Cell lineage analysis of the maroon-like mutation of Drosophila melanogaster revealed the most extensive degree of non-autonomy reported to date in Drosophila: all 1454 gynandromorphs in which X chromosome loss uncovered the ma-l mutation had ma-l+. eye colour. In contrast, among 331 gynandromorphs in which X chromosome loss simultaneously uncovered the vermilion and maroon-like mutations, approximately 16% had v phenotype but with one possible exception all gynandromorphs again had ma-l+ eye colour. These results suggest that very small amounts of the ma-l+ gene product are necessary for wild-type eye colour development and they are therefore compatible with the one cistron–allelic complementation model that has been proposed for the ma-l locus. They also provide the best estimate available to date of In(1)wvc-induced internal mosaicism: 7%. A preliminary attempt to detect DNA-induced transformants among 6 DNA-injected preblastoderm ma-l embryos and at least 80000 of their F1 to F4 descendants has yielded completely negative results. An investigation of the maternal effect which ma-l+ mothers exert on the eye colour of their genetically ma-l offspring revealed that, in contrast to earlier observations, this effect is not universal: some phenotypically ma-l and intermediate ma-l flies were observed in young cultures. The discrepancy between this and earner observations is probably attributable to as yet uncharacterized nutritional deficiencies in the diet of flies used in this experiment. Cytoplasm drawn from blastoderm ma-l+ embryos and injected into the posterior region of ma-l preblastoderm embryos failed to induce eye-colour alterations in all seven flies which survived the treatment. Injection of the contents of embryos of certain genotypes and developmental stages into ma-l pupae 24–48 h old did alter in some instances the eye colour of treated ma-l flies. Various tests strongly suggest that these alterations are not due to injection of a substance that has been stored in the egg during oogensis or that has been produced by the embryo itself prior to injection and they therefore preclude the possibility that a simple in vivo bioassay for the ma-l+ substance has been achieved. Rather, they indicate that the observed eye-colour alterations are due to transplantation of blastoderm-stage embryos which remain active long enough within ma-l hosts to produce and release a substance into the hosts' haemolymph and that this substance in turn induces phenotypic alterations in the hosts' eye colour. When v and ma-l eye colour changes are simultaneously monitored, it appears that injection of embryonic contents into pupae is equally or more effective in modifying the v phenotype than in modifying the ma-l phenotype. Based on these observations, a tentative hypothesis regarding the time of activation of the ma-l+ gene and the relationship between the immediate product of this gene, the maternal substance stored in the egg and the substance released by tissue transplants is proposed.
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