Antonio García-Bellido

Developmental analysis of some mutants of the bithorax system ofDrosophila

SummaryMutants in the bithorax system ofDrosophila produce homeotic transformations that affect the mesothoracic, metathoracic and first abdominal segments. In the present report we describe a clonal analysis of the development of those mutants transforming the metathorax and first abdominal segments into mesothorax.The main results indicate that (1) The normal dorsal metathoracic (haltere) disk has similar developmental parameters to the dorsal mesothoracic disk. The main difference is that the initial and final numbers of cells are different in both disks. (2) In flies mutant forBithorax andpostbithorax (which transform the haltere into wing) the transformed haltere disk has the same initial and final number of cells as the normal wing disk. (3) In morphogenetic mosaics homozygousbithorax (andpostbithorax) clones express their genotype autonomously regardless of the genotype of surrounding haltere cells. This autonomy is expressed in a regulation of the number of adult cells per compartment, typical cell affinities and final cuticular differentiation.

Induction, detection and characterization of cell differentiation mutants in Drosophila.

SummaryMitotic recombination has been used as a tool for detecting cell differentiation mutants in clones of heterozygous individuals. With this method, previously mutagenized (ethyl methanesulfonate) genomes can be screened for mutants, induced anywhere in the genome, in a first generation, and irrespectively of being lethal themselves or present ina lethal chromosome. Induced mitotic recombination was used again in order to give the chromo-some arm location and recombinational locus.In this way 13 new cell differentiation mutants out of 1504 studied genomes have been detected, meiotically mapped and isolated. They affect the process of cell differentiation of chaetes and/or trichomes in the adult cuticle of the thorax and/or the tergites.A mitotic chromosome map has been constructed using the mitotic recombination data of these new mutants and those of other cell differentiation mutants already available.The applicability of this method for the detection of other cell mutants in other systems is discussed.

Gene-dose titration analysis in the search of trans-regulatory genes in Drosophila.

We have searched for trans‐regulatory genes in two genetic systems in Drosophila, the bithorax complex (BX‐C) and the achaete‐scute complex (AS‐C). Previous genetic evidence suggests that the activation of both BX‐C and AS‐C, depends on trans‐regulatory genes (Polycomb, Pc, in the former and hairy, h, in the latter) acting in a negative type of control. Mutants of these regulatory genes in heterozygous condition have dominant derepression phenotypes in flies with extra doses of the corresponding gene complexes. We have searched for new loci, with similar gene‐dose relationships. We have isolated only new alleles (six) of Pc in the BX‐C experiment. In the AS‐C experiment four h alleles, and 13 alleles of a new locus (extramacrochaetae, emc) have been discovered. Whereas the h locus shows specific interactions upon achaete, the new locus, emc, is specific for the scute part of the AS‐C. Statistical analysis suggests that these are the only loci in the genome with those dose‐dependent properties in the two systems.

Cell proliferation patterns in the wing imaginal disc of Drosophila

Morphogenetic processes, based on the temporal and spatial control of cell proliferation, are involved in determining the size and shape of an organism. We have used clonal analysis, employing X-ray-induced mitotic recombination, to study cell proliferation and differentiation processes in the developing wing imaginal disc of Drosophila. Our results show a non-uniform distribution of mitotic activities during different stages of wing development. This may reflect waves of cell proliferation which derive from distinct centers of cell proliferation within the growing wing imaginal disc. These proliferation centers are located within the major wing compartments (i.e. the anterior, posterior, dorsal and ventral compartments) and they are restricted to the areas which give rise to the intervein regions of the adult wing. The mitotic recombination analysis, combined with the study of Minute and gynandromorph mosaics, show that the presumptive vein regions of the wing represent distinct boundaries which delimit the proliferation centers to the intervein regions. We present a generative model of wing morphogenesis that is consistent with our results.

Behaviour of cells mutant for an EGF receptor homologue of Drosophila in genetic mosaics

The Drosophila homologue of the epidermal growth factor receptor (DEGFr or DER, also called torpedo or top) has many mutant alleles that cause either embryonic lethality (both early and late), pupal lethality or female sterility, possibly corresponding to degrees of hypomorphism. We have studied the clonal behaviour of some lethal alleles in genetic mosaics in the imaginal development of thorax, head and tergite epidermis. These alleles cause reduced cell viability to different degrees (measured in frequency and size of clones), smaller cell sizes, abnormal patterning of sensory-organ differentiation and lack of differentiation of macro-chaetae and veins. These effects are cellautonomous but also cause abnormal differentiation in wild-type cells surrounding the clones. In addition, we have studied the phenotypes of double mutant combinations of viable top alleles with wingpattern mutants, some related to other Drosophila proto-oncogenes, to reveal gene interactions in the role(s) of DER in cell proliferation and differentiation. We discuss how those complex cell-behaviour phenotypes and genetic interactions are related to the molecular nature of the DER.

