Sex chromosomes in Anguilliformes: a ZZ/ZW sex determination system in Dalophis imberbis (Ophichthidae).

Sex chromosomes in Anguilliformes: a ZZ/ZW sex determination system in Dalophis imberbis (Ophichthidae) S. Salvadori, E. Coluccia, R. Cannas, S. Buccoli, A. Cau and A.M. Deiana Dipartimento di Biologia Animale ed Ecologia, Università di Cagliari, Cagliari, Italy. Among Anguilliformes, morphologically differentiated sex chromosomes have been described in 25% of the species studied. Interestingly, in this taxon the occurrence of different sex chromosome systems, both male and female heterogamety as well as a multiple chromosome system, has been reported. In this work we describe the karyotype and demonstrate the presence of a ZZ/ZW sex determination system in Dalophis imberbis (Delaroche, 1809). The chromosome preparations have been obtained from blood cell cultures. Dalophis imberbis has 46 chromosomes; its karyotype is made up of 4 metacentric and submetacentric pairs, 18 acrocentric and subtelocentric pairs and a sex chromosome pair, heteromorphic in females. By measuring with the Cromowin System software the relative length and the centromeric index of each chromosome pair, we have constructed the idiogram of this species. By C-banding we have located the constitutive heterochromatin in all centromeres; some chromosomes show peculiar banding patterns, in particular Z and W chromosomes present large heterochromatic blocks, probably related to mechanisms that lead to the inactivation and differentiation of sex chromosomes. Moreover, by replication banding, which represents the most effective karyotyping method in fishes, we have characterized each chromosome pair of D. imberbis. Furthermore, the 45S rDNA FISH has allowed to locate the ribosomal genes in two pairs of autosomes and in the subtelomeric region of the chromosome W. Ag-staining has shown in different individuals, three-four transcriptionally active NORs; the ribosomal clusters present on the W chromosome are often Ag-positive.