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BackgroundArthropods comprise the largest and most diverse phylum on Earth and play vital roles in nearly every ecosystem. Their diversity stems in part from variations on a conserved body plan, resulting from and recorded in adaptive changes in the genome. Dissection of the genomic record of sequence change enables broad questions regarding genome evolution to be addressed, even across hyper-diverse taxa within arthropods.ResultsUsing 76 whole genome sequences representing 21 orders spanning more than 500 million years of arthropod evolution, we document changes in gene and protein domain content and provide temporal and phylogenetic context for interpreting these innovations. We identify many novel gene families that arose early in the evolution of arthropods and during the diversification of insects into modern orders. We reveal unexpected variation in patterns of DNA methylation across arthropods and examples of gene family and protein domain evolution coincident with the appearance of notable phenotypic and physiological adaptations such as flight, metamorphosis, sociality, and chemoperception.ConclusionsThese analyses demonstrate how large-scale comparative genomics can provide broad new insights into the genotype to phenotype map and generate testable hypotheses about the evolution of animal diversity.

BackgroundThe duplication of genes can occur through various mechanisms and is thought to make a major contribution to the evolutionary diversification of organisms. There is increasing evidence for a large-scale duplication of genes in some chelicerate lineages including two rounds of whole genome duplication (WGD) in horseshoe crabs. To investigate this further, we sequenced and analyzed the genome of the common house spider Parasteatoda tepidariorum.ResultsWe found pervasive duplication of both coding and non-coding genes in this spider, including two clusters of Hox genes. Analysis of synteny conservation across the P. tepidariorum genome suggests that there has been an ancient WGD in spiders. Comparison with the genomes of other chelicerates, including that of the newly sequenced bark scorpion Centruroides sculpturatus, suggests that this event occurred in the common ancestor of spiders and scorpions, and is probably independent of the WGDs in horseshoe crabs. Furthermore, characterization of the sequence and expression of the Hox paralogs in P. tepidariorum suggests that many have been subject to neo-functionalization and/or sub-functionalization since their duplication.ConclusionsOur results reveal that spiders and scorpions are likely the descendants of a polyploid ancestor that lived more than 450 MYA. Given the extensive morphological diversity and ecological adaptations found among these animals, rivaling those of vertebrates, our study of the ancient WGD event in Arachnopulmonata provides a new comparative platform to explore common and divergent evolutionary outcomes of polyploidization events across eukaryotes.

MicroRNAs are small (∼22 nt) noncoding RNAs that repress translation and therefore regulate the production of proteins from specific target mRNAs. microRNAs have been found to function in diverse aspects of gene regulation within animal development and many other processes. Among invertebrates, both conserved and novel, lineage specific, microRNAs have been extensively studied predominantly in holometabolous insects such as Drosophila melanogaster However little is known about microRNA repertoires in other arthropod lineages such as the chelicerates. To understand the evolution of microRNAs in this poorly sampled subphylum, we characterized the microRNA repertoire expressed during embryogenesis of the common house spider Parasteatoda tepidariorum We identified a total of 148 microRNAs in P. tepidariorum representing 66 families. Approximately half of these microRNA families are conserved in other metazoans, while the remainder are specific to this spider. Of the 35 conserved microRNAs families 15 had at least two copies in the P. tepidariorum genome. A BLAST-based approach revealed a similar pattern of duplication in other spiders and a scorpion, but not among other chelicerates and arthropods, with the exception of a horseshoe crab. Among the duplicated microRNAs we found examples of lineage-specific tandem duplications, and the duplication of entire microRNA clusters in three spiders, a scorpion, and in a horseshoe crab. Furthermore, we found that paralogs of many P. tepidariorum microRNA families exhibit arm switching, which suggests that duplication was often followed by sub- or neofunctionalization. Our work shows that understanding the evolution of microRNAs in the chelicerates has great potential to provide insights into the process of microRNA duplication and divergence and the evolution of animal development.

