Structural mutants of the W chromosome that are kept in laboratory strains are described using light and electron microscopic techniques. The series comprises a translocation, a deficiency, fusions, and W fragments. -Conclusions drawn from the mutants are the following: At least one half of the W serves no vital function and is dispensable. Female sex determination, if the W is involved at all, rests on the other half. The amount and form of heterochromatin in somatic interphase nuclei depends on the amount and location of the W chromosome material. The W inactivation in somatic cells does not spread to attached autosomal segments.

We describe six new mutants, spontaneous or EMS-induced, and a previously isolated radiation-induced mutant of Megaselia scalaris Loew (Diptera, Phoridae). Five are eye-color mutants, one affects the segmentation of the abdomen, and one disturbs the regular form and arrangement of ommatidia in the compound eye. A complementation test exposed two pairs of alleles among the five eye-color mutants, leaving three different loci that affect eye pigmentation. Two of them influence the amount of xanthommatin synthesized, and one blocks the ommochrome pathway at the kynurenine step. We established chromosome assignment of the mutants by crossbreeding them with Y chromosome-autosome translocation strains.

No abstract