Chromosome-level genome assemblies of 2 hemichordates provide new insights into deuterostome origin and chromosome evolution.

Deuterostomes are a monophyletic group of animals that includes Hemichordata, Echinodermata (together called Ambulacraria), and Chordata. The diversity of deuterostome body plans has made it challenging to reconstruct their ancestral condition and to decipher the genetic changes that drove the diversification of deuterostome lineages. Here, we generate chromosome-level genome assemblies of 2 hemichordate species, Ptychodera flava and Schizocardium californicum, and use comparative genomic approaches to infer the chromosomal architecture of the deuterostome common ancestor and delineate lineage-specific chromosomal modifications. We show that hemichordate chromosomes (1N = 23) exhibit remarkable chromosome-scale macrosynteny when compared to other deuterostomes and can be derived from 24 deuterostome ancestral linkage groups (ALGs). These deuterostome ALGs in turn match previously inferred bilaterian ALGs, consistent with a relatively short transition from the last common bilaterian ancestor to the origin of deuterostomes. Based on this deuterostome ALG complement, we deduced chromosomal rearrangement events that occurred in different lineages. For example, a fusion-with-mixing event produced an Ambulacraria-specific ALG that subsequently split into 2 chromosomes in extant hemichordates, while this homologous ALG further fused with another chromosome in sea urchins. Orthologous genes distributed in these rearranged chromosomes are enriched for functions in various developmental processes. We found that the deeply conserved Hox clusters are located in highly rearranged chromosomes and that maintenance of the clusters are likely due to lower densities of transposable elements within the clusters. We also provide evidence that the deuterostome-specific pharyngeal gene cluster was established via the combination of 3 pre-assembled microsyntenic blocks. We suggest that since chromosomal rearrangement events and formation of new gene clusters may change the regulatory controls of developmental genes, these events may have contributed to the evolution of diverse body plans among deuterostomes.

The Patterns of Codon Usage between Chordates and Arthropods are Different but Co-evolving with Mutational Biases.

Different frequencies amongst codons that encode the same amino acid (i.e. synonymous codons) have been observed in multiple species. Studies focused on uncovering the forces that drive such codon usage showed that a combined effect of mutational biases and translational selection works to produce different frequencies of synonymous codons. However, only few have been able to measure and distinguish between these forces that may leave similar traces on the coding regions. Here, we have developed a codon model that allows the disentangling of mutation, selection on amino acids and synonymous codons, and GC-biased gene conversion (gBGC) which we employed on an extensive dataset of 415 chordates and 191 arthropods. We found that chordates need 15 more synonymous codon categories than arthropods to explain the empirical codon frequencies, which suggests that the extent of codon usage can vary greatly between animal phyla. Moreover, methylation at CpG sites seems to partially explain these patterns of codon usage in chordates but not in arthropods. Despite the differences between the two phyla, our findings demonstrate that in both, GC-rich codons are disfavored when mutations are GC-biased, and the opposite is true when mutations are AT-biased. This indicates that selection on the genomic coding regions might act primarily to stabilize its GC/AT content on a genome-wide level. Our study shows that the degree of synonymous codon usage varies considerably among animals, but is likely governed by a common underlying dynamic.

El Genoma en los Cordados: Introducción a la Genómica Comparada / The Genome in the Chordates: Introduction to Comparative Genomics

El objetivo del presente trabajo es describir brevemente conceptos de la genómica de los cordados así como de la genómica comparada y sus aspectos éticos. El genoma de los cordados cambió ligeramente dando lugar al genoma de los vertebrados, entre ellos el de los mamíferos, entre los que se incluye el ser humano. La genómica comparada estudia las semejanzas y diferencias entre genomas de diferentes organismos; trata de explicar la información proporcionada por la selección natural para entender la función y los procesos evolutivos que actúan sobre los genomas. La complejidad de los procesos evolutivos constituye todo un desafío a la hora de analizar e interpretar la información biológica generada; en este sentido, la Bioinformática y la Biología Computacional proporcionan un amplio abanico de técnicas estadísticas, matemáticas y algorítmicas para el análisis de datos biológicos. Un reto clave es analizar el caudal de datos de secuencias de ADN con el fin de comprender la información almacenada en términos de estructura, función y evolución proteicas. En Biología computacional BLAST y ClustalW2 son las herramientas más empleadas para el análisis de alineamientos múltiples de secuencias. La Genómica está resultando clave también en el campo de la medicina; conocer la cartografía del genoma humano proporciona una valiosa información a tener en cuenta a la hora de detectar genes implicados en ciertas enfermedades. Esto conlleva que en la actualidad nos centremos más en la predicción de patologías que en la prevención, por lo que la tendencia es que en el futuro la Medicina Genómica acabe desbancando a la Medicina Preventiva. El Proyecto Genoma Humano presenta diversas aplicaciones que, al no tener una clara cobertura legal, traen consigo un nuevo paradigma con problemas éticos, sociales y legales que la comunidad científica trata de resolver para compaginar los aspectos morales con el progreso en la investigación.

The aim of this paper is to briefly describe concepts of genomics of chordates and comparative genomics and its ethical aspects. The genome of chordates changed slightly resulting in the genome of vertebrates, including mammals. The human being belongs to the phylum chordates. Comparative genomics studies the similarities and differences between genomes of different organisms, trying to explain the information provided by natural selection to understand the function and evolutionary processes that act on genomes. Evolutive processes' complexity is considered a major challenge in terms of analyzing and interpreting all the biological information generated; in this sense, Bioinformatics and Computational Biology provide an enormous range of statistical, mathematical and algorythmical techniques for biological data analyses. A key challenge for bioinformatics is to analyze the flow of DNA sequence data to understand the information stored in terms of structure, function and protein evolution. BLAST and ClustalW2 tools are used for the analysis of multiple sequence alignments in Computational Biology. Genomics is also playing a key role in Medicine; human genome's cartography provides valuable information to detect genes involved in certain diseases. It entails that nowadays it is better to focus on prediction much more than on prevention. The current tendency is that in the future Genomic Medicine will displace Preventive Medicine. The Human Genome Project implies diverse applications that do not have clear legal coverage. This brings a new paradigm with ethical, social and legal problems that the scientific community is trying to resolve in order to combine morality and research progress.