Early replication banding in Leporinus species (Osteichthyes, Characiformes) bearing differentiated sex chromosomes (ZW).

There are few examples of differentiated sex chromosomes in fishes. In the genus Leporinus, seven species present a highly differentiated ZW system, derived from heterochromatinization process. Cytogenetic analyses carried out in three of these fish species, Leporinus obtusidens, L. elongatus and L. reinhardti, through RBG-banding, showed late replication bands, coincident with heterochromatic regions in both Z and W chromosomes. A similar interstitial early replication segment was observed in the complex heterochromatic region along the Wq arms in the three species, which might correspond to a pseudoautosomal segment (SD, sex determining locus). Asynchrony related to the replication pattern among different Z chromosomes was not observed. When the identification of nuclear organizer regions by silver nitrate was performed over chromosomal preparations previously exposed to 5-bromo-2'-deoxyuridine (BrdU), remarkable positive signals at interstitial and telomeric position were observed on the q arms of W chromosomes in the species L. elongatus and L. reinhardti. The absence of 18S ribosomal RNA gene loci in this region, formerly demonstrated by FISH, indicates that this argentophilic behavior is putatively due to heterochromatin decondensation caused by BrdU incorporation, favoring such Ag+ reaction. Early and late replication bands were also observed in the heterochromatic portions of Z and W chromosomes, indicating that euchromatic and heterochromatic regions are interspersed. The present data suggest a significant level of heterochromatic complexity in the sex chromosomes of each species. On the other hand, the replication pattern shared by them supports a monophyletic origin.