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Expression and Embryonic Function of Empty Spiracles: a Drosophila Homeo Box Gene with Two Patterning Functions on the Anterior-Posterior Axis of the Embryo

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Expression and Embryonic Function of Empty Spiracles: a Drosophila Homeo Box Gene with Two Patterning Functions on the Anterior-Posterior Axis of the Embryo Downloaded from genesdev.cshlp.org on September 28, 2021 - Published by Cold Spring Harbor Laboratory Press Expression and embryonic function of empty spiracles: a Drosophila homeo box gene with two patterning functions on the anterior-posterior axis of the embryo Dyana Dalton/'^ Robin Chadwick,^''* and William McGinnis^ ^Departments of Human Genetics and ^Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06511 USA Using the even-skipped homeo box as a probe to identify diverged homeo box genes in the Drosophila genome, we isolated the empty spiracles {ems) gene. Structural and functional comparisons between ems and other embryonic patterning genes of Drosophila suggest that ems acts, in part, as a homeotic selector gene, specifying the identity of some of the most anterior head segments. Mutant embryos lacking ems protein have severe patterning defects in the anterior head and are missing tracheal structures, including the filzkorper, which are normally developed by the eighth abdominal segment, ems has two different spatial patterns of expression during embryogenesis. The early, head-specific pattern consists of a single anterior stripe at the syncytial and cellular blastoderm stages. The later, metameric pattern consists of bilateral patches of ems expression in neural and ectodermal cells of every head and body segment. Variations of the ems expression pattern in bicoid mutants suggests that the morphogen protein produced by bicoid has a concentration-dependent regulatory role in the establishment of head-specific ems expression. In contrast, the metameric ems pattern is initiated independently of bicoid protein, and ems becomes expressed at high levels in the primordia of the duplicated filzkorper that develop in the anterior half of bicoid mutant embryos. [Key Words: Drosophila; empty spiracles-, homeo box; pattern formation] Received April 20, 1989; revised version accepted September 25, 1989. Most of the genes required for spatial patterning on the Many of the Drosophila embryonic patterning genes anterior-posterior (AP) axis of the Drosophila embryo are members of multigene families that share conserved have been identified in genetic screens for mutations protein-coding regions (McGinnis et al. 1984b; Frigerio that result in cuticular defects. Analysis of these genes et al. 1986). Thus, a method to isolate cloned copies of and their interactions has led to a tentative model of em­ potential embryonic patterning genes has been to use bryonic patterning on the AP axis. Establishment of these conserved protein-coding regions as probes to iso­ overall AP polarity requires the expression of maternal late and identify additional family members. This ap­ coordinate genes (Frohnhofer et al. 1986; MacDonald proach has been exploited most productively with the and Struhl 1986; Schupbach and Wieschaus 1986; Nuss- use of a conserved 180-bp DNA sequence called the lein-Volhard et al. 1987; Berleth et al. 1988). The meta­ homeo box, which has identified the coding regions of meric pattern of the embryo is then established by three many previously known Drosophila patterning genes classes of segmentation genes. The gap genes, the pair- (McGinnis et al. 1984a; Scott and Weiner 1984; Levine rule genes, and the segment polarity genes divide the et al. 1985) and at least one previously unidentified pat­ embryo into successively more discrete domains, re­ terning gene, caudal (Mlodzik et al. 1985; McDonald sulting in segmentation (Nusslein-Volhard and Wies­ and Struhl 1986). Homeo box sequences encode amino chaus 1980; for review, see Akam 1987). Finally, the ho­ acid domains of —60 residues called homeo domains, meotic selector genes, most of which are found in the which are part of proteins that range in size from —25 to Antennapedia and Bithorax complexes, are required to 75 kD. All homeo domains tested to date contain a se­ assign unique identities to the segments (Lewis 1978; quence-specific DNA binding activity (Desplan et al. Kaufman 1983). 1985, 1988; Hoey and Levine 1988; for review, see Scott et al. 1989). The DNA-binding and transcriptional regu­ ^Cuiient address: Genentech, Department of Developmental Biology, latory functions encoded in Drosophila homeo domain South San Francisco, California 94080 USA.; ^Department of Brain and proteins (Han et al. 1989; Jaynes and OTarrell 1988; Cognitive Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 USA. Driever and Nusslein-Volhard 1989; Krasnow et al. 1940 GENES & DEVELOPMENT 3:1940-1956 © 1989 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/89 $1.00 Downloaded from genesdev.cshlp.org on September 28, 2021 - Published by Cold Spring Harbor Laboratory Press Sttuctuie and function of empty spiiacles 1989; Winslow et al. 1989) are believed to be the prin­ Table 1. Cytological locations of the new homeo box clones cipal biochemical mechanisms through which homeo Clone Cytological location