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Experiments are described that provide an opportunity to estimate the genetic limits of the structural (amino acid coding) portion of the rosy locus (3:52.0) in Drosophila melanogaster, which controls the enzyme, xanthine dehydrogenase (XDH). This is accomplished by mapping experiments which localize sites responsible for electrophoretic variation in the enzyme on the known genetic map of null-XDH rosy mutants. Electrophoretic sites are distributed along a large portion of the null mutant map. A cis-trans test involving electrophoretic variants in the left- and right-hand portions of the map leads to the conclusion that the entire region between these variants is also structural. Hence most, if not all, of the null mutant map of the rosy locus contains structural information for the amino acid sequence of the XDH polypeptide. Consideration is given to the significance of the present results for the general problem of gene organization in higher eukaryotes.

A new mutant, mit (mitotic loss inducer), is described. The mutant is recessive and maternal in action, producing gynandromorphs and haplo-4 mosaics among the progeny of homozygous mit females. Mosaic loss of maternal or paternal chromosomes can occur. The probabilities of either maternal or paternal X chromosome loss are equal. mit has been mapped to approximately 57 on the standard X chromosome map.-Using gyandromorphs generated by mit, a morphogenetic fate map, placing the origins of 40 cuticular structures on the blastoderm surface, has been constructed. This fate map is consistent with embryological data and with the two other fate maps generated in different ways.

A key feature of the exosome model of transformation in Drosophila melanogaster is that the DNA segments responsible for the phenomenon are not integrated into the linear structure of the chromosomes. This feature is confirmed by the failure to find whole-body transformants in a series of large-scale experiments, and by the absence of evidence for integration under conditions favoring genetic exchange between chromosomes and introduced DNA.

The exosome model of transformation in Drosophila melanogaster proposes that DNA fragments enter the cells of treated individuals and become firmly associated with their homologous chromosome segments, but are never integrated into the linear structure of the chromosome. This paper reports results that validate two central predictions of the model: (a) the introduced genetic information usually maps precisely at the site of the target locus, and (b) the original chromosomal information is still present at that site.