The origin and evolution of stereotyped patterns of macrochaetes on the nota of cyclorraphous Diptera

Development and evolution of color patterns in ladybird beetles: A case study in Harmonia axyridis

Many organisms show various geometric color patterns on their bodies, and the developmental, evolutionary, genetic, and ecological bases of these patterns have been intensely studied in various organisms. Ladybird beetles display highly diverse patterns of wing (elytral) color and are one of the most attractive model organisms for studying these characteristics. In this study, we reviewed the genetic history of elytral color patterns in the Asian multicolored ladybird beetle Harmonia axyridis from the classical genetic studies led by the pupils of Thomas Hunt Morgan and Theodosius Dobzhansky to recent genomic studies that revealed that a single GATA transcription factor gene, pannier, regulates the highly diverse elytral color patterns in this species. We also reviewed and discussed the developmental and evolutionary mechanisms driven by the pannier locus in H. axyridis. In the development sections, we focused on the following two topics: (a) how the red (carotenoid) and black (melanin) pigmentation of elytra is regulated by the pannier and pigmentation genes and (b) how the diverse color patterns are formed by integrating regulatory inputs from other genes involved in wing development. In the evolution section, we subsequently focused on the highly diversified DNA sequences within the first intron of pannier that are 56-76 kb long and that were generated through recurrent multiple inversions. Furthermore, we discussed how these recurrent inversions have driven the diversification of color patterns throughout evolution.

A model study of the morphogenesis of D. melanogaster mechanoreceptors: the central regulatory circuit

Macrochaetes (large bristles) are sensor organs of the Drosophila peripheral nervous system with a function of mechanoreceptors. An adult mechanoreceptor comprises four specialized cells: shaft (trichogen), socket (tormogen), neuron, and glial cell (thecogen). All these cells originate from a single cell, the so-called sensor organ precursor (SOP) cell. Separation of the SOP cell from the encompassing cells of the imaginal disc initiates a multistage process of sensory organ development. A characteristic feature of the SOP cell is the highest amount of the proneural proteins AS-C as compared with the encompassing ectodermal cells. The accumulation of proneural proteins and maintenance of their amount in the SOP cell at a necessary level is provided by the gene network with the achaete-scute gene complex (AS-C) as its key component. The activity of this complex is controlled by the central regulatory circuit (CRC). The CRC comprises the genes hairy, senseless (sens), charlatan (chn), scratch (scrt), daughterless (da), extramacrochaete (emc), and groucho (gro), coding for the transcription factors involved in the system of direct links and feedbacks and implementation of activation-repression relationships between the CRC components. The gene phyllopod (phyl), involved in degradation of the AS-C proteins, is also associated with the CRC functioning. In this paper, we propose a mathematical model for the CRC functioning as a regulator of the amount of proneural AS-C proteins in the SOP cell taking into account their degradation. The modeling has demonstrated that a change in the amount of proneural proteins in the SOP cell is stepwise rather than strictly monotonic. This prediction can be tested experimentally.

The achaete-scute complex in Diptera: patterns of noncoding sequence evolution

The achaete‐scute complex (AS‐C) has been a useful paradigm for the study of pattern formation and its evolution. achaete‐scute genes have duplicated and evolved distinct expression patterns during the evolution of cyclorraphous Diptera. Are the expression patterns in different species driven by conserved regulatory elements? If so, when did such regulatory elements arise? Here, we have sequenced most of the AS‐C of the fly Calliphora vicina (including the genes achaete, scute and lethal of scute) to compare noncoding sequences with known cis‐regulatory sequences in Drosophila. The organization of the complex is conserved with respect to Drosophila species. There are numerous small stretches of conserved noncoding sequence that, in spite of high sequence turnover, display binding sites for known transcription factors. Synteny of the blocks of conserved noncoding sequences is maintained suggesting not only conservation of the position of regulatory elements but also an origin prior to the divergence between these two species. We propose that some of these enhancers originated by duplication with their target genes.

Salvador-Warts-Hippo pathway in a developmental checkpoint monitoring helix-loop-helix proteins

The E proteins and Id proteins are, respectively, the positive and negative heterodimer partners for the basic-helix-loop-helix protein family and as such contribute to a remarkably large number of cell-fate decisions. E proteins and Id proteins also function to inhibit or promote cell proliferation and cancer. Using a genetic modifier screen in Drosophila, we show that the Id protein Extramacrochaetae enables growth by suppressing activation of the Salvador-Warts-Hippo pathway of tumor suppressors, activation that requires transcriptional activation of the expanded gene by the E protein Daughterless. Daughterless protein binds to an intronic enhancer in the expanded gene, both activating the SWH pathway independently of the transmembrane protein Crumbs and bypassing the negative feedback regulation that targets the same expanded enhancer. Thus, the Salvador-Warts-Hippo pathway has a cell-autonomous function to prevent inappropriate differentiation due to transcription factor imbalance and monitors the intrinsic developmental status of progenitor cells, distinct from any responses to cell-cell interactions.

