Chromosome markers and karyology of selachians

Among vertebrates, chondrichthyans exhibit peculiar karyotypes and total amount and composition of DNA very different from those of tetrapods and teleosteans. Selachians have relatively large genome sizes (more than 30 pg/N), which are inversely related to the fraction rich in adenine-thymine. Moreover, they show a high chromosome number (2n = 60-100), decreasing in the most specialized species. The karyotypes of Scyliorhinus stellaris, Torpedo ocellata, and T. marmorata have been investigated by several techniques in order to distinguish particular genome fractions along the chromosome arms. C-banding appears to be positive in most telomeric regions in Scyliorhinus and at the interstitial level in the two torpedo species. Studies with restriction enzymes (RE) have just been started in selachians, employing ALU I and HIND III in Scyliorhinus stellaris. The former digests the whole chromosome except the telomeric regions, revealing patterns similar to the C-bands. The latter cuts the chromosomes into several interstitial regions, producing G-bands. Other RE are being studied, which will allow identification in situ of qualitative differences in the various DNAs.