Q. Explain Cytological Evidence?

Cytology is the study of the morphology and physiology of cells. The information about the chromosome number, shape and pairing at meiosis is used for classification purpose. Cytotaxonomy refers to the use of chromo, some number and morphology as data for classification. Cytogenetics includes those studies dealing with observations of chromosome pairing or behaviour at meiosis. In angiosperms haploid number of chromosomes ranges from n=2 in Haplopapus gracilis (compositae) to around n= 132 in Poa littoroa (Gramineae). Most angiosperms have chromosome numbers ranging between n=7 and n=12. About 30 to 40 per cent of flowering plants are polyploids. Polyploids are organisms that have higher chromosome numbers because of multiplication of chromosome sets. In flowering plants there are several kinds of 26 polyploids number relationship, Pinus (Pinaceae) is homoploid with n=12, while in composite family the different species having n=90, 18 and 27. are present. There are still other genera with numbers that show no simple numerical relationship to one another (aneuploids) e.g. Brassica (Cruciferae) with n=6,7,8,9 or 10. Base number and chromosome size is useful in understanding relationships in the grass family. Because relatedness of taxa is often reflected in homology of the chromosomes, pairing at meiosis in hybrids of two species helps in understanding relationships of closely related species. You should know that higher the percentage of pairing at meiosis in hybrids of two species the more closely the plants are presumed to be related. In animals there are now more reliable karyotypes for about 1,000 species of mammals, birds and insects.