Cytology as an Important Tool for Solving Evolutionary Problems in Angiosperms

Abstract

The usefulness and reliability of cytological methods for studying evolutionary problems has been recognized for a long time. Cytological characters used for determining similarities/differences between various taxa include chromosome number, their form and behaviour. While information on chromosome number pinpoints the role of numerical variations, karyotypic studies help in understanding the role of structural changes in the evolutionary process. Important karyotypic aspects used to evaluate relationships between different taxa include absolute/relative size of chromosomes, position of primary constrictions and number of chromosomes with nucleolus-organizing regions. While chromosomes of different size/form can be studied by simple squashing and solid staining techniques, for establishing relationship between taxa having uniform karyotypes, chromosome banding and fluorescence in situ hybridization techniques are employed. Putative grouping of chromosomes and their behaviour during reduction division has been used in many cases for determining base number, ploidy level, type of ploidy and also for detecting structural chromosomal changes that a taxon might have undergone during evolution. Information on chromosome pairing in allopolyploid crop plants also assist in elucidating the evolutionary phenomenon involved in their production. Despite the advantages conferred by chromosomal information, it has rarely been used as an independent indicator of relationships between different groups. It is because the phenotypic aspects of the chromosomes cannot be more informative than the expression of genes as seen in the morphology of a plant. For determining with great certainty the relationship between various taxa, chromosomes need to be viewed in conjunction with other aspects of an organism as breeding system, habit, morphology, distribution and biochemistry.