Drosophila eye development: Notch and Delta amplify a neurogenic pattern conferred on the morphogenetic furrow by scabrous.

Loss of function mutations of scabrous and conditional alleles of Notch and Delta affect the pattern of morphogenetic furrow development. By studying differentiation of R8 cells, the first photoreceptor neuron subtype to differentiate, we show that all furrow cells pass through an R8-competent stage. Function of Notch and scabrous is necessary if most of these cells are to attain other cell fates. The scabrous gene confers a regular pattern on the morphogenetic furrow, restricting R8 differentiation to alternating groups of cells. Notch and Delta function to restrict the R8 fate to a single cell in each group. Without scabrous gene function, action of Notch and Delta on the entire morphogenetic furrow results in a disorganised pattern of ommatidia arising from a disorganised array of single R8 cells. Aspects of the scabrous mutant phenotype also suggest a secondary role in selecting a single R8 cell from competent clusters. We show that scabrous expression preceeds changes in the apical profiles of morphogenetic furrow cells that identify ommatidial precurf1p4cells, and also preceeds changes in levels of Notch and Delta expression. The pattern of initiation of sca expression depends on sca gene function, indicating that patterning of the morphogenetic furrow depends on the pattern of posterior columns. Our results suggest that in the eye, Notch and Delta amplify and refine a morphogenetic landscape generated by scabrous. Cell determination in other tissues and organisms might also be molded in a two-step process where initial inhomogeneities determined by one protein provide a context for subsequent development.

There are two mechanisms of achiasmate segregation in Drosophila females, one of which requires heterochromatic homology.

There are numerous examples of the regular segregation of achiasmate chromosomes at meiosis I in Drosophila melanogaster females. Classically, the choice of achiasmate segregational partners has been thought to be independent of homology, but rather made on the basis of availability or similarities in size and shape. To the contrary, we show here that heterochromatic homology plays a primary role in ensuring the proper segregation of achiasmate homologs. We observe that the heterochromatin of chromosome 4 functions as, or contains, a meiotic pairing site. We show that free duplications carrying the 4th chromosome pericentric heterochromatin induce high frequencies of 4th chromosome nondisjunction regardless of their size. Moreover, a duplication from which some of the 4th chromosome heterochromatin has been removed is unable to induce 4th chromosome nondisjunction. Similarly, in the absence of either euchromatic homology or a size similarity, duplications bearing the X chromosome heterochromatin also disrupt the segregation of two achiasmate X chromosome centromeres. Although heterochromatic regions are sufficient to conjoin nonexchange homologues, we confirm that the segregation of heterologous chromosomes is determined by size, shape, and availability. The meiotic mutation Axs differentiates between these two processes of achiasmate centromere coorientation by disrupting only the homology-dependent mechanism. Thus there are two different mechanisms by which achiasmate segregational partners are chosen. We propose that the absence of diplotene-diakinesis during female meiosis allows heterochromatic pairings to persist until prometaphase and thus to co-orient homologous centromeres. We also propose that heterologous disjunctions result from a separate and homology-independent process that likely occurs during prometaphase. The latter process, which may not require the physical association of segregational partners, is similar to those observed in many insects, in Saccharomyces cerevisiae and in C. elegans males. We also suggest that the physical basis of this process may reflect known properties of the Drosophila meiotic spindle.

The genetic analysis of distributive segregation in Drosophila melanogaster. I. Isolation and characterization of Aberrant X segregation (Axs), a mutation defective in chromosome partner choice.

