Dominant lethal mutations in insects with holokinetic chromosomes. I. Irradiation of Oncopeltus (Hemiptera: Lygaeidae) sperm and oocytes

Sexually mature 7- to 8-day-old Oncopeltus fasciatus (Dallas) males and females were irradiated and outcrossed to untreated partners. The males received a dose of 10 krad and the females 1 krad. Egg hatch of females mated to irradiated males was reduced to 2.4% and that of irradiated females to 3.3% (only Metaphase I oocytes sampled). Eggs of different ages were dechorionated, fixed, and stained for cytological examination. Most eggs fertilized with irradiated sperm showed a decreased rate of embryonic development, but at least 70% reached the blastoderm stage. In eggs from irradiated females, chromosome aberrations were evident as early as the meiotic divisions that preceded syngamy. Again, early embryonic development was slow but otherwise essentially normal until the blastoderm stage. Thereafter developmental abnormalities occurred, and death ensued. Cells with more or less than the normal 16 chromosomes were not unusual in 18- to 20-hour embryos with irradiated parents. Different cells in the same embryo often had a wide range of chromosome numbers. Apparently, in species with holokinetic chromosomes, most dominant lethal mutations cause death after blastoderm formation. In dipteran and hymenopteran species, dominant lethal mutations are expressed during the early cleavage divisions.