Genome size variations are related to X-chromosome heterochromatin polymorphism in Arvicanthis sp. from Benin (West Africa)

The African grass rat, Arvicanthis sp., from Benin shows sex-chromosome pol ymorphism for the presence in the karyotype of three X-chromosome forms: su btelocentric (Xst), submetacentric (Xsm), and metacentric (Xm). Here we rep ort statistically significant differences of diploid genome size (g.s.) amo ng individuals, from 6.22 +/-. 0.06 pg to 6.80 +/- 0.10 pg, depending on th e S-chromosome form. The evaluation of the total and X-chromosome heterochr omatin content alone (Xst: 2.16 +/- 0.22%; Xsm: 5.02 +/-. 0.22%; Xm: 6.54 /- 0.28%), supports this suggestion. DNA base-pair preferential staining re vealed that heterochromatin of both autosome and sex chromosomes is AT-rich . Digestion of genomic DNA with sixteen restriction enzymes produced DNA ba nds representing the repetitive fraction of the genome. When these were use d as probes for fluorescence in situ hybridization (FISH), two hybridizatio n patterns were found: a) FISH positive signals localized in the pericentro meric areas of every chromosome of the complement and b) chromosome specifi c signals co-localizing with both the pericentromeric regions of every chro mosome and the heterochromatic regions of Xsm and Xm, but not on the Xst ch romosome. FISH performed with a (TTAGGG)n probe revealed a telomere-only pa ttern.