Regulatory divergence modifies limb length between mammals

The case against simplistic genetic explanations of evolution

Humans are curious to understand the causes of traits that distinguish us from other animals and that distinguish vastly different species from one another. We also have a proclivity for simple stories and sometimes tend toward seeking and accepting simple genetic explanations for large evolutionary shifts, even to a single gene. Here, I reveal how a biased expectation of mechanistic simplicity threads through the long history of evolutionary and developmental genetics. I argue, however, that expecting a simple mechanism threatens a deeper understanding of evolution, and I define the limitations for interpreting experimental evidence in evolutionary developmental genetics.

Electrical stimulation of chicken embryo development supports the Inside story scenario of human development and evolution

Animal evolution is driven by random mutations at the genome level. However, it has long been suggested that there exist physical constraints which limit the set of possible outcomes. In craniate evolution, it has been observed that head features, notably in the genus homo, can be ordered in a morphological diagram such that, as the brain expands, the head rocks more forward, face features become less prognathous and the mouth tends to recede. One school of paleontologists suggests that this trend is wired somewhere structurally inside the anatomy, and that random modifications of genes push up or down animal forms along a pre-determined path. No actual experiment has been able to settle the dispute. I present here an experiment of electric stimulation of the head in the chicken embryo which is able to enhance the magnitude of tension forces during development. This experimental intervention causes a correlated brain shrinkage and rotatory movement of the head, congruent with tissue texture, which proves that head dilation and flexure are intimately linked. Numerical modelling explains why the brain curls when it dilates. This gives support to the idea that there exists, in the texture of the vertebrate embryo, a latent dynamic pattern for the observed paleontological trends in craniates towards homo, a concept known as Inside story.

Palaeoatmosphere facilitates a gliding transition to powered flight in the Eocene bat, Onychonycteris finneyi

The evolutionary transition to powered flight remains controversial in bats, the only flying mammals. We applied aerodynamic modeling to reconstruct flight in the oldest complete fossil bat, the archaic Onychonycteris finneyi from the early Eocene of North America. Results indicate that Onychonycteris was capable of both gliding and powered flight either in a standard normodense aerial medium or in the hyperdense atmosphere that we estimate for the Eocene from two independent palaeogeochemical proxies. Aerodynamic continuity across a morphological gradient is further demonstrated by modeled intermediate forms with increasing aspect ratio (AR) produced by digital elongation based on chiropteran developmental data. Here a gliding performance gradient emerged of decreasing sink rate with increasing AR that eventually allowed applying available muscle power to achieve level flight using flapping, which is greatly facilitated in hyperdense air. This gradient strongly supports a gliding (trees-down) transition to powered flight in bats.

Functional analysis of a first hindlimb positioning enhancer via Gdf11 expression

During the early development of tetrapods, including humans, the embryonic body elongates caudally once the anterior-posterior axis is established. During this process, region-specific vertebral morphogenesis occurs, with the determination of limb positioning along the anterior-posterior axis. We previously reported that Gdf11 functions as an anatomical integration system that determines the positioning of hindlimbs and sacral vertebrae where Gdf11 is expressed. However, the molecular mechanisms underlying induction of Gdf11 expression remain unclear. In this study, we searched for non-coding regions near the Gdf11 locus that were conserved across species to elucidate the regulatory mechanisms of Gdf11 expression. We identified an enhancer of the Gdf11 gene in intron 1 and named it highly conserved region (HCR). In HCR knockout mice, the expression level of endogenous Gdf11 was decreased, and the position of the sacral-hindlimb unit was shifted posteriorly. We also searched for factors upstream of Gdf11 based on the predicted transcription factor binding sites within the HCR. We found that inhibition of FGF signaling increased endogenous Gdf11 expression, suggesting that FGF signaling negatively regulates Gdf11 expression. However, FGF signaling does not regulate HCR activity. Our results suggest that there are species-specific Gdf11 enhancers other than HCR and that FGF signaling regulates Gdf11 expression independent of HCR.

Resolving the mechanisms underlying epithelial-to-mesenchymal transition of the lateral plate mesoderm

Formation of the vertebrate limb buds begins with a localized epithelial-to-mesenchymal transition (EMT) of the somatic lateral plate mesoderm (LPM). While the processes that drive proliferation and outgrowth of the limb mesenchyme are well established, the fundamental mechanisms that precede this process and initiate EMT are less understood. In this review, we outline putative drivers of EMT of the LPM, drawing from analyses across a range of vertebrates and developmental models. We detail the expression patterns of key EMT transcriptional regulators in the somatic LPM of the presumptive limb fields, and their potential role in producing a mesenchymal cell fate. These include a putative cooperative role between the EMT inducers PRRX1 and TWIST1, supported by evidence in zebrafish and chicken models but unconfirmed data from mice. As such, additional functional data are required to definitively determine the mechanisms that initiate and drive EMT of the somatic LPM, a critical transition preceding formation of the limb bud mesenchyme.

Identification and characterization of Hand2 upstream genomic enhancers active in developing stomach and limbs

BACKGROUND

The bHLH transcription factor HAND2 plays important roles in the development of the embryonic heart, face, limbs, and sympathetic and enteric nervous systems. To define how and when HAND2 regulates these developmental systems, requires understanding the transcriptional regulation of Hand2.

RESULTS

Remarkably, Hand2 is flanked by an extensive upstream gene desert containing a potentially diverse enhancer landscape. Here, we screened the regulatory interval 200 kb proximal to Hand2 for putative enhancers using evolutionary conservation and histone marks in Hand2-expressing tissues. H3K27ac signatures across embryonic tissues pointed to only two putative enhancer regions showing deep sequence conservation. Assessment of the transcriptional enhancer potential of these elements using transgenic reporter lines uncovered distinct in vivo enhancer activities in embryonic stomach and limb mesenchyme, respectively. Activity of the identified stomach enhancer was restricted to the developing antrum and showed expression within the smooth muscle and enteric neurons. Surprisingly, the activity pattern of the limb enhancer did not overlap Hand2 mRNA but consistently yielded a defined subectodermal anterior expression pattern within multiple transgenic lines.

CONCLUSIONS

Together, these results start to uncover the diverse regulatory potential inherent to the Hand2 upstream regulatory interval.

DNA-guided transcription factor cooperativity shapes face and limb mesenchyme

Transcription factors (TFs) can define distinct cellular identities despite nearly identical DNA-binding specificities. One mechanism for achieving regulatory specificity is DNA-guided TF cooperativity. Although in vitro studies suggest that it may be common, examples of such cooperativity remain scarce in cellular contexts. Here, we demonstrate how "Coordinator," a long DNA motif composed of common motifs bound by many basic helix-loop-helix (bHLH) and homeodomain (HD) TFs, uniquely defines the regulatory regions of embryonic face and limb mesenchyme. Coordinator guides cooperative and selective binding between the bHLH family mesenchymal regulator TWIST1 and a collective of HD factors associated with regional identities in the face and limb. TWIST1 is required for HD binding and open chromatin at Coordinator sites, whereas HD factors stabilize TWIST1 occupancy at Coordinator and titrate it away from HD-independent sites. This cooperativity results in the shared regulation of genes involved in cell-type and positional identities and ultimately shapes facial morphology and evolution.

