1,721,570 research outputs found
Flexoelectricity in Liquid Crystals
No full textA rich variety of symmetric and non-symmetric liquid crystal dimers have been synthesised . and studied in order to investigate their structure property relations; in particular their flexoelectric properties. The project is introduced in Chapters 1 and 2 where Chapter 1 gives a brief general background to liquid crystals and liquid crystal synthe~is, and Chapter 2 gives a more detailed background to the flexoelectric effect. The research can be broadly divided into two parts. Chapters 3 and 4 focus on seven conventional non-symmetric liquid crystal dimer series containing two mesogenic groups ether linked through a flexible spacer. These non-symmetric dimers all contain a ( . cyanobiphenyl mesogenic group at one end and either a poly-fluorinated biphenyl or a phenyl-cyclohexyl-alkane mesogenic group at the other. Almost all these materials possess nematic phases (and in one series a smectic phase) and show strong flexoelectric coupling to an applied field with some experiments yielding very large values for the flexoelastic ratio. Chapter 5 focuses on two series of bent core liquid crystal dimers, wher~ the long chain spacer is divided by a catechol or resorcinol based disruptor group. The disruptor is located in the centre of the molecule and symmetric cyanobiphenyl mesogenic groups are at each end. These materials show a curious odd-even effect in the nematic-isotropic transition temperatures which is further investigated in Chapter 6 by reducing the symmetry of the compounds and studying the changes in the transitional ·properties. 1.ills was achieved in one series by changing the relative position of the catechol linking group along the spac~r chain or, in a different series, by altering one of the mesogenic end groups to a di-fluorinated biphenyl (a mesogenic moiety studied in Chapter 3). The liquid crystal properties were investigated by·optical microscopy, differential scanning calorimetry and deuterium NMR spectroscopy. The flexoelastic ratios were determined from thed~pendence of the tilt in the optic axis on an applied electric fieldEThOS - Electronic Theses Online ServiceGBUnited Kingdo
The evolution of an ancient metazoan biomineralization strategy was supported by a horizontal gene transfer
No full textThe molecular mechanisms that generate morphological novelty are of great interest to evolutionary biologists because these are the processes that can explain how the diversity of life on earth arose. With advances in sequencing technologies, the high-throughput analysis and comparison of entire genomes is now possible. Bioinformatic mining of such genome-wide data sets often includes a search for horizontal gene transfers (HGTs) as these events can provide exciting insight into how morphological, or physiological novelties may have arisen. A recent paper by Jackson et al.1 demonstrates that a HGT into the genome of the sponge Astrosclera willeyana likely supported the evolution of this animal's biomineralization strategy. This HGT, which occurred deep in time, was perhaps a key event in the evolution of this animal's body form and would not have been detected by certain in silico methods commonly used to screen large data sets
Mantle Modularity Underlies the Plasticity of the Molluscan Shell: Supporting Data From Cepaea nemoralis
No full textMolluscs have evolved the capacity to fabricate a wide variety of shells over their 540+ million-year history. While modern sequencing and proteomic technologies continue to expand the catalog of molluscan shell-forming proteins, a complete functional understanding of how any mollusc constructs its shell remains an ambitious goal. This lack of understanding also constrains our understanding of how evolution has generated a plethora of molluscan shell morphologies. Taking advantage of a previous expression atlas for shell-forming genes in Lymnaea stagnalis, I have characterized the spatial expression patterns of seven shell-forming genes in the terrestrial gastropod Cepaea nemoralis, with the aim of comparing and contrasting their expression patterns between the two species. Four of these genes were selected from a previous proteomic screen of the C. nemoralis shell, two were targeted by bioinformatics criteria designed to identify likely shell-forming gene products, and the final one was a clear homolog of a peroxidase sequence in the L. stagnalis dataset. While the spatial expression patterns of all seven C. nemoralis genes could be recognized as falling into distinct zones within the mantle tissue similar to those established in L. stagnalis, some zones have apparently been modified. These similarities and differences hint at a modularity to the molluscan mantle that may provide a mechanistic explanation as to how evolution has efficiently generated a diversity of molluscan shells.Open-Access-Publikationsfonds 202