We describe the isolation and characterization of Aberrant X segregation (Axs), a dominant female-specific meiotic mutation. Although Axs has little or no effect on the frequency or distribution of exchange, or on the disjunction of exchange bivalents, nonexchange X chromosomes undergo nondisjunction at high frequencies in Axs/+ and Axs/Axs females. This increased X chromosome nondisjunction is shown to be a consequence of an Axs-induced defect in distributive segregation. In Axs-bearing females, fourth chromosome nondisjunction is observed only in the presence of nonexchange X chromosomes and is argued to be the result of improper X and fourth chromosome associations within the distributive system. In XX females bearing a compound fourth chromosome, the frequency of nonhomologous disjunction of the X chromosomes from the compound fourth chromosome is shown to account for at least 80% of the total X nondisjunction observed. In addition, Axs diminishes or ablates the capacity of nonexchange X chromosomes to form trivalents in females bearing either a Y chromosome or a small free duplication for the X. Axs also impairs compound X from Y segregation. The effect of Axs on these segregations parallels the defects observed for homologous nonexchange X chromosome disjunction in Axs females. In addition to its dramatic effects on the X chromosome, Axs exerts a similar effect on the segregation of a major autosome. We conclude that Axs defines a locus required for proper homolog disjunction within the distributive system.

Molecular analysis of an unstable P element insertion at the singed locus of Drosophila melanogaster: evidence for intracistronic transposition of a P element.

In a companion study, a number of P element insertions into the singed locus were characterized. Here is reported a detailed analysis of the structure and mutability of another P element insertion at sn, known as sncm. Under conditions which mobilize P elements, sncm mutates at high frequency to both wild-type (sn+) and to a much more extreme allele (snext). Wild-type revertants appear to represent precise or nearly precise excisions of the P element. Certainly two, and most likely all five, of the snext alleles studied result from the insertion of a duplicate copy of this P element into a nearby site in an inverted orientation. We propose a model in which both the sn+ and snext mutational events can be explained by excision of the P element from one chromatid followed by reintegration into the sister chromatid at a nearby site (intracistronic transposition). Finally, it is shown that the snext alleles are themselves unstable and the structure of a resulting chromosome aberration is examined.

Autosomal modifiers of the bobbed phenotype are a major component of the rDNA magnification paradox in Drosophila melanogaster.

rDNA magnification in Drosophila melanogaster is defined experimentally as the ability of bb/Ybb- males to produce exceptional progeny that are wild type with respect to rDNA associated phenotypes. Here, we show that some of these bobbed-plus progeny result not from genetic reversion at the bb locus but rather from variants at two or more autosomal loci that ameliorate the bobbed phenotype of rDNA deficient males in Drosophila. In doing so we resolve several aspects of a long-standing paradox concerning the phenomenon of rDNA magnification. This problem arose from the use of two genetic assays, which were presumed to be identical, but paradoxically, produced conflicting data on both the kinetics of reversion and the stability of magnified bb+ chromosomes. We resolve this problem by demonstrating that in one assay bobbed-plus progeny arise primarily by genetic reversion at the bobbed locus, whereas in the other assay bobbed-plus progeny arise both by reversion and by an epistatic effect of autosomal modifiers on the bobbed phenotype. We further show that such modifiers can facilitate the appearance of phenotypically bobbed-plus progeny even under conditions where genetic reversion is blocked by magnification defective mutants. Finally, we present a speculative model relating the action of these modifiers to the large increases in rDNA content observed in males undergoing magnification.

Repair-defect mutations inhibit rDNA magnification in Drosophila and discriminate between meiotic and premeiotic magnification.

We have examined rDNA magnification in Drosophila melanogaster males carrying one of 11 recombination- or repair-defective mutations representing seven loci. We show that mutations defined by a defect in postreplication repair (mus-101, mei-41, and mus-108) are also defective in rDNA magnification, whereas mutations that do not affect postreplication repair have little or no effect on magnification. mei-41 inhibits only premeiotic magnification events, while mus-108 blocks both premeiotic and meiotic events. This suggests that meiotic and premeiotic events share some but not all functions. A molecular analysis of rDNA magnification reveals that in mus-108 males, changes in the rDNA restriction pattern can occur within one or a few generations under magnifying conditions. We interpret these data in terms of the role of DNA repair systems in rDNA magnification and in terms of stable maintenance of tandemly repeated genes.