Comparative analyses of heterochromatin in Microtus: sequence heterogeneity and localized expansion and contraction of satellite DNA arrays

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Abstract
Southern blotting, C-banding, base-specific fluorochrome staining, and fluorescence in situ hybridization were used to analyze the constitutive heterochromatin in eight species and subspecies of arvicolid rodents (genus Microtus). Autosomal centromeric regions portrayed considerable variability between species in the amount of C-band-positive material present; e.g., M. chrotorrhinus showed relatively little, whereas M. cabrerae exhibited extensive centromeric staining. Autoso-mal interstitial C-bands were noted in M. guentheri and two subspecies of M. ochrogaster. All Y chromosomes examined were predominately or completely heterochromatic, as were substantial portions of the giant X chromosomes in three species (M. agrestis, M. cabrerae, and M. chrotorrhinus). Hoechst 33258 staining (with its affinity for AT binding sites) showed bright fluorescence on the heterochromatin of the sex chromosomes of M. agrestis and moderate fluorescence on those of M. cabrerae and M. chrotorrhinus; however, only the heterochromatin of M. cabrerae and M. chrotorrhinus hybridized with an AT-rich satellite DNA probe (MSAT-160) isolated from M. chrotorrhinus. Hoechst 33258-bright autosomal centromeres of M. arvalis and M. cabrerae also hybridized to the probe, whereas the Hoechst 33258-bright Y chromosomes of M. arvalis and M. guentheri did not. Two pairs of autosomes in M. guentheri are comprised of six distinct regions, based upon C-banding, Hoechst 33258 staining, chromomycin A3/distamy-cin A staining, and in situ hybridization. The centromeric regions of acrocentric autosomes known to retain conserved G-banding patterns may exhibit variable hybridization intensity when different species or subspecies are compared. M. ochrogaster portrays considerable intersubspecific variability in the size and location of autosomal telomeric and interstitial C-bands that are also sites of hybridization. These latter two findings illustrate that dramatic differences in copy number of the tandem satellite array can exist at homologous chromosomal positions both within and between species.

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