The aim of this investigation was to isolate structural chromosome mutations causing "semi"-sterility which can be used for genetic control of the onion fly Hylemya antiqua
(Meigen). For the induction, X-rays or fast neutrons were applied in different doses on males and females. "Semi"-sterile families were screened cytologically for translocations and inversions. It appeared that high doses 0.5 krad of X-rays) tended to yield complex rearrangements which present difficulties in practical programs, in addition to the useful less complicated rearrangements. They also produce more genetic background damage than lower doses (0-5 krad), which still yield acceptable numbers of uncomplicated rearrangements and therefore are advised for a genetic control program. Because of strongly reduced fecundity of irradiated females these are less suitable for induction of rearrangements than males, although older females because of higher fecundity give better results than young females. Fast neutrons on young males were useful and a low dose of 0.25 krad appeared to be as good as 1.0 krad.
Seventeen different translocations and two pericentric inversions were observed. Most "breakpoints" are located on the longer chromosome arms. Inversion heterozygous males were as fertile as the control which confirms their achiasmate meiosis. Eight translocation stocks were sibcrossed. In five of these, translocation homozygotes were found as larvae and in three out of these five even as adults. The occurrence of duplication/deficiency (from adjacent I orientation) larvae in the five stocks mentioned was remarkable.
From experiments with an X-linked translocation it was concluded that the acrocentric chromosomes are the sex-chromosomes. The alternate and adjacent I orientation frequencies in translocation heterozygotes (X-linked) were shown to be equal in each sex, while no adjacent II was found. In this translocation meiotic numerical non- disjunction occurred in 18.7% of the tranlocation heterozygous females but in 2.0% of the males. This difference is probably a consequence of the difference in meiotic behaviour between the sexes. A good relationship was found between the degree of chromosomal unbalance and the frequency of late embryonic lethals (brown eggs) which was used as a graduator in the selection for "semi"-sterility.
Adult fertile translocation trisomics and adult sterile tertiary trisomics were obtained (both sexes) after meiotic numerical non- disjunction. In translocation trisomic males the X-, Y- and translocated (extra) X-chromosome were shown to disjoin at random. In females the two normal X-chromosomes almost (95%) preferentially disjoin, while the translocated X-chromosome goes to either one of the poles. Primary trisomic males (XXY) and females (XXX) were obtained from testcrossed translocation trisomic parents. XXY males produced four types of gametes XY, X, Y and XX in equal numbers. XXX females only gave XX and X gametes in an equal number. Succesfull attempts to obtain homozygotes for this X-linked translocation are reported. The theoretical background of genetic insect control is discussed.