XO Type of Sex Determination in the Genus Chymomyza (Diptera : Drosophilidae)

The karyotypes of four species of the genus Chymomyza were studied with a larval brain cell-Giemsa staining technique. C. obscuroides possessed 2n=7 in male and 2n=8 in female, consisting of 3 pairs of metacentric autosomes and a single or 1 pair of acrocentric X chromosomes. C. japonica and C. caudatula showed the same chromosome numbers, 2n=7 in male and 2n=8 in female, but with a different composition : 2 pairs of metacentric autosomes, 1 pair of microchromosomes and a single or 1 pair of acrocentric X chromosomes. These three species of the costata species-group had XO type of the sex determination. On the other hand, C. pararufithorax of the procnemis group retained 2n=8 in both sexes, with 2 pairs of metacentric autosomes, 1 pair of microchromosomes and 1 unequal pair of sex chromosomes, acrocentric X and submetacentric Y.