We describe a collection of P-element insertions that have considerable utility for generating custom chromosomal aberrations in Drosophila melanogaster. We have mobilized a pair of engineered P elements, p{RS3} and p{RS5}, to collect 3243 lines unambiguously mapped to the Drosophila genome sequence. The collection contains, on average, an element every 35 kb. We demonstrate the utility of the collection for generating custom chromosomal deletions that have their end points mapped, with base-pair resolution, to the genome sequence. The collection was generated in an isogenic strain, thus affording a uniform background for screens where sensitivity to genetic background is high. The entire collection, along with a computational and genetic toolbox for designing and generating custom deletions, is publicly available. Using the collection it is theoretically possible to generate Ͼ12,000 deletions between 1 bp and 1 Mb in size by simple eye color selection. In addition, a further 37,000 deletions, selectable by molecular screening, may be generated. We are now using the collection to generate a second-generation deficiency kit that is precisely mapped to the genome sequence. G ENETICALLY tractable model organisms are valufor components that function in particular pathways and characterize how individual genes participate in able research tools for uncovering basic biological such pathways. principles that are conserved through evolution. ManyThe fruit fly, Drosophila melanogaster, is one such tractamolecular pathways, such as signaling cascades, gene ble model that has been used extensively to elucidate regulatory pathways, and cell cycle control circuits, were many conserved genetic hierarchies. One particularly first characterized genetically in model systems. The powerful approach with Drosophila is the ability to rapsubsequent molecular cloning of the genes involved in idly carry out focused genome-wide screens for pathsuch pathways has shown how evolution has utilized way components by identifying loci that modify specific basic molecular building blocks to control a wide variety phenotypes (see St. Johnston 2002 for review). In this of biological processes. Key to the success of such apapproach, a sensitized genetic background, most comproaches has been the ability to carry out genetic screens monly exhibiting an easily scored adult phenotype such as rough eyes or a wing defect, is used to search for mutations in genes that make the phenotype more se- sensitized background and the phenotype is assessed. specific recombinase (FRT site) placed within intron one. In the case of RS3, a second FRT site is placed Importantly, the mutagenized chromosome is heterozygous, allowing genetic interactions between the sensiupstream of the first of the mini-white exons; in the case of RS5 the second FRT site is located downstream of tized background and mutations that are homozygous lethal to be detected. Particularly useful tools for such the mini-white exons. Golic and Golic demonstrated how a pair of RS3 and RS5 e...

We describe a second-generation deficiency kit for Drosophila melanogaster composed of molecularly mapped deletions on an isogenic background, covering 77% of the Release 5.1 genome. Using a previously reported collection of FRT-bearing P-element insertions, we have generated 655 new deletions and verified a set of 209 deletion-bearing fly stocks. In addition to deletions, we demonstrate how the P elements may also be used to generate a set of custom inversions and duplications, particularly useful for balancing difficult regions of the genome carrying haplo-insufficient loci. We describe a simple computational resource that facilitates selection of appropriate elements for generating custom deletions. Finally, we provide a computational resource that facilitates selection of other mapped FRT-bearing elements that, when combined with the DrosDel collection, can theoretically generate over half a million precisely mapped deletions.T HE availability of chromosomal deletion collections is of considerable benefit to the Drosophila research community for gene mapping, the phenotypic characterization of alleles, and genomewide genetic interaction screens. A core deficiency kit, composed of 270 genetically heterogeneous deletions covering 92% of the genome, has been built up over many years by the Bloomington Drosophila Stock Center (BDSC; http:/ / flystocks.bio.indiana.edu/Browse/df-dp/dfkit-info.htm). Continuing efforts by the Bloomington Center are currently focused on expanding genome coverage by recovering deletions in the vicinity of haplo-insufficient regions (K. Cook, personal communication). Despite the considerable utility of this collection, it does, by its very nature, suffer from a number of limitations. These include a heterogeneous genetic background, the presence of uncharacterized second-site mutations, and, for most deletions, molecularly undefined breakpoints. More recently, two groups have taken advantage of two key technologies: large collections of transposon insertions precisely mapped to the Drosophila genome sequence and site-specific recombination, to develop tools for producing custom chromosomal deletions in homogeneous genetic backgrounds that are mapped to the genome sequence with single-base-pair resolution (Parks et al. 2004;Ryder et al. 2004;Thibault et al. 2004).Sequence data from this article have been deposited with the EMBL/ GenBank data libraries under accession nos. AJ545047-AJ547612 and AJ622065-AJ622812. In both cases, the new deletion collections are generated using FLP-mediated recombination between pairs of transposon-borne FRT sites, a method originally developed in Drosophila by Golic and Golic (1996). In one case (Parks et al. 2004), a set of .29,000 P-element and piggyBac insertions (Thibault et al. 2004) were used to generate 519 deletions covering 56% of the euchromatic genome (the Exelixis collection). The high number of starting insertions used by this group allows fine-scale coverage of the genome with relatively small deletions; the average size of the exist...

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