Gene name - araucan and caupolican
Cytological map position - 69D1-D6
Function - Transcription factor
Keyword(s) - wing pattern formation
Symbol - ara and caup
Genetic map position - 3-
Classification - Homeodomain Pbx class
Cellular location - nuclear
Drosophila is the organism of choice for uncovering and analyzing genetic basis of development. Analysis of the iroquois mutation, and the discovery of three homeodomain proteins associated with what has become known as the Iroquois group, illustrates the power of Drosophila genetics. The iroquois mutation was isolated in a search for mutations that alter the pattern of macrochaetae. The gene-dose titration method is based on the idea that changing the gene dosage of two interacting genes may sometimes result in an abnormal phenotype, even when changing the gene dosage of either of the genes individually has no detectable effect. In the isolation of iroquois, mutations were selected which alter the pattern of bristles when doubly heterozygous with the deletion Df(4)M62f. This deletion removes the cubitus interruptus gene which is involved in the patterning of adult sense organs in addition to its role in the segmentation of the embryo. Two mutations were isolated which proved to be allelic, and these were termed iro1 and iro2 (Leyns, 1995).
Initially, two transcriptions units were detected within the iroquois locus. Following the practice of honoring Amerindian tribes at this locus, the genes have been named araucan and caupolican. Caupolicán was an Araucanian chief and a leader of the Indian resistance to the Spanish invaders of Chile. From 1553 to 1558 Caupolicán fought the Spanish, ultimately taking sole command of the Indian resistance. After several victories, Caupolicán suffered three disastrous routs at the hands of forces led by Don Garcia Hertado de Mendoza, losing more than 6,000 men in one of the defeats. Caupolicán was barbarously murdered in 1558. Although Caupolicán was a man of great skill and valor, his fame rests primarily on the verses dedicated to him by the poet Alonso de Ercilla y Zúñiga in his long poem La Araucana. The poet was with the army of Mendoza and apparently a witness to the deeds of Caupolicán.
Because the iroquois locus codes for more than one protein, the name iroquois, as in the iro complex (IROC), is reserved for the locus (Gómez-Skarmeta, 1996a). The proteins turn out to possess homeodomains similar to those of Drosophila Extradenticle and mammalian Pbx1. Subsequently the gene mirror, closely linked to ara and caup, was identified. Mirror also proves to be a homeoprotein, and is the third PBX-class homeoprotein of the IROC complex (McNeill, 1997).
The expression pattern of ara and caup in the wing confirms, amplifies, and clarifies an idea that is deeply rooted in the developmental biology of Drosophila: gene expression establishes a prepattern that determines the size, shape and number of sensory organ mother cells (SMC) in imaginal discs, the larval structures that give rise to most of the adult epidermis (Ghysen, 1988). The epidermis of the wings and most of the mesothorax is derived from a pair of imaginal wing discs. In mature larvae, each wing disc displays approximately 20 distinct proneural clusters that develop into precisely positioned SMCs. The pattern of proneural clusters in the wing is constructed piece-meal by enhancer-like, cis-regulatory elements regulating achaete and scute expression into complex spatio-temporal patterns (Gómez-Skarmeta, 1995).
But what establishes the domains of transcription for achaete and scute? Part of this pattern is established by the action of ara and caup on achaete and scute enhancers. But this merely begs the question of what establishes the pattern for ara and caup transcription. ara-caup expression in the wing is restricted to two symmetrical patches, one at each side of the dorsoventral compartment border. ara-caup expression in these patches is necessary for the specification of the prospective vein L3 and associated sensory organs. Here, ara-caup expression is mediated by the Hedgehog signal through its induction of high levels of Cubitus interruptus in anterior cells near the AP compartment border. The high levels of CI activate decapentaplegic expression, and together, CI and DPP positively control ara-caup. It is unlikely that Optomotor blind and Spalt mediate the dpp requirement because mutant cells lacking either omb or spalt can still differentiate vein L3. Another candidate is the zinc finger protein encoded by the schnurri locus, which acts in the Dpp signaling pathway and whose removal suppresses vein formation. dpp by itself is insufficient to account for ara-caup expression. wingless is expressed in a narrow strip of cells straddling the DV compartment boundary of the wing disc, corresponding to the prospective wing margin. The dorsal and ventral ara-caup L3 patches are separated by a gap that corresponds to the cells that accumulate detectable amounts of WG. Clones of mutant wg expressing cells spanning the gap between the L3 patches extend these patches toward the DV border, leaving a narrow gap of only one or two cell diameters. Thus WG represses ara-caup expression at the prospective wing margin domain. Therefore, one aspect of the prepattern in wing discs is established by segment polarity genes, which in turn target homeodomain proteins (ara and caup), which regulate the proneural genes achaete and scute (Gómez-Skarmeta, 1996b).
