HETEROCHROMATIN HETEROGENEITY IN THE HOLOCENTRIC X CHROMATIN OF MEGOURA-VICIAE (HOMOPTERA, APHIDIDAE)

Holocentric chromosomes, prepared by spreading embryo cells obtained f rom Megoura viciae parthenogenetic females, have been C-banded, enzyma tically digested in situ using the specific endonucleases DdeI (C down arrow TNAG), DraI (TTT down arrow AAA), Tru9I (TT down arrow AA), and CfoI (GCG down arrow C), and subsequently stained with Giemsa, DAPI, CMA(3), and AgNO3. We observed that the X chromosome had the best defi ned banding patterns. In the M. viciae X chromosome there is a certain amount of heterogeneity in heterochromatic DNA composition. In fact, the CC-rich NOR-associated heterochromatin differs from other heteroch romatic bands that are characterized by AT-rich DNAs. Our data also in dicate that, in M. viciae holocentric chromosomes, all heterochromatic blocks are accessible to in situ enzyme attack, the only limit to the digestion being the presence or absence of recognition targets. This is an interesting point, since, in monocentric chromosomes, it is well known that in situ endonuclease digestion is heavily affected not onl y by DNA base composition but also by chromatin compactness that may l imit enzyme accessibility to their specific targets.