INTRODUCTION The Hox genes, which encode homeodomain-containing transcription factors, are involved in the specification of body plans of multicellular organisms (reviews, Kenyon et al., 1997; Lawrence and Morata, 1994; McGinnis and Krumlauf, 1992). In both insects and vertebrates, these genes are organized into a conserved chromosomal cluster from which they are segmentally expressed in a temporal and spatial order, termed colinearity, along the anterior-posterior (A-P) body axis of the embryo (Duboule and Morata, 1994). Lack of expression or ectopic expression of Hox genes can cause the change of one segment identity to another, a phenomenon referred to as homeotic transformation. The C. elegans Hox genes, lin-39, ceh-13, mab-5 and egl-5, are not tightly clustered, unlike the Hox genes in Drosophila and vertebrates (Bürglin and Ruvkun, 1993). However, functional conservation has been demonstrated between lin-39 and the Drosophila Hox gene Scr, as well as between mab-5 and Drosophila Antp in larvae and adult worms (Hunter and Kenyon, 1995). In addition, region-specific expression of the genes lin-39, mab-5 and egl-5 has been observed in embryos and larvae. Unlike their insect and vertebrate counterparts, however, the lin-39, mab-5 and egl-5 genes do not appear to be essential for embryogenesis. Instead, mutants defective for these genes show abnormal postembryonic development and sexual maturation (Chisholm, 1991; Chow and Emmons, 1994; Clark et al., 1993; Wang et al., 1993) Analyses of these genes have demonstrated that they are necessary to specify cell fates along the A-P axis of the worm, as well as to act as a developmental switch that controls cell migration (op. cit.; Salser and Kenyon, 1992). CEH-13 possesses structural features typical of the labial or HOX1 class of proteins including characteristic residues inside and outside the homeodomain (Schaller et al., 1990; Sharkey et al., 1997). In insects and chordates, genes of the labial class have been shown to have multiple functions in the anterior part of the embryo (Carpenter et al., 1993; Dollé et al., 1993; Gavalas et al., 1998; Goddard et al., 1996; Hirth et al., 1998; Mark et al., 1993; Merrill et al., 1989; Studer et al., 1996) . Here we show that CEH-13 is present in many different cell types and its rostral boundary of expression is located anteriorly to those of the other C. elegans Hox genes. Furthermore, we describe the phenotypic analysis of ceh- 13(sw1) mutants during C. elegans embryogenesis. Using a ‘four-dimensional’ (4D) recording system and immunocytochemistry, we demonstrate that ceh-13 mutants show a disorganization of epidermal and mesodermal cells as well as adhesion defects in the anterior part of the embryo. These results suggest that the ceh-13 gene is required to organize the anterior part of the C. elegans embryo. 1537 Development 126, 1537-1546 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 DEV7709 The Caenorhabditis elegans lin-39, mab-5 and egl-5 Hox genes specify cell fates along the anterior-posterior body axis of the nematode during postembryonic development, but little is known about Hox gene functions during embryogenesis. Here, we show that the C. elegans labial- like gene ceh-13 is expressed in cells of many different tissues and lineages and that the rostral boundary of its expression domain is anterior to those of the other Hox genes. By transposon-mediated mutagenesis, we isolated a zygotic recessive ceh-13 loss-of-function allele, sw1, that exhibits an embryonic sublethal phenotype. Lineage analyses and immunostainings revealed defects in the organization of the anterior lateral epidermis and anterior body wall muscle cells. The epidermal and mesodermal identity of these cells, however, is correctly specified. ceh- 13(sw1) mutant embryos also show fusion and adhesion defects in ectodermal cells. This suggests that ceh-13 plays a role in the anterior organization of the C. elegans embryo and is involved in the regulation of cell affinities. Key words: Caenorhabditis elegans, Hox gene, labial class, Morphogenesis, Pattern formation, Cell affinity SUMMARY Anterior organization of the Caenorhabditis elegans embryo by the labial-like Hox gene ceh-13 Karin Brunschwig 1 , Claudia Wittmann 1 , Ralf Schnabel 2, *, Thomas R. Bürglin 3 , Heinz Tobler 1 and Fritz Müller 1,‡ 1 Institute of Zoology, University of Fribourg, Pérolles, CH-1700 Fribourg, Switzerland 2 Max-Planck-Institut für Biochemie, 82152 Martinsried, Germany 3 Department of Cell Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland *Present address: Institut für Genetik, TU Braunschweig, Spielmannstrasse 7, D-38106 Braunschweig, Germany ‡ Author for correspondence (e-mail: fritz.mueller@unifr.ch) Accepted 18 January; published on WWW 3 March 1999