Polar Biol (1996) 16: 559—564  Springer-Verlag 1996 ORIGINAL PAPER A. Morescalchi · M.A. Morescalchi · G. Odierna V. Stingo · T. Capriglione Karyotype and genome size of zoarcids and notothenioids (Teleostei, Perciformes) from the Ross Sea: cytotaxonomic implications Received: 7 June 1995/Accepted: 27 November 1995 Abstract In the absence of fossils, the origin of Notothenioidei, a perciform suborder dominating the fish fauna of the Southern Ocean, remains conjectural; some morphoecological evidence suggests relationships to zoarcoids. To test this point we have compared the karyotype morphology and genome size of two species of zoarcids from the Ross Sea to those of one species each of the notothenioid families Artedidraconidae, Bathydraconidae, Channichthyidae and Nototheniidae from the same region. A karyotype of 48, mostly acro- centric, chromosomes, localization of nucleolar organ- izers on a pair of small dibrachial chromosomes, a genome size of about 3 pg of DNA, characterize both zoarcids; similar features can be found in the karyology of the notothenioids (especially the Nototheniidae). However, all shared characters appear as plesio- morphic in teleost karyology, which does not help in producing new data on the problem of notothenioid relationships. Introduction In association with species of the families Liparidae and Zoarcidae, the members of the perciform suborder Notothenioidei dominate the recent Antarctic ich- thyofauna. Notothenioids could be monophyletic (Iwami 1985) and include six families whose stem is per- haps to be found among Bovichtidae, a small, mostly South Pacific, taxon with representatives exhibiting an M.A. Morescalchi ( ) Institute of Comparative Anatomy, The University, Viale Benedetto XV 5, I-16132 Genova, Italy G. Odierna · V. Stingo · T. Capriglione Department of Evolutionary and Comparative Biology, The University, Via Mezzocannone 8, I-80134 Napoli, Italy array of plesiomorphic characters in the suborder (see in Gon and Heemstra 1990; Eastman 1993). Due to the absence of fossils (Grande and Eastman 1986; Young 1991) and to the fact that the characterization of Notothenioidei often utilizes physiological rather than morphological apomorphies, the origin of this sub- order remains uncertain (Eastman 1991, 1993; Miller 1993). The Blennioidei has been considered as an out- group of Notothenioidei (Gosline 1968; Eakin 1981); however, Anderson (1984, 1990) noted that Gosline’s Blennioidei is paraphyletic and suggested that Zoar- coidei, whose members have a long history of isolation in Southern Ocean waters, could share ancestry with the Notothenioidei. Among living zoarcoids, the family Zoarcidae (eelpouts) has many representatives in the Antarctic Region (22 species), some of which live in the most southern waters of the Ross Sea: the benthic ¸ycodich- thys dearborni and Pachycara brachycephalum, are both equipped with blood antifreeze peptides (DeVries 1988; Cheng and DeVries 1991). Here we have compared the karyotype morphology and the genome size (nuclear DNA amount) of the Ross Sea zoarcids to those of representatives of four notothenioid families from the same area. We hoped to find cytogenetic apomorphies useful to test the hypotheses of relationships between zoarcoids and notothenioids. Materials and methods Specimens of fish different species were caught in the waters sur- rounding the Italian Base of Terra Nova Bay (notothenioids) and in McMurdo Sound (zoarcids) during the last two missions to Antarc- tica by biologists of the Italian P.N.R.A; the zoarcids were donated by Drs. A.L. DeVries and G. Reinhart. The number of sex of the specimens studied were as follows: Zoarcidae: Pachycara brachycephalum Pappenheim 1912: one fe- male, two males; ¸ycodichthys dearborni DeWitt 1962; two females, one male;