Expression Of Spalt Research Articles

Spalt expression and the development of melanic color patterns in pierid butterflies

BackgroundLittle is currently known about wing pattern development in the butterfly family Pieridae, which consists mostly of black melanized elements on white or yellow/orange backgrounds. A single transcription factor, Spalt (Sal), has been previously associated with the development of some pattern elements in Pieris rapae, but it is unclear to what extent Sal is associated with patterns in other pierid species.ResultsWe use immunohistochemistry targeting Sal proteins across several pierids and show that Sal is associated with dense patches of melanization across species but is not associated with vein-melanization or diffuse melanization on the wing. In addition, Sal is expressed along cross-veins and wing compartment midlines that do not develop melanization. Male and female P. rapae spots are sexually dimorphic in size and this dimorphism is also present in the domains of Sal expression. Finally, by disrupting cells positioned in the center of the anterior black spots of P. rapae, before and during the time of Sal expression, spot size was reduced.ConclusionsOur results suggest, but do not conclusively show, that pierid spots may develop in a manner similar to that of nymphalid eyespots, that is, containing a group of signaling cells at the center of the pattern responsible for the differentiation of the complete spot, and that spots and eyespots share at least one signal-response gene in common, the transcription factor Sal. We propose that focal differentiation and focal signaling mechanisms evolved prior to the split of the nymphalid and pierid lineages.

Knockdown of spalt function by RNAi causes de-repression of Hox genes and homeotic transformations in the crustacean Artemia franciscana

Hox genes play a central role in the specification of distinct segmental identities in the body of arthropods. The specificity of Hox genes depends on their restricted expression domains, their interaction with specific cofactors and selectivity for particular target genes. spalt genes are associated with the function of Hox genes in diverse species, but the nature of this association varies: in some cases, spalt collaborates with Hox genes to specify segmental identities, in others, it regulates Hox gene expression or acts as their target. Here we study the role of spalt in the branchiopod crustacean Artemia franciscana. We find that Artemia spalt is expressed in the pre-segmental ‘growth zone’ and in stripes in each of the trunk (thoracic, genital and post-genital) segments that emerge from this zone. Using RNA interference (RNAi), we show that knocking down the expression of spalt has pleiotropic effects, which include thoracic to genital (T → G), genital to thoracic (G → T) and post-genital to thoracic (PG → T) homeotic transformations. These transformations are associated with a stochastic de-repression of Hox genes in the corresponding segments of RNAi-treated animals (AbdB for T → G and Ubx/AbdA for G → T and PG → T transformations). We discuss a possible role of spalt in the maintenance of Hox gene repression in Artemia and in other animals.

Ultrabithorax is required for membranous wing identity in the beetle Tribolium castaneum

The two pairs of wings that are characteristic of ancestral pterygotes (winged insects) have often undergone evolutionary modification. In the fruitfly, Drosophila melanogaster, differences between the membranous forewings and the modified hindwings (halteres) depend on the Hox gene Ultrabithorax (Ubx). The Drosophila forewings develop without Hox input, while Ubx represses genes that are important for wing development, promoting haltere identity. However, the idea that Hox input is important to the morphologically specialized wing derivatives such as halteres, and not the more ancestral wings, requires examination in other insect orders. In beetles, such as Tribolium castaneum, it is the forewings that are modified (to form elytra), while the hindwings retain a morphologically more ancestral identity. Here we show that in this beetle Ubx 'de-specializes' the hindwings, which are transformed to elytra when the gene is knocked down. We also show evidence that elytra result from a Hox-free state, despite their diverged morphology. Ubx function in the hindwing seems necessary for a change in the expression of spalt, iroquois and achaete-scute homologues from elytron-like to more typical wing-like patterns. This counteracting effect of Ubx in beetle hindwings represents a previously unknown mode of wing diversification in insects.

The Drosophila tumor suppressor gene lethal(2)giant larvae is required for the emission of the Decapentaplegic signal.

The Drosophila tumor suppressor gene lethal(2) giant larvae (lgl) encodes a cytoskeletal protein required for the change in shape and polarity acquisition of epithelial cells, and also for asymmetric division of neuroblasts. We show here that lgl participates in the emission of Decapentaplegic (Dpp), a member of the transforming growth factor beta (TGFbeta) family, in various developmental processes. During embryogenesis, lgl is required for the dpp-dependent transcriptional activation of zipper (zip), which encodes the non-muscle myosin heavy chain (NMHC), in the dorsalmost ectodermal cells - the leading edge cells. The embryonic expression of known targets of the dpp signaling pathway, such as labial or tinman was abolished or strongly reduced in lgl mutants. lgl mutant cuticles exhibited phenotypes resembling those observed in mutated partners of the dpp signaling pathway. In addition, lgl was required downstream of dpp and upstream of its receptor Thickveins (Tkv) for the dorsoventral patterning of the ectoderm. During larval development, the expression of spalt, a dpp target, was abolished in mutant wing discs, while it was restored by a constitutively activated form of Tkv (Tkv(Q253D)). Taking into account that the activation of dpp expression was unaffected in the mutant, this suggests that lgl function is not required downstream of the Dpp receptor. Finally, the function of lgl responsible for the activation of Spalt expression appeared to be required only in the cells that produce Dpp, and lgl mutant somatic clones behaved non autonomously. We therefore position the activity of lgl in the cells that produce Dpp, and not in those that respond to the Dpp signal. These results are consistent with a same role for lgl in exocytosis and secretion as that proposed for its yeast ortholog sro7/77 and lgl might function in parallel or independently of its well-documented role in the control of epithelial cell polarity.

The transcription factors KNIRPS and KNIRPS RELATED control cell migration and branch morphogenesis during Drosophila tracheal development.

Cell migration during embryonic tracheal system development in Drosophila requires DPP and EGF signaling to generate the archetypal branching pattern. We show that two genes encoding the transcription factors KNIRPS and KNIRPS RELATED possess multiple and redundant functions during tracheal development. knirps/knirps related activity is necessary to mediate DPP signaling which is required for tracheal cell migration and formation of the dorsal and ventral branches. Ectopic knirps or knirps related expression in lateral tracheal cells respecifies their anteroposterior to a dorsoventral migration behavior, similar to that observed in the case of ectopic DPP expression. In dorsal tracheal cells knirps/knirps related activity represses the transcription factor SPALT; this repression is essential for secondary and terminal branch formation. However, in cells of the dorsal trunk, spalt expression is required for normal anteroposterior cell migration and morphogenesis. spalt expression is maintained by the EGF receptor pathway and, hence, some of the opposing activities of the EGF and DPP signaling pathways are mediated by spalt and knirps/knirps related. Furthermore, we provide evidence that the border between cells acquiring dorsal branch and dorsal trunk identity is established by the direct interaction of KNIRPS with a spalt cis-regulatory element.