Nonrandom Distribution of Lethals Induced by Tritiated Thymidine in Drosophila Melanogaster.

HE use of tritiated thymidine (H3T) has, in recent years, become quite widespread in many types of biological research. Because thymidine is a specific precursor of DNA, its direct and exclusive incorporation into chromosomes permits analysis of their behavior and metabolism during the mitotic cycle. Its incorporation is revealed through the use of radioautography, for which purpose tritiated thymidine has exceptional qualities, permitting resolutions at the chromatid level. The excellent radioautographs reflect the physical properties of the radioactive H3 atom. A comparison of the characteristics of the beta emission of C1" and H3 provides an insight into the relevant properties of the tritium atom. The beta particle emitted by H3 has a maximum energy of 18 kev, and an average energy of 6 kev, while the average energy of the C1' beta particle is 50 kev. The corresponding path lengths in water are 0.8 to 1.0 p for the H3, and approximately 50 p for the C1' beta particle (NELMS 1956). The amount of energy necessary for the formation of one ion pair is estimated at 0.0325 kev (LAPP and ANDREWS 1954). The H3 particle can thus lead, on the average, to the formation of about 180 ion pairs over a distance of 0.9 p. BACQ and ALEXANDER (1955) list the ionization density of H3 beta particles at 2 0 0 / p , whereas for C" the density is 3 0 / p . When H3 atoms are present in the molecular structure of the chromosomes, each beta emission will accordingly produce 10 to 20 ionizations within the first 0.1 p of path traversed. According to LEA (1955), 15 to 20 ionizations within 0.1 p of a chromatin thread are required to produce a chromatid break with a probability of one. Thus, although in terms of energy levels the beta particles emitted by C" would be expected to have greater genetic effect, the special properties of the H3 beta particle would seem to prove more interesting. LAJTHA and OLIVER (1959) and PLAUT (1959) have already pointed this out in stressing the fact that virtually all the H3 beta energy will be dissipated within the nucleus of origin. The ability of tritiated thymidine to produce sex-linked recessive lethals in Drosophila males has already been reported (KAPLAN and SISKEN 1960), Since then its mutagenic properties have been further investigated: in mice, by BATEMAN and CHANDLEY (1962) who measured the frequency of induced dominant