Nucleolar Organizer Region (NOR) Polymorphism in Relation to Interspecific and Intraspecific Variability in Some Indian Anurans-A Review

The amphibians being originated from Crossopterygian fishes in the late Devonian constitute one of the most interesting groups of vertebrates. Indian subcontinent is rich in amphibian fauna but the cytogenetical studies in this field are scanty. The present review has been oriented to analyse the nucleolar organizer region polymorphism in some Indian anuran bufonids, ranids and rhacophorid. Conventional staining of mitotic metaphase stages revealed that three Bufonid species studied possess symmetrical karyotypes with 2n=22 and NF=44. The four ranid species and a single sp. of Rhacophorus studied possess symmetrical karyotypes with 2n=26 and NF=52. Nucleolar organizer region or secondary constriction (SC) region staining by HCl-trypsin digestion and silver impregnation technique revealed that the number and position of NORs are variable not only among the different species but also in different individuals of the same species. Considerable variation in the length of NOR or SC has been noted in different specimens of the same species. An uneven distribution of NOR has also been found in Bufo stomaticus where males possess one NOR bearing chromosome in the chromosome pair # 5 and female with two NOR bearing chromosome that is in the chromosome pair # 7. The NOR polymorphism in relation to interspecific and intraspecific variability in the size and number is interesting which would be of cytotaxonomic value to trace the phylogenetic relationship among the different species.