X-IRRADIATION-INDUCED CONGENITAL ANOMALIES IN HYBRID MICE

¹ Radiological Research Laboratory, Department of Radiology, College of Physicians & Surgeons, Columbia University, New York 32, N. Y.

1. Increased radioresistance is manifested in hybrid embryos exposed to 200 r at 8.5 days of embryonic development when divergent stocks of mice are crossbred, as determined by the incidence of congenital anomalies. This is evidence of embryonic heterosis.

2. When hybrid mice are mated with hybrids (of the same crosses) or with either of the parental stocks, it is apparent that the presence of CF1 influence (genes?) affords the embryos more radioresistance only when the cross is with the CF1 line. The presence of CF1 genes brings the results up to the pure CF1 data so that it appears that there is a sort of dominance effect which becomes heterosis when the proportion of CF1 genes exceeds this minimum in these heterozygous embryos.

3. In hybrid combinations the development of CNS congenital anomalies appears to be more frequently related to the presence of CF1 genes than to those from the C57 strain. Conversely, resorptions appear to be more directly related to the presence of the C57 genes. Even in the pure strains this is substantiated because 14% of the CF1 embryos and none of the C57 embryos showed CNS anomalies following 200 r x-rays at 8.5 days' gestation. Likewise, the C57 strain embryos reacted to this exposure by producing 92% resorptions while the CF1 embryos showed only 32%.

4. The data on congenital anomalies seem at first to be confused. This is due to the simple fact of relative radioresistance of the CF1 embryos, which allows them to survive and hence to develop x-irradiation-induced congenital anomalies, particularly of the central nervous system. The radiosensitivity of the C57 strain reduced their survival. As a result of this, there are fewer survivors to develop CNS anomalies.

5. As in other studies in heterosis, it appears that this embryonic hybrid vigor is correlated with maximum heterozygosity and is reduced as this condition is diluted toward either of the parental conditions.

6. Radioresistance (or, conversely, radiosensitivity) of the embryo appears to be closely allied to inherent genetic factors quite different in the two strains of mice. In one strain there is the greater tendency to resorption and death and in the other to survival with attendant development of congenital anomalies.