| Research Abstract |
On the basis of the data accumulated during the last ten years by examining more than 900,000 embryos corresponding to approx.2,400,000 loci, we consider that the basis of the Medaka specific-locus test system has been well setablished. Because the treatment so far adopted in our study was exposure to a single agent including gamma-rays, ENU or reactor radiation, we decided to precede to examine the genetic effects of combined gamma-rays-- ENU treatments by using the Medaka specific-locus test system.
Fifty wild type adult males of the Sakura strain were exposed at room temperature to 4.75 Gy of gamma-rays followed 20 to 30 min.later by an exposure to 0.5mM ENU for 2 hours at 27ﾟC.Each treated male was mated to a female of our tester strain AA2. Extensive sequential collection of embryos was done from the first day through 30 days after the combined treatments. The genetic endpoints examined were dominant lethals, total mutations and viable mutations. Observed results were compared with
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those already obtained for separate treatments with gamma-rays or ENU.The additivity or synergism was tentatively calculated on the basis of fold-increase over spontaneous rates [90% upper confidence limit/lower limit], which were 0.0557 [0.0566/0.0550]/gamete, 3.79 [5.51/2.61]x10^<-5>/locus, and 4.17 [12.6/1.38]x 10^<-6>/locus, respectively, for dominant lethals, total mutation and viable mutation.
The observed dominant lethal rates for sperm, spermatids, spermatocytes, differentiating spermatogonia and stem spermatogonia were, respectively, 0.584 [0.605/0.565], 0.550 [0.563/0.537], 0.394 [0.413/0.375], 0.123 [0.134/0.114], and 0.0493 [0.0525/0.464]. The observed values for the postmeiotics including sperm, spermatids and spermatocytes were higher than the value expected on the basis of simple additivity, but lower than that expected on the basis of synergism. With regard to the premeiotics including differentiating and stem spermatogonia, however, the observed dominant lethal rates were lower than both the two expected rates. The situation between the observed and expected rates of total mutations was almost the same as that for dominant lethals. The observed viable mutation frequencies for the postmeiotics as well as the premeiotics were not significantly different from that expected on the basis of simple additivity, the rates expected on the basis of synergism being almost 10-fold higher than the previous two sets of rates. The ratio of total mutation rate to viable one changed from radiation-type to ENU-type with changes from postmeitic exposure to premeiotic exposure. Less
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