An expanded Notch-Delta model exhibiting long-range patterning and incorporating MicroRNA regulation

Notch-Delta signaling is a fundamental cell-cell communication mechanism that governs the differentiation of many cell types. Most existing mathematical models of Notch-Delta signaling are based on a feedback loop between Notch and Delta leading to lateral inhibition of neighboring cells. These models result in a checkerboard spatial pattern whereby adjacent cells express opposing levels of Notch and Delta, leading to alternate cell fates. However, a growing body of biological evidence suggests that Notch-Delta signaling produces other patterns that are not checkerboard, and therefore a new model is needed. Here, we present an expanded Notch-Delta model that builds upon previous models, adding a local Notch activity gradient, which affects long-range patterning, and the activity of a regulatory microRNA. This model is motivated by our experiments in the ascidian Ciona intestinalis showing that the peripheral sensory neurons, whose specification is in part regulated by the coordinate activity of Notch-Delta signaling and the microRNA miR-124, exhibit a sparse spatial pattern whereby consecutive neurons may be spaced over a dozen cells apart. We perform rigorous stability and bifurcation analyses, and demonstrate that our model is able to accurately explain and reproduce the neuronal pattern in Ciona. Using Monte Carlo simulations of our model along with miR-124 transgene over-expression assays, we demonstrate that the activity of miR-124 can be incorporated into the Notch decay rate parameter of our model. Finally, we motivate the general applicability of our model to Notch-Delta signaling in other animals by providing evidence that microRNAs regulate Notch-Delta signaling in analogous cell types in other organisms, and by discussing evidence in other organisms of sparse spatial patterns in tissues where Notch-Delta signaling is active.

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.

The Enhancer of split complex arose prior to the diversification of schizophoran flies and is strongly conserved between Drosophila and stalk-eyed flies (Diopsidae)

Background

In Drosophila, the Enhancer of split complex (E(spl)-C) comprises 11 bHLH and Bearded genes that function during Notch signaling to repress proneural identity in the developing peripheral nervous system. Comparison with other insects indicates that the basal state for Diptera is a single bHLH and Bearded homolog and that the expansion of the gene complex occurred in the lineage leading to Drosophila. However, comparative genomic data from other fly species that would elucidate the origin and sequence of gene duplication for the complex is lacking. Therefore, in order to examine the evolutionary history of the complex within Diptera, we reconstructed, using several fosmid clones, the entire E(spl)-complex in the stalk-eyed fly, Teleopsis dalmanni and collected additional homologs of E(spl)-C genes from searches of dipteran EST databases and the Glossina morsitans genome assembly.

Results

Comparison of the Teleopsis E(spl)-C gene organization with Drosophila indicates complete conservation in gene number and orientation between the species except that T. dalmanni contains a duplicated copy of E(spl)m5 that is not present in Drosophila. Phylogenetic analysis of E(spl)-complex bHLH and Bearded genes for several dipteran species clearly demonstrates that all members of the complex were present prior to the diversification of schizophoran flies. Comparison of upstream regulatory elements and 3' UTR domains between the species also reveals strong conservation for many of the genes and identifies several novel characteristics of E(spl)-C regulatory evolution including the discovery of a previously unidentified, highly conserved SPS+A domain between E(spl)mγ and E(spl)mβ.

Conclusion

Identifying the phylogenetic origin of E(spl)-C genes and their associated regulatory DNA is essential to understanding the functional significance of this well-studied gene complex. Results from this study provide numerous insights into the evolutionary history of the complex and will help refine the focus of studies examining the adaptive consequences of this gene expansion.

The kinase Sgg modulates temporal development of macrochaetes in Drosophila by phosphorylation of Scute and Pannier

Evolution of novel structures is often made possible by changes in the timing or spatial expression of genes regulating development. Macrochaetes, large sensory bristles arranged into species-specific stereotypical patterns, are an evolutionary novelty of cyclorraphous flies and are associated with changes in both the temporal and spatial expression of the proneural genes achaete (ac) and scute (sc). Changes in spatial expression are associated with the evolution of cis-regulatory sequences, but it is not known how temporal regulation is achieved. One factor required for ac-sc expression, the expression of which coincides temporally with that of ac-sc in the notum, is Wingless (Wg; also known as Wnt). Wingless downregulates the activity of the serine/threonine kinase Shaggy (Sgg; also known as GSK-3). We demonstrate that Scute is phosphorylated by Sgg on a serine residue and that mutation of this residue results in a form of Sc with heightened proneural activity that can rescue the loss of bristles characteristic of wg mutants. We suggest that the phosphorylated form of Sc has reduced transcriptional activity such that sc is unable to autoregulate, an essential function for the segregation of bristle precursors. Sgg also phosphorylates Pannier, a transcriptional activator of ac-sc, the activity of which is similarly dampened when in the phosphorylated state. Furthermore, we show that Wg signalling does not act directly via a cis-regulatory element of the ac-sc genes. We suggest that temporal control of ac-sc activity in cyclorraphous flies is likely to be regulated by permissive factors and might therefore not be encoded at the level of ac-sc gene sequences.