The molecular mechanisms underlying evolution of morphological novelties are not well understood, although co-option is often inferred. A new study demonstrates that the partial co-option of the trichome gene regulatory network underlies the evolution of novel projections on Drosophila male genitalia. [ABSTRACT FROM AUTHOR]

Morphology evolves often through changes in developmental genes, but the causal mutations, and their effects, remain largely unknown. The evolution of naked cuticle on larvae of Drosophila sechellia resulted from changes in five transcriptional enhancers of shavenbaby (svb), a transcript of the ovo locus that encodes a transcription factor that governs morphogenesis of microtrichiae, hereafter called 'trichomes'. Here we show that the function of one of these enhancers evolved through multiple single-nucleotide substitutions that altered both the timing and level of svb expression. The consequences of these nucleotide substitutions on larval morphology were quantified with a novel functional assay. We found that each substitution had a relatively small phenotypic effect, and that many nucleotide changes account for this large morphological difference. In addition, we observed that the substitutions had non-additive effects. These data provide unprecedented resolution of the phenotypic effects of substitutions and show how individual nucleotide changes in a transcriptional enhancer have caused morphological evolution., The genetic mechanisms underlying morphological evolution remain largely unknown (1,2). Comparative studies indicate that changes in the timing (heterochrony), location (heterotopy), and level of gene expression have caused much of [...]

The spiders Cuplennius salei and Achaearanea tepidariorum are the laboratory examples that answers to the questions related to evolutionary development. Recent studies of axis formation, segmentation and neurogenesis in these spiders and reviewed to better understand their contribution.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.cell.2007.05.026 Byline: Thomas Gregor (1)(2)(3)(4), Eric F. Wieschaus (3)(4), Alistair P. McGregor (5), William Bialek (1)(2), David W. Tank (1)(2)(3) Keywords: DEVBIO Abstract: Patterning in multicellular organisms results from spatial gradients in morphogen concentration, but the dynamics of these gradients remain largely unexplored. We characterize, through in vivo optical imaging, the development and stability of the Bicoid morphogen gradient in Drosophila embryos that express a Bicoid-eGFP fusion protein. The gradient is established rapidly ([approximately equal to]1 hr after fertilization), with nuclear Bicoid concentration rising and falling during mitosis. Interphase levels result from a rapid equilibrium between Bicoid uptake and removal. Initial interphase concentration in nuclei in successive cycles is constant ([+ or -]10%), demonstrating a form of gradient stability, but it subsequently decays by approximately 30%. Both direct photobleaching measurements and indirect estimates of Bicoid-eGFP diffusion constants (D [less than or equal to] 1 [mu]m.sup.2/s) provide a consistent picture of Bicoid transport on short ([approximately equal to]min) time scales but challenge traditional models of long-range gradient formation. A new model is presented emphasizing the possible role of nuclear dynamics in shaping and scaling the gradient. Author Affiliation: (1) Lewis-Sigler Institute for Integrative Genomics, Princeton University, Princeton, New Jersey 08544, USA (2) Joseph Henry Laboratories of Physics, Princeton University, Princeton, New Jersey 08544, USA (3) Department of Molecular Biology, Princeton University, Princeton, New Jersey 08544, USA (4) Howard Hughes Medical Institute, Princeton University, Princeton, New Jersey 08544, USA (5) Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544, USA Article History: Received 15 October 2006; Revised 15 February 2007; Accepted 3 May 2007 Article Note: (miscellaneous) Published: July 12, 2007

The investigation of the genes regulating early embryogenesis in the wasp Nasonia vitripennis and the beetle Tribollum castaneum have provided some clues as to how early development is controlled in insects other than higher dipterans such as Drosophila melanogaster. The results have shown that during the evolution of long-germ-band development, Nasonia and Drosophila might have evolved different mechanisms to pattern posterior segments, marginalizing the important role of the terminal system in short-germ-band embryos.

The existing understanding of how a gradient of bicoid (Bcd) protein regulates gene expression along the anteroposterior axis of the Drosophila embryo, which is necessary for the correct development of the head and thorax, is studied. Embryos produced by bcd mutant mothers develop with ectopic post-erior structures such as an extra telson and spiracles in place of a proper head and thorax, and also exhibit abdominal defects.