Ribosomal DNA is frequently concentrated on only one X chromosome in permanently apomictic aphids, but this does not inhibit male determination.

The ribosomal DNA arrays in the nucleolar organizer regions (NORs) of aphids are generally-located in a telomeric or subtelomeric position on the X chromosomes. In aphid populations or species that have lost the sexual part of their life cycle and become permanent apomicts, multiple rDNA copies are often concentrated on only one of the original X chromosomes. This situation has been found in apomictic members of three aphid subfamilies, although not as yet in the tribe Aphidini, which includes several permanent apomicts that are important pests. Some clonal cultures of Acyrthosiphon pisum and Myzus persicae reared for many years in conditions that prolong apomixis and inhibit sexual reproduction also have their rDNA arrays concentrated on one X chromosome. A 50-year-old clone of Aphis fabae, however, still retains a pair of similar-sized rDNA arrays on its X chromosomes. Although pairing of the X chromosomes by their NORs during prophase of the maturation division seems to be required for determination of XO males in aphids, a clone of A. pisum with one rDNA array was nevertheless able to produce males when subjected to appropriate environmental conditions.