Differential transmission of translocations induced in spermatogonia of mice by irradiation.

Abstract

The hindquarters of two batches each of five young adult male mice were exposed to an acute dose of 1,200 R of 250-kV X-rays in two fractions of 600 R, separated by a period of eight weeks. Means of 41.2% and 32.6% spermatocytes with one or more multivalent configuration were found when the mice were killed 13 to 18 and 59 weeks, respectively, after the second dose. The multivalent configurations were attributed to reciprocal transloeations induced in spermatogonia. Other types of chromosomal abnormality were rare. 150 sons were tested for semi-sterility and subsequently examined cytologically. Five were found to be translocation heterozygotes. One other was cytologically normal but nevertheless exhibited semi-sterility and transmitted this character to some of its descendents. The frequencies of spermatocytes with various numbers and types of multivalents were used to estimate the proportions of sperm with normal, balanced translocated and unbalanced haploid genomes, and hence the expected frequencies of zygotes with abnormal karyo types. The expected frequency of semi-steriles (or translocation heterozygotes) in the progeny proved to be about twice as great as that observed in parallel genetic experiments (Lyon <i>et al</i>., 1964; Searle, 1964) and in the sons of the irradiated males. Expected dominant lethals were also about twice as frequent as observed by Lyon <i>et al</i>. Whatever assumptions are made regarding the distribution of chiasmata in the multivalents and their manner of disjunction, the observed frequencies of translocation heterozygotes or dominant lethals or both are much less than the expected frequencies. The evidence is considered to favour a selective process operating on diploid rather than haploid genomes but perhaps taking effect on the haploid spermatids or sperm.