NOR heteromorphism within a parthenogenetic lineage of the aphid Megoura viciae

In parthenogenetic females of a clone of the aphid Megoura viciae (Homopter a, Aphididae), more than 50% of the cells show heteromorphism between homol ogous NORs which are located on one telomeric region of the two X chromosom es. Using different techniques, such as staining with the CG-specific fluor ochrome chromomycin A(3), silver staining and in-situ hybridization with an rDNA probe, we have shown that the observed heteromorphism is due to an un equal distribution of ribosomal genes between homologous NOR regions. The t otal number of rDNA genes per individual aphid remained constant. Moreover, the analysis of cells from single embroys has shown that the observed hete romorphism is not only intraclonal but also intraindividual. These data, to gether with the finding of X chromosomes connected by chromatin bridges bet ween their NORs, allow us to suggest that mitotic unequal crossing over cou ld be the main cause of NOR heteomorphism in this taxon.