A mammalian spot test: Induction of genetic alterations in pigment cells of mouse embryos with X-rays and chemical mutagens

Summary

Embryos heterozygous for five recessive coat-color genes from the cross C 57 BL/6 J Han x T-stock were X-irradiated with 100 r or treated in utero with 50 mg/kg methyl methanesulfonate (MMS) and ethyl methanesulfonate (EMS), respectively. Controls consisted of irradiated embryos of C 57 BL x C 57 BL matings homozygous wild-type for the genes under study, and non-treated offspring of both types of mating.

The colors of the spots observed in the adult fur were either due to expression of the recessive coat genes or were white.

• 1. Irradiated and mutagen-treated offspring of C 57 BL x T-stock matings had almost exclusively nonwhite spots, distributed randomly over the mouse surface.

• 2. Irradiated offspring of C 57 BL x C 57 BL matings had only white spots which were always midventral.

• 3. In non-treated offspring of both types of mating no spot could be observed.

After correcting for white midventral spots observed in the one type of control, the frequency of expression of one or the other of the recessive color genes is calculated to be about 11% for embryos irradiated with 100 r at 11 days postconception, about 8% for embryos irradiated with 100 r at 10${\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}$ days postconception, about 1% for embryos irradiated with 100 r at 9 days postconception, about 7% for embryos treated with 50 mg MMS/kg at 10${\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}$ days postconception, and about 8% for embryos treated with 50 mg EMS/kg at 10${\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}$ days postconception.

It is discussed that the white midventral spots are preferentially the result of pigment cell killing, while the nonwhite spots are preferentially the result of gene mutations or recombinational processes like mitotic crossing over and mitotic gene conversion. Of numerical and structural chromosome aberrations only those come into question which are able to pass the filter of several mitoses. Therefore, the test system described is supposed to cover only heritable DNA-alterations, but the whole spectrum of them.