Gonadal Mosaicism Induced by Chemical Treatment of Sperm inDrosophila melanogaster

Abstract

Accurate interpretation of forward genetic screens of chromosomes exposed in mature spermatozoa to a mutagenic chemical requires understanding-incomplete to date-of how exposed chromosomes and their replicas proceed through early development stages from the fertilized ovum to establishment of the germline of the treated male's offspring. We describe a model for early embryonic development and establishment of the germline of Drosophila melanogaster and a model-validating experiment. Our model proposes that, barring repair, DNA strands modified by treatment with alkylating agents are stable and mutagenic. Each replication of an alkylated strand can result in misreplication and a mutant-bearing daughter nucleus. Daughter nuclei thenceforth replicate faithfully and their descendants comprise the embryonic syncytium. Of the 256 nuclei present after the eighth division, several migrate into the polar plasm at the posterior end of the embryo to found the germline. Based upon distribution of descendants of the alkylated strands, the misreplication rate, and the number of nuclei selected as germline progenitors, the frequency of gonadal mosaicism is predictable. Experimentally, we tracked chromosomes 2 and 3 from EMS-treated sperm through a number of generations, to characterize autosomal recessive lethal mutations and infer gonadal genetic content of the sons of treated males. Over 50% of 106 sons bore germlines that were singly, doubly, or triply mosaic for chromosome 2 or chromosome 3. These findings were consistent with our model, assuming a rate of misreplication between 0.65 and 0.80 at each replication of an alkylated strand. Crossing treated males to mismatch-repair-deficient females had no apparent effect on mutation rate.KEYWORDS male sterility; meiotic mutants; paternal effect mutants; spermatogenesis S INCE "The artificial transmutation of the gene" (Muller 1926), it has been noted that newly induced mutations occasionally appear as mosaics in which only part of the mutant animal expresses the new mutation. The mutant tissue sometimes extends into the germline. Muller attributed this to a duality of the genetic material in the sperm, when only one of the two components is mutated. During the Second World War, Charlotte Auerbach (1946a,b) performed classified experiments using Drosophila melanogaster looking for genetic effects of mustard gas. Hers was the first demonstration of chemical mutagenesis. She also found mosaics, and with significantly higher frequencies than found following X irradiation. The nature of the duality of the genetic contents of mature spermatozoa remained enigmatic until the discovery of the double helix (Watson and Crick 1953a,b). Since that time many studies have attempted to explain results of chemical mutagenesis in terms of alterations of the DNA at the level of its individual stands, but have not fully characterized the effects on the germline. Such characterization is critical to the accurate interpretation of experimental data from forward genetic screens.Mosaicism ap...

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