The role of rDNA genes in X chromosome association in the aphid Acyrthosiphon pisum

Silver staining of mitotic metaphases of the aphid A. pisum reveals the pre sence of argentophilic bridges connecting the two X chromosomes. The presen ce of nucleolar material connecting sex chromosomes seems to be quite a com mon phenomenon in organisms belonging to very different phyla, and suggests a role of nucleolar proteins in chromosome association and disjunction. In somatic cells of A. pisum, bridges connecting X chromosomes are detectable not only after silver staining but also after CMA(3) staining. This findin g suggests that GC rich DNA is involved in this type of association. Molecu lar analysis of rDNA intergenic spacers shows several 247 bp repeats contai ning short sequences having a high level of homology with the X sequence of Escherichia coli and with the consensus core region of human hypervariable minisatellites. Moreover, each 247 bp repeat presents a perfect, copy of a promoter sequence for polymerase I. These aphid repeats show structural ho mologies with a 240 bp repeat, which is considered to be responsible for se x chromosome pairing in Drosophila, not only in view of their common presen ce within rDNA spacers but also for their length and structure. The presenc e of X sequences in the IGS of A. pisum, by promoting unequal crossing-over between rDNA genes, could thus give rise to the nucleolar organizing regio n (NOR) heteromorphism described in different aphid species. Although X pai ring at NORs is fundamental in aphid male determination, the presence of he teromorphism of rDNA genes does not inhibit male determination in the A. pi sum clone utilized for our experiments.