Breaking up the chromosomes of Baetica ustulata by in situ treatments with restriction endonucleases

Abstract

Fixed chromosomes of Baetica ustulata were treated with restriction endonucleases and subsequently stained with either Giemsa or the DNA-specific dye propidium iodide. Each enzyme produces a specific banding pattern with both dyes, which demonstrates the value of restriction endonucleases for chromosome banding in this species. The results obtained agree with the hypothesis that the action of restriction enzymes is based on cutting and extraction of DNA and is essentially determined by DNA base composition rather than by chromatin structure. However, strictly centric bands could be an exception. At least nine chromatin types can be distinguished in B. ustulata according to their different response to enzyme digestion and to the fluorescence banding patterns. Differential digestion of specific heterochromatic areas in band 4.4 and in the supernumerary segment of M5 by the HpaII – MspI enzyme pair suggests a high level of DNA methylation at these regions. The distribution of the different classes of DNA repeated sequences in the chromosome complement of B. ustulata indicates that some bands seem to follow the general principles of heterochromatin equilocality, while conceited evolution of heterochromatic DNA in other regions might have different rates of convergence in this species.Key words: restriction endonucleases, chromatin heterogeneity, methylated bands, equilocality, concerted evolution.