Genetic regulation of theAchaete-scute complex ofDrosophila melanogaster

SummaryMutants in two loci,hairy (h+) andextramacrochaetae (emc+), produce phenotypes corresponding to an excess of function of theachaete-scute complex (AS-C), that is, they cause the appearance of extra chaetae. These mutants, although recessive in normal flies, become dominant in the presence of extra doses of AS-C. Here we study the interactions between these three genes, in an attempt to elucidate their relationships. The results show that the insufficiency produced byh oremc mutants can be titrated by altering the number of copies of AS-C. Moreover, excess of function of AS-C produced by derepression mutants within the complex (Hairy-wing) can also be titrated by altering the number of wild type copies of+ oremc+. These specific interactions indicate that bothh+ andemc+ code for “repressors” of AS-C that interact with theachaete andscute region of the complex respectively.

The relative expression amounts of apterous and its co‐factor dLdb/Chip are critical for dorso‐ventral compartmentalization in the Drosophila wing

Dorso‐ventral axis formation in the Drosophila wing requires the localized accumulation of the Apterous LIM/homeodomain protein (Ap) in dorsal cells. Here we report that dLdb/Chip encodes a LIM‐binding cofactor that controls Ap activity. Both lack and excess of dLdb/Chip function cause the same phenotype as apterous (ap) lack of function; i.e. dorsal to ventral transformations, generation of new wing margins, and wing outgrowths. These results indicate that the normal function of Ap in dorso‐ventral compartmentalization requires the correct amount of the DLDB/CHIP co‐factor, and suggest that the Ap and DLDB/CHIP proteins form a multimeric functional complex. In support of this model, we show that the dLdb/Chip excess‐of‐function phenotypes can be rescued by ap overexpression.

Genetic analysis of the wing vein pattern of Drosophila

SummaryOf the many mutations known to affect the wing vein pattern we have selected the most extreme in 29 genes for study. Their phenotype can be classified in two major classes: lack-of-veins and excess-of-veins, and in several internally coherent groups. The study of multiple mutant combinations, within groups and between groups, reveals several genetic operations at work in the generation of the vein pattern. The finding that some of these mutations also affect cell proliferation in characteristic ways has prompted a generative model of wing morphogenetic and pattern formation based on cell behaviour properties defined by the corresponding wild-type genes.

Determining the role of patterned cell proliferation in the shape and size of the Drosophila wing

The present work is a detailed analysis of the numerical and positional parameters of cell proliferation in all of the derivatives of the wing disk. We have made use of twin clones resulting from mitotic recombination events at three different ages of development. The interfaces between twin clones indicate the relative position in the anlage of the mother cells. Interface types vary with age of clone initiation and with wing regions. They are indicative of the main allocation of postmitotic cells of the growing clones. Growth is exponential and intercalar, i.e., the progeny of ancestor cells becomes more and more separated. Clones are compact, indicating that daughter cells tend to remain side by side. The shape of the clones is wing region characteristic. Subpopulations of cells grow preferentially along veins and wing margins and show characteristic shapes in different pleural regions. The shape and size of the adult wing regions largely depend on the shape of clones and hence of the allocation of successive rounds of daughter cells. The role of mitogenic morphogens in wing size and shape is discussed.

Genes involved in the activation of the bithorax complex ofDrosophila

SummaryWe have studied the genetic properties and developmental effects of several mutations, in eight different loci, which alter the specification of embryonic segments. All of the changes are related to the transformations caused by mutants of the bithorax complex.Several properties: the interactions of these mutants with different mutants of the bithorax complex, the interactions between themselves, the effect of changing dosages of their wildtype alleles and their response to ether induction of phenocopies permit one to distinguish between those mutations which affect activation of the bithorax genes in early embryogenesis and those which affect expression of the bithorax genes during development. This paper deals mainly with the Rg-bx locus and its interactions with other loci which affect segment specification.In particular two loci show genetic and developmental characteristics which seem to conform to those expected for a repressor coding gene (Polycomb) and for an inducer synthesizing gene (Rg-bx) active in a negative control system of the genes in the bithorax complex. A model for the interaction of the wildtype products of these genes to determine the segmental characteristics of both the thorax and abdomen is proposed.