PATTERNS OF DNASE-I SENSITIVITY IN THE HOLOCENTRIC CHROMOSOMES OF THEAPHID MEGOURA-VICIAE

Using the in situ nick translation technique, we looked for the presen ce of DNase I sensitive sites in Megoura viciae chromosomes, to study the distribution of active or potentially active genes in aphids, a gr oup of insects possessing holocentric chromosomes. Cytological prepara tions obtained by the spreading of embryo cells were treated in situ w ith increasing concentrations (ranging from 5 to 200 ng/mL) of DNase I . At DNase I concentrations below 50 ng/mL, only one hypersensitive si te was observed, and this was located on a telomeric region of the X c hromosome that contains transcriptionally active nucleolar organizing regions, as assayed by silver staining. Interestingly, at intermediate concentrations of DNase. the incorporation of biotinylated nucleotide occurred uniformly throughout all chromosomes, whereas at concentrati ons above 100 ng/mL, a C-like banding pattern was produced. Our data d iffer from results obtained with mammalian, frog, and grasshopper chro mosomes, where it was found that DNase I nicking is concentrated at th e distal regions of all chromosomes.