The two genes are transcribed in the same direction on the chromosome, separated by 12 kb. ara transcripts consist of a principle 3.0 kb form and two other less abundant forms (1.8 and 0.6 kb). caup is present in two equally abundant transcripts of 4.7 and 3.4 kb, present throughout development, and a 1.3 kb mRNA, characteristic of adults. The differently sized transcripts open the possibility of alternative processing or transcription initiation/termination sites (or both) for both genes (Gómez-Skarmeta, 1996a).
Exons - 5 for ara and 4 for caup
Six of the seven amino acids most conserved in a sample of 246 homeodomain proteins are found in the Ara and Caup sequences. Predictions of secondary structure indicate a conservation of the four alpha helices characteristic of most homeodomains. The homeodomains most similar to those of Ara and Caup (80% identity) have been found in the human sequence R46202 and in several mouse proteins (up to 100% identity). The next most similar homeodomains, those of the human Pbx and Drosophila Extradenticle proteins, have only 37% identity. Ara and Caup homeodomains are nearly identical. The homeodomain of Drosophila Mirror (McNeill, 1997) has 57 out of 60 amino acids identical to Caup and Ara. In the amino-terminal domain, these proteins possess a highly conserved Notch interaction domain, which has been proposed to be involved in protein-protein interactions. The Notch interaction domain of the homeodomain proteins is homologous to a stretch of amino acids in Xenopus, rat and Drosophila Notch. This putative protein interaction domain is similar to the central part of the epidermal growth factor repeats of the Notch protein. C-terminal to the novel homology region are two potential phosphorylation sites for mitogen-activated kinase (Rolled in Drosophila). Mirror, Caup and Ara also share a novel homology region that is found halfway between the homeodomains and C-terminus of the proteins (Gómez-Skarmeta, 1996a and McNeill, 1997).
A new Caenorhabditis elegans homeobox gene, ceh-25, is described that belongs to the TALE superclass of atypical homeodomains, which are characterized by three extra residues between helix 1 and helix 2. ORF and PCR analysis reveals a novel type of alternative splicing within the homeobox. The alternative splicing occurs such that two different homeodomains can be generated, which differ in their first 25 amino acids. ceh-25 is an ortholog of the vertebrate Meis genes and it shares a new conserved domain of 130 amino acids with them. A thorough analysis of all TALE homeobox genes was performed and a new classification is presented. Four TALE classes are identified in animals: PBC, MEIS, TGIF and IRO (Iroquois); two types in fungi: the mating type genes (M-ATYP) and the CUP genes; and two types in plants: KNOX and BEL. The IRO class has a new conserved motif downstream of the homeodomain. For the KNOX class, a conserved domain, the KNOX domain, was defined upstream of the homeodomain. Comparison of the KNOX domain and the MEIS domain shows significant sequence similarity revealing the existence of an archetypal group of homeobox genes that encode two associated conserved domains. Thus TALE homeobox genes were already present in the common ancestor of plants, fungi and animals and represent a branch distinct from the typical homeobox genes (Burglin, 1999).
date revised: 26 May 97
Home page: The Interactive Fly © 1995, 1996 Thomas B. Brody, Ph.D.
The Interactive Fly resides on the
Society for Developmental Biology's Web server.