A context-dependent bifurcation in the Pointed transcriptional effector network contributes specificity and robustness to retinal cell fate acquisition.

Spatiotemporally precise and robust cell fate transitions, which depend on specific signaling cues, are fundamental to the development of appropriately patterned tissues. The fidelity and precision with which photoreceptor fates are recruited in the Drosophila eye exemplifies these principles. The fly eye consists of a highly ordered array of ~750 ommatidia, each of which contains eight distinct photoreceptors, R1-R8, specified sequentially in a precise spatial pattern.

Kibra and Merlin Activate the Hippo Pathway Spatially Distinct from and Independent of Expanded.

The Hippo pathway is emerging as a key evolutionarily conserved signaling mechanism that controls organ size. Three membrane-associated proteins, Kibra, Merlin, and Expanded, regulate pathway activity, but the precise molecular mechanism by which they function is still poorly understood. Here we provide evidence that Merlin and Kibra activate Hippo signaling in parallel to Expanded at a spatially distinct cellular domain, the medial apical cortex.

Natural variation of the expression pattern of the segmentation gene even-skipped in melanogaster.

The evolution of canalized traits is a central question in evolutionary biology. Natural variation in highly conserved traits can provide clues about their evolutionary potential. Here we investigate natural variation in a conserved trait-even-skipped (eve) expression at the cellular blastoderm stage of embryonic development in Drosophila melanogaster.

Naturally occurring deletions of hunchback binding sites in the even-skipped stripe 3+7 enhancer.

Changes in regulatory DNA contribute to phenotypic differences within and between taxa. Comparative studies show that many transcription factor binding sites (TFBS) are conserved between species whereas functional studies reveal that some mutations segregating within species alter TFBS function. Consistently, in this analysis of 13 regulatory elements in Drosophila melanogaster populations, single base and insertion/deletion polymorphism are rare in characterized regulatory elements.

Effect of genetic variation in a Drosophila model of diabetes-associated misfolded human proinsulin.

The identification and validation of gene-gene interactions is a major challenge in human studies. Here, we explore an approach for studying epistasis in humans using a Drosophila melanogaster model of neonatal diabetes mellitus. Expression of the mutant preproinsulin (hINS(C96Y)) in the eye imaginal disc mimics the human disease: it activates conserved stress-response pathways and leads to cell death (reduction in eye area).

Rearrangements of 2.5 kilobases of noncoding DNA from the Drosophila even-skipped locus define predictive rules of genomic cis-regulatory logic.

Rearrangements of about 2.5 kilobases of regulatory DNA located 5' of the transcription start site of the Drosophila even-skipped locus generate large-scale changes in the expression of even-skipped stripes 2, 3, and 7. The most radical effects are generated by juxtaposing the minimal stripe enhancers MSE2 and MSE3 for stripes 2 and 3 with and without small "spacer" segments less than 360 bp in length.

Sex-specific pattern formation during early Drosophila development.

The deleterious effects of different X-chromosome dosage in males and females are buffered by a process called dosage compensation, which in Drosophila is achieved through a doubling of X-linked transcription in males. The male-specific lethal complex mediates this process, but is known to act only after gastrulation. Recent work has shown that the transcription of X-linked genes is also upregulated in males prior to gastrulation; whether it results in functional dosage compensation is not known.

Consequences of eukaryotic enhancer architecture for gene expression dynamics, development, and fitness.

The regulatory logic of time- and tissue-specific gene expression has mostly been dissected in the context of the smallest DNA fragments that, when isolated, recapitulate native expression in reporter assays. It is not known if the genomic sequences surrounding such fragments, often evolutionarily conserved, have any biological function or not. Using an enhancer of the even-skipped gene of Drosophila as a model, we investigate the functional significance of the genomic sequences surrounding empirically identified enhancers.

Evolution and inheritance of early embryonic patterning in Drosophila simulans and D. sechellia.

Pattern formation in Drosophila is a widely studied example of a robust developmental system. Such robust systems pose a challenge to adaptive evolution, as they mask variation that selection may otherwise act upon. Yet we find variation in the localization of expression domains (henceforth "stripe allometry") in the pattern formation pathway.

Artificial selection on egg size perturbs early pattern formation in Drosophila melanogaster.

Pattern formation in Drosophila embryogenesis has been widely investigated as a developmental and evolutionary model of robustness. To ask whether genetic variation for pattern formation is suppressed in this system, artificial selection for divergent egg size was used to challenge the scaling of even-skipped (eve) pattern formation in mitotic cycle 14 (stage 5) embryos of Drosophila melanogaster. Three-dimensional confocal imaging revealed shifts in the allometry of eve pair-rule stripes along both anterior–posterior (A–P) and dorsoventral (D–V) axes as a correlated response to egg size selection, indicating the availability of genetic variation for this buffered trait.

Canalization of segmentation and its evolution in Drosophila.

Segmentation in Drosophila embryogenesis occurs through a hierarchical cascade of regulatory gene expression driven by the establishment of a diffusion-mediated morphogen gradient. Here, we investigate the response of this pattern formation process to genetic variation and evolution in egg size. Specifically, we ask whether spatial localization of gap genes Kruppel (Kr) and giant (gt) and the pair-rule gene even-skipped (eve) during cellularization is robust to genetic variation in embryo length in three Drosophila melanogaster isolines and two closely related species.

Functional evolution of a cis-regulatory module.

Lack of knowledge about how regulatory regions evolve in relation to their structure-function may limit the utility of comparative sequence analysis in deciphering cis-regulatory sequences. To address this we applied reverse genetics to carry out a functional genetic complementation analysis of a eukaryotic cis-regulatory module-the even-skipped stripe 2 enhancer-from four Drosophila species. The evolution of this enhancer is non-clock-like, with important functional differences between closely related species and functional convergence between distantly related species.

Nanos plays a conserved role in axial patterning outside of the Diptera.

Axial patterning is a fundamental event in early development, and molecules involved in determining the body axes provide a coordinate system for subsequent patterning. While orthologs of Drosophila bicoid and nanos play a conserved role in anteroposterior (AP) patterning within at least a subset of Diptera, conservation of this process has not yet been demonstrated outside of the flies. Indeed, it has been argued that bicoid, an instrumental "anterior" factor in Drosophila melanogaster, acquired this role during the evolution of more-derived dipterans.

Functional evolution of noncoding DNA.

Noncoding DNA in eukaryotes encodes functionally important signals for the regulation of chromosome assembly, DNA replication, and gene expression. The increasing availability of whole-genome sequences of related taxa has led to interest in the evolution of these signals, and the phylogenetic footprints they produce. Cis-regulatory sequences controlling gene expression are often conserved among related species, but are rarely conserved between distantly related taxa.