El petit amfiox: el miratge de l'origen dels vertebrats

L'origen dels vertebrats és un dels centres d'interès de molts científics i de la societat, i el nostre animal favorit, el petit i amigable amfiox, fa més d'un segle que ocupa aquesta posició privilegiada, i reflecteix en un miratge, com a fòssil vivent, el que va ocórrer en aquell transcendental moment de l'evolució. No és només morfològicament el representant viu de l'ancestre de tots els cordats, sinó que a més posseeix un genoma preduplicatiu, un model en versió simple del genoma de tots els vertebrats, incloent-hi l'humà. L'anàlisi del genoma de l'amfiox ha permès clarificar l'origen del nostre genoma propi, alhora que l'ha situat en una posició clau per entendre l'origen i funcionament de famílies i xarxes gèniques importants per al desenvolupament embrionari i la fisiologia. A més, els avenços tècnics en la reproducció i experimentació permeten raonadament somiar a resoldre els secrets dels nostres orígens i del funcionament del genoma.The little amphious: a mirage to the origen of vertebrates. The origin of vertebrates is one of the top interests for many scientists and the society, and our favorite animal has occupied this privileged position for more than a century, reflecting in a mirage, as a living fossil, what occurred in this critical landmark of evolution. It is not only morphologically the closest living relative of Chordates, but also possesses a preduplicative genome, a simple model for the genome of all vertebrates, including humans. The analysis of the amphioxus genome has served to clarify the origin of our own genome, and as showed it is in a key position to understand the origin and functioning of gene families and gene networks critical for embryonic development and physiology. Besides, technical advances in its reproduction and experimentation make plausible to dream in illuminating the secrets of our origins and the functioning of the genome

The ADAR Family in Amphioxus: RNA Editing and Conserved Orthologous Site Predictions.

RNA editing is a relatively unexplored process in which transcribed RNA is modified at specific nucleotides before translation, adding another level of regulation of gene expression. Cephalopods use it extensively to increase the regulatory complexity of their nervous systems, and mammals use it too, but less prominently. Nevertheless, little is known about the specifics of RNA editing in most of the other clades and the relevance of RNA editing from an evolutionary perspective remains unknown. Here we analyze a key element of the editing machinery, the ADAR (adenosine deaminase acting on RNA) gene family, in an animal with a key phylogenetic position at the root of chordates: the cephalochordate amphioxus. We show, that as in cephalopods, ADAR genes in amphioxus are predominantly expressed in the nervous system; we identify a number of RNA editing events in amphioxus; and we provide a newly developed method to identify RNA editing events in highly polymorphic genomes using orthology as a guide. Overall, our work lays the foundations for future comparative analysis of RNA-editing events across the metazoan tree

The dorsoanterior brain of adult amphioxus shares similarities in expression profile and neuronal composition with the vertebrate telencephalon

Funder: FP7 People: Marie-Curie Actions; doi: http://dx.doi.org/10.13039/100011264; Grant(s): 229597Abstract: Background: The evolutionary origin of the telencephalon, the most anterior part of the vertebrate brain, remains obscure. Since no obvious counterpart to the telencephalon has yet been identified in invertebrate chordates, it is difficult to trace telencephalic origins. One way to identify homologous brain parts between distantly related animal groups is to focus on the combinatorial expression of conserved regionalisation genes that specify brain regions. Results: Here, we report the combined expression of conserved transcription factors known to specify the telencephalon in the vertebrates in the chordate amphioxus. Focusing on adult specimens, we detect specific co-expression of these factors in the dorsal part of the anterior brain vesicle, which we refer to as Pars anterodorsalis (PAD). As in vertebrates, expression of the transcription factors FoxG1, Emx and Lhx2/9 overlaps that of Pax4/6 dorsally and of Nkx2.1 ventrally, where we also detect expression of the Hedgehog ligand. This specific pattern of co-expression is not observed prior to metamorphosis. Similar to the vertebrate telencephalon, the amphioxus PAD is characterised by the presence of GABAergic neurons and dorsal accumulations of glutamatergic as well as dopaminergic neurons. We also observe sustained proliferation of neuronal progenitors at the ventricular zone of the amphioxus brain vesicle, as observed in the vertebrate brain. Conclusions: Our findings suggest that the PAD in the adult amphioxus brain vesicle and the vertebrate telencephalon evolved from the same brain precursor region in ancestral chordates, which would imply homology of these structures. Our comparative data also indicate that this ancestral brain already contained GABA-, glutamatergic and dopaminergic neurons, as is characteristic for the olfactory bulb of the vertebrate telencephalon. We further speculate that the telencephalon might have evolved in vertebrates via a heterochronic shift in developmental timing

The amphioxus genome and the evolution of the chordate karyotype

Lancelets ('amphioxus') are the modern survivors of an ancient chordate lineage, with a fossil record dating back to the Cambrian period. Here we describe the structure and gene content of the highly polymorphic approx520-megabase genome of the Florida lancelet Branchiostoma floridae, and analyse it in the context of chordate evolution. Whole-genome comparisons illuminate the murky relationships among the three chordate groups (tunicates, lancelets and vertebrates), and allow not only reconstruction of the gene complement of the last common chordate ancestor but also partial reconstruction of its genomic organization, as well as a description of two genome-wide duplications and subsequent reorganizations in the vertebrate lineage. These genome-scale events shaped the vertebrate genome and provided additional genetic variation for exploitation during vertebrate evolution

Amphioxus functional genomics and the origins of vertebrate gene regulation.

