Members of the Wnt family of secreted signaling proteins are key regulators of cell migration and axon guidance. In the nematode C. elegans, the migration of the QR neuroblast descendants requires multiple Wnt ligands and receptors. We found that the migration of the QR descendants is divided into three sequential phases that are each mediated by a distinct Wnt signaling mechanism. Importantly, the transition from the first to the second phase, which is the main determinant of the final position of the QR descendants along the anteroposterior body axis, is mediated through a cell-autonomous process in which the time-dependent expression of a Wnt receptor turns on the canonical Wnt/β-catenin signaling response that is required to terminate long-range anterior migration. Our results show that, in addition to direct guidance of cell migration by Wnt morphogenic gradients, cell migration can also be controlled indirectly through cell-intrinsic modulation of Wnt signaling responses.

Variability in gene expression contributes to phenotypic heterogeneity even in isogenic populations. Here, we used the stereotyped, Wnt signaling-dependent development of the Caenorhabditis elegans Q neuroblast to probe endogenous mechanisms that control gene expression variability. We found that the key Hox gene that orients Q neuroblast migration exhibits increased gene expression variability in mutants in which Wnt pathway activity has been perturbed. Distinct features of the gene expression distributions prompted us on a systematic search for regulatory interactions, revealing a network of interlocked positive and negative feedback loops. Interestingly, positive feedback appeared to cooperate with negative feedback to reduce variability while keeping the Hox gene expression at elevated levels. A minimal model correctly predicts the increased gene expression variability across mutants. Our results highlight the influence of gene network architecture on expression variability and implicate feedback regulation as an effective mechanism to ensure developmental robustness.

Highlights d WOX1, 3, and 5 are required redundantly for lateral leaf growth and auxin biosynthesis d YUC1 expression can partially bypass the requirement for WOX genes in leaf growth d Time-lapse imaging allows quantitation of WOX effects on leaf growth d WOXes shape a growth gradient that underlies the A. thaliana ellipsoid leaf shape

The origin of a terrestrial flora in the Ordovician required adaptation to novel biotic and abiotic stressors. Oil bodies, a synapomorphy of liverworts, accumulate secondary metabolites, but their function and development are poorly understood. Oil bodies of Marchantia polymorpha develop within specialized cells as one single large organelle. Here, we show that a CLASS I HOMEODOMAIN LEUCINE-ZIPPER (C1HDZ) transcription factor controls the differentiation of oil body cells in two different ecotypes of the liverwort M. polymorpha, a model genetic system for early divergent land plants. In flowering plants, these transcription factors primarily modulate responses to abiotic stresss including drought. However, loss-of-function alleles of the single ortholog gene, MpC1HDZ, in M. polymorpha did not exhibit phenotypes associated with abiotic stress. Rather Mpc1hdz mutant plants were more susceptible to herbivory and total plant extracts of the mutant exhibited reduced antibacterial activity. Transcriptomic analysis of the mutant revealed a reduction in expression of genes related to secondary metabolism that was accompanied by a specific depletion of oil body terpenoid compounds. Through time lapse imaging we observed that MpC1HDZ expression maxima precede oil body formation indicating that MpC1HDZ mediates differentiation of oil body cells. Our results indicate that M. polymorpha oil bodies, and MpC1HDZ, are critical for defense against herbivory but not for abiotic stress-tolerance. Thus, C1HDZ genes were co-opted to regulate separate responses to biotic and abiotic stressors in two distinct land plant lineages..

