In the genus Drosophila, numerous species have evolved in such a way that interspecific differences may be found at any imaginable level of the genetic organization. The literature reports divergences in the chromosome sets, in the gene sequences, and in the length of recombination

maps of corresponding chromosomal elements. This investigation aimed at gathering data in Drosophila hydei, concerning mutagenesis. Such data, in comparison with existing data from other species, in particular Drosophila melanogaster, were expected to throw light upon the bearing which a specific type of genetic organization has on experimental manipulations of the genome.

A brood pattern method has been devised for D. hydei, which allows to assess the mutagenic action of X-rays on the various stages of spermatogenesis. Male germ cell development in D. hydei was found to require twice as much time as in D. melanogaster. Individual stages are prolongated to the same proportion. Their recognition was based on stage-specific mutational properties which coincided well with those reported from D. melanogaster.

A method is presented which permits the scoring of sex-linked recessive lethals in D. hydei. The method was used to determine the frequency of X-ray induced lethal mutations. This turned out to be 2.3x10^-2/1000 R in submature sperm and advanced spermatids, on the average. Measurements were made over a wide dose-range, between 1.5 und 12 kR. No noticeable difference was observed between D. melanogaster and D. hydei with respect to the rate of induced lethals. The mutation rates in D. funebris and D. subobscura seem to agree with this same figure.

D. hydei differs from D. melanogaster in the architecture of the X chromosome. Presumably because of this difference, two particular mutational events registered most clearly in the experirnents with D. hydei: (1) In the first generation progeny of irradiated males, a dose-dependent shift of the sex ratio was observed. This is due most likely to sex-linked types of mutation with dominant lethal effect, induced during spermiogenesis. The estimated rate of the dominant lethal effect is about twice that of recessive lethals. (2) Early stages of spermatogenesis were found to be particularly susceptible to the induction of bobbed deletions. This mutational event is interpreted as a breakage phenomenon whereby the nucleolar organizer region gets separated from the X chromosome and is subsequently lost.