COPY NUMBERS OF A CLUSTERED LONG-RANGE REPEAT DETERMINE C-BAND STAINING

A cluster of long-range repeats (LRRs) with a repeat size of roughly 1 00 kb is part of band D of chromosome 1 of the house mouse, Mus muscul us. The cluster is cytogenetically polymorphic: it is either C-band ne gative or C-band positive. Our results show that the differential stai ning behavior depends on the LRR copy number and not on differences in DNA composition. There is a threshold between 105 and 175 LRR copies per haploid genome; clusters with lower copy numbers stain C-band nega tive, whereas those with higher copy numbers are C-band positive. Abov e this threshold, the size of the C-band is linearly correlated with t he LRR copy number. The results imply that sequences capable of formin g heterochromatin may be dispersed throughout the genome but are not r ecognized as such by cytogenetic techniques, unless they reach the thr eshold amount and concentration.