Mutation induction (resistance to 6-thioguanine) in Chinese hamster fibroblasts (V79) by exposure to accelerated heavy ions (O, Ne, Ca, Ti, Ni, Xe, Pb and U with energies between 5 and 14.8 MeV/u) was investigated, covering a range of LET from 300 to about 15,700 KeV/micron. The LET-dependence of the mutation induction cross-section (sigma m) has, in a similar way to inactivation (sigma i), to be described by separate curves for each ion. Both sigma m and mutagenicity (sigma m/sigma i) decrease with increasing specific energy for any given ion. Relative biological effectiveness for mutation induction was found to be significantly smaller than unity for the ions and energies investigated.

Flow cytometric analysis of enzymatically decon-densed, DAPI-stained spermatozoa was performed to confirm the suspected production of unbalanced spermatozoa in heterozygous rams carrying a 1;20 translocation. High-precision flow cytometry (coefficient of variation, 0.6–0.8 %) with a PAS II flow cytometer depicted Y- and X-chromosome-bearing spermatozoa from three cytogenetically normal rams as two distinct peaks. The difference in DNA fluorescence intensity between the gonosomes averaged 4.8 %. Analysis of sperm samples from three heterozygous 1;20 translocation carriers yielded histograms with five peak distributions. The individual peaks were attributed to spermatozoa with a normal, balanced, and unbalanced chromosomal status. Peaks within Y- and X-spermatozoa populations were distributed in a ratio of 1:2:1 and were almost completely separated, with a coefficient of variation of 0.5–0.6%. Owing to the relative size of the translocated chromosomal segment (2.4% of the total DNA content, as determined from the flow cytometric data), histograms with five instead of the expected six peaks were observed.

The induction of resistance to 6-thioguanine by heavy ion exposure was investigated with various accelerated ions (oxygen-uranium) up to linear energy transfer (LET) values of about 15,000 keV/microns. Survival curves are exponential with fluence; mutation induction shows a linear dependence. Cross-sections (sigma i: inactivation, sigma m: mutation) were derived from the respective slopes. Generally, sigma i rises over the whole LET range, but separates into different declining curves for single ions with LET values above 200 keV/microns. Similar behaviour is seen for sigma m. The new SIS facility at GSI, Darmstadt, makes it possible to study the effects of ions with the same LET but very different energies and track structures. Experiments using nickel and oxygen ions (up to 400 MeV/u) showed that inactivation cross-sections do not depend very much on track structure, i.e. similar values are found with different ions at the same LET. This is not the case for mutation induction, where very energetic ions display considerably smaller induction cross-sections, compared with low-energy ions of identical LET. Preliminary analyses using the polymerase chain reaction (PCR) demonstrate that even heavy ions cause "small alterations" (small deletions or base changes). The proportion of the total deletions seems to increase with LET.

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