Ag-detected NOR and C-banding patterns in Bacillus rossius (Insecta Phasmatodea) from Sicily

SUMMARY

The Giemsa karyotype of Sicilian Bacillus rossius, belonging to both bisexual and unisexual demes, has been defined. Heterochromosomes (XX, ♀; XO, ♂) rank as third elements in size—not fourth ones—and no heteromorphic pairs exist, on the contrary of what stated in a previous report. C-banding has revealed that six chromosome pairs have positive pericentromeric blocks and that a given number of additional C-banded areas are found; these positive non-centromeric C-banded areas, which often appear as satellites, can be heteromorphic in size and usually correspond to an Ag-detectable NOR location. Comparative Ag-NOR and C-banding pattern analyses have revealed that in bisexuals NOR sites are fewer, have constant localizations and always are heterozygous for their activity, while in unisexuals NORs and the associated C-banded areas are more numerous, variably localized and always homozygous. Pair 13 constantly bears an Ag-detectable NOR in both bisexual and unisexual samples. Heterozygous versus homozygous Ag-NOR staining has been explained in terms of either differential or similar rDNA transcriptional activity in homologous chromosomes, while fixed versus numerically variable condition has been related to different cytological mechanisms of reproduction; parthenogenesis well explains the trend towards homozygosity and clonal maintenance of new NOR locations. The rare Ag-negative non-centromeric C-banded areas have been envisaged as silent rDNA cistrons. A link emerges between non-centromeric C-positive blocks and rDNA cistrons. Finally, in B. rossius Ag-NORs may represent cytological features of low taxonomical relevance expecially when unisexual samples are involved.