Heiko A. Schmidt

Unexpected complexity of the Wnt gene family in a sea anemone

The Wnt gene family encodes secreted signalling molecules that control cell fate in animal development and human diseases. Despite its significance, the evolution of this metazoan-specific protein family is unclear. In vertebrates, twelve Wnt subfamilies were defined, of which only six have counterparts in Ecdysozoa (for example, Drosophila and Caenorhabditis). Here, we report the isolation of twelve Wnt genes from the sea anemone Nematostella vectensis , a species representing the basal group within cnidarians. Cnidarians are diploblastic animals and the sister-group to bilaterian metazoans. Phylogenetic analyses of N. vectensis Wnt genes reveal a thus far unpredicted ancestral diversity within the Wnt family. Cnidarians and bilaterians have at least eleven of the twelve known Wnt gene subfamilies in common; five subfamilies appear to be lost in the protostome lineage. Expression patterns of Wnt genes during N. vectensis embryogenesis indicate distinct roles of Wnts in gastrulation, resulting in serial overlapping expression domains along the primary axis of the planula larva. This unexpectedly complex inventory of Wnt family signalling factors evolved in early multi-cellular animals about 650 million years (Myr) ago, predating the Cambrian explosion by at least 100 Myr (refs 5, 8). It emphasizes the crucial function of Wnt genes in the diversification of eumetazoan body plans.

An ancient Wnt-Dickkopf antagonism in Hydra

The dickkopf (dkk) gene family encodes secreted antagonists of Wnt signalling proteins, which have important functions in the control of cell fate, proliferation, and cell polarity during development. Here, we report the isolation, from a regeneration-specific signal peptide screen, of a novel dickkopf gene from the fresh water cnidarian Hydra. Comparative sequence analysis demonstrates that the Wnt antagonistic subfamily Dkk1/Dkk2/Dkk4 and the non-modulating subfamily Dkk3 separated prior to the divergence of cnidarians and bilaterians. In steady-state Hydra, hydkk1/2/4-expression is inversely related to that of hywnt3a. hydkk1/2/4 is an early injury and regeneration responsive gene, and hydkk1/2/4-expressing gland cells are essential for head regeneration in Hydra, although once the head has regenerated they are excluded from it. Activation of Wnt/β-Catenin signalling leads to the complete downregulation of hydkk1/2/4 transcripts. When overexpressed in Xenopus, HyDkk1/2/4 has similar Wnt-antagonizing activity to the Xenopus gene Dkk1. Based on the corresponding expression patterns of hydkk1/2/4 and neuronal genes, we suggest that the body column of Hydra is a neurogenic environment suppressing Wnt signalling and facilitating neurogenesis.

Genetic interaction between distinct dobrava hantavirus subtypes in Apodemus agrarius and A. flavicollis in nature

ABSTRACT Dobrava virus (DOBV) occurs in two different rodent species, Apodemus flavicollis (DOBV-Af) and A. agrarius (DOBV-Aa). We sequenced the S and M genomic segments from sympatric DOBV-Af and DOBV-Aa strains which fell into two distinct genetic lineages. Molecular phylogenetic analyses gave evidence for genetic reassortment between S and M segments of DOBV-Af and DOBV-Aa and indicated homologous recombination events in DOBV evolution. DOBV-Af and DOBV-Aa are distinct but also subject to genetic exchanges that affect their evolutionary trajectories.

Maximum‐Likelihood Analysis Using TREE‐PUZZLE

TREE‐PUZZLE provides means to analyze and reconstruct evolutionary relationships and trees based on quartets, i.e. groups of 4 sequences. explains how to reconstruct trees based on the maximum‐likelihood principle and quartet puzzling. discusses likelihood mapping, a method to visualize phylogenetic content in a multiple sequence alignment. explains how to compare tree topologies using different tests.

Nodal signalling determines biradial asymmetry in Hydra

In bilaterians, three orthogonal body axes define the animal form, with distinct anterior–posterior, dorsal–ventral and left–right asymmetries. The key signalling factors are Wnt family proteins for the anterior–posterior axis, Bmp family proteins for the dorsal–ventral axis and Nodal for the left–right axis. Cnidarians, the sister group to bilaterians, are characterized by one oral–aboral body axis, which exhibits a distinct biradiality of unknown molecular nature. Here we analysed the biradial growth pattern in the radially symmetrical cnidarian polyp Hydra, and we report evidence of Nodal in a pre-bilaterian clade. We identified a Nodal-related gene (Ndr) in Hydra magnipapillata, and this gene is essential for setting up an axial asymmetry along the main body axis. This asymmetry defines a lateral signalling centre, inducing a new body axis of a budding polyp orthogonal to the mother polyp’s axis. Ndr is expressed exclusively in the lateral bud anlage and induces Pitx, which encodes an evolutionarily conserved transcription factor that functions downstream of Nodal. Reminiscent of its function in vertebrates, Nodal acts downstream of β-Catenin signalling. Our data support an evolutionary scenario in which a ‘core-signalling cassette’ consisting of β-Catenin, Nodal and Pitx pre-dated the cnidarian–bilaterian split. We presume that this cassette was co-opted for various modes of axial patterning: for example, for lateral branching in cnidarians and left–right patterning in bilaterians.

