Revisiting Spalax: Mitotic and meiotic chromosome variability

Further cytological studies on Israeli mole rats (Spalax) were motivated by their postulated active speciation. Four major chromosomal forms were characterized by Wahrman and collaborators in 1969. The differences between their chromosome numbers, 2n = 52, 54, 58 and 60, were then attributed to 1-4 Robertsonian changes, and the differences in the number of chromosome arms were assumed to be due to pericentric inversions. The new results obtained by differential staining techniques agree with the earlier interpretations. C-banding has shown that some of the evolutionary changes were accompanied by changes in the quantity and distribution of constitutive heterochromatin. All chromosomal forms also possess a considerable amount of chromosome micro-changes, including variation in the length of a C-negative, heterochromatic modification, at the base of the long arm of Chromosome I. Four chromosomes may carry nucleolus-organizing regions (NORs), one of them in a distal position. Each of five individuals had a morphologically different NOR spectrum. Chiasma frequency, determined in males, showed some variation. Since a chromosomally-derived type of sterility has, in other mammals, been attributed to the consequences of associations between unpaired autosomal segments and the differential segments of the sex chromosomes, an electron microscopic search for such associations in pachytene nuclei of a hybrid (2n = 52 × 58) male was initiated. Unpaired, non-homologous autosomal regions were seen in early pachytene, but not enough cells were available for detecting autosome-sex chromosome associations, if they exist. By late pachytene only completely paired synaptonemal complexes were observed. The possibility that the development of reproductive barriers in the Spa/ax system partly depends on the presence of unpaired autosomal segments which originated from chromosomal micro-changes and/or Robertsonian stranslocations deserves further study.