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Here we investigate mechanisms underlying the diversification of biological forms using crucifer leaf shape as an example. We show that evolution of an enhancer element in the homeobox gene REDUCED COMPLEXITY (RCO) altered leaf shape by changing gene expression from the distal leaf blade to its base. A single amino acid substitution evolved together with this regulatory change, which reduced RCO protein stability, preventing pleiotropic effects caused by its altered gene expression. We detected hallmarks of positive selection in these evolved regulatory and coding sequence variants and showed that modulating RCO activity can improve plant physiological performance. Therefore, interplay between enhancer and coding sequence evolution created a potentially adaptive path for morphological evolution.Supplemental material is available for this article.Received September 29, 2016; revised version accepted October 25, 2016. Understanding the genetic basis for evolutionary change is a fundamental problem in biology. Morphological diversity is often underpinned by cis-regulatory divergence of developmental genes and consequent spatiotemporal modification of their expression (Gompel et al. 2005;Hay and Tsiantis 2006;Prud'homme et al. 2006;Carroll 2008;Chan et al. 2010;Frankel et al. 2011;Studer et al. 2011;Arnoult et al. 2013;Rast-Somssich et al. 2015;Indjeian et al. 2016). However, the origin of specific cisregulatory elements underlying morphological diversity is still poorly understood (Rebeiz et al. 2015). For example, it is unclear whether such cis elements tend to arise de novo from rapidly evolving sequences or through the cooption of existing conserved regulatory sequences (Rebeiz et al. 2011;Boyd et al. 2015;Villar et al. 2015). Furthermore, it has not been investigated whether and how coding sequences evolve in concert with regulatory changes to optimize gene function in a new expression domain. Finally, links between regulatory changes underlying morphological change and organismal physiology and fitness remain scarce.Plant leaves present a useful genetic model to tackle these questions because they show substantial morphological variation (Shleizer-Burko et al. 2011;Bar and Ori 2014) and have considerable eco-physiological importance as the major site of photosynthetic carbon fixation in terrestrial ecosystems (Givnish 1978). The REDUCED COMPLEXITY (RCO) gene played a key role in leaf shape diversification in the crucifer family (Sicard et al. 2014;Vlad et al. 2014), to which the reference plant Arabidopsis thaliana belongs. RCO arose through gene duplication and encodes a class I homeobox leucine zipper protein.Its function was discovered in Cardamine hirsuta, where it acts to divide the leaf into distinct leaflets by locally repressing growth at the leaf margin, creating a complex shape. This species-specific activity of RCO arose by neofunctionalization following gene duplication of its ancestral paralog, LMI1, which is conserved in seed plants. Specifically, RCO acquired a novel expression domain within...

Regulation of gene expression by signaling pathways often occurs through a transcriptional switch, where the transcription factor responsible for signal-dependent gene activation represses the same targets in the absence of signaling. T-cell factors (TCFs) are transcription factors in the Wnt/ß-catenin pathway, which control numerous cell fate specification events in metazoans. The TCF transcriptional switch is mediated by many co-regulators that contribute to repression or activation of Wnt target genes. It is typically assumed that DNA recognition by TCFs is important for target gene location, but plays no role in the actual switch. TCF/Pangolin (the fly TCF) and some vertebrate TCF isoforms bind DNA through two distinct domains, a High Mobility Group (HMG) domain and a C-clamp, which recognize DNA motifs known as HMG and Helper sites, respectively. Here, we demonstrate that POP-1 (the C. elegans TCF) also activates target genes through HMG and Helper site interactions. Helper sites enhanced the ability of a synthetic enhancer to detect Wnt/ß-catenin signaling in several tissues and revealed an unsuspected role for POP-1 in regulating the C. elegans defecation cycle. Searching for HMG-Helper site clusters allowed the identification of a new POP-1 target gene active in the head muscles and gut. While Helper sites and the C-clamp are essential for activation of worm and fly Wnt targets, they are dispensable for TCF-dependent repression of targets in the absence of Wnt signaling. These data suggest that a fundamental change in TCF-DNA binding contributes to the transcriptional switch that occurs upon Wnt stimulation.

We have previously described InvEE transgenic mice in which non-dividing, differentiating epidermal cells express oncogenically activated MAPK kinase 1 (MEK1). Skin wounding triggers tumour formation in InvEE mice via a mechanism that involves epidermal release of IL-1a and attraction of a pro-tumorigenic inflammatory infiltrate. To look for potential effects on the underlying connective tissue, we screened InvEE and wild-type epidermis for differential expression of cytokines and immune modulators. We identified a single protein, CD26 (dipeptidyl peptidase-4). CD26 serum levels were not increased in InvEE mice. In contrast, CD26 was upregulated in keratinocytes expressing mutant MEK1 and in the epithelial compartment of InvEE tumours, where it accumulated at cell-cell borders. CD26 expression was increased in dermal fibroblasts following skin wounding but was downregulated in tumour stroma. CD26 activity was stimulated by calcium-induced intercellular adhesion in keratinocytes, suggesting that the upregulation of CD26 in InvEE epidermis is due to expansion of the differentiated cell layers. IL-1a treatment of dermal fibroblasts stimulated CD26 activity, and therefore epidermal IL-1a release may contribute to the upregulation of CD26 expression in wounded dermis. Pharmacological blockade of CD26, via Sitagliptin, reduced growth of InvEE tumours, while combined inhibition of IL-1a and CD26 delayed tumour onset and reduced tumour incidence. Our results demonstrate that inappropriate activation of MEK1 in the epidermis leads to changes in dermal fibroblasts that, like the skin inflammatory infiltrate, contribute to tumour formation.