Stepwise emergence of the neuronal gene expression program in early animal evolution.

The assembly of the neuronal and other major cell type programs occurred early in animal evolution. We can reconstruct this process by studying non-bilaterians like placozoans. These small disc-shaped animals not only have nine morphologically described cell types and no neurons but also show coordinated behaviors triggered by peptide-secreting cells.

Studying Placozoa WBR in the Simplest Metazoan Animal, Trichoplax adhaerens.