[Epigenetics, principles and examples of applications]

Since the discovery of DNA as the support of genetic information, the challenge for generations of life scientists was to understand the mechanisms underlying the process that translate the sequence of a gene to a phenotype. In the 1950s, the concept of epigenetics was defined by the British biologist Conrad H. Waddington as the study of "epigenesis" that governs the biological processes involved in the development of any organism. The term epigenetics, now best defined as "above the DNA sequence" reflects the gene-environment interactions by which genes determine traits. Since, its first description, studies underlying the mechanisms involved in these processes has led to an increasing understanding of the regulation all genome transactions such as transcription, replication, repair and the biological pathways coordinated by these mechanisms. We will discuss here the main principles regulating epigenetic processes, their roles in physiology, their evolution over the life time and their implications in medicine.

Genomic Basis of Adaptive Divergence in Leg Length between Ground- and Tree-Dwelling Species within a Bird Family

Hind limbs of tetrapods vary greatly in length and the variability can be associated with locomotor adaptation. Although the phenotypic evolution has been well documented, the underlying genetic basis remains poorly understood. We address this issue by integrating comparative genomics and functional prediction with a study system consisting of ground-dwelling, long-legged and tree-dwelling, short-legged species within the avian family Paridae. Genome-wide divergence and phenotypic correlation analyses jointly identified five highly divergent genomic regions that are significantly related with the difference in leg length between these two groups. Gene annotation for these regions detected three genes involved in skeletal development, that is, PTPA, BRINP1, and MIGA2, with the first one being under the strongest selection. Furthermore, four single nucleotide polymorphisms (SNPs) in the coding region of PTPA can well distinguish the two groups with distinct leg length. Among the four SNPs, one is non-synonymous mutation, and according to the prediction for protein structure and function, it can affect the 3D structure of the encoded protein by altering the corresponding amino acid's position. The alleles of PTPA were found in all sequenced species of the orders Palaeognathae and Psittaciformes, which typically take a ground locomotion style. A whole-genome scanning across bird species uncovered that the four SNPs are more likely to be present in resident passerines with increased leg length/wing length ratios (a proxy of leg-dependent locomotion efficiency). Our findings provide insight into the molecular evolution of locomotion performance based on leg morphology in birds.

Chromatin accessibility in gill tissue identifies candidate genes and loci associated with aquaculture relevant traits in tilapia

The Nile tilapia (Oreochromis niloticus) accounts for ∼9% of global freshwater finfish production however, extreme cold weather and decreasing freshwater resources has created the need to develop resilient strains. By determining the genetic bases of aquaculture relevant traits, we can genotype and breed desirable traits into farmed strains. We generated ATAC-seq and gene expression data from O. niloticus gill tissues, and through the integration of SNPs from 27 tilapia species, identified 1168 highly expressed genes (4% of all Nile tilapia genes) with highly accessible promoter regions with functional variation at transcription factor binding sites (TFBSs). Regulatory variation at these TFBSs is likely driving gene expression differences associated with tilapia gill adaptations, and differentially segregate in freshwater and euryhaline tilapia species. The generation of novel integrative data revealed candidate genes e.g., prolactin receptor 1 and claudin-h, genetic relationships, and loci associated with aquaculture relevant traits like salinity and osmotic stress acclimation.

DNA-guided transcription factor cooperativity shapes face and limb mesenchyme

Transcription factors (TFs) can define distinct cellular identities despite nearly identical DNA-binding specificities. One mechanism for achieving regulatory specificity is DNA-guided TF cooperativity. Although in vitro studies suggest it may be common, examples of such cooperativity remain scarce in cellular contexts. Here, we demonstrate how 'Coordinator', a long DNA motif comprised of common motifs bound by many basic helix-loop-helix (bHLH) and homeodomain (HD) TFs, uniquely defines regulatory regions of embryonic face and limb mesenchyme. Coordinator guides cooperative and selective binding between the bHLH family mesenchymal regulator TWIST1 and a collective of HD factors associated with regional identities in the face and limb. TWIST1 is required for HD binding and open chromatin at Coordinator sites, while HD factors stabilize TWIST1 occupancy at Coordinator and titrate it away from HD-independent sites. This cooperativity results in shared regulation of genes involved in cell-type and positional identities, and ultimately shapes facial morphology and evolution.

Insights into the formation and diversification of a novel chiropteran wing membrane from embryonic development

BACKGROUND

Through the evolution of novel wing structures, bats (Order Chiroptera) became the only mammalian group to achieve powered flight. This achievement preceded the massive adaptive radiation of bats into diverse ecological niches. We investigate some of the developmental processes that underlie the origin and subsequent diversification of one of the novel membranes of the bat wing: the plagiopatagium, which connects the fore- and hind limb in all bat species.

RESULTS

Our results suggest that the plagiopatagium initially arises through novel outgrowths from the body flank that subsequently merge with the limbs to generate the wing airfoil. Our findings further suggest that this merging process, which is highly conserved across bats, occurs through modulation of the programs controlling the development of the periderm of the epidermal epithelium. Finally, our results suggest that the shape of the plagiopatagium begins to diversify in bats only after this merging has occurred.

CONCLUSIONS

This study demonstrates how focusing on the evolution of cellular processes can inform an understanding of the developmental factors shaping the evolution of novel, highly adaptive structures.

Timing of organogenesis underscores the evolution of neonatal life histories and powered flight in bats

Bats have undergone one of the most drastic limb innovations in vertebrate history, associated with the evolution of powered flight. Knowledge of the genetic basis of limb organogenesis in bats has increased but little has been documented regarding the differences between limb organogenesis in bats and that of other vertebrates. We conducted embryological comparisons of the timelines of limb organogenesis in 24 bat species and 72 non-bat amniotes. In bats, the time invested for forelimb organogenesis has been considerably extended and the appearance timing of the forelimb ridge has been significantly accelerated, whereas the timing of the finger and first appearance of the claw development has been delayed, facilitating the enlargement of the manus. Furthermore, we discovered that bats initiate the development of their hindlimbs earlier than their forelimbs compared with other placentals. Bat neonates are known to be able to cling continuously with their well-developed foot to the maternal bodies or habitat substrates soon after birth. We suggest that this unique life history of neonates, which possibly coevolved with powered flight, has driven the accelerated development of the hindlimb and precocious foot.

Gene expression changes during the evolution of the tetrapod limb

Major changes in the vertebrate anatomy have preceded the conquest of land by the members of this taxon, and continuous changes in limb shape and use have occurred during the later radiation of tetrapods. While the main, conserved mechanisms of limb development have been discerned over the past century using a combination of classical embryological and molecular methods, only recent advances made it possible to identify and study the regulatory changes that have contributed to the evolution of the tetrapod appendage. These advances include the expansion of the model repertoire from traditional genetic model species to non-conventional ones, a proliferation of predictive mathematical models that describe gene interactions, an explosion in genomic data and the development of high-throughput methodologies. These revolutionary innovations make it possible to identify specific mutations that are behind specific transitions in limb evolution. Also, as we continue to apply them to more and more extant species, we can expect to gain a fine-grained view of this evolutionary transition that has been so consequential for our species as well.

