Nucleotide variation and physical mapping of ribosomal genes using FISH in genus Tor (Pisces, Cyprinidae)

Molecular cytogenetic studies were carried out for localization of 18S and 5S ribosomal DNAs on chromosomes of three cyprinid fish species viz., T. khudree, T. mussullah and T. mosal mahanadicus using two color fluorescence in situ hybridization (FISH). All the species typically possessed 100 diploid chromosomes with minor variation in karyo-morphology. The 18S rDNA signals were observed on two pair of chromosomes in T. khudree and T. mussullah, and three pairs in T. mosal mahanadicus. The location of 18S signals also showed affinity to silver nitrate and chromomycin A3 staining. Similarly, variation in localization of 5S rDNA among the three species has been detected with the presence of FISH signals on one pair of chromosome in T. khudree and T. mussullah, and on two pairs in T. mosal mahanadicus. These molecular markers could be used as species specific markers for taxonomic identification and can further add in understanding the dynamics of genome organization and karyotypic evolution of these species. The 18S rDNA region was sequenced that generated 1811, 1810 and 1776 bp long 18S sequence in T. khudree, T. mussullah and T. mosal mahanadicus, respectively. The 18S rDNA sequence showed 95-98% identity among the subject species. Similarly, 5S sequencing generated 203 bp long fragments in these species with 100% identity in coding and 9.63% variability in non-transcribed spacer regions. The nucleotide sequence variations could be used for understanding the genetic diversity and will add new informative characters in comparative genomics. These results, in general, would enhance the value and interpretation of ecological assessment data for conservation of Tor species.