Dan L. Lindsley

Toward a comprehensive genetic analysis of male fertility in Drosophila melanogaster.

Drosophila melanogaster is a widely used model organism for genetic dissection of developmental processes. To exploit its full potential for studying the genetic basis of male fertility, we performed a large-scale screen for male-sterile (ms) mutations. From a collection of 12,326 strains carrying ethyl-methanesulfonatetreated, homozygous viable second or third chromosomes, 2216 ms lines were identified, constituting the largest collection of ms mutations described to date for any organism. Over 2000 lines were cytologically characterized and, of these, 81% failed during spermatogenesis while 19% manifested postspermatogenic processes. Of the phenotypic categories used to classify the mutants, the largest groups were those that showed visible defects in meiotic chromosome segregation or cytokinesis and those that failed in sperm individualization. We also identified 62 fertile or subfertile lines that showed high levels of chromosome loss due to abnormal mitotic or meiotic chromosome transmission in the male germ line or due to paternal chromosome loss in the early embryo. We argue that the majority of autosomal genes that function in male fertility in Drosophila are represented by one or more alleles in the ms collection. Given the conservation of molecular mechanisms underlying important cellular processes, analysis of these mutations should provide insight into the genetic networks that control male fertility in Drosophila and other organisms, including humans.

Anent the genomics of spermatogenesis in Drosophila melanogaster.

An appreciable fraction of the Drosophila melanogaster genome is dedicated to male fertility. One approach to characterizing this subset of the genome is through the study of male-sterile mutations. We studied the relation between vital and male-fertility genes in three large autosomal regions that were saturated for lethal and male-sterile mutations. The majority of male-sterile mutations affect genes that are exclusively expressed in males. These genes are required only for male fertility, and several mutant alleles of each such gene were encountered. A few male-sterile mutations were alleles of vital genes that are expressed in both males and females. About one-fifth of the genes in Drosophila melanogaster show male-specific expression in adults. Although some earlier studies found a paucity of genes on the X chromosome showing male-biased expression, we did not find any significant differences between the X chromosome and the autosomes either in the relative frequencies of mutations to male sterility or in the frequencies of genes with male-specific expression in adults. Our results suggest that as much as 25% of the Drosophila genome may be dedicated to male fertility.

Edward Novitski: Drosophila Virtuoso

EDWARD Novitski, 1918–2006, was the acknowledged master of that special art of manipulating chromosomes during what Lucchesi (1994) called “the age of Drosophila chromosome mechanics.” Following the Sturtevant tradition, his guiding principle was to derive as much information as possible from breeding experiments with minimum use of direct cytological examination. Nobody could perform this kind of chromosome manipulation as well as Ed and he relished new challenges. (Novitskis closest friends and relatives, especially in later years, called him Eddie, a name he seems to have preferred. Those, such as ourselves, whose acquaintance extends over many years, knew him as Ed, which we shall adopt here.) He continued this kind of work long after the development of microbial and molecular genetics had carried the field in new directions. Another side of Novitski was a love of pranks and practical jokes, which were indeed clever, sometimes diabolically so. He reached what must be the pinnacle of achievement in this rarified atmosphere, for his reputation was such that pranks that he never carried out were attributed to him nonetheless. He admired Richard Feynman, a physics genius who also enjoyed such things as safe cracking, bongo drums, and practical jokes. Both men took great pleasure in “finding things out” and shared highly creative minds and a love of pranks. Fortunately, Ed completed an autobiographical memoir not long before his death (Novitski 2005). He was unusual; so, naturally, the book is unusual. It has four main sections. The first is “Fun and Games,” a series of anecdotes and practical jokes. The first prank is his most famous, perpetrated on Herschel Roman. Ed manipulated the seminar clock to run slowly so that Herschel, after finishing his talk, seemed to have an embarrassingly long period of time remaining to be filled. His attempts to extricate himself make an amusing story. The second part of Eds book is the story of his own life. The third is his account of the rift between Alfred Sturtevant and Theodosius Dobzhansky. Novitski had the unique vantage point of having been successively a student of each of them, and he is clearly in Sturtevants corner. The fourth part is titled “The Pleasure of Finding Things Out.” It recounts some of his most interesting intellectual challenges. The book is as idiosyncratic as Ed himself, a mixture of deep science, anecdotes, intellectual depth, and whimsy.

Gonadal Mosaicism Induced by Chemical Treatment of Sperm in Drosophila melanogaster

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.