Somatic Cell Mutation Induced by Sunlight in Drosophila

There is ample epidemiological evidence showing that sunlight can cause skin cancer in the human. In experimental studies, simulated sunlight or UV lamps are used for demonstrating carcinogenesis and other biological effects. Little studies, however, have been performed using natural sunlight itself. In this work, we have examined the mutagenicity of natural sunlight in Drosophila. The Drosophila wing spot test is useful to detect somatic cell mutations. Third instar larvae in petri dishes were exposed to sunlight (ultraviolet region with <290 nm wavelength cut off by a plastic cover) in the yard of Okayama University campus (north latitude: 34* 39', east longitude: 133°*55'). The sunlight was mutagenic in Drosophila larvae and produced pyrimidine dimers in their DNA. In the observed mutagenicity, there was dependence on the exposure period and UV fluence. During the two-year monitoring, the highest induction of mutant spot observed was 1.98 total spots/wing on June 25, 1998, and the lowest was 0.64 on December 29, 1998, while non-exposure spontaneous spots were 0.29 and 0.32 on these days, respectively. Thus, solar radiation was mutagenic both in summer and in winter. J Epidemiol, 1999 ; 9 : S66-S71.


Somatic
Cell Mutation Induced by Sunlight in Drosophila There is ample epidemiological evidence showing that sunlight can cause skin cancer in the human.In experimental studies, simulated sunlight or UV lamps are used for demonstrating carcinogenesis and other biological effects.Little studies, however, have been performed using natural sunlight itself.In this work, we have examined the mutagenicity of natural sunlight in Drosophila.The Drosophila wing spot test is useful to detect somatic cell mutations.Third instar larvae in petri dishes were exposed to sunlight (ultraviolet region with <290 nm wavelength cut off by a plastic cover) in the yard of Okayama University campus (north latitude: 34* 39', east longitude: 133°*55').The sunlight was mutagenic in Drosophila larvae and produced pyrimidine dimers in their DNA.In the observed mutagenicity, there was dependence on the exposure period and UV fluence.During the two-year monitoring, the highest induction of mutant spot observed was 1.98 total spots/wing on June 25, 1998, and the lowest was 0.64 on December 29, 1998, while non-exposure spontaneous spots were 0.29 and 0.32 on these days, respectively.Thus, solar radiation was mutagenic both in summer and in winter.J Epidemiol, 1999 ; 9 : S66-S71.natural sunlight, somatic mutation, pyrimidine dimers, Drosophila Epidemiological data show that solar radiation is carcinogenic in humans, and there are experimental results demonstrating that sunlight is indeed carcinogenic in rodents 1-3) An increased risk of skin cancer, possibly due to the increased UV radiation that was derived from the ozone layer-depletion, is a current issue in Australia 4,5) There are numerous reports on experimental photocarcinogenesis 6).Although there are studies demonstrating mutagenicity and cytotoxicity of sunlight in cultured cells 7), bacte-riophage8,9), Bacillus subtilis spores 10) and yeast 11), no data are available on the sunlight mutagenicity in somatic cells.The Drosophila wing spot test is useful to detect the somatic cell mutation including gene mutation, chromosomal recombination, segmental chromosomal deletions, and nondisjunction 12).In this study, we have examined the mutagenic activity and lethality of sunlight using the Drosophila wing spot test.We have been able to detect somatic cell mutations in the Drosophila larvae that have been exposed to sunlight in the field of Okayama University.We have monitored the mutagenicity for 2 years, measuring the UV insolation alongside.
We also measured the amount of cyclobutane thymine dimers and (6-4)photoproducts in the DNA extracted from the sunexposed Drosophila larvae; these DNA lesions are believed to be responsible for photocarcinogenesis 13,14)

RESULTS AND DISCUSSION
We have monitored the mutagenicity caused by solar radiation in 1997 (Figure 2) and in 1998 (Figure 3).The mutagenicities increased depending on the exposure period and UV fluence within each day tested.The highest mutagenicity found was about 2 total spots/wing.We reported previously that a similar extent of mutagenesis was induced by UVA irradiation (320-400 nm, with black light) at about 200 kJ/m221) On some clear days, when the sunlight was very strong (UVA 200 kJ/m2; UVB 100 kJ/m2), larvae were killed after a 4 hrexposure.The lowest mutagenicity, observed in December, was found to be still statistically positive.As Figure 4 shows, both of the photoproducts in the larval DNA increased with the elongation of the exposure time up to 4 hr.On exposure for 6 hr, the photoproducts decreased.It is conceivable that the rate of photoproduct formation was reduced as the passage of day-time due to the decline of solar radiation, while the repair of photodamage stayed constant.
The mutagenicities were in a stationary phase after the 6 hrexposure.
During the two-year monitoring, sunlight in the summer was about 3-5 times more mutagenic than that in the winter.
Although the mutagenicities increased depending on the UV fluences within a day examined, the correlation between the mutagenicities and UV fluences among 14 experiments for two years was not significant, as shown in Figure 5.It is suggested that the causes of mutagenic events are different among individual days.One possibility is that the light composition in sunlight may vary depending on the season and weather2,4).
The UV dose declines in late afternoon, and yet the mutagenicity stayed constant (Figure 2, 3).
In this study, we demonstrated that sunlight causes somatic mutations in Drosophila.We consider that this Drosophila test may be useful as a model for higher organisms.