A cytogenetic analysis of Gambusia holbrooki (Cyprinodontiformes, Poecilidae) from the River Sarno

The mosquitofish, Gambusia holbrooki, is one of the few teleostean fish sti ll present in the polluted waters of the River Sarno. The specimens taken i n this river had 48 chromosomes, an acrocentric, gradually decreasing in le ngth in both sexes. However, several (12%) female prates showed a chromosom e with a unilateral gap often accompanied by a chromosome number reduced to 47, due to the presence of a biarmed element derived by centric fusion of two uniarmed elements. The nuclear organizer regions (NORs) were localized on two of the smallest chromosome pairs by Ag NOR banding. C banding + DAPI revealed centromeric AT-rich heterochromatin on all elements. The results also suggest a possible mutagenic and toxic activity of the substances pres ent in the polluted waters of the River Sarno.