Comparative genomics uncovers the evolutionary history, demography, and molecular adaptations of South American canids

The remarkable radiation of South American (SA) canids produced 10 extant species distributed across diverse habitats, including disparate forms such as the short-legged, hypercarnivorous bush dog and the long-legged, largely frugivorous maned wolf. Despite considerable research spanning nearly two centuries, many aspects of their evolutionary history remain unknown. Here, we analyzed 31 whole genomes encompassing all extant SA canid species to assess phylogenetic relationships, interspecific hybridization, historical demography, current genetic diversity, and the molecular bases of adaptations in the bush dog and maned wolf. We found that SA canids originated from a single ancestor that colonized South America 3.9 to 3.5 Mya, followed by diversification east of the Andes and then a single colonization event and radiation of Lycalopex species west of the Andes. We detected extensive historical gene flow between recently diverged lineages and observed distinct patterns of genomic diversity and demographic history in SA canids, likely induced by past climatic cycles compounded by human-induced population declines. Genome-wide scans of selection showed that disparate limb proportions in the bush dog and maned wolf may derive from mutations in genes regulating chondrocyte proliferation and enlargement. Further, frugivory in the maned wolf may have been enabled by variants in genes associated with energy intake from short-chain fatty acids. In contrast, unique genetic variants detected in the bush dog may underlie interdigital webbing and dental adaptations for hypercarnivory. Our analyses shed light on the evolution of a unique carnivoran radiation and how it was shaped by South American topography and climate change.

Developmental biology is poised to discover altogether new principles in biology in the 21st century

In the 20th century, developmental biology spearheaded a revolution in our understanding of complex biological problems. Its success rests in great part on a truly unique approach that has recruited a diversity of systems and research organisms rather than focusing on isolated cells or molecules, while also employing a wide variety of technological and intellectual approaches. But what will developmental biology contribute to this century? Advances in technology and instrumentation are presently moving at neck-breaking speed and herald the advent of an age of technological wonders in which previously inaccessible biology is now tangibly within our grasps. For instance, single-cell RNAseq has revealed novel, transient cell states in both stem and differentiated cells that are specified by defined changes in gene expression frequency during regeneration. Additionally, genome-wide epigenetic analyses combined with gene editing and transgenic methodologies have identified the existence of regeneration responsive enhancers in adult vertebrate tissues. These circumstances combined with our discipline’s diversity of experimental and intellectual approaches offer unimaginable opportunities for developmental biologists not only to discover new biology but also to reveal entirely new principles of biology.

Interspecies transcriptomics identify genes that underlie disproportionate foot growth in jerboas

Despite the great diversity of vertebrate limb proportion and our deep understanding of the genetic mechanisms that drive skeletal elongation, little is known about how individual bones reach different lengths in any species. Here, we directly compare the transcriptomes of homologous growth cartilages of the mouse (Mus musculus) and bipedal jerboa (Jaculus jaculus), the latter of which has "mouse-like" arms but extremely long metatarsals of the feet. Intersecting gene-expression differences in metatarsals and forearms of the two species revealed that about 10% of orthologous genes are associated with the disproportionately rapid elongation of neonatal jerboa feet. These include genes and enriched pathways not previously associated with endochondral elongation as well as those that might diversify skeletal proportion in addition to their known requirements for bone growth throughout the skeleton. We also identified transcription regulators that might act as "nodes" for sweeping differences in genome expression between species. Among these, Shox2, which is necessary for proximal limb elongation, has gained expression in jerboa metatarsals where it has not been detected in other vertebrates. We show that Shox2 is sufficient to increase mouse distal limb length, and a nearby putative cis-regulatory region is preferentially accessible in jerboa metatarsals. In addition to mechanisms that might directly promote growth, we found evidence that jerboa foot elongation may occur in part by de-repressing latent growth potential. The genes and pathways that we identified here provide a framework to understand the modular genetic control of skeletal growth and the remarkable malleability of vertebrate limb proportion.

Modeling uniquely human gene regulatory function via targeted humanization of the mouse genome

The evolution of uniquely human traits likely entailed changes in developmental gene regulation. Human Accelerated Regions (HARs), which include transcriptional enhancers harboring a significant excess of human-specific sequence changes, are leading candidates for driving gene regulatory modifications in human development. However, insight into whether HARs alter the level, distribution, and timing of endogenous gene expression remains limited. We examined the role of the HAR HACNS1 (HAR2) in human evolution by interrogating its molecular functions in a genetically humanized mouse model. We find that HACNS1 maintains its human-specific enhancer activity in the mouse embryo and modifies expression of Gbx2, which encodes a transcription factor, during limb development. Using single-cell RNA-sequencing, we demonstrate that Gbx2 is upregulated in the limb chondrogenic mesenchyme of HACNS1 homozygous embryos, supporting that HACNS1 alters gene expression in cell types involved in skeletal patterning. Our findings illustrate that humanized mouse models provide mechanistic insight into how HARs modified gene expression in human evolution.

The Regulation of Growth in Developing, Homeostatic, and Regenerating Tetrapod Limbs: A Minireview

The size and shape of the tetrapod limb play central roles in their functionality and the overall physiology of the organism. In this minireview we will discuss observations on mutant animal models and humans, which show that the growth and final size of the limb is most impacted by factors that regulate either limb bud patterning or the elongation of the long bones. We will also apply the lessons that have been learned from embryos to how growth could be regulated in regenerating limb structures and outline the challenges that are unique to regenerating animals.

Time to synchronize our clocks: Connecting developmental mechanisms and evolutionary consequences of heterochrony

Heterochrony, defined as a change in the timing of developmental events altering the course of evolution, was first recognized by Ernst Haeckel in 1866. Haeckel's original definition was meant to explain the observed parallels between ontogeny and phylogeny, but the interpretation of his work became a source of controversy over time. Heterochrony took its modern meaning following the now classical work in the 1970-80s by Steven J. Gould, Pere Alberch, and co-workers. Predicted and described heterochronic scenarios emphasize the many ways in which developmental changes can influence evolution. However, while important examples of heterochrony detected with comparative morphological methods have multiplied, the more mechanistic understanding of this phenomenon lagged conspicuously behind. Considering the rapid progress in imaging and molecular tools available now for developmental biologists, this review aims to stress the need to take heterochrony research to the next level. It is time to synchronize the different levels of heterochrony research into a single analysis flow: from studies on organismal-level morphology to cells to molecules and genes, using complementary techniques. To illustrate how to achieve a more comprehensive understanding of phyletic morphological diversification associated with heterochrony, we discuss several recent case studies at various phylogenetic scales that combine morphological, cellular, and molecular analyses. Such a synergistic approach offers to more fully integrate phylogenetic and ontogenetic dimensions of the fascinating evolutionary phenomenon of heterochrony.

