Polyploidy, increased sets of chromosomes, occurs during development, cellular stress, disease and evolution. Despite its prevalence, little is known about the physiological alterations that accompany polyploidy. We previously described 'ploidy-specific lethality', where a gene deletion that is not lethal in haploid or diploid budding yeast causes lethality in triploids or tetraploids. Here we report a genome-wide screen to identify ploidy-specific lethal functions. Only 39 out of 3,740 mutations screened exhibited ploidy-specific lethality. Almost all of these mutations affect genomic stability by impairing homologous recombination, sister chromatid cohesion, or mitotic spindle function. We uncovered defects in wild-type tetraploids predicted by the screen, and identified mechanisms by which tetraploidization affects genomic stability. We show that tetraploids have a high incidence of syntelic/monopolar kinetochore attachments to the spindle pole. We suggest that this defect can be explained by mismatches in the ability to scale the size of the spindle pole body, spindle and kinetochores. Thus, geometric constraints may have profound effects on genome stability; the phenomenon described here may be relevant in a variety of biological contexts, including disease states such as cancer.

In most animals, the brain makes behavioral decisions that are transmitted by descending neurons to the nerve cord circuitry that produces behaviors. In insects, only a few descending neurons have been associated with specific behaviors. To explore how descending neurons control an insect’s movements, we developed a novel method to systematically assay the behavioral effects of activating individual neurons on freely behaving terrestrial D. melanogaster. We calculated a two-dimensional representation of the entire behavior space explored by these flies, and we associated descending neurons with specific behaviors by identifying regions of this space that were visited with increased frequency during optogenetic activation. Applying this approach across a large collection of descending neurons, we found that (1) activation of most of the descending neurons drove stereotyped behaviors, (2) in many cases multiple descending neurons activated similar behaviors, and (3) optogenetically activated behaviors were often dependent on the behavioral state prior to activation.

Highlights d Genetic reagents target homologous neurons in multiple Drosophila species d Homologous descending neurons drive distinct fly songs in a similar social context d Evolutionary changes downstream of the homologous neurons cause song differences d Courtship song circuit multifunctionality may facilitate rapid fly song evolution

Wolbachia is an intracellular microbe harbored by a wide variety of arthropods (including Drosophila) and filarial nematodes. Employing several different strategies including male killing, induced parthenogenesis, cytoplasmic incompatibility, and feminization, and acting by as-yet-unknown mechanisms, Wolbachia alters host reproduction to increase its representation within a population. Wolbachia is closely associated with gametic incompatibility but also interacts with Drosophila in other, little understood ways. We report here significant and widespread infection of Wolbachia within laboratory stocks and its real and potential impact on Drosophila research. We describe the results of a survey indicating that ‫%03ف‬ of stocks currently housed at the Bloomington Drosophila Stock Center are infected with Wolbachia. Cells of both reproductive tissues and numerous somatic organs harbor Wolbachia and display considerable variation in infection levels within and between both tissue types. These results are discussed from the perspective of Wolbachia's potential confounding effects on both host fitness and phenotypic analyses. In addition to this cautionary message, the infection status of stock centers may provide further opportunities to study the genetic basis of host/symbiosis. and Nasonia vitripennis (Reed and Werren 1995) is charCulex pipiens, a rickettsia-like microorganism, Wolbachia pipientis was determined to be the agent responsible for acterized by asynchronous mitotic divisions and chromatin bridges between nuclei, defects that accumulate with a form of inherited reproductive failure (Laven 1959; subsequent mitotic divisions, resulting in embryo lethalYen and Barr 1973). This phenotype, termed cytoplasity. The earliest CI defect, observed in both Nasonia and mic incompatibility (CI), is manifest when a WolbachiaDrosophila, is delayed paternal pronuclear breakdown infected male mates with an uninfected female ( Figure 1) and entry into mitosis (Reed and Werren 1995; Calor with a female infected with a different Wolbachia type. laini et al. 1997;Tram and Sullivan 2002). In addition to Diptera, CI has since been found to be a In addition to CI, Wolbachia has been found to maWolbachia-induced trait in a wide diversity of arthropod nipulate host reproduction in other ways, including femorders, including Acarina (Vela et al. 2000), Coleoptera inization, male killing, and parthenogenesis. Each of these (Wade and Stevens 1985), Homoptera (Hoshizaki and Wolbachia-induced phenotypes serves the same purpose: Shimada 1995), Hymenoptera (Breeuwer and Werren to increase the prevalence of Wolbachia-infected indi-1990), Isoptera (Bandi et al. 1997), Lepidoptera (Brower viduals (Stouthamer et al. 1999). Although CI is by 1976), and Orthoptera (Kamoda et al. 2000). Since all Wolfar the most prevalent effect of Wolbachia infection in bachia are removed from spermatids prior to the compleDrosophila, male-killing Wolbachia have recently been tion of spermatogenesis (Bressac and Rousset 1993; described in Drosophil...

