NOR and heterochromatin analysis in two cartilaginous fishes by C-, Ag- and RE (restriction endonuclease)-banding.

Selachian chromosomes were investigated for the first time by C-, Ag- and RE (restriction endonuclease)-banding. A pair of NOR-bearing acrocentric chromosomes, as well as a third extra site, were demonstrated in two species of Torpedo. In situ digestion with restriction endonucleases revealed a peculiar arrangement of constitutive heterochromatin: in Torpedo ocellata the centromere never appeared C-banded, and was always digested by AluI, PstI, DdeI and HaeIII; in Torpedo marmorata, instead, heterochromatin was present in the centromere, and was not digested by the same enzymes. The occurrence of different types of centromeric heterochromatin suggests an involvement of highly repeated DNA fractions in the Robertsonian phenomena that occurred in chromosome rearrangements in the genus Torpedo.