Contribution of the geneextramacrochaetae to the precise positioning of bristles inDrosophila

The bristle patterning genes hairy and extramacrochaetae regulate the development of structures required for flight in Diptera

The distribution of sensory bristles on the thorax of Diptera (true flies) provides a useful model for the study of the evolution of spatial patterns. Large bristles called macrochaetes are arranged into species-specific stereotypical patterns determined via spatially discrete expression of the proneural genes achaete–scute (ac–sc). In Drosophila ac-sc expression is regulated by transcriptional activation at sites where bristle precursors develop and by repression outside of these sites. Three genes, extramacrochaetae (emc), hairy (h) and stripe (sr), involved in repression have been documented. Here we demonstrate that in Drosophila, the repressor genes emc and h, like sr, play an essential role in the development of structures forming part of the flight apparatus. In addition we find that, in Calliphora vicina a species diverged from D. melanogaster by about 100 Myr, spatial expression of emc, h and sr is conserved at the location of development of those structures. Based on these findings we argue, first, that the role emc, h and sr in development of the flight apparatus preceded their activities for macrochaete patterning; second, that species-specific variation in activation and repression of ac-sc expression is evolving in parallel to establish a unique distribution of macrochaetes in each species.

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The distribution of sensory bristles is a useful model to study spatial patterns.

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In Drosophila melanogaster the genes emc, h and sr repress bristle formation.

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In D. melanogaster emc and h are essential for flight apparatus development.

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Notably, in Calliphora vicina emc, h and sr are expressed in the flight apparatus.

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We argue that emc, h and sr had an early role in flight apparatus development.

Redundant mechanisms mediate bristle patterning on the Drosophila thorax

The thoracic bristle pattern of Drosophila results from the spatially restricted expression of the achaete-scute (ac-sc) genes in clusters of cells, mediated by the activity of many discrete cis-regulatory sequences. However, ubiquitous expression of sc or asense (ase) achieved with a heterologous promoter, in the absence of endogenous ac-sc expression, and the activity of the cis-regulatory elements, allows the development of bristles positioned at wild-type locations. We demonstrate that the products of the genes stripe, hairy, and extramacrochaetae contribute to rescue by antagonizing the activity of Sc and Ase. The three genes are expressed in specific but overlapping spatial domains of expression that form a prepattern that allows precise positioning of bristles. The redundant mechanisms might contribute to the robustness of the pattern. We discuss the possibility that patterning in trans by antagonism is ancestral and that the positional cis-regulatory sequences might be of recent origin.

Two or four bristles: functional evolution of an enhancer of scute in Drosophilidae

Changes in cis-regulatory sequences are proposed to underlie much of morphological evolution. Yet, little is known about how such modifications translate into phenotypic differences. To address this problem, we focus on the dorsocentral bristles of Drosophilidae. In Drosophila melanogaster, development of these bristles depends on a cis-regulatory element, the dorsocentral enhancer, to activate scute in a cluster of cells from which two bristles on the posterior scutum arise. A few species however, such as D. quadrilineata, bear anterior dorsocentral bristles as well as posterior ones, a derived feature. This correlates with an anterior expansion of the scute expression domain. Here, we show that the D. quadrilineata enhancer has evolved, and is now active in more anterior regions. When used to rescue scute expression in transgenic D. melanogaster, the D. quadrilineata enhancer is able to induce anterior bristles. Importantly, these properties are not displayed by homologous enhancers from control species bearing only two posterior bristles. We also provide evidence that upstream regulation of the enhancer, by the GATA transcription factor Pannier, has been evolutionarily conserved. This work illustrates how, in the context of a conserved trans-regulatory landscape, evolutionary tinkering of pre-existing enhancers can modify gene expression patterns and contribute to morphological diversification.

Evolutionary change in function of the dorsocentral enhancer (DCE) of scute has resulted in altered bristle formation between two species of Drosophila.

emc has a role in dorsal appendage fate formation in Drosophila oogenesis

extramacrochaetae (emc) functions during many developmental processes in Drosophila, such as sensory organ formation, sex determination, wing vein differentiation, regulation of eye photoreceptor differentiation, cell proliferation and development of the Malpighian tubules, trachea and muscles in the embryo. It encodes a Helix-Loop-Helix transcription factor that negatively regulates bHLH proteins. We show here that emc mRNA and protein are present throughout oogenesis in a dynamic expression pattern and that emc is involved in the regulation of chorionic appendage formation during late oogenesis. Expression of sense and antisense emc constructs as well as emc follicle cell clones leads to eggs with shorter, thicker dorsal appendages that are closer together at base than in the wild type. We demonstrate that emc lies downstream of fs(1)K10, gurken and EGFR in the Grk/EGFR signalling pathway and that it participates in controlling Broad-Complex expression at late stages of oogenesis.

Notch signalling in development: on equivalence groups and asymmetric developmental potential

Notch-mediated lateral signalling takes place between equivalent cells that then adopt alternative fates. An example is the epidermal-neural choice in the Drosophila neuro-ectoderm. Sometimes this choice is random. Sometimes it is biased to a preferred cell by an extrinsic signal. Sometimes the neural precursor is predetermined. A stochastic outcome is probably older in evolutionary terms.

Control of cell fate choices by lateral signaling in the adult peripheral nervous system of Drosophila melanogaster

The thoracic integument of the adult fruit fly is a relatively simple but highly patterned structure. It is composed of sensory organ cells distributed within a monolayer of epidermal cells. Both cell types are easily detected at the cuticular surface, as each external sense organ forms a sensory bristle and each epidermal cell secretes a small nonsensory hair. Inhibitory cell-cell interactions play a key role in regulating the distribution as well as the formation of the sense organs. This review focuses on the role of these cell-cell interactions in the adoption of alternative cell fates. We also show that Notch, Hairless, and Suppressor of Hairless, three components of this intercellular signaling pathway, exhibit dose-dependent genetic interactions. Finally we address how this intercellular signaling mechanism may be modulated to result in highly reproducible outcomes.