The HSR and heterochromatin distribution in natural populations of Mus musculus domesticus (Rodentia: Murinae) from Greece

The natural populations of the house mouse, Mus musculus domesticus, of the Euro-Mediterranean region have been under extensive scrutiny for several years, thanks to an extensive Robertsonian chromosomal variation. However, this subspecies is also known for the occasional appearance of Homogenously Staining Regions (HSRs) in chromosome 1 of the karyotype. Based on the extensive C-banding chromosomal study of 236 house mice from 32 localities of Peloponnisos, S Greece, eight Rb individuals from two localities were found with HSRs in their karyotype (HSR incidence: 3.4%), thus constituting the first record of this chromosomal trait in house mouse populations from Greece. Moreover in the context of this study, we came across five individuals with significant heterochromatin amplification below the centromere of the X chromosome and of two autosomal pairs (incidence: 2.1%). Notably, four of these individuals also carried HSRs. This finding is important because increase of heterochromatin content has only rarely been documented for M. in. domesticus and this is in fact the first report of such prominent heterochromatin amplification in individuals of natural populations of this subspecies.