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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 presence of argentophilic bridges connecting the two X chromosomes. The presence of nucleolar material connecting sex chromosomes seems to be quite a common 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 CMA3 staining. This finding suggests that GC rich DNA is involved in this type of association. Molecular analysis of rDNA intergenic spacers shows several 247 bp repeats containing short sequences having a high level of homology with the chi 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 homologies with a 240 bp repeat, which is considered to be responsible for sex chromosome pairing in Drosophila, not only in view of their common presence within rDNA spacers but also for their length and structure. The presence of chi sequences in the IGS of A. pisum, by promoting unequal crossing-over between rDNA genes, could thus give rise to the nucleolar organizing region (NOR) heteromorphism described in different aphid species. Although X pairing at NORs is fundamental in aphid male determination, the presence of heteromorphism of rDNA genes does not inhibit male determination in the A. pisum clone utilized for our experiments.