Egg Morphology Mutants

We have conducted a hybrid dysgenic screen of the X chromosome for mutations affecting female fertility, with particular attention to those causing abnormal egg and eggshell morphology. In a screen of 4017 dysgenic strains, 398 mutants derived from 168 different germ lines were isolated and assigned to eight classes according to their diverse phenotypes. One interesting class consists of mutants that block oogenesis at specific stages. Our analysis has focused on mutations affecting eggshell formation, including mutants that lay morphologically abnormal sterile eggs as well as those that lay no eggs but exhibit blocks in the late stages of oogenesis. A subset of 48 mutants was assorted into 30 allelic groups by inter se complementation and genetically localized by interval mapping. Two multiallele complementation groups, del (7 alleles) and ne1 (8 alleles), were identified as well as five two-allele complementation groups. A search for alleles among mutants generated in other female sterile screens was unsuccessful, pointing to the distinctive nature of the dysgenic mutant collection. The single case of allelism determined in this study was one with a lethal allele of the Broad-Complex, l ( l )npr, suggesting a possible involvement of ecdysone in choriogenesis. A subset of 18 dysgenic strains was analyzed for P element hybridization and 16 of these were found to have hybridization signals in the appropriate cytogenetic interval. By examining these signals in two or more alleles of the same complementation group, we have been able to tentatively localize two mutations. Light and eiectron microscopy of the eggshell in 43 different strains has revealed a variety of effects. The respiratory appendages were defective in 27 of these mutants. Effects on the ultrastructure of the main body of the endochorion were not strongly correlated with the appendage defects, and could be classified as minor (1 4 mutants) or major (16 mutants). Although 13 mutants showed no ultrastructural chorion defects, six of these had defective respiratory appendages. I N Drosophila, the events of oogenesis take place over approximately 80 hr in follicles (egg chambers) which consist of the oocyte, 15 nurse cells and ca. 1000 follicular epithelial cells. Reflecting the morphology of the maturing follicle, oogenesis has been divided into 14 stages (KING 1970). It is initiated when primordial follicle cells of mesodermal origin envelop the interconnected cyst of germ cell derivatives, the oocyte plus associated nurse cells, to form the stage 1 egg chamber. Later, landmark events of oogenesis include the deposition of yolk proteins within the oocyte (vitellogenesis) and the elaboration of the eggshell (vitelline membrane formation and choriogenesis). During oogenesis the framework for early embryonic development is also laid down, as indicated by maternal effect mutations (NUSSLEIN-VOLHARD 1979). Mutagenesis experiments have provided a rich res' To whom correspondence should be addressed. of page charges. This article must therefore be hereby marked "advertisement" The publication costs of this article were partly defrayed by the payment in accordance with 18 U.S.C $1734 solely to indicate this fact. Genetics 122: 847-858 (August, 1989) ervoir of female sterile mutants to aid in the identification of genes that function during oogenesis and embryogenesis (summarized in PERRIMON et al. 1986). We have been particularly interested in identifying genes that affect eggshell formation. To this end we have conducted a P-M hybrid dysgenic screen of the X chromosome for eggshell defect mutants. Ourintention was to generate alleles that might be tagged by insertion of a P element and thus be amenable to a molecular analysis, once initial characterization of the mutants had been carried out. In this report we summarize the results of this mutagenesis and present preliminary genetic and morphological characterization of the 398 mutant strains obtained. We note that this collection exhibits a significant diversity of mutant effects (eight different categories). Moreover, examination of mutants representing two of these categories (those laying sterile or semisterile eggs of abnormal morphology and those laying no eggs) has permitted identification of a subset of mutations having distinct effects during oogenesis.