Genes Controlling Segmental Specification in the Drosophila Thorax
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Proc. NatL Acad. Sci. USA Vol. 79, pp. 7380-7384, December 1982 Developmental Biology Genes controlling segmental specification in the Drosophila thorax (homeosis/insect segments/determination/clonal analysis) GARY STRUHL* Medical Research Council Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, England Communicated by M. F. Perutz, August 25, 1982 ABSTRACT The roles of three homeotic genes, Ubx+, Scr+, METHODS and Antp+, in the Drosophila thorax have been studied by deter- Genotypes Employed. Except where otherwise indicated, mining the cellular phenotypes of mutations resulting in loss of all mutations and chromosomes are described in ref. 13. The gene function. The principal results are: (i) The Scr+ and Ubx+ Ubx' mutation behaves as a hypomorph, resulting in the loss genes are required in the prothorax and metathorax, respectively; of most, but probably not all, of the wild-type gene function in the absence ofthese genes, both segments develop like the me- (14, 15). The AntpNs+RC3 mutation is an apparent null allele of sothorax. (ii) The Antp+ gene is required in all three thoracic seg- the Antp+ gene (7). The Scrl3A mutation is a recessive lethal ments: in its absence, parts ofthe mesothorax are transformed into allele kindly provided by C. Nusslein-Volhard. The ScrCl mu- corresponding parts of the antenna, and similar transformations tation was induced on a chromosome already bearing the to antenna are found in the prothorax and metathorax if the Scr+ AntpNS+RC3 mutation with the mutagen ethyl methanesulfonate and Ubx+ genes also are absent. (iii) Loss of the Ubx+ gene early (unpublished data). Both the Scrl3A and ScrCl mutations cause in embryogenesis, but not later, leads to the inappropriate activity embryonic lethality when trans to other Scr mutations; also, of the Scr+ gene in the meso- and metathorax. Results i and ii such mutant embryos show the characteristic homeotic phe- argue strongly that segmental determination is specified in a com- notype resultingfrom loss ofthe Scr+ gene described previously binatorial fashion in the head and thorax by the selective activities (6). of the Scr+, Ubx+, Antp+, and putative head-determining genes. Analysis of Mutant Clones. Marked clones of cells homo- Result iii suggests that a product of the Ubx+ gene also plays an zygous for mutations of the Scr, Antp, and Ubx loci were ob- early, regulatory role in ensuring the correct spatial expression tained as described below for the triple mutant combination of the Scr+ gene during subsequent development. ScrCl AntpNs+RC3 Ubx'. A chromosome carrying all three mu- tations as well as the ell mutation was constructed, and embryos Insects are composed of a series ofhead, thoracic, and abdom- or larvae of the genotype Scrcl AntpNs+RC3 Ubx' e"/Ki Sb63b inal segments, each displaying unique and sometimes dramat- M(3)w"24 were irradiated at appropriate times after egg laying ically different characteristics. Yet, in the absence ofparticular (3 ± 1 hr = blastoderm stage and 24-48 hr = first larval instar; homeotic genes, one or more segments can be transformed into all experiments were performed at 25°C). All the mutations are other segments (1-4). Thus, segments are homologous units in located on the right arm of the third chromosome; because Ki which diverse cell patterns develop under discrete genetic is positioned closest to the centromere, mitotic recombinations controls. that occur between Ki and the centromere can give rise to Of the many homeotic genes that have been described in homozygous Ki+ Sb+ M(3)w+ cells, which must also be homozy- Drosophila, several have the distinguishing feature that they gous for the Scrl3A, AntpNs+RC3, Ubxl, and ell mutations. The must be active in some segments and inactive in others [e.g., Kinked, Stubble b, and Minute(3)w"2" mutations are dominant genes of the bithorax-complex (1, 2) and the Antp+ (5-7) and and affect bristle morphology; the ebony" mutation is recessive Scr+ (8-10, 6) genes]. Such segment-specific genes appear to and causes cuticle, especially bristles, to be heavily pigmented. the Thus, clones ofhomozygous cells bear blackened, but otherwise dictate the particular developmental pathways followed by normal, bristles, in contrast to surrounding heterozygous cells different segments. Recently, a second class ofhomeotic genes that bear gnarled, stunted bristles. In addition to affecting bris- has been identified whose members appear to be required in tle morphology, the M(3)w"24 mutation also causes heterozy- most, or all, segments. The products oftwo ofthese genes [Pc+ gous cells to divide at slower than normal rates (16). Conse- (2, 11) and esc+ (12)] appear to be required for the selective quently, homozygous Ki+ cells carrying two copies of the expression of the bithorax-complex genes and possibly other M(3)w+ allele are able to grow faster than surrounding hetero- segment-specific