D U R I N G larval and early pupal stages, the testes of Drosophila melamgmter are ellipsoidal in form. Following attachment of the developing seminal vesicle to the testes, the latter elongate and coil. The seminal vesicles are derived from the genital disc. If attachment does not occur the testes do not develop in this manner, even though they may increase in size. A defect of the genital disc, resulting in a n absence of the seminal vesicles, occurs in over 60% of the male progeny when

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... s with specific attached-X chromosomes are backcrossed to males from the tumorous-head strain (symbolized by tu-h) for several generations (WOOLF 1966). IJndeveloped (bean-shaped) testes are the most obvious manifestation of this genital disc defect. The genetic mechanism causing this defect is complex consisting of a third chromosome gene from flies of the tu-h strain, a maternal effect attributable to chromosome 1, and polygenic modifiers. Flies from the tu-h strain contain a set of these modifiers, primarily on chromosome 2. Seven of nine different attached-X chromosomes tested were shown to contain the gene producing the maternal effect ( WOOLF 1966). The tumorous-head strain is characterized by flies with amorphous head growths (NEWBY 1949) . The abnormality results from a third chromosome gene (tu-3) showing reduced penetrance. The frequency of affected flies is increased by a sex-linked recessive gene (tu-I) responsible for a maternal effect and polygenic modifiers mainly on chromosome 2. When tu-1 and tu-3 are homozygous, and modifiers are present at a high frequency, over 80% of the flies of the strain show an amorphous head growth (GARDNER and WOOLF 1949; GARDNER, STOTT and DEARDEN 1952) . The sex-linked gene tu-I occurs at a high frequency in some natural populations (JOHNSON and GARDNER 1965) . The objectives of this paper are to present evidence that (1) tu-3 is the gene responsible for the male genital disc defect, (2) the sex-linked gene producing the maternal effect involved in the male genital disc defect is tu-I+, ( 3 ) modifiers have a profound influence on the frequency of males with the genital disc defect, and (4) even though the genital disc defect is sex-limited, it occurs in transformed females (XXY flies homlozygous for tra) . PROCEDURE AND RESULTS All progeny were reared at 25°C in half-pint bottles containing the standard agar-cornmealbrewers ycast-molasses-propionic acid medium. Flies were dissected in insect saline solution.