Reversal of heterochromatization and the activity of the paternal chromosome set in the male mealy bug.

N male mealy bugs with the lecanoid chromosome system, one haploid set of I chromosomes becomes heterochromatic early in embryogeny, whereas both haploid sets of female nuclei remain euchromatic. The heterochromatic set was first described by SCHRADER (1921 ) . Later, HUGHES-SCHRADER (1935) showed that this set was not transmitted by the male to its offspring. SCHRADER and HUGHES-SCHRADER (1 931 ) suggested that the heterochromatic set was inactive genetically and HUGHES-SCHRADER (1 948) suggested that it was of paternal origin. BROWN and NELSON-REES (1961 ) analyzed chromosomes for aberrations in embryos which developed after paternal and maternal irradiation and demonstrated that the heterochromatic set was indeed of paternal origin. They also found that, after maternal irradiation of 8000r, very few sons and daughters survived. On the other hand, after paternal irradiation with up to 30,000 rep, all the zygotic daughters died, but all the sons survived. The death of the daughters after either maternal or paternal irradiation must have resulted from the induction of dominant lethals in the irradiated chromosome set. The survival of sons after paternal irradiation ‘with up to 30,000 rep indicated that the induced dominant lethals were not expressed. BROWN and NELSON-REES’ interpretation of these results was that the paternal set was genetically inactive when it was in the heterochromatic state. However, following paternal irradiation with doses above 30,000 rep, the survival of the sons declined sharply, suggesting some residual activity of the heterochromatic (H) set. This residual activity was studied further by NELSON-REES (1962) who showed that the sons surviving high doses of paternal irradiation were sterile. He also showed that at doses above 30,000 rep the irradiated paternal set was highly rearranged and contained fragments and translocations. Loss and nondisjunction of the irradiated chromosomes caused the amount of heterochromatic chromosomal material to vary greatly in young embryos, and only male embryos possessing an approximately normal bulk of the heterochromatic material survived. NELSON-REES’S results suggested that the H set performed a function which was essential for male fertility and that the residual activity of the H set expressed itself as a bulk requirement for the H set. Another observation which suggested that the H set may be genetically active