Abstract
The distribution of silver-stained nucleolus-organizing regions (NORs) in fibroblast chromosomes from all seven extant species of Equidae are described. There are variations in the number and locations of silver-stained NORs in different species but most of the cells in an individual of any speies had only 2 to 4 silver-stained NORs. In Equus przewalskii and E. caballus NORs were detected on up to three different chromosomes. In E. asinus 11 different chromosomes were observed to possess NOR sites. E. hemionus kulan and E. hemionus onager had NORs on 2 large metacentric pairs and a small acrocentric pair. In E. grevyi and E. burchelli NORs were located on 3 to 4 different pairs. E. zebra hartmannae had silver-staining over the telomere regions of the short arms of the chromosomes 1, 3 and 4. A comparison of G-banded chromosomes and silver-stained NORs has revealed one autosome with conserved G-band patterns and possessing a silver-staining NOR in all the species except E. asinus. Variations in the number and multichromosomal locations of NORs in various species could have evolved by pairing and exchanges between non-homologous chromosomal heterochromatin having similar satellite DNA sequences.
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