Shaping modern human skull through epigenetic, transcriptional and post-transcriptional regulation of the RUNX2 master bone gene

RUNX2 encodes the master bone transcription factor driving skeletal development in vertebrates, and playing a specific role in craniofacial and skull morphogenesis. The anatomically modern human (AMH) features sequence changes in the RUNX2 locus compared with archaic hominins' species. We aimed to understand how these changes may have contributed to human skull globularization occurred in recent evolution. We compared in silico AMH and archaic hominins' genomes, and used mesenchymal stromal cells isolated from skull sutures of craniosynostosis patients for in vitro functional assays. We detected 459 and 470 nucleotide changes in noncoding regions of the AMH RUNX2 locus, compared with the Neandertal and Denisovan genomes, respectively. Three nucleotide changes in the proximal promoter were predicted to alter the binding of the zinc finger protein Znf263 and long-distance interactions with other cis-regulatory regions. By surface plasmon resonance, we selected nucleotide substitutions in the 3'UTRs able to affect miRNA binding affinity. Specifically, miR-3150a-3p and miR-6785-5p expression inversely correlated with RUNX2 expression during in vitro osteogenic differentiation. The expression of two long non-coding RNAs, AL096865.1 and RUNX2-AS1, within the same locus, was modulated during in vitro osteogenic differentiation and correlated with the expression of specific RUNX2 isoforms. Our data suggest that RUNX2 may have undergone adaptive phenotypic evolution caused by epigenetic and post-transcriptional regulatory mechanisms, which may explain the delayed suture fusion leading to the present-day globular skull shape.

Non-model systems in mammalian forelimb evo-devo

Mammal forelimbs are highly diverse, ranging from the elongated wing of a bat to the stout limb of the mole. The mammal forelimb has been a long-standing system for the study of early developmental patterning, proportional variation, shape change, and the reduction of elements. However, most of this work has been performed in mice, which neglects the wide variation present across mammal forelimbs. This review emphasizes the critical role of non-model systems in limb evo-devo and highlights new emerging models and their potential. We discuss the role of gene networks in limb evolution, and touch on functional analyses that lay the groundwork for further developmental studies. Mammal limb evo-devo is a rich field, and here we aim to synthesize the findings of key recent works and the questions to which they lead.

Recapitulating Evolutionary Divergence in a Single Cis-Regulatory Element Is Sufficient to Cause Expression Changes of the Lens Gene Tdrd7

Mutations in cis-regulatory elements play important roles for phenotypic changes during evolution. Eye degeneration in the blind mole rat (BMR; Nannospalax galili) and other subterranean mammals is significantly associated with widespread divergence of eye regulatory elements, but the effect of these regulatory mutations on eye development and function has not been explored. Here, we investigate the effect of mutations observed in the BMR sequence of a conserved noncoding element upstream of Tdrd7, a pleiotropic gene required for lens development and spermatogenesis. We first show that this conserved element is a transcriptional repressor in lens cells and that the BMR sequence partially lost repressor activity. Next, we recapitulated evolutionary changes in this element by precisely replacing the endogenous regulatory element in a mouse line by the orthologous BMR sequence with CRISPR-Cas9. Strikingly, this repressor replacement caused a more than 2-fold upregulation of Tdrd7 in the developing lens; however, increased mRNA level does not result in a corresponding increase in TDRD7 protein nor an obvious lens phenotype, possibly explained by buffering at the posttranscriptional level. Our results are consistent with eye degeneration in subterranean mammals having a polygenic basis where many small-effect mutations in different eye-regulatory elements collectively contribute to phenotypic differences.

Prenatal Developmental Trajectories of Fluctuating Asymmetry in Bat Humeri

Fluctuating asymmetry (random fluctuations between the left and right sides of the body) has been interpreted as an index to quantify both the developmental instabilities and homeostatic capabilities of organisms, linking the phenotypic and genotypic aspects of morphogenesis. However, studying the ontogenesis of fluctuating asymmetry has been limited to mostly model organisms in postnatal stages, missing prenatal trajectories of asymmetry that could better elucidate decoupled developmental pathways controlling symmetric bone elongation and thickening. In this study, we quantified the presence and magnitude of asymmetry during the prenatal development of bats, focusing on the humerus, a highly specialized bone adapted in bats to perform under multiple functional demands. We deconstructed levels of asymmetry by measuring the longitudinal and cross-sectional asymmetry of the humerus using a combination of linear measurements and geometric morphometrics. We tested the presence of different types of asymmetry and calculated the magnitude of size-controlled fluctuating asymmetry to assess developmental instability. Statistical support for the presence of fluctuating asymmetry was found for both longitudinal and cross-sectional asymmetry, explaining on average 16% of asymmetric variation. Significant directional asymmetry accounted for less than 6.6% of asymmetric variation. Both measures of fluctuating asymmetry remained relatively stable throughout ontogeny, but cross-sectional asymmetry was significantly different across developmental stages. Finally, we did not find a correspondence between developmental patterns of longitudinal and cross-sectional asymmetry, indicating that processes promoting symmetrical bone elongation and thickening work independently. We suggest various functional pressures linked to newborn bats’ ecology associated with longitudinal (altricial flight capabilities) and cross-sectional (precocial clinging ability) developmental asymmetry differentially. We hypothesize that stable magnitudes of fluctuating asymmetry across development could indicate the presence of developmental mechanisms buffering developmental instability.

Enhancer redundancy in development and disease

Shadow enhancers are seemingly redundant transcriptional cis-regulatory elements that regulate the same gene and drive overlapping expression patterns. Recent studies have shown that shadow enhancers are remarkably abundant and control most developmental gene expression in both invertebrates and vertebrates, including mammals. Shadow enhancers might provide an important mechanism for buffering gene expression against mutations in non-coding regulatory regions of genes implicated in human disease. Technological advances in genome editing and live imaging have shed light on how shadow enhancers establish precise gene expression patterns and confer phenotypic robustness. Shadow enhancers can interact in complex ways and may also help to drive the formation of transcriptional hubs within the nucleus. Despite their apparent redundancy, the prevalence and evolutionary conservation of shadow enhancers underscore their key role in emerging metazoan gene regulatory networks.

Amplicon genome fishing (AGF): a rapid and efficient method for sequencing target cis-regulatory regions in nonmodel organisms

Cis-regulatory sequences play a crucial role in regulating gene expression and are evolutionary hot spots that drive phenotypic divergence among organisms. Sequencing some cis-regulatory regions of interest in many different species is common in comparative genetic studies. For nonmodel organisms lacking genomic data, genome walking is often the preferred method for this type of application. However, applying genome walking will be laborious and time-consuming when the number of cis-regulatory regions and species to be analyzed is large. In this study, we propose a novel method called amplicon genome fishing (AGF), which can isolate and sequence cis-regulatory regions of interest for any organism. The main idea of the AGF method is to use fragments amplified from the target cis-regulatory regions as enrichment baits to capture and sequence the whole target cis-regulatory regions from genomic library pools. Unlike genome walking, the AGF method is based on hybridization capture and high-throughput sequencing, which makes this method rapid and efficient for projects where some cis-regulatory regions have to be sequenced for many species. We used human amplicons as capture baits and successfully sequenced five target enhancer regions of Homo sapiens, Mus musculus, Gallus gallus, and Xenopus tropicalis, proving the feasibility and repeatability of AGF. To show the utility of the AGF method in real studies, we used it to sequence the ZRS enhancer, a cis-regulatory region associated with the limb loss of snakes, for twenty-three vertebrate species (includes many limbless species never sequenced before). The newly obtained ZRS sequences provide new perspectives into the relationship between the ZRS enhancer's evolution and limb loss in major tetrapod lineages.