Characterization of the pigmented shell-forming proteome of the common grove snail Cepaea nemoralis
Full text linkBackground With a diversity of pigmented shell morphotypes governed by Mendelian patterns of inheritance, the common grove snail, Cepaea nemoralis, has served as a model for evolutionary biologists and population geneticists for decades. Surprisingly, the molecular mechanisms by which C. nemoralis generates this pigmented shelled diversity, and the degree of evolutionary conservation present between molluscan shell-forming proteomes, remain unknown. Results Here, using next generation sequencing and high throughput proteomics, we identify and characterize the major proteinaceous components of the C. nemoralis shell, the first shell-proteome for a pulmonate mollusc. The recent availability of several marine molluscan shell-proteomes, and the dataset we report here, allow us to identify 59 evolutionarily conserved and novel shell-forming proteins. While the C. nemoralis dataset is dominated by proteins that share little to no similarity with proteins in public databases, almost half of it shares similarity with proteins present in other molluscan shells. In addition, we could not find any indication that a protein (or class of proteins) is directly associated with shell pigmentation in C. nemoralis. This is in contrast to the only other partially characterized molluscan-shell pigmentation mechanism employed by the tropical abalone Haliotis asinina. Conclusions The unique pulmonate shell-forming proteome that we report here reveals an abundance of both mollusc-specific and pulmonate-specific proteins, suggesting that novel coding sequences, and/or the extensive divergence of these sequences from ancestral sequences, supported the innovation of new shell types within the Conchifera. In addition, we report here the first evidence that molluscs use independently evolved mechanisms to pigment their shells. This proteome provides a solid foundation from which further studies aimed at the functional characterization of these shell-forming proteins can be conducted
An ancient process in a modern mollusc: early development of the shell in Lymnaea stagnalis
No full textBackground The morphological variety displayed by the molluscan shell underlies much of the evolutionary success of this phylum. However, the broad diversity of shell forms, sizes, ornamentations and functions contrasts with a deep conservation of early cell movements associated with the initiation of shell construction. This process begins during early embryogenesis with a thickening of an ectodermal, ‘dorsal’ (opposite the blastopore) population of cells, which then invaginates into the blastocoel to form the shell gland. The shell gland evaginates to form the shell field, which then expands and further differentiates to eventually become the adult shell-secreting organ commonly known as the mantle. Despite the deep conservation of the early shell forming developmental program across molluscan classes, little is known about the fine-scale cellular or molecular processes that underlie molluscan shell development. Results Using modern imaging techniques we provide here a description of the morphogenesis of a gastropod shell gland and shell field using the pulmonate gastropod Lymnaea stagnalis as a model. We find supporting evidence for a hypothesis of molluscan shell gland specification proposed over 60 years ago, and present histochemical assays that can be used to identify a variety of larval shell stages and distinct cell populations in whole mounts. Conclusions By providing a detailed spatial and temporal map of cell movements and differentiation events during early shell development in L. stagnalis we have established a platform for future work aimed at elucidation of the molecular mechanisms and regulatory networks that underlie the evo-devo of the molluscan shell
Transposable Elements: From DNA Parasites to Architects of Metazoan Evolution
No full textOne of the most unexpected insights that followed from the completion of the human genome a decade ago was that more than half of our DNA is derived from transposable elements (TEs). Due to advances in high throughput sequencing technologies it is now clear that TEs comprise the largest molecular class within most metazoan genomes. TEs, once categorised as "junk DNA", are now known to influence genomic structure and function by increasing the coding and non-coding genetic repertoire of the host. In this way TEs are key elements that stimulate the evolution of metazoan genomes. This review highlights several lines of TE research including the horizontal transfer of TEs through host-parasite interactions, the vertical maintenance of TEs over long periods of evolutionary time, and the direct role that TEs have played in generating morphological novelty
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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