Body plans, which characterize the anatomical organization of animal groups of high taxonomic rank, often evolve by the reduction or loss of appendages (limbs in vertebrates and legs and wings in insects, for example). In contrast, the addition of new features is extremely rare and is thought to be heavily constrained, although the nature of the constraints remains elusive. Here we show that the treehopper (Membracidae) 'helmet' is actually an appendage, a wing serial homologue on the first thoracic segment. This innovation in the insect body plan is an unprecedented situation in 250 Myr of insect evolution. We provide evidence suggesting that the helmet arose by escaping the ancestral repression of wing formation imparted by a member of the Hox gene family, which sculpts the number and pattern of appendages along the body axis. Moreover, we propose that the exceptional morphological diversification of the helmet was possible because, in contrast to the wings, it escaped the stringent functional requirements imposed by flight. This example illustrates how complex morphological structures can arise by the expression of ancestral developmental potentials and fuel the morphological diversification of an evolutionary lineage.

Many developmental control genes contain stalled RNA Polymerase II (Pol II) in the early Drosophila embryo, including four of the eight Hox genes. Here, we present evidence that the stalled Hox promoters possess an intrinsic insulator activity. The enhancer-blocking activities of these promoters are dependent on general transcription factors that inhibit Pol II elongation, including components of the DSIF and NELF complexes. The activities of conventional insulators are also impaired in embryos containing reduced levels of DSIF and NELF. Thus, promoter-proximal stalling factors might help promote insulator-promoter interactions. We propose that stalled promoters help organize gene complexes within chromosomal loop domains.Supplemental material is available at http://www.genesdev.org.

The ventral midline is a source of signals that pattern the nerve cord of insect embryos. In dipterans such as the fruitfly Drosophila melanogaster (D. mel.) and the mosquito Anopheles gambiae (A. gam.), the midline is narrow and spans just 1-2 cells. However, in the honeybee, Apis mellifera (A. mel.), the ventral midline is broad and encompasses 5-6 cells. slit and other midline-patterning genes display a corresponding expansion in expression. Evidence is presented that this difference is due to divergent cis regulation of the single-minded (sim) gene, which encodes a bHLH-PAS transcription factor essential for midline differentiation. sim is regulated by a combination of Notch signaling and a Twist (Twi) activator gradient in D. mel., but it is activated solely by Twi in A. mel. We suggest that the Twi-only mode of regulation--and the broad ventral midline--represents the ancestral form of CNS patterning in Holometabolous insects.

Dorsoventral (DV) patterning of the Drosophila embryo is controlled by a concentration gradient of Dorsal, a sequence-specific transcription factor related to mammalian NF-B. The Dorsal gradient generates at least 3 distinct thresholds of gene activity and tissue specification by the differential regulation of target enhancers containing distinctive combinations of binding sites for Dorsal, Twist, Snail, and other DV determinants. To understand the evolution of DV patterning mechanisms, we identified and characterized Dorsal target enhancers from the mosquito Anopheles gambiae and the flour beetle Tribolium castaneum. Putative orthologous enhancers are located in similar positions relative to the target genes they control, even though they lack sequence conservation and sometimes produce divergent patterns of gene expression. The most dramatic example of this conservation is seen for the ''shadow'' enhancer regulating brinker: It is conserved within the intron of the neighboring Atg5 locus of both flies and mosquitoes. These results suggest that, like exons, an enhancer position might be subject to constraint. Thus, novel patterns of gene expression might arise from the modification of conserved enhancers rather than the invention of new ones. We propose that this enhancer constancy might be a general property of regulatory evolution, and should facilitate enhancer discovery in nonmodel organisms.