Making a bat: The developmental basis of bat evolution

Bats are incredibly diverse, both morphologically and taxonomically. Bats are the only mammalian group to have achieved powered flight, an adaptation that is hypothesized to have allowed them to colonize various and diverse ecological niches. However, the lack of fossils capturing the transition from terrestrial mammal to volant chiropteran has obscured much of our understanding of bat evolution. Over the last 20 years, the emergence of evo-devo in non-model species has started to fill this gap by uncovering some developmental mechanisms at the origin of bat diversification. In this review, we highlight key aspects of studies that have used bats as a model for morphological adaptations, diversification during adaptive radiations, and morphological novelty. To do so, we review current and ongoing studies on bat evolution. We first investigate morphological specialization by reviewing current knowledge about wing and face evolution. Then, we explore the mechanisms behind adaptive diversification in various ecological contexts using vision and dentition. Finally, we highlight the emerging work into morphological novelties using bat wing membranes.

Cis-acting variation is common across regulatory layers but is often buffered during embryonic development

Precise patterns of gene expression are driven by interactions between transcription factors, regulatory DNA sequences, and chromatin. How DNA mutations affecting any one of these regulatory "layers" are buffered or propagated to gene expression remains unclear. To address this, we quantified allele-specific changes in chromatin accessibility, histone modifications, and gene expression in F1 embryos generated from eight Drosophila crosses at three embryonic stages, yielding a comprehensive data set of 240 samples spanning multiple regulatory layers. Genetic variation (allelic imbalance) impacts gene expression more frequently than chromatin features, with metabolic and environmental response genes being most often affected. Allelic imbalance in cis-regulatory elements (enhancers) is common and highly heritable, yet its functional impact does not generally propagate to gene expression. When it does, genetic variation impacts RNA levels through two alternative mechanisms involving either H3K4me3 or chromatin accessibility and H3K27ac. Changes in RNA are more predictive of variation in H3K4me3 than vice versa, suggesting a role for H3K4me3 downstream from transcription. The impact of a substantial proportion of genetic variation is consistent across embryonic stages, with 50% of allelic imbalanced features at one stage being also imbalanced at subsequent developmental stages. Crucially, buffering, as well as the magnitude and evolutionary impact of genetic variants, is influenced by regulatory complexity (i.e., number of enhancers regulating a gene), with transcription factors being most robust to cis-acting, but most influenced by trans-acting, variation.

Dissection of the Fgf8 regulatory landscape by in vivo CRISPR-editing reveals extensive intra- and inter-enhancer redundancy

Developmental genes are often regulated by multiple elements with overlapping activity. Yet, in most cases, the relative function of those elements and their contribution to endogenous gene expression remain poorly characterized. An example of this phenomenon is that distinct sets of enhancers have been proposed to direct Fgf8 in the limb apical ectodermal ridge and the midbrain-hindbrain boundary. Using in vivo CRISPR/Cas9 genome engineering, we functionally dissect this complex regulatory ensemble and demonstrate two distinct regulatory logics. In the apical ectodermal ridge, the control of Fgf8 expression appears distributed between different enhancers. In contrast, we find that in the midbrain-hindbrain boundary, one of the three active enhancers is essential while the other two are dispensable. We further dissect the essential midbrain-hindbrain boundary enhancer to reveal that it is also composed by a mixture of essential and dispensable modules. Cross-species transgenic analysis of this enhancer suggests that its composition may have changed in the vertebrate lineage.

Regulation of vertebrate forelimb development and wing reduction in the flightless emu

The vertebrate limb is a dynamic structure which has evolved into many diverse forms to facilitate complex behavioral adaptations. The principle molecular and cellular processes that underlie development of the vertebrate limb are well characterized. However, how these processes are altered to drive differential limb development between vertebrates is less well understood. Several vertebrate models are being utilized to determine the developmental basis of differential limb morphogenesis, though these typically focus on later patterning of the established limb bud and may not represent the complete developmental trajectory. Particularly, heterochronic limb development can occur prior to limb outgrowth and patterning but receives little attention. This review summarizes the genetic regulation of vertebrate forelimb diversity, with particular focus on wing reduction in the flightless emu as a model for examining limb heterochrony. These studies highlight that wing reduction is complex, with heterochronic cellular and genetic events influencing the major stages of limb development. Together, these studies provide a broader picture of how different limb morphologies may be established during development.

Changes in regeneration-responsive enhancers shape regenerative capacities in vertebrates

Vertebrates vary in their ability to regenerate, and the genetic mechanisms underlying such disparity remain elusive. Comparative epigenomic profiling and single-cell sequencing of two related teleost fish uncovered species-specific and evolutionarily conserved genomic responses to regeneration. The conserved response revealed several regeneration-responsive enhancers (RREs), including an element upstream to inhibin beta A (inhba), a known effector of vertebrate regeneration. This element activated expression in regenerating transgenic fish, and its genomic deletion perturbed caudal fin regeneration and abrogated cardiac regeneration altogether. The enhancer is present in mammals, shares functionally essential activator protein 1 (AP-1)-binding motifs, and responds to injury, but it cannot rescue regeneration in fish. This work suggests that changes in AP-1-enriched RREs are likely a crucial source of loss of regenerative capacities in vertebrates.

Evaluating the probability of CRISPR-based gene drive contaminating another species

The probability D that a given clustered regularly interspaced short palindromic repeats (CRISPR)-based gene drive element contaminates another, nontarget species can be estimated by the following Drive Risk Assessment Quantitative Estimate (DRAQUE) Equation: D = h y b + t r a n s f × e x p r e s s × c u t × f l a n k × i m m u n e × n o n e x t i n c t with hyb = probability of hybridization between the target species and a nontarget species; transf = probability of horizontal transfer of a piece of DNA containing the gene drive cassette from the target species to a nontarget species (with no hybridization); express = probability that the Cas9 and guide RNA genes are expressed; cut = probability that the CRISPR-guide RNA recognizes and cuts at a DNA site in the new host; flank = probability that the gene drive cassette inserts at the cut site; immune = probability that the immune system does not reject Cas9-expressing cells; nonextinct = probability of invasion of the drive within the population. We discuss and estimate each of the seven parameters of the equation, with particular emphasis on possible transfers within insects, and between rodents and humans. We conclude from current data that the probability of a gene drive cassette to contaminate another species is not insignificant. We propose strategies to reduce this risk and call for more work on estimating all the parameters of the formula.

A 17q24.3 duplication identified in a large Chinese family with brachydactyly-anonychia

Background

Brachydactyly (BD) is a rare autosomal dominant inherited disease characterized by shortness of the fingers and/or toes, which has been classified into the subtypes A–E. However, the exact cause and mechanism of BD remain to be illuminated. Here, we aim to reveal the clinical and genetic characteristics of a subtype of BD, brachydactyly‐anonychia.

