Résumé :
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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 CMA(3) staining. This finding suggests that GC rich DNA is involved in this type of association. Molecular analysis of r DNA intergenic spacers shows several 247 bp repeats containing 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 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 r DNA spacers but also for their length and structure. The presence of X sequences in the IGS of A. pisum, by promoting unequal crossing-over between r DNA 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 r DNA genes does not inhibit male determination in the A. pisum clone utilized for our experiments.
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