Equilocality and heterogeneity of constitutive heterochromatin: in situ localization of two families of highly repetitive DNA in Dociostaurus genei (Orthoptera).

The chromosome complement of the grasshopper Dociostaurus genei is characterized by the presence of constitutive heterochromatin (C-bands) located in the centromeric regions of all the chromosomes and in the distal regions of some autosomes in the form of supernumerary segments. A sequence analysis was carried out to obtain information about the molecular characteristics of both heterochromatic regions. Two families of tandemly repetitive DNA (DgT2 and DgA3) from D. genei were cloned and characterized. Data obtained from in situ hybridization indicate that these families are located solely in the regions of constitutive heterochromatin. The DgT2 clone is representative of a family of sequences which mainly forms the centromeric C-bands in each chromosome of the complement. The DgA3 family is the major component of the distal C-bands (supernumerary segments) present in most of the autosomal pairs. These results show the existence in D. genei of two different families of repetitive DNA restricted to different chromosomal domains. We discuss these results in the light of the possible role of chromosomal disposition in the maintenance of the differences between heterochromatic DNA from different chromosomal regions and the homogenization of DNA sequences from equilocal chromosomal domains.