Vertebrates have greatly elaborated the basic chordate body plan and evolved highly distinctive genomes that have been sculpted by two whole-genome duplications. Here we sequence the genome of the Mediterranean amphioxus (Branchiostoma lanceolatum) and characterize DNA methylation, chromatin accessibility, histone modifications and transcriptomes across multiple developmental stages and adult tissues to investigate the evolution of the regulation of the chordate genome. Comparisons with vertebrates identify an intermediate stage in the evolution of differentially methylated enhancers, and a high conservation of gene expression and its cis-regulatory logic between amphioxus and vertebrates that occurs maximally at an earlier mid-embryonic phylotypic period. We analyse regulatory evolution after whole-genome duplications, and find that-in vertebrates-over 80% of broadly expressed gene families with multiple paralogues derived from whole-genome duplications have members that restricted their ancestral expression, and underwent specialization rather than subfunctionalization. Counter-intuitively, paralogues that restricted their expression increased the complexity of their regulatory landscapes. These data pave the way for a better understanding of the regulatory principles that underlie key vertebrate innovations

Factores y receptores neurotróficos en el pre-vertebrado arquetípico anfioxo

[spa] Los factores y receptores neurotróficos son esenciales para el desarrollo y mantenimiento del sistema nervioso vertebrado y contribuyen de manera decisiva en funciones neuronales complejas tales como el aprendizaje, la memoria y el comportamiento social complejo. Dichos genes han sido estudiados en el anfioxo, un invertebrado marino que, por su plano corporal, desarrollo embrionario y genoma prototípico respecto a los vertebrados, es actualmente considerado el pariente vivo más cercano al que fue el ancestro de los vertebrados. Este estudio describe la caracterización a nivel molecular y funcional del primer receptor Trk jamás aislado en un invertebrado, rebatiendo la idea de que los receptores Trk son una innovación de los vertebrados, y sugieriendo que se originó por barajado de exones contenedores de dominios proteicos. Asímismo la expresión de este gen durante el desarrollo embrionario del anfioxo ha revelado ciertas características novedosas respecto al desarrollo de su sistema nervioso. Durante la neurulación se ha provado la migración individual de células ectodérmicas, lo que podría reflejar un comportamiento parecido al de las células de la cresta neural de vertebrados, un tipo de células pluripotentes que dan lugar a un gran número de estructuras únicamente presentes en los vertebrados y totalmente ausentes en el anfioxo. Este trabajo también incluye la caracterización del primer miembro conocido de la familia de las caspasas en el anfioxo, involucradas en muerte neuronal y, además las primeras técnicas desarrolladas para la obtención rutinaria de embriones, con la potencialidad que ello comporta para su manipulación "in vivo", con el objetivo de realizar ensayos funcionales y abrir las puertas a los estudios de Evo-Devo experimental.[eng] Neurotrophic factors and their receptors play an essential role in the development and maintenance of the vertebrate nervous system, where they crucially contribute to higher neuronal functions such as learning, memory and complex social behaviour. These genes have been studied for the first time in the cephalochordate amphioxus, which is presently the best stand in for the study of one of the major events in evolution: the invertebrate-vertebrate transition. This work describes the molecular and functional characterisation of the first Trk receptor ever isolated from an invertebrate, refuting the idea that Trk receptors are a vertebrate evolutionary novelty. The results obtained suggest that exon shuffling was a key mechanism to generate a unique ProtoTrk gene, similar to that of amphioxus, which subsequently expanded by gene duplication during vertebrate evolution giving rise to all members of the Trk gene family presently known in higher vertebrates. Furthermore, the expression pattern of this Trk receptor throughout embryonic development revealed certain particularities of the amphioxus nervous system never described before. The embryonic epidermis of amphioxus neurulae contains a cell population able to migrate individually, mimicking the migratory behaviour of the vertebrate neural crest cells, a pluripotent cell type only present in vertebrates and totally absent in amphioxus. This work also includes the characterisation of the first caspase ever isolated in amphioxus and the first approaches towards experimental Evo-Devo through the development of new techniques to obtain amphioxus embryos "a la carte" and to manipulate them "in vivo"

Restricted Proliferation During Neurogenesis Contributes to Regionalisation of the Amphioxus Nervous System.

The central nervous system of the cephalochordate amphioxus consists of a dorsal neural tube with an anterior brain. Two decades of gene expression analyses in developing amphioxus embryos have shown that, despite apparent morphological simplicity, the amphioxus neural tube is highly regionalised at the molecular level. However, little is known about the morphogenetic mechanisms regulating the spatiotemporal emergence of cell types at distinct sites of the neural axis and how their arrangements contribute to the overall neural architecture. In vertebrates, proliferation is key to provide appropriate cell numbers of specific types to particular areas of the nervous system as development proceeds, but in amphioxus proliferation has never been studied at this level of detail, nor in the specific context of neurogenesis. Here, we describe the dynamics of cell division during the formation of the central nervous system in amphioxus embryos, and identify specific regions of the nervous system that depend on proliferation of neuronal precursors at precise time-points for their maturation. By labelling proliferating cells in vivo at specific time points in development, and inhibiting cell division during neurulation, we demonstrate that localised proliferation in the anterior cerebral vesicle is required to establish the full cell type repertoire of the frontal eye complex and the putative hypothalamic region of the amphioxus brain, while posterior proliferating progenitors, which were found here to derive from the dorsal lip of the blastopore, contribute to elongation of the caudal floor plate. Between these proliferative domains, we find that trunk nervous system differentiation is independent from cell division, in which proliferation decreases during neurulation and resumes at the early larval stage. Taken together, our results highlight the importance of proliferation as a tightly controlled mechanism for shaping and regionalising the amphioxus neural axis during development, by addition of new cells fated to particular types, or by influencing tissue geometry