Sex determination of the onion fly,Hylemya antiqua (Meigen)

X and Y chromosomes of Hylemya antiqua occur in two forms each. X_L and X_S, and Y₁ and Y₂. The larger X_L has an intercalary proximal segment which is absent in the more common smaller X_S. The acrocentric Y chromosome (Y₁), does not differ morphologically from X_S. A smaller metacentric Y₂ is apparently not homologous with Y₁. Two types of males, XY₁ and XXY₂, coexist in at least one Dutch population. XY₂ has been observed in one individual only. In larval ganglion cells an association has been observed between chromosome Y₂ and a probably non-homologous, intercalary segment of autosome 4. A numerical somatic variation of Y₂ can lead to gynandromorphs and sex ratios significantly different from 1∶1. XX cells can differentiate into functional spermatozoa in XX/XXY₂ mosaic testes. This indicates the presence of a diffusable male determining substance, which can reverse the “genotypic” sex of a cell. The occurrence of some spermatozoa-containing “cysts” in ovaries of two gynanders suggests a more or less autonomous (independent of the gonadal environment) differentiation of XXY₂ germ cells. XXXY₂ males and XXX females do not show a serious reduction in fertility. Even XXXXY₂ males do not exhibit any sign of intersexuality and spermatogenesis seems unaffected. All 62 scored M II cells of X-tetrasomic males contained 2 Xs.