An Amphioxus Gli Gene Reveals Conservation of Midline Patterning and the Evolution of Hedgehog Signalling Diversity in Chordates

Background. Hedgehog signalling, interpreted in receiving cells by Gli transcription factors, plays a central role in the development of vertebrate and Drosphila embryos. Many aspects of the signalling pathway are conserved between these lineages, however vertebrates have diverged in at least one key aspect: they have evolved multiple Gli genes encoding functionally-distinct proteins, increasing the complexity of the hedgehog-dependent transcriptional response. Amphioxus is one of the closest living relatives of the vertebrates, having split from the vertebrate lineage prior to the widespread gene duplication prominent in early vertebrate evolution. Principal findings. We show that amphioxus has a single Gli gene, which is deployed in tissues adjacent to sources of hedgehog signalling derived from the midline and anterior endoderm. This shows the duplication and divergence of the Gli family, and hence the origin of vertebrate Gli functional diversity, was specific to the vertebrate lineage. However we also show that the single amphioxus Gli gene produces two distinct transcripts encoding different proteins. We utilise three tests of Gli function to examine the transcription regulatory capacities of these different proteins, demonstrating one has activating activity similar to Gli2, while the other acts as a weak repressor, similar to Gli3. Conclusions. These data show that the vertebrates and amphioxus have evolved functionally-similar repertoires of Gli proteins using parallel molecular routes; vertebrates via gene duplication and divergence, and amphioxus via alternate splicing of a single gene. Our results demonstrate that similar functional complexity of intercellular signalling can be achieved via different evolutionary pathways

New spinosaurids from the Wessex Formation (Early Cretaceous, UK) and the European origins of Spinosauridae

Spinosaurids are among the most distinctive and yet poorly-known of large-bodied theropod dinosaurs, a situation exacerbated by their mostly fragmentary fossil record and competing views regarding their palaeobiology. Here, we report two new Early Cretaceous spinosaurid specimens from the Wessex Formation (Barremian) of the Isle of Wight. Large-scale phylogenetic analyses using parsimony and Bayesian techniques recover the pair in a new clade within Baryonychinae that also includes the hypodigm of the African spinosaurid Suchomimus. Both specimens represent distinct and novel taxa, herein named Ceratosuchops inferodios gen. et sp. nov. and Riparovenator milnerae gen. et sp. nov. A palaeogeographic reconstruction suggests a European origin for Spinosauridae, with at least two dispersal events into Africa. These new finds provide welcome information on poorly sampled areas of spinosaurid anatomy, suggest that sympatry was present and potentially common in baryonychines and spinosaurids as a whole, and contribute to updated palaeobiogeographic reconstructions for the clade

Deciphering the fossil record of early bilaterian embryonic development in light of experimental taphonomy

Experimental analyses of decay in a tunicate deuterostome and three lophotrochozoans indicate that the controls on decay and preservation of embryos, identified previously based on echinoids, are more generally applicable. Four stages of decay are identified regardless of the environment of death and decay. Embryos decay rapidly in oxic and anoxic conditions, although the gross morphology of embryos is maintained for longer under anoxic conditions. Under anoxic reducing conditions, the gross morphology of the embryos is maintained for the longest period of time, compatible with the timescale required for bacterially mediated mineralization of soft tissues. All four stages of decay were encountered under all environmental conditions, matching the spectrum of preservational qualities encountered in all fossil embryo assemblages. The preservation potential of embryos of deuterostomes and lophotrochozoans is at odds with the lack of such embryos in the fossil record. Rather, the fossil record of embryos, as sparse as it is, is dominated by forms interpreted as ecdysozoans, cnidarians, and stem-metazoans. The dearth of deuterostome and lophotrochozoan embryos may be explained by the fact that ecdysozoans, at least, tend to deposit their eggs in the sediment rather than through broadcast spawning. However, fossil embryos remain very rare and the main controlling factor on their fossilization may be the unique conspiracy of environmental conditions at a couple of sites. The preponderance of fossilized embryos of direct developers should not be used in evidence against the existence of indirect development at this time in animal evolutionary history

Cellular and Subcellular Structure of Neoproterozoic Animal Embryos

Stereoblastic embryos from the Doushantuo Formation of China exhibit occasional asynchronous cell division, with diminishing blastomere volume as cleavage proceeded. Asynchronous cell division is common in modern embryos, implying that sophisticated mechanisms for differential cell division timing and embryonic cell lineage differentiation evolved before 551 million years ago. Subcellular structures akin to organelles, coated yolk granules, or lipid vesicles occur in these embryos. Paired reniform structures within embryo cells may represent fossil evidence of cells about to undergo division. Embryos exhibit no evidence of epithelial organization, even in embryos composed of ?1000 cells. Many of these features are compatible with metazoans, but the absence of epithelialization is consistent only with a stem-metazoan affinity for Doushantuo embryos

Molecular regionalization of the developing amphioxus neural tube challenges major partitions of the vertebrate brain

All vertebrate brains develop following a common Bauplan defined by anteroposterior (AP) and dorsoventral (DV) subdivisions, characterized by largely conserved differential expression of gene markers. However, it is still unclear how this Bauplan originated during evolution. We studied the relative expression of 48 genes with key roles in vertebrate neural patterning in a representative amphioxus embryonic stage. Unlike nonchordates, amphioxus develops its central nervous system (CNS) from a neural plate that is homologous to that of vertebrates, allowing direct topological comparisons. The resulting genoarchitectonic model revealed that the amphioxus incipient neural tube is unexpectedly complex, consisting of several AP and DV molecular partitions. Strikingly, comparison with vertebrates indicates that the vertebrate thalamus, pretectum, and midbrain domains jointly correspond to a single amphioxus region, which we termed Di-Mesencephalic primordium (DiMes). This suggests that these domains have a common developmental and evolutionary origin, as supported by functional experiments manipulating secondary organizers in zebrafish and mice.Spanish Ministry of Economy and Competitiveness and European FEDER funds (grant number BFU2014-57516-P). To Luis Puelles and Jose Luis Ferran. European Research Council (grant number ERC-StG-LS2-637591). To Manuel Irimia. Spanish Ministry of Economy and Competitiveness (grant number SEV-2012-0208).Centro de Excelencia Severo Ochoa (to CRG, Manuel Irimia). Spanish Ministry of Economy and Competitiveness (grant number BFU2014-58908-P). To Jordi Garcia-Fernadez. Seneca Foundation, Comunidad de Murcia (grant number 19904/GERM/15). To Luis Puelles. Generalitat de Catalunya (grant number). ICREA Academia Prize to Jordi Garcia-Fernandez. Spanish Ministry of Economy and Competitiveness (grant number BFU2013-43213-P). To Paola Bovolenta. Spanish Ministry of Economy and Competitiveness (grant number BFU2014-55076-P). To Manuel Irimia. Including an FPI PhD fellowship to Laura Lopez-Blanch. Marine Alliance for Science and Technology Scotland (MASTS) (grant number). To Ildiko Somorjai