mosomes in the larval salivary glands so that clones can be assigned positions in the genome by means of in situ hybridization. There are about 5000 polytene bands, most ranging in DNA content from 5 to 50 kilobase pairs (kb), with an average DNA content of 20 kb (Heino et al., 1994). The limit of cytological resolution with in situ hybridization is approximately 20 kb (Merriam et al., 1991). One advantage of the hybridization approach is that the approximate locations of clones covering much of the genome can be assembled relatively rapidly (yielding a ''framework map"), although clones that appear to be adjacent in the framework map need not necessarily overlap at the molecular level. The in situ mapping strategy therefore requires that molecular overlaps be determined after the framework map is completed rather than during assembly.

A physical map of the Drosophila genome based on yeast artificial chromosomes (YACs) ordered by in situ hybridization has been reported previously (Ajioka et al., 1991; Merriam et al., 1991; Hartl, 1992; Hartl and Lozovskaya, 1992; Lozovskaya et al., 1993; Cai et al., 1994). The YAC map includes 1200 clones with inserts averaging 200 kb that cover 90% of the euchromatic part of the genome. These clones have been grouped into 150 "cytological contigs," averaging 650 kb in extent, based on apparent overlaps detected by means of in situ hybridization; the gaps between cytological contigs average 50 kb (Hartl, 1992). The YAC clones provide molecular access to much of the Drosophila genome, but the vector has a low copy number and it is difficult to separate large quantities of YAC from contaminating yeast genome.

A second-level framework map based on bacteriophage P1 clones has now been assembled and is reported here. The map is based on 2461 clones with insert sizes averaging 80 kb that have been ordered by in situ hybridization. The P1 map includes an estimated 85% of the sites in the euchromatic genome with an average depth of coverage of 1.8. The mapped P1 clones, as well as 6755 additional clones, are being screened with STS markers in order to be arrayed in molecular contigs. The localizations of P1 clones and other information, as well as sources from which P1 clones may be obtained, are available to the scientific community by anonymous file-transfer protocol from ftp.bio.indiana.edu in the directory flybase or by electronic mail requests to flybase@morgan.harvard.edu.

Drosophila Strains and Procedures. Drosophila P1 clones were produced from DNA extracted from nuclei of adult D. melanogaster flies from an