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 Hind III 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 restricti on enzymes, followed by in situ nick translation, showed that ApaI targets are localized at the nucleolus organizer-bearing X telomeric region, wherea s HindIII restriction sites are clustered in intercalary C-positive areas o n the same X chromosome. Fluorescent in situ hybridization (FISH) carried o ut by using digoxygenin-labeled HindIII repeats as probe fully confirmed ov erlapping between the hybridization sites of this probe and the AT-rich int ercalary heterochromatic bands on the X chromosome. These findings, togethe r with published data, allow us to conclude that the M. persicae genome pos sesses three classes of C-positive heterochromatin: (i) a GC-rich argentoph ilic band located on one telomere of the X chromosome that contains ApaI ta rgets; (ii) AT-rich intercalary bands located on the X chromosome containin g clustered HindIII fragments; (iii) AT-rich telomeric bands, located on au tosomes, consisting of Haem repeats. Molecular analysis has shown that the length of the HindIII repeat consensus sequence is 189 bp with an AT conten t 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 eukar yotic 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 regi on whereas they proved to be highly conservative in the first 50 bp, thus s uggesting that this portion of the repeat might be functionally important.