Cytogenetic and molecular characterization of a highly repeated DNA sequence in the peach potato aphid Myzus persicae

Electrophoresis following digestion of Myzus persicae genomic DNA with HindIII showed the presence of a prominent band of approximately 200 bp whereas a faint electrophoretic band corresponding to DNA fragments of about 3000 bp was observed after digestion with ApaI. In situ digestion with restriction enzymes, followed by in situ nick translation, showed that ApaI targets are localized at the nucleolus organizer-bearing X telomeric region, whereas HindIII restriction sites are clustered in intercalary C-positive areas on the same X chromosome. Fluorescent in situ hybridization (FISH) carried out by using digoxygenin-labeled HindIII repeats as probe fully confirmed overlapping between the hybridization sites of this probe and the AT-rich intercalary heterochromatic bands on the X chromosome. These findings, together with published data, allow us to conclude that the M. persicae genome possesses three classes of C-positive heterochromatin: (i) a GC-rich argentophilic band located on one telomere of the X chromosome that contains ApaI targets; (ii) AT-rich intercalary bands located on the X chromosome containing clustered HindIII fragments; (iii) AT-rich telomeric bands, located on autosomes, consisting of Haem repeats. Molecular analysis has shown that the length of the HindIII repeat consensus sequence is 189 bp with an AT content of 67%. Southern blotting with HindIII monomers revealed a regular ladder of bands composed of multimers of basic length that are characteristic of satellite DNAs. The HindIII repeat displays other features typical of eukaryotic satellite arrays such as overlapping with heterochromatic bands and a high degree of sequence similarity among monomers (84%-94%). A similarity plot showed that sequences were particularly variable in the 50-100 bp region whereas they proved to be highly conservative in the first 50 bp, thus suggesting that this portion of the repeat might be functionally important.