochimeras can be regarded as genetic mosaics. Regarding the distribution of the different genotypes, it is distinguished between sectorial and periclinal chimeras.
Often, all nuclei of an individual (or a species) are polyploid. This is the case with many known cultured plants and also with 30-50 percent of all the wild growing angiosperms of temperate climates. The next two sections show that polyploidy is subdivided into autopolyploidy and allopolyploidy.
© Peter v. Sengbusch - b-online@botanik.uni-hamburg.de
