DNA sequence analysis of mitochondrial Cyt-b and the species status ofLaniarius dubiosus (Rchw. 1899)

In 1899Reichenow described an african bushshrike in juvenile plumage as a new species. He named itLaniarius dubiosus. DNA from type material (feathers and skin) was extracted and DNA sequences from the mitochondrial Cyt-b gene were analysed. Comparisons of DNA-sequences from other bushshrikes (including the type-specimen from Luehder's bushshrikeLaniarius lühderi) support the judgement that dubiosus does not represent a full species rather than a representative of the western subspecies of the Luehder's bushshrikeLaniarius luehderi.

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Zusammenfassung Der vonReichenow (1899) als neue Art beschriebene, sich im Jugendkleid befindliche BuschwürgerLaniarius dubiosus ist nach DNA-Sequenzanalysen des mitochondrialen Cyt-b Genes mit hoher Wahrscheinlichkeit ein Jungvogel der westlichen Rasse des BraunscheitelwürgersLaniarius lühderi. Auf der Basis eines DNA-Stammbaumes von sieben verschiedenen Buschwürgern ist der 1991 neu beschriebeneLaniarius liberatus am wenigsten eng mitL. dubiosus verwandt.

     

Ancient linkage of a POU class 6 and an anterior Hox-like gene in cnidaria: implications for the evolution of homeobox genes

Linkage analyses in metazoan genomes suggest two ancestral arrays for the majority of homeobox genes. The related homeobox genes and chromosomal regions that are dispersed in extant species derived possibly from only two single common ancestor regions. One proposed ancestral array, designated as ANTP mega-array, contains most of the ANTP class homeobox genes; the second, named the contraHox super-paralogon, would consist of the classes PRD, POU, LIM, CUT, prospero, TALE and SIX. Here, we report the tight linkage of a POU class 6 gene to an anterior Hox-like gene in the hydrozoan Eleutheria dichotoma and discuss its possible significance for the evolution of homeobox genes. POU class 6 genes also seem to be ancestrally linked to the HoxC and A clusters in vertebrates, despite POU homeobox genes belonging to the contraHox paralogon. Hence, the much tighter linkage of a POU class 6 gene to an anterior Hox-like gene in a cnidarian is possibly the evolutionary echo of an ancestral genomic region from which most metazoan homeobox classes emerged.     

An automated phylogenetic key for classifying homeoboxes

When novel gene sequences are discovered, they are usually identified, classified, and annotated based on aggregate measures of sequence similarity. This method is prone to errors, however. Phylogenetic analysis is a more accurate basis for gene classification and ortholog identification, but it is relatively labor-intensive and computationally demanding. Here we report and demonstrate a rapid new method for gene classification based on phylogenetic principles. Given the phylogeny of a minimal sample of gene family members, our method automatically identifies amino acids that are phylogenetically characteristic of each class of sequences in the family; it then classifies a novel sequence based on the presence of these characteristic attributes in its sequence. Using a subset of homeobox protein sequences as a test case, we show that our method approximates classification based on full-scale phylogenetic analysis with very high accuracy in a tiny fraction of the time.     

Homology of Hox Genes and the Zootype Concept in Early Metazoan Evolution

The correct identification of homologous Hox genes within and between diplo- and triploblastic animals is of crucial importance for recent hypotheses on the anagenetic evolution of animal bauplans. While the homology discussion in general has reached new heights, we apply traditional homology criteria to assign homology to Hox genes from diploblastic animals. Comparison of theTrox-2gene from the presumably most basal metazoan animal, the placozoanTrichoplax adhaerens,to other Hox genes suggests the presence of unambiguous homologs in Hydrozoa and Scyphozoa and the absence of any specific homolog in triploblasts. Furthermore, the comparisons provide support for the idea that Hox genes—at least in diploblastic animals—are composed of functional subunits (modules), which to some degree have undergone independent evolution. The findings are not readily compatible with the existence of the “zootype” in diploblastic animals.     