Methods

In this study, a large Chinese family with three members affected by brachydactyly‐anonychia was investigated. Both whole‐exome sequencing and microarray‐based comparative genomic hybridization (CGH) were performed on this family and the results of copy number variation (CNV) were verified by quantitative real‐time PCR (qPCR).

Results

All the affected individuals showed short fingers and toes as well as missing nails; and the absence of middle phalanges in figure II‐V of the upper and lower extremities was observed by X‐ray examination. A duplication involving in the region of 17q24.3 was detected by CGH. The results of qPCR also represented this duplication in 17q24.3 in all the patients.

Conclusion

In summary, our findings suggest that 17q24.3 duplication is the genetic cause of brachydactyly‐anonychia in this family, which support the prior report that brachydactyly‐anonychia is associated with 17q24.3 duplication, and further indicates the pathogenic correlation between BD and CNVs.

Interaction between oocytes, cortical germ cells and granulosa cells of the mouse and bat, following the dissociation-re-aggregation of adult ovaries

It is widely accepted that the oocyte plays a very active role in promoting the growth of the follicle by directing the differentiation of granulosa cells and secreting paracrine growth factors. In turn, granulosa cells regulate the development of the oocytes, establishing close bidirectional communication between germ and somatic cells. The presence of cortical cells with morphological characteristics, similar to primordial germ cells that express specific germline markers, stem cells and cell proliferation, known as adult cortical germ cells (ACGC) have been reported in phyllostomid bats. Using magnetic cell separation techniques, dissociation-cellular re-aggregation and organ culture, the behaviour of oocytes and ACGC was analyzed by interacting in vitro with mouse ovarian cells. Bat ACGC was mixed with disaggregated ovaries from a transgenic mouse that expressed green fluorescent protein. The in vitro reconstruction of the re-aggregates was evaluated. We examined the viability, integration, cellular interaction and ovarian morphogenesis by detecting the expression of Vasa, pH3, Cx43 and Laminin. Our results showed that the interaction between ovarian cells is carried out in the adult ovary of two species, without them losing their capacity to form follicular structures, even after having been enzymatically dissociated.

A complex regulatory landscape involved in the development of mammalian external genitals

Developmental genes are often controlled by large regulatory landscapes matching topologically associating domains (TADs). In various contexts, the associated chromatin backbone is modified by specific enhancer-enhancer and enhancer-promoter interactions. We used a TAD flanking the mouse HoxD cluster to study how these regulatory architectures are formed and deconstructed once their function achieved. We describe this TAD as a functional unit, with several regulatory sequences acting together to elicit a transcriptional response. With one exception, deletion of these sequences didn't modify the transcriptional outcome, a result at odds with a conventional view of enhancer function. The deletion and inversion of a CTCF site located near these regulatory sequences did not affect transcription of the target gene. Slight modifications were nevertheless observed, in agreement with the loop extrusion model. We discuss these unexpected results considering both conventional and alternative explanations relying on the accumulation of poorly specific factors within the TAD backbone.

CHD9 upregulates RUNX2 and has a potential role in skeletal evolution

BACKGROUND

Changes in gene regulation are widely recognized as an important driver of adaptive phenotypic evolution. However, the specific molecular mechanisms that underpin such changes are still poorly understood. Chromatin state plays an essential role in gene regulation, by influencing the accessibility of coding loci to the transcriptional machinery. Changes in the function of chromatin remodellers are therefore strong candidates to drive changes in gene expression associated with phenotypic adaptation. Here, we identify amino acid homoplasies in the chromatin remodeller CHD9, shared between the extinct marsupial thylacine and eutherian wolf which show remarkable skull convergence. CHD9 is involved in osteogenesis, though its role in the process is still poorly understood. We examine whether CHD9 is able to regulate the expression of osteogenic target genes and examine the function of a key substitution in the CHD9 DNA binding domain.

RESULTS

We examined whether CHD9 was able to upregulate its osteogenic target genes, RUNX2, Osteocalcin (OC) and ALP in HEK293T cells. We found that overexpression of CHD9 upregulated RUNX2, the master regulator of osteoblast cell fate, but not the downstream genes OC or ALP, supporting the idea that CHD9 regulates osteogenic progenitors rather than terminal osteoblasts. We also found that the evolutionary substitution in the CHD9 DNA binding domain does not alter protein secondary structure, but was able to drive a small but insignificant increase in RUNX2 activation. Finally, CHD9 was unable to activate an episomal RUNX2 promoter-reporter construct, suggesting that CHD9 requires the full chromatin complement for its function.

CONCLUSIONS

We provide new evidence to the role of CHD9 in osteogenic differentiation through its newly observed ability to upregulate the expression of RUNX2. Though we were unable to identify significant functional consequences of the evolutionary substitution in HEK293T cells, our study provides important steps forward in the functional investigation of protein homoplasy and its role in developmental processes. Mutations in coding genes may be a mechanism for driving adaptive changes in gene expression, and their validation is essential towards determining the functional consequences of evolutionary homoplasy.

Endochondral ossification and the evolution of limb proportions

Mammals have remarkably diverse limb proportions hypothesized to have evolved adaptively in the context of locomotion and other behaviors. Mechanistically, evolutionary diversity in limb proportions is the result of differential limb bone growth. Longitudinal limb bone growth is driven by the process of endochondral ossification, under the control of the growth plates. In growth plates, chondrocytes undergo a tightly orchestrated life cycle of proliferation, matrix production, hypertrophy, and cell death/transdifferentiation. This life cycle is highly conserved, both among the long bones of an individual, and among homologous bones of distantly related taxa, leading to a finite number of complementary cell mechanisms that can generate heritable phenotype variation in limb bone size and shape. The most important of these mechanisms are chondrocyte population size in chondrogenesis and in individual growth plates, proliferation rates, and hypertrophic chondrocyte size. Comparative evidence in mammals and birds suggests the existence of developmental biases that favor evolutionary changes in some of these cellular mechanisms over others in driving limb allometry. Specifically, chondrocyte population size may evolve more readily in response to selection than hypertrophic chondrocyte size, and extreme hypertrophy may be a rarer evolutionary phenomenon associated with highly specialized modes of locomotion in mammals (e.g., powered flight, ricochetal bipedal hopping). Physical and physiological constraints at multiple levels of biological organization may also have influenced the cell developmental mechanisms that have evolved to produce the highly diverse limb proportions in extant mammals. This article is categorized under: Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Comparative Development and Evolution > Regulation of Organ Diversity Comparative Development and Evolution > Organ System Comparisons Between Species.

Functional Consequences of Chromosomal Rearrangements on Gene Expression: Not So Deleterious After All?

Chromosomes are folded and organized into topologically associating domains (TADs) which provide a framework for the interaction of enhancers with the promoter of their target gene(s). Structural rearrangements observed during evolution or in disease contexts suggest that changes in genome organization strongly affect gene expression and can have drastic phenotypic effects. In this review, I will discuss how recent genomic engineering experiments reveal a more contrasted picture, suggesting that TADs are important but not always essential for gene expression regulation.