box genes assign cell, segmental, or regional fate. Because the homeo box is found in many develop­ E4 88A1,2 mental patterning genes, we have been isolating diver­ E5 88A gent members of the Diosophila homeo box gene family E81 69C/D with the expectation that some may control additional E86 38AI-3 developmental patterning functions. For example, sev­ E97 57B5,6 E103 90F eral known homeotic selector genes assign unique seg­ Elll 73A mental identities to thoracic and abdominal regions on the AP axis. However, no known selectors would assign the diverse head segment identities anterior to those as­ development. As is described in more detail below, the signed by Deformed {Dfd) and labial (Merrill et al. 1987; E4 homeo box clone detects transcripts and proteins at Regulski et al. 1987; Diederich et al. 1989). As is de­ the cellular blastoderm that are localized in a stripe in scribed here, one of the divergent homeo box genes that the primordia of the anterior head. A mutant with defec­ has been isolated, empty spiracles [ems], is required for tive head development, ems, had been mapped pre­ the development of structures that derive from some of viously to roughly the same cytogenetic position as E4 the most anterior head segments of the Drosophila em­ (Jiirgens et al. 1984). These initial results suggested that bryo. In addition, ems is required for development of the the E4 homeo box clone might encode ems. To test this tracheal system in the eighth abdominal segment. Com­ possibility, we performed a detailed molecular analysis parisons with other patterning genes lead us to propose of the E4 homeo box gene in wild-type chromosomes, as that the principal embryonic role of ems is to act as a well as in ems mutant chromosomes. homeotic selector gene controlling antennal and man­ dibular segment identity. Cytological location of the E4 clone Results The cytological location of E4 was found to be within the 88A1,2 doublet by in situ localization of a biotiny- Genomic library screen with the even-skipped homeo lated E4 probe in polytene chromosome preparations. box probe Several deficiency chromosomes with breakpoints in the To test whether the even-skipped {eve) homeo box could 88A region were used to provide a more detailed cyto­ detect more divergent members of the homeo box gene genetic localization for both E4 sequences and ems mu­ family, we hybridized an eve homeobox probe to a tant alleles. The deficiency chromosomes were first Southern blot of £coRI-digested Drosophila genomic tested by in situ hybridization for the presence or ab­ DNA. Under reduced stringency conditions, multiple sence of E4 DNA sequences on the deleted chromatid. £coRI fragments in the Drosophila genome hybridize Deficiency chromosomes were also placed in trans to re­ the eve homeo box probe. Most of the hybridizing frag­ cessive lethal ems alleles to determine whether each de­ ments differ in size from the £coRI fragments, repre­ ficiency uncovered the ems gene. The deficiency chro­ senting previously isolated homeo box genes (data not mosomes Df(3R)ry^^ (Hall and Kankel 1976) and shown), suggesting that they contain new members of Df(3R)redP^^ (Capdevila and Garcia-Belhdo 1981; R. the Drosophila homeo box family. Kelley^ pers. comm.), with breakpoints at 88A1 and To isolate clones containing these new homeo boxes, 88A4, respectively, contain E4 DNA sequences and do a Drosophila genomic library was screened under re­ not uncover the ems mutants (Fig. 1). The deficiency duced stringency conditions with the eve homeo box chromosomes Df(3R)fs293~^^ (R. Kelley, pers. comm.), probe. A total of 81 phage clones were isolated and sepa­ Df(3R)red^ (Cans et al. 1980; R. Kelley, pers. comm.), and rated into 12 different classes, based on high-stringency Df(3R)red^' (Hall and Kankel 1976), which have dele­ hybridization to each other and to previously isolated tions that eliminate the 88A1-88A4 region, are all homeo box genes. Five of the classes correspond to the missing E4 sequences and all uncover the ems muta­ previously isolated homeo box genes eve, Sex combs re­ tions. These results place both the E4 sequences and the duced [Scr], labial, H2.0, and bicoid [bed] (Kuroiwa et al. ems gene in the cytogenetic interval 88A1-88A4 (Fig. 1985; MacDonald et al. 1986b; Frasch et al. 1987; Barad 1). et al. 1988; Berleth et al. 1988; Diederich et al. 1989). The remaining seven classes, designated E4, E5, £81, Molecular structure of the E4 locus E86, E97, E103, and El 11, represent new Drosophila homeo box sequences mapping to the cytogenetic loca­ The eve homeo box screen of a XEMBL4 library led to tions shown in Table 1. the isolation of a 14-kb E4 genomic insert in the clone The new homeo box genes were initially characterized \E4, shown in Figure 2A. To test the number of tran­ by testing the temporal and spatial expression of their script species that include the E4 homeo box, a 4.1-kb transcripts during embryogenesis. We were particularly £coRI-Hi22dIII fragment, which contains the homeo box interested in any with potential roles in anterior head exon of the E4 transcription unit, was hybridized to a GENES & DEVELOPMENT 1941 Downloaded from genesdev.cshlp.org on September 28, 2021 - Published by Cold Spring Harbor Laboratory Press Dalton et al.
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