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

In this paper we review recent work concerning both cytogenetic and molecular features of constitutive heterochromatin in the holocentric chromosomes of different aphid species. Sections include an extensive survey regarding chromosomal localization and DNA base composition of C-positive areas together with a cytological and molecular analysis of nucleolar organizer regions. The possible role of mitotic crossing over on genetic variability in parthenogenetic lineages is also briefly reviewed.