To examine possible effects of space radiation on living organism, we have analyzed two types of mutations, sex-linked recessive lethal mutations and somatic mutations, in fruit fly of the species Drosophila melanogaster . Drosophila strains used were wild type strains and a radiation-sensitive strain mei-41. Two different developmental stages of samples were sent into space; young adult males to analyze sexlinked recessive lethal mutations and about 30hr-old larvae to detect somatic mutations in wing epidermal cells. For wild type and mei-41 strains each, about 200 adult male flies and about 6,000 larvae were loaded on space shuttle Endeavour. The male flies returned from space were mated to virgin female flies of a tester strain, and the presence of the lethal mutations was analyzed at F2 generation. The frequencies of sex-linked recessive lethal mutations in flight groups were 2 and 3 times higher for wild type Canton-S and mei-41, respectively, than those in ground control groups. Most larvae sent to space emerged as adult flies within about 10 days after the landing. The presence of wing-hair somatic mutations, which give morphological change in hairs growing on the surface of wing epidermal cells, was analyzed under microscope. In wild type strain Muller-5, the frequency of wing hair mutant spots in flight group was about 1.5-fold higher than that in ground control, and in Canton-S-derived wild type strain the frequencies were similar between the two groups. By contrast, for mei-41 strain the mutation frequency was lower in flight group than in control group. The observed higher frequency of lethal mutations in the flight group might be due to a possibility that radiation effects on reproductive cells could be greatly enhanced under micro gravity. However, if this would be the case, we do not have appropriate explanation for the apparent absence of such synergistic effects on somatic wing-hair mutation system.

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Rhythms of 24 hr periodicity underlie most aspects of daily life, although we are rarely aware of its presence. This circadian rhythmicity is a consequence of evolution of the living organism, which takes place on the earth for millions of years. Some of these rhythms are actually derived from an endogenous oscillating mechanism whose inherent period is not exactly 24 hrs. Therefore, it must be constantly adjusted to the solar cycle of day and night. It has been firmly established that the suparachiasmatic nucleus (SCN) of the hypothalamus is the highest center of these endogenous circadian rhythms. Lesions of the SCN eliminates most behavioral and physiological rhythms and actvities of the SCN exhibited a dramatic change between day and night. Furthermore, rhythms of SCN neurons persist under in vitro conditions. The cellular or molecular mechanism for these neurons to generate a 24 hr rhythm is unclear at present. The SCN contains many neuropeptides and some of them display circadian rhythms, while others do not. We found that this distinct behavior of peptides in the SCN was due to different regulation of mRNA or gene transcription. This observation indicate the possibility that genes are involved in the generation of circadian rhythms in the SCN.

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