Occurrence of NORs heteromorphism of the two X chromosomes in different aphid species.

The chromosomal localization of the rDNA has been determined in embryo cells of four aphid species (Megoura viciae, Acyrthosiphon pisum, Myzus persicae, Aphis sambuci) by staining with Chromomycin A3, a fluorochrome specific for GC-rich DNA, by silver staining and by in situ hybridization with a probe obtained by PCR amplification of 28S rDNA obtained from A. pisum. All these techniques showed that in all the examined species the ribosomal cistrons are located in one telomeric position on the X chromosomes. This localization of NORs seems to be a general phenomenon in aphids whereas autosomal NORs have been reported for Schoutedenia ralumensis only. Our data show a certain amount of heteromorphism between homologous NORs. This condition, already reported for some aphid species, is an unusual phenomenon in other animal species. The origin of this rDNA heteromorphism is still unclear. It may be caused by differential gene amplification, unequal crossing-over, unequal sister chromatid exchange or unequal mitotic crossing-over. The last hypothesis may be supported by the frequent observation of X connected by NORs bearing telomeres in mitotic metaphases.