Nucleospora salmonis (Hedrick, Groff et Baxa, 1991), an intranuclear microsporidian parasite of marine and freshwater fish, causes diseases mainly in salmonid species. Losses have been reported in stocks of salmonid fish reared in the region of Auvergne (France). The cause of chronic mortalities in the local host species raised in aquaculture and destined for supplementation of the river system Loire-Allier was examined. The presence of N. salmonis was confirmed by PCR and histology in Salmo salar L. previously and in newly investigated salmonid species, Salmo salar, Salmo trutta fario L., Thymallus thymallus (L.) and Salvelinus alpinus (L.), present in European streams. The infection by N. salmonis was consistent in all cases with characteristic symptoms of the disease in deceased or moribund fish. The small subunit ribosomal DNA from N. salmonis was partially sequenced and compared to previously characterised N. salmonis isolates. As a result, a genotype, or clonal entity, was attributed to N. salmonis among Atlantic salmon found along the Northern Atlantic coastal lines and other salmonid species co-inhabiting or co-cultivated in the Auvergne region.
The present paper reports some cytogenetic peculiarities observed in the Ag-NORs of Pamphagus ortolaniae chromosomes, the unusual behaviour of ribosomal sites after silver staining and the intense Ag-positive reaction of centromeric regions at spermatogonial metaphase and spermatocyte metaphase I and II. Moreover, a conclusive identification and localization of all the ribosomal clusters is provided by using heterologous rDNA FISH on spermatocyte chromosomes. 18S-28S rDNA mapped on a single chromosome pair and resulted multiclustered along the chromosomal body in three distinct serial regions, r1, r2 and r3. Surprisingly, these areas were scarcely (r1) or never (r2 and r3) detectable by silver impregnation. As in other Orthoptera and many groups of arthropods, FISH with the pentamer (TTAGG)n as the probe labelled the telomeres of all chromosomes.