Chromosomes as Sources of Taxonomic Information for Plant Systematics and Evolution

Chromosomes are the units of inheritance within the nuclei of all eukaryote cells and carry the genes as DNA packaged in the chromosomes forming the chromatin fibril with histone proteins. The DNA content of each chromosome is a single linear DNA double helix, which corresponds genetically to a linkage group. The chromosomes are only seen during cell division because of coiling up as chromatin in the non-dividing nucleus. In somatic cells, chromosomes occur as pairs of homologous chromosomes or homologs. During cell division, the chromosomes, condense into individual chromosomes during the stages of division (prophase, metaphase, anaphase, and telophase). The chromosomes are best studied at metaphase in somatic cells as each chromosome appears as two longitudinal chromatids. Each chromosome is composed of two arms separated by the centromere and terminated at the ends by telomeres. One member of each homologous chromosome is inherited from either the male or female plants, these homologs pair as bivalents in the first meiotic prophase. In about 45% of plants, more than two sets of chromosomes have been reported, these are known as polyploid numbers. The chromosome features that have been used as sources of taxonomic information include various attributes of chromosomes; the most common features are: 1) chromosome number, 2) chromosome size and gross morphology, 3) chromosomal variations in number and shape, 4) chromosome's behavior at meiosis, 5) chromosome banding as revealed by differential staining of some chromosome parts and 6) in situ hybridization and chromosome painting to locate genes on chromosomes. A special role is claimed for chromosomal data in taxonomy since chromosomes are the seats of genetic information that guides plant species evolution. The recent molecular cytogenetics approaches are producing a wealth of new data of enormous taxonomic and evolutionary importance, that must be fully integrated with traditional taxonomic data. In this article, the taxonomic attributes of chromosomes are described and examples of their role of chromosomes in flowering plant systematics and evolution are outlined