REforge Associates Transcription Factor Binding Site Divergence in Regulatory Elements with Phenotypic Differences between Species

Elucidating the genomic determinants of morphological differences between species is key to understanding how morphological diversity evolved. While differences in cis-regulatory elements are an important genetic source for morphological evolution, it remains challenging to identify regulatory elements involved in phenotypic differences. Here, we present Regulatory Element forward genomics (REforge), a computational approach that detects associations between transcription factor binding site divergence in putative regulatory elements and phenotypic differences between species. By simulating regulatory element evolution in silico, we show that this approach has substantial power to detect such associations. To validate REforge on real data, we used known binding motifs for eye-related transcription factors and identified significant binding site divergence in vision-impaired subterranean mammals in 1% of all conserved noncoding elements. We show that these genomic regions are significantly enriched in regulatory elements that are specifically active in mouse eye tissues, and that several of them are located near genes, which are required for eye development and photoreceptor function and are implicated in human eye disorders. Thus, our genome-wide screen detects widespread divergence of eye-regulatory elements and highlights regulatory regions that likely contributed to eye degeneration in subterranean mammals. REforge has broad applicability to detect regulatory elements that could be involved in many other phenotypes, which will help to reveal the genomic basis of morphological diversity.

Scientific statement on the coverage of bats by the current pesticide risk assessment for birds and mammals

Bats are an important group of mammals, frequently foraging in farmland and potentially exposed to pesticides. This statement considers whether the current risk assessment performed for birds and ground dwelling mammals exposed to pesticides is also protective of bats. Three main issues were addressed. Firstly, whether bats are toxicologically more or less sensitive than the most sensitive birds and mammals. Secondly, whether oral exposure of bats to pesticides is greater or lower than in ground dwelling mammals and birds. Thirdly, whether there are other important exposure routes relevant to bats. A large variation in toxicological sensitivity and no relationship between sensitivity of bats and bird or mammal test-species to pesticides could be found. In addition, bats have unique traits, such as echolocation and torpor which can be adversely affected by exposure to pesticides and which are not covered by the endpoints currently selected for wild mammal risk assessment. The current exposure assessment methodology was used for oral exposure and adapted to bats using bat-specific parameters. For oral exposure, it was concluded that for most standard risk assessment scenarios the current approach did not cover exposure of bats to pesticide residues in food. Calculations of potential dermal exposure for bats foraging during spraying operations suggest that this may be a very important exposure route. Dermal routes of exposure should be combined with inhalation and oral exposure. Based on the evidence compiled, the Panel concludes that bats are not adequately covered by the current risk assessment approach, and that there is a need to develop a bat-specific risk assessment scheme. In general, there was scarcity of data to assess the risks for bat exposed to pesticides. Recommendations for research are made, including identification of alternatives to laboratory testing of bats to assess toxicological effects.

Functional Conservation of a Developmental Switch in Mammals since the Jurassic Age

ThPOK is a "master regulator" of T lymphocyte lineage choice, whose presence or absence is sufficient to dictate development to the CD4 or CD8 lineages, respectively. Induction of ThPOK is transcriptionally regulated, via a lineage-specific silencer element, SilThPOK. Here, we take advantage of the available genome sequence data as well as site-specific gene targeting technology, to evaluate the functional conservation of ThPOK regulation across mammalian evolution, and assess the importance of motif grammar (order and orientation of TF binding sites) on SilThPOK function in vivo. We make three important points: First, the SilThPOK is present in marsupial and placental mammals, but is not found in available genome assemblies of nonmammalian vertebrates, indicating that it arose after divergence of mammals from other vertebrates. Secondly, by replacing the murine SilThPOK in situ with its marsupial equivalent using a knockin approach, we demonstrate that the marsupial SilThPOK supports correct CD4 T lymphocyte lineage-specification in mice. To our knowledge, this is the first in vivo demonstration of functional equivalency for a silencer element between marsupial and placental mammals using a definitive knockin approach. Finally, we show that alteration of the position/orientation of a highly conserved region within the murine SilThPOK is sufficient to destroy silencer activity in vivo, demonstrating that motif grammar of this "solid" synteny block is critical for silencer function. Dependence of SilThPOK function on motif grammar conserved since the mid-Jurassic age, 165 Ma, suggests that the SilThPOK operates as a silenceosome, by analogy with the previously proposed enhanceosome model.

Postcranial heterochrony, modularity, integration and disparity in the prenatal ossification in bats (Chiroptera)

BACKGROUND

Self-powered flight is one of the most energy-intensive types of locomotion found in vertebrates. It is also associated with a range of extreme morpho-physiological adaptations that evolved independently in three different vertebrate groups. Considering that development acts as a bridge between the genotype and phenotype on which selection acts, studying the ossification of the postcranium can potentially illuminate our understanding of bat flight evolution. However, the ontogenetic basis of vertebrate flight remains largely understudied. Advances in quantitative analysis of sequence heterochrony and morphogenetic growth have created novel approaches to study the developmental basis of diversification and the evolvability of skeletal morphogenesis. Assessing the presence of ontogenetic disparity, integration and modularity from an evolutionary approach allows assessing whether flight may have resulted in evolutionary differences in the magnitude and mode of development in bats.

RESULTS

We quantitatively compared the prenatal ossification of the postcranium (24 bones) between bats (14 species), non-volant mammals (11 species) and birds (14 species), combining for the first time prenatal sequence heterochrony and developmental growth data. Sequence heterochrony was found across groups, showing that bat postcranial development shares patterns found in other flying vertebrates but also those in non-volant mammals. In bats, modularity was found as an axial-appendicular partition, resembling a mammalian pattern of developmental modularity and suggesting flight did not repattern prenatal postcranial covariance in bats.

CONCLUSIONS

Combining prenatal data from 14 bat species, this study represents the most comprehensive quantitative analysis of chiropteran ossification to date. Heterochrony between the wing and leg in bats could reflect functional needs of the newborn, rather than ecological aspects of the adult. Bats share similarities with birds in the development of structures involved in flight (i.e. handwing and sternum), suggesting that flight altriciality and early ossification of pedal phalanges and sternum are common across flying vertebrates. These results indicate that the developmental modularity found in bats facilitates intramodular phenotypic diversification of the skeleton. Integration and disparity increased across developmental time in bats. We also found a delay in the ossification of highly adaptable and evolvable regions (e.g. handwing and sternum) that are directly associated with flight performance.

gga-mir-133a-3p Regulates Myoblasts Proliferation and Differentiation by Targeting PRRX1

Non-coding RNAs play a regulatory role in the growth and development of skeletal muscle. Our previous study suggested that gga-mir-133a-3p was a potential candidate for regulating myoblast proliferation and differentiation in skeletal muscle. The purpose of our study was to reveal the regulatory mechanism of gga-mir-133a-3p in the proliferation and differentiation of chicken myoblasts. Through the detection of cell proliferation activity, cell cycle progression and EdU, we found that gga-mir-133a-3p can significantly inhibit the proliferation of myoblasts. In the process of myogenic differentiation, gga-mir-133a-3p is up-regulated, while gga-mir-133a-3p can significantly promote the up-regulation of differentiation-related muscle-derived factors, indicating that gga-mir-133a-3p can promote the differentiation of myoblasts. Validation at the transcriptional level and protein level proved that gga-mir-133a-3p can inhibit the expression of PRRX1, and the dual-luciferase assay also showed their direct targeting relationship. Correspondingly, PRRX1 can significantly promote myoblast proliferation and inhibit myoblast differentiation. In our study, we confirmed that gga-mir-133a-3p participates in the regulation of proliferation and differentiation of myoblasts by targeting PRRX1.