Dipteran-associated Harpellales from lowland and submontane tropical rain forests of Veracruz (Mexico)

We report on the species of Harpellales found in dipteran hosts during two surveys (32 field d) in the state of Veracruz, Mexico. One new morphospecies, Genistellospora dorsicaudata, is described with particular attention to the position of the terminal cell associated with fully developed fertile thalli bearing sexual spores. We emend the description of G. guanacastensis to include morphometrics on the zygospores, based on discovery of the sexual spores for that species in our collections. Thirteen other previously described species, which are new for Mexico, include G. homothallica, Pennella montana, Simuliomyces microsporus, Smittium aciculare, S. brasiliense (in a new host type), S. culisetae, S. dipterorum, S. microsporum, S. simulii and the unbranched species Harpella melusinae, H. tica, Stachylina grandispora and S. paucispora. Some species have been described but not named, specifically one each of Harpella, Pennella and Smittium. All taxa are identified morphologically, illustrated and additional details on their ecology are provided.

An arthropod cis-regulatory element functioning in sensory organ precursor development dates back to the Cambrian

Background

An increasing number of publications demonstrate conservation of function of cis-regulatory elements without sequence similarity. In invertebrates such functional conservation has only been shown for closely related species. Here we demonstrate the existence of an ancient arthropod regulatory element that functions during the selection of neural precursors. The activity of genes of the achaete-scute (ac-sc) family endows cells with neural potential. An essential, conserved characteristic of proneural genes is their ability to restrict their own activity to single or a small number of progenitor cells from their initially broad domains of expression. This is achieved through a process called lateral inhibition. A regulatory element, the sensory organ precursor enhancer (SOPE), is required for this process. First identified in Drosophila, the SOPE contains discrete binding sites for four regulatory factors. The SOPE of the Drosophila asense gene is situated in the 5' UTR.

Results

Through a manual comparison of consensus binding site sequences we have been able to identify a SOPE in UTR sequences of asense-like genes in species belonging to all four arthropod groups (Crustacea, Myriapoda, Chelicerata and Insecta). The SOPEs of the spider Cupiennius salei and the insect Tribolium castaneum are shown to be functional in transgenic Drosophila. This would place the origin of this regulatory sequence as far back as the last common ancestor of the Arthropoda, that is, in the Cambrian, 550 million years ago.

Conclusions

The SOPE is not detectable by inter-specific sequence comparison, raising the possibility that other ancient regulatory modules in invertebrates might have escaped detection.

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.

How Drosophila melanogaster Forms its Mechanoreceptors

A strictly determined number of external sensory organs, macrochaetes, acting as mechanoreceptors, are orderly located on drosophila head and body. Totally, they form the bristle pattern, which is a species-specific characteristic of drosophila.Each mechanoreceptor comprises four specialized cells derived from the single sensory organ precursor (SOP) cell. The conserved bristle pattern combined with a comparatively simple structure of each mechanosensory organ makes macrochaetes a convenient model for studying the formation spatial structures with a fixed number of elements at certain positions and the mechanism underlying cell differentiation.The macrochaete morphogenesis consists of three stages. At the first stage, the proneural clusters segregate from the massive of ectodermal cells of the wing imaginal disc. At the second stage, the SOP cell is determined and its position in the cluster is specified. At the third stage, the SOP cell undergoes two asymmetric divisions, and the daughter cells differentiate into the components of mechanoreceptor: shaft, socket, bipolar neuron, and sheath.The critical factor determining the neural pathway of cell development is the content of proneural proteins, products of the achaete-scute (AS-C) gene complex, reaching its maximum in the SOP cell.The experimental data on the main genes and their products involved in the control of bristle pattern formation are systematized. The roles of achaete-scute complex, EGFR and Notch signaling pathways, and selector genes in these processes are considered. An integral scheme describing the functioning of the system controlling macrochaete development in D. melanogaster is proposed based on analysis of literature data.

The stars and stripes of animal bodies: evolution of regulatory elements mediating pigment and bristle patterns in Drosophila

Evolution has generated enormous morphological diversity in animals and one of the genetic processes that might have contributed to this is evolution of the cis-regulatory sequences responsible for the temporal and spatial expression of genes regulating embryonic development. This could be particularly relevant to pleiotropic genes with multiple independently acting regulatory modules. Loss or gain of modules enables altered expression without loss of other functions. Here I focus on recent studies correlating differences in morphological traits between related species of Drosophila to changes in cis-regulatory sequences. They show that ancestral regulatory modules have evolved to mediate different transcriptional outputs and suggest that evolution of cis-regulatory sequences might reflect a general mechanism driving evolutionary change.