Identification of a polytene chromosome band containing a male sex determiner of Chironomus thummi thummi

Hybrid males of Chironomus thummi piger x Ch. th. thummi crosses were backcrossed with females of both parental stocks. Fourth-instar larvae of these backcrosses showed sex specific differences in the pairing behavior of region D₃d-g in chromosome arm F of salivary gland chromosome III. — Analysis of the banding pattern of region D₃d-g after RB and quinacrine staining demonstrated that in piger x thummi hybrid males a single selectively stained band occurs within this region in the heterozygous condition at map position D₃e₁. This band could only be found in the thummi chromosome partner, it is heterochromatic and contains AT-rich DNA. In female hybrid larvae, however, such a selectively stained band is present in neither the thummi nor the piger chromosome region D₃d-g. From these results it is concluded that the selectively stained band D₃e₁ represents the male sex determiner of our Ch. th. thummi stock and that the male is the heterogametic sex.