Factors affecting the frequency and pattern of mitomycin C-induced chromatid exchanges

Data are reported on the frequency and pattern of Mitomycin C-induced exchanges in lymphocytes of a marsupial hybrid Macropus rufogriseus female x Wallabia bicolor male and in four individuals, two male and two female, of M. rufogriseus. In the hybrid, exchanges are found almost exclusively between the M. rufogriseus chromosomes and the breakpoints are in the C-banded pericentromeric regions. In the four M. rufogriseus animals the data show no evidence for preferential exchanges between homologues and so provide no justification for postulating somatic pairing between homologues. Comparisons of the frequency of exchanges between non-homologous chromosomes in the hybrid and in the four individuals of M. rufogriseus show that there is no difference in the frequency of exchange for any given chromosome but that the specific pattern of exchanges may differ between animals. The data are interpreted as providing evidence for different types of heterochromatin and possible bases for these differences are discussed.