Yannick Le Parco

Phylogeny and Evolution of Calcareous Sponges: Monophyly of Calcinea and Calcaronea, High Level of Morphological Homoplasy, and the Primitive Nature of Axial Symmetry

Because calcareous sponges are triggering renewed interest with respect to basal metazoan evolution, a phylogenetic framework of their internal relationships is needed to clarify the evolutionary history of key morphological characters. Morphological variation was scored at the suprageneric level within Calcispongia, but little phylogenetic information could be retrieved from morphological characters. For the main subdivision of Calcispongia, the analysis of morphological data weakly supports a classification based upon cytological and embryological characters (Calcinea/Calcaronea) rather than the older classification scheme based upon the aquiferous system (Homocoela/Heterocoela). The 18S ribosomal RNA data were then analyzed, both alone and in combination with morphological characters. The monophyly of Calcispongia is highly supported, but the position of this group with respect to other sponge lineages and to eumetazoan taxa is not resolved. The monophyly of both Calcinea and Calcaronea is retrieved, and the data strongly rejected the competing Homocoela/Heterocoela hypothesis. The phylogeny implies that characters of the skeleton architecture are highly homoplastic, as are characters of the aquiferous system. However, axial symmetry seems to be primitive for all Calcispongia, a conclusion that has potentially far-reaching implications for hypotheses of early body plan evolution in Metazoa.

The Comparison of β-Thymosin Homologues Among Metazoa Supports an Arthropod-Nematode Clade

Abstract. The definition of an Ecdysozoa clade among the protostomians, including all phyla with a regularly molted α-chitin-rich cuticle, has been one of the most provocative hypotheses to arise from recent investigations on animal phylogeny. Here we present evidence in favor of an arthropod-nematode clade, from the comparison of β-thymosin homologues among the Metazoa. Arthropods and nematodes share the absence of the highly conserved β-thymosin form found in all other documented bilaterian phyla as well as sponges, and the possession of a very unusual, internally triplicated homologue of the β-thymosin protein, unknown in other phyla. We argue that such discrete molecular character is phylogenetically very powerful and provides strong evidence for the monophyly of an arthropod-nematode clade.

Haemocytes accumulate collagen transcripts during Drosophila melanogaster metamorphosis

SummaryWe report a direct examination of the expression of one collagen gene (DCg1) during Drosophila melanogaster metamorphosis, based on data from in situ hybridization. The transcripts of this gene, thought to encode a basement membrane type IV collagen, are mainly accumulated during ecdysis in wandering haemocytes. Our results demonstrate that haemocytes contribute to extracellular matrix deposition and seem to perform a fibroblastic function during Drosophila development.

Hox gene survey in the chaetognath Spadella cephaloptera: evolutionary implications

We present the isolation of six Hox genes in the chaetognath Spadella cephaloptera. We identified one member of the paralogy group 3, four median genes and a mosaic gene that shares features of both median and posterior classes (SceMedPost). Several hypotheses may account for the presence of a mosaic Hox gene in this animal. Here we propose that SceMedPost may represent an ancestral gene, which has not diverged totally into a posterior or a median one. This hypothesis has interesting implications for the reconstruction of the evolutionary history of Hox genes and suggests that Chaetognatha lineage divergence could predate the deuterostome/protostome split. Such a phylogenetic position is considered in the light of their embryological and morphological characters.

DCg1 αIV collagen chain of Drosophila melanogaster is synthesized during embryonic organogenesis by mesenchymal cells and is deposited in muscle basement membranes

Abstract We report the purification and characterization of three sequence-specific polyclonal antibodies raised against specific portions of the Drosophila αIV collagen chain produced from the gene DCg1. These antibodies were used for immunolocalization experiments on tissue sections from embryonic organogenesis stages (13–17) and first larval stages. This analysis was paralleled by in situ hybridization experiments with a labeled fragment of the gene DCg1. We demonstrated that, by late embryogenesis, the DCg1 αIV chain was synthesized by individual mesoblasts and deposited in basement membranes of skeletal and visceral muscles. These sites of αIV collagen deposition were the same, by first and second instars, but the protein was then synthesized by fat body cells. Our results were reminiscent of those obtained for vertebrate in vitro myogenesis, they suggested, moreover, a tissue-specific composition of basement membranes in Drosophila melanogaster .

Restricted expression of a median Hox gene in the central nervous system of chaetognaths

Hox genes encode a set of evolutionarily conserved transcription factors that regulate anterior–posterior patterning. Here we report the first developmental expression of a Hox gene from Chaetognatha. These metazoans have been shown recently to be part of the protostome group of bilaterians. We describe the analysis of the SceMed4 gene (a Spadella cephaloptera Median Hox gene) including its expression from late stages of egg development to 7 days after hatching. In all of these stages, SceMed4 is expressed in two lateral stripes in a restricted region of the developing ventral ganglion.

High Level of Structural Polymorphism Driven by Mobile Elements in the Hox Genomic Region of the Chaetognath Spadella cephaloptera

Little is known about the relationships between genome polymorphism, mobile element dynamics, and population size among animal populations. The chaetognath species Spadella cephaloptera offers a unique perspective to examine this issue because they display a high level of genetic polymorphism at the population level. Here, we have investigated in detail the extent of nucleotide and structural polymorphism in a region harboring Hox1 and several coding genes and presumptive functional elements. Sequencing of several bacterial artificial chromosome inserts representative of this nuclear region uncovered a high level of structural heterogeneity, which is mainly caused by the polymorphic insertion of a diversity of genetic mobile elements. By anchoring this variation through individual genotyping, we demonstrated that sequence diversity could be attributed to the allelic pool of a single population, which was confirmed by detection of extensive recombination within the genomic region studied. The high average level of nucleotide heterozygosity provides clues of selection in both coding and noncoding domains. This pattern stresses how selective processes remarkably cope with intense sequence turnover due to substitutions, mobile element insertions, and recombination to preserve the integrity of functional landscape. These findings suggest that genome polymorphism could provide pivotal information for future functional annotation of genomes.

Phylogeny of Animals: Genomes Have a Lot to Say

Multiple lines of evidence have been proposed to resolve the tree of metazoans. Views based on morphology and development were often questioned because they relied on characters whose evolutionary orientation is difficult. Molecules offer an independent perspective and the employment of some genes, such as ribosomal RNA subunits (SSU/LSU) or Hox genes, have led to a profound reshaping of the metazoan tree, leading to the “New View” of animal phylogeny. However, classical molecular approaches have not succeeded in settling some long-standing issues in animal relationships. Recently, extensive genome data have been collected for a large set of organisms including several evolutionarily relevant marine species. This has allowed the development of phylogenomic approaches, which have the potential to overcome the limitations of single-gene phylogenies by inferring trees on the base of whole genome evidence. This new approach has triggered several advances regarding metazoan relationships such as the reevaluation of chordate relationships and the re-positioning of some problematic minor phyla with improved accuracy. These studies have also raised new questions about the processes that underlie morphological evolution, as well as the future of molecular phylogenetics.

Variations of chromatin-associated proteins during the prereplicative phase of traumatic regeneration of the polychaete annelid Owenia fusiformis

Abstract We previously demonstrated, during the prereplicative phase of traumatic regeneration, an increase of transcription of poly(A) + -RNA which was correlated to variations in the accessibility of DNA in the chromatin to different probes. In this paper, we study the variations at the chromatin-associated protein level. In chromatin extracted from normal and from 12 h regenerating animals, quantitative and qualitative changes (modifications of the electrophoretic pattern and phosphorylation) were found in non-histone proteins extractable by low ionic concentration (probably ribonucleoproteins).