Molecular phylogenetics: parallelized parameter estimation and quartet puzzling

Exponential growth of the data available for molecular sequence analysis causes eminent need for methods to analyze large datasets in reasonable time. In molecular phylogenetics maximum-likelihood methods became very popular despite their vast need for computational power. During the last decades parallel computing has proven to be a valuable way to decrease running time of computationally intensive analyses. In this paper we suggest to parallelize the estimation of parameters for evolutionary models and the quartet puzzling algorithm to reconstruct phylogenetic trees from DNA and protein sequences applying the maximum-likelihood principle. Furthermore, we discuss effects of the different parallel granularities of the algorithms.

UNIT 6.6 Maximum-Likelihood Analysis Using TREE-PUZZLE

TREE‐PUZZLE provides a means to analyze and reconstruct evolutionary relationships and trees based on quartets, i.e., groups of four sequences. Basic Protocol 1 explains how to reconstruct trees based on the maximum‐likelihood principle and quartet puzzling. Basic Protocol 2 discusses likelihood mapping, a method to visualize phylogenetic content in a multiple sequence alignment. Basic Protocol 3 explains how to compare tree topologies using different tests.

PhyNav: A Novel Approach to Reconstruct Large Phylogenies

A novel method, PhyNav, is introduced to reconstruct the evolutionary relationship among contemporary species based on their genetic data. The key idea is the definition of the so-called minimal κ-distance subset which contains most of the relevant phylogenetic information from the whole dataset. For this reduced subset the subtree is created faster and serves as a scaffold to construct the full tree. Because many minimal subsets exist the procedure is repeated several times and the best tree with respect to some optimality criterion is considered as the inferred phylogenetic tree. PhyNav gives encouraging results compared to other programs on both simulated and real datasets.

In silico analysis of gene expression patterns during early development of Xenopus laevis.

The information as to where and when a mRNA is present in a given cell is essential to bridge the gap between the DNA sequence of a gene and its physiological function. Therefore, a major component of functional genomics is to characterize the levels and the spatio-temporal domains of gene expression. Currently, there is just a few specialised public databases available storing the data on gene expression while they are needed as a resource for the field. Moreover, there is a need to develop and assess computational tools to compare and analyse expression profiles in a suitable way for biological interpretation. Here we describe our recent work on developing a database on gene expression for the frog Xenopus laevis, and on setting up and using new tools for the analysis and comparison of gene expression patterns. We used histogram clustering to compare expression profiles at both gene and tissue levels using a set of data coming from the characterization of the expression of genes during early development of Xenopus. This enabled us to draw a tree of tissue relatedness and to identify coexpressed genes by in silico analysis.

The Phylogeny of Thermophiles and Hyperthermophiles and the Three Domains of Life

The nature of the first cells and the environment in which they lived are two of the most interesting problems in evolutionary biology. All living things are descendents of these primordial cells and are divided into three fundamental lineages or domains, Archaea (formerly known as Archaebacteria), Bacteria (formerly known as Eubacteria), and the Eucarya (formerly known as Eukaryotes, Woese et al. 1990). The Archaea and Bacteria are prokaryotic domains whereas the Eucarya includes all other living things that have a nucleus (i.e., the genetic material is separated from the cytoplasm by a nuclear envelope). The observation of the three primary domains, first made on the basis of small subunit (i.e., 16S, 18S) ribosomal DNA (rDNA) sequence comparisons (Woese 1987), has created a framework with which the nature of the last common ancestor (LCA) can be addressed. In this review we present phylogenies of the prokaryotic domains to understand the origin and distribution of the thermophiles (organisms able to grow in temperatures > 45°C) and the hyperthermophiles (organisms able to grow in temperatures > 80°C). Hyperthermophiles are limited to the Archaea and Bacteria. In addition, we inspect the distribution of extremophiles within the cyanobacteria. The cyanobacteria are unique in being able to tolerate rapidly fluctuating environmental conditions. This capacity has presumably allowed some cyanobacteria to survive in niches unexplored by other prokaryotes and eukaryotes which have a more restricted environmental tolerance. We do not deal with eukaryotic extremophiles in this chapter because no multicellular plant or animal yet found can survive in temperatures of about 50°C and no protists can tolerate long-term exposure to temperatures in excess of 60°C (Madigan and Marrs 1997).