genes. Thus, these gene products may have zygous cells (17) and hence, can form large portions of the regulatory roles in ensuring the correct expression of segment- adult compartments (18) to which they contribute. specific homeotic genes. Clones were identified either under the dissecting or com- In this report I describe the interdependent roles of three pound microscope by the appearance of marked bristles and segment-specific homeotic genes (Ubx+, Antp+, and Scr+) in because they frequently disrupted the normal morphology. the cells giving rise to the adult thorax of Drosophila. The re- Clones in several parts ofthe fly that normally carry few, or no, sults argue strongly that these genes act in a combinatorial fash- large bristles were difficult to detect and hence, may have been ion to specify segmental determination. In addition, they sug- missed. In addition, because of the large numbers of flies in- gest that the products of some of these genes also have volved (ca. 15,000), it was not possible to screen all parts of all regulatory roles in controlling the selective expression ofothers. flies with equal thoroughness. For these reasons, the frequen- cies ofclones in different parts ofthe fly, or ofclones ofdifferent The publication costs ofthis article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertise- * Present address: Dept. of Biochemistry and Molecular Biology, Har- ment" in accordance with 18 U. S. C. §1734 solely to indicate this fact. vard University, 7 Divinity Ave., Cambridge, MA 02138. 7380 Downloaded by guest on September 30, 2021 Developmental Biology: Struhl Proc. Natl. Acad. Sci. USA 79 (1982) 7381 genotypes in the same part, cannot be usefully compared. viable when trans to other recessive mutations ofthe gene; such All clones identified under the dissecting microscope were mutant flies show transformations of the first leg to second leg subsequently examined under the compound microscope. and of the proboscis to maxillary palp, but the meso- and me- tathoracic segments are normal (10). Clones ofcells homozygous RESULTS for the Scr 3Amutation were induced around the blastoderm stage or during subsequent development. These clones are nor- The independent roles of the Ultrabithorax+ (Ubx+) and An- mal in both the meso- and metathoracic segments but cause tennapedia+ (Antp') genes have been studied previously by autonomous transformation of all portions of the first leg into inducing marked clones ofcells homozygous or hemizggous for corresponding portions ofthe second leg (Fig. 1). A few Scrl3A mutations of either locus (e.g., Ubx1 and AntpNs+Rc ) in em- clones were observed in the dorsal derivative of the prothorax, bryos or larvae that are otherwise heterozygous (1, 7, 15, 19). the humerus. However, these clones were small, marking one, Here, this approach has been applied to the Sex combs reduced' or at most a few bristles, and hence, difficult to interpret. Fi- (Scr') gene (8-10) and to all possible combinations of the nally, clones in the proboscis autonomously transformed the Antp', Scr', and Ubx+ genes by generating marked clones of labial palps into maxillary palps, but clones in the eye-antenna the following mutant genotypes: Scrl3A, Scrcl Ant Ns+RC3 3 were normal. These results indicate that the Scr' gene is not Scr3A Ubx1, AntpNs+RC3 Ubxl, and Scrcl AntpNs+R Ubx1 required either in the meso- or metathorax or in the eye-an- The phenotypes of such clones are summarized in Table 1, il- tenna but that it is required in at least the ventral prothorax for lustrated in Fig. 1, and described below. specifying pro- as opposed to mesothoracic development. In Ubx1 Clones. Clones induced after 8 hr following fertilization addition, they indicate that the Scr+ gene is required in the are normal in the pro- and mesothoracic segments but transform proboscis for specifying labial as opposed to maxillary devel- the dorsal and ventral appendages of the metathorax (haltere opment. and third leg) into their counterparts in the mesothorax (wing Scrl3A Ubxl Clones. The phenotype of Scr BA Ubx' clones and second leg) (1, 15, 19). Clones induced earlier are identical larval devel- except that they transform the posterior compartments of both induced around the blastoderm stage and during the second and third legs into the posterior compartment ofthe opment is the additive phenotype of Scrl3A and Ubx1 clones first leg (refs. 15 and 20; e.g., Fig. 1). Thus, the Ubx+ gene is induced during larval development (Fig. 1). Thus, in the thorax, required in the metathorax to specify meta- as opposed to me- doubly mutant cells transform the pro- and metathorax into sothoracic development. In addition, it appears to have an early mesothorax but do not affect the mesothorax itself. This result function in portions of the posterior meso- and metathorax in indicates that the transformation of portions of the meso- and preventing prothoracic development. metathorax to prothorax caused by early loss of the Ubx+ gene Scrl3A Clones.