Embryonic Mortality in Mouse Populations after Eleven Generations of Spermatogonial Irradiation.

WE have empirically approached the question of what will happen to the genetic constitution of an irradiated population of mice. That is, we have exposed populations of mice to radiation in a certain way for several successive generations and are now prepared to compare the progeny in the irradiated and nonirradiated populations by a variety of criteria. The empirical and analytical approaches to this question are complementary, not mutually exclusive. The approach we have used draws heavily upon the results of previous experiments of the analytical type (GREEN 1959, 1960). It has the potentiality of revealing any effect of long-continued irradiation of a genetic system, but it suffers from the disadvantage of not yielding by itself a clear-cut genetic interpretation of any difference that may be found. The analytical approach has the advantage of working with known types of mutations and aberrations, but the disadvantage of not being able to predict what will happen if those same changes are suffused into a population. In 1956 we started 12 populations of mice each with 16 mated pairs per generation with the mates chosen at random from the mice available in each generation except that brother-sister matings were excluded. The mice of the 192 initial matings were all descended from a single pair in an inbred strain, C57BL/lOGn. For brevity this is called the “inbred” experiment and is denoted by RX-ML. The 12 populations represent all possible combinations of three levels of radiation exposure with four levels of inbreeding. The radiation exposures were 0, 50, and 100r applied only to the gonads of young adult males with bodies shielded, seven weeks before mating. The four inbreeding levels were determined by the number of interbreeding pairs of mice in sublines within each population. Another experiment with exactly the same radiation and breeding plan was started in 1958 with 192 matings between mice descended from a 4-way cross of the C3HeB/FeJ, C57BL/6J, DBA/2J, and BALB/cJ strains. This is called the “hybrid” experiment and is denoted by DX-ML. Further details about the plan of the experiments appear elsewhere (GREEN 1964a). Waiting for seven weeks after irradiating the testes before making up the matings ensures that the sperm are descended from irradiated spermatogonia ( OAKBERG 1957). This means, further, that any mutations induced in the spermatogonia and subsequently transmitted to the progeny will most probably be recessive and that a large fraction of these, maybe as many as 75 percent, will be lethal when homozygous (RUSSELL and RUSSELL 1959). The probability for two mutant alleles, identical by descent from a single mutation, to unite at fertiliza-