Phylogenetic context and Basal metazoan model systems

In comparative studies using model organisms, extant taxa areoften referred to as basal. The term suggests that such taxaare descendants of lineages that diverged early in the historyof some larger taxon. By this usage, the basal metazoans comprisejust four phyla (Placozoa, Porifera, Cnidaria, and Ctenophora)and the large clade Bilateria. We advise against this practicebecause basal refers to a region at the base or root of a phylogenetictree. Thus, referring to an extant taxon or species as basal,or as more basal than another, can be misleading. While muchprogress has been made toward understanding some of the phylogeneticrelationships within these groups, the relationships among themare still largely not known with certainty. Thus, sound inferencesfrom comparative studies of model organisms demand continuedillumination of phylogeny. Hypotheses about the mechanisms underlyingmetazoan evolution can be drawn from the study of model organismsin Cnidaria, Ctenophora, Placozoa, and Porifera, but it is clearthat these model organisms are likely to be derived in manyrespects. Therefore, testing these hypotheses requires the studyof yet additional model organisms. The most effective testsare those that investigate model organisms with phylogeneticpositions among two sister groups comprising a larger cladeof interest.     

The most primitive metazoan animals,the placozoans,show high sensitivity to increasing ocean temperatures and acidities

The increase in atmospheric carbon dioxide (CO₂) leads to rising temperatures and acidification in the oceans, which directly or indirectly affects all marine organisms, from bacteria to animals. We here ask whether the simplest—and possibly also the oldest—metazoan animals, the placozoans, are particularly sensitive to ocean warming and acidification. Placozoans are found in all warm and temperate oceans and are soft‐bodied, microscopic invertebrates lacking any calcified structures, organs, or symmetry. We here show that placozoans respond highly sensitive to temperature and acidity stress. The data reveal differential responses in different placozoan lineages and encourage efforts to develop placozoans as a potential biomarker system.     

Placozoa and the evolution of Metazoa and intrasomatic cell differentiation

The multicellular Metazoa evolved from single-celled organisms (Protozoa) and usually – but not necessarily – consist of more cells than Protozoa. In all cases, and thus by definition, Metazoa possess more than one somatic cell type, i.e. they show-in sharp contrast to protists–intrasomatic differentiation. Placozoa have the lowest degree of intrasomatic variation; the number of somatic cell types according to text books is four (but see also Jakob W, Sagasser S, Dellaporta S, Holland P, Kuhn K, and Schierwater B. The Trox-2 Hox/ParaHox gene of Trichoplax (Placozoa) marks an epithelial boundary. Dev Genes Evol 2004;214:170–5). For this and several other reasons Placozoa have been regarded by many as the most basal metazoan phylum. Thus, the morphologically most simply organized metazoan animal, the placozoan Trichoplax adhaerens, resembles a unique model system for cell differentiation studies and also an intriguing model for a prominent “urmetazoon” hypotheses—the placula hypothesis. A basal position of Placozoa would provide answers to several key issues of metazoan-specific inventions (including for example different lines of somatic cell differentiation leading to organ development and axis formation) and would determine a root for unraveling their evolution. However, the phylogenetic relationships at the base of Metazoa are controversial and a basal position of Placozoa is not generally accepted (e.g. Schierwater B, DeSalle R. Can we ever identify the Urmetazoan? Integr Comp Biol 2007;47:670–76; DeSalle R, Schierwater B. An even “newer” animal phylogeny. Bioessays 2008;30:1043–47). Here we review and discuss (i) long-standing morphological evidence for the simple placozoan bauplan resembling an ancestral metazoan stage, (ii) some rapidly changing alternative hypotheses derived from molecular analyses, (iii) the surprising idea that triploblasts (Bilateria) and diploblasts may be sister groups, and (iv) the presence of genes involved in cell differentiation and signaling pathways in the placozoan genome.