Ancient mechanisms for the evolution of the bicoid homeodomain's function in fly development

The ancient mechanisms that caused developmental gene regulatory networks to diversify among distantly related taxa are not well understood. Here we use ancestral protein reconstruction, biochemical experiments, and developmental assays of transgenic animals carrying reconstructed ancestral genes to investigate how the transcription factor Bicoid (Bcd) evolved its central role in anterior-posterior patterning in flies. We show that most of Bcd's derived functions are attributable to evolutionary changes within its homeodomain (HD) during a phylogenetic interval >140 million years ago. A single substitution from this period (Q50K) accounts almost entirely for the evolution of Bcd's derived DNA specificity in vitro. In transgenic embryos expressing the reconstructed ancestral HD, however, Q50K confers activation of only a few of Bcd's transcriptional targets and yields a very partial rescue of anterior development. Adding a second historical substitution (M54R) confers regulation of additional Bcd targets and further rescues anterior development. These results indicate that two epistatically interacting mutations played a major role in the evolution of Bcd's controlling regulatory role in early development. They also show how ancestral sequence reconstruction can be combined with in vivo characterization of transgenic animals to illuminate the historical mechanisms of developmental evolution.

The Combinatorial Creature: Cortical Phenotypes within and across Lifetimes

The neocortex is one of the most distinctive structures of the mammalian brain, yet also one of the most varied in terms of both size and organization. Multiple processes have contributed to this variability, including evolutionary mechanisms (i.e., alterations in gene sequence) that alter the size, organization, and connections of neocortex, and activity dependent mechanisms that can also modify these same features. Thus, changes to the neocortex can occur over different time-scales, including within a single generation. This combination of genetic and activity dependent mechanisms that create a given cortical phenotype allows the mammalian neocortex to rapidly and flexibly adjust to different body and environmental contexts, and in humans permits culture to impact brain construction.

bric à brac (bab), a central player in the gene regulatory network that mediates thermal plasticity of pigmentation in Drosophila melanogaster

Drosophila body pigmentation has emerged as a major Evo-Devo model. Using two Drosophila melanogaster lines, Dark and Pale, selected from a natural population, we analyse here the interaction between genetic variation and environmental factors to produce this complex trait. Indeed, pigmentation varies with genotype in natural populations and is sensitive to temperature during development. We demonstrate that the bric à brac (bab) genes, that are differentially expressed between the two lines and whose expression levels vary with temperature, participate in the pigmentation difference between the Dark and Pale lines. The two lines differ in a bab regulatory sequence, the dimorphic element (called here bDE). Both bDE alleles are temperature-sensitive, but the activity of the bDE allele from the Dark line is lower than that of the bDE allele from the Pale line. Our results suggest that this difference could partly be due to differential regulation by AbdB. bab has been previously reported to be a repressor of abdominal pigmentation. We show here that one of its targets in this process is the pigmentation gene tan (t), regulated via the tan abdominal enhancer (t_MSE). Furthermore, t expression is strongly modulated by temperature in the two lines. Thus, temperature sensitivity of t expression is at least partly a consequence of bab thermal transcriptional plasticity. We therefore propose that a gene regulatory network integrating both genetic variation and temperature sensitivity modulates female abdominal pigmentation. Interestingly, both bDE and t_MSE were previously shown to have been recurrently involved in abdominal pigmentation evolution in drosophilids. We propose that the environmental sensitivity of these enhancers has turned them into evolutionary hotspots.

Complex traits such as size or disease susceptibility are typically modulated by both genetic variation and environmental conditions. Model organisms such as fruit flies (Drosophila) are particularly appropriate to analyse the interactions between genetic variation and environmental factors during the development of complex phenotypes. Natural populations carry high genetic variation and can be grown in controlled conditions in the laboratory. Here, we use Drosophila melanogaster female abdominal pigmentation, which is both genetically variable and modulated by the environment (temperature) to dissect this kind of interaction. We show that the pigmentation difference between two inbred fly lines is caused by genetic variation in an enhancer of the bab locus, which encodes two transcription factors controlling abdominal pigmentation. Indeed, this enhancer drives differential expression between the two lines. Interestingly, this enhancer is sensitive to temperature in both lines. We show that the effect of bab on pigmentation is mediated by the pigmentation gene tan (t) that is repressed by bab. Thus, the previously reported temperature-sensitive expression of t is a direct consequence of bab transcriptional plasticity.

Changing While Staying the Same: Preservation of Structural Continuity During Limb Evolution by Developmental Integration

More than 150 years since Charles Darwin published "On the Origin of Species", gradual evolution by natural selection is still not fully reconciled with the apparent sudden appearance of complex structures, such as the bat wing, with highly derived functions. This is in part because developmental genetics has not yet identified the number and types of mutations that accumulated to drive complex morphological evolution. Here, we consider the experimental manipulations in laboratory model systems that suggest tissue interdependence and mechanical responsiveness during limb development conceptually reduce the genetic complexity required to reshape the structure as a whole. It is an exciting time in the field of evolutionary developmental biology as emerging technical approaches in a variety of non-traditional laboratory species are on the verge of filling the gaps between theory and evidence to resolve this sesquicentennial debate.

Gene(s) and individual feeding behavior: Exploring eco-evolutionary dynamics underlying left-right asymmetry in the scale-eating cichlid fish Perissodus microlepis

The scale‐eating cichlid fish Perissodus microlepis is a textbook example of bilateral asymmetry due to its left or right‐bending heads and of negative frequency‐dependent selection, which is proposed to maintain this stable polymorphism. The mechanisms that underlie this asymmetry remain elusive. Several studies had initially postulated a simple genetic basis for this trait, but this explanation has been questioned, particularly by reports observing a unimodal distribution of mouth shapes. We hypothesize that this unimodal distribution might be due to a combination of genetic and phenotypically plastic components. Here, we expanded on previous work by investigating a formerly identified candidate SNP associated to mouth laterality, documenting inter‐individual variation in feeding preference using stable isotope analyses, and testing their association with mouth asymmetry. Our results suggest that this polymorphism is influenced by both a polygenic basis and inter‐individual non‐genetic variation, possibly due to feeding experience, individual specialization, and intraspecific competition. We introduce a hypothesis potentially explaining the simultaneous maintenance of left, right, asymmetric and symmetric mouth phenotypes due to the interaction between diverse eco‐evolutionary dynamics including niche construction and balancing selection. Future studies will have to further tease apart the relative contribution of genetic and environmental factors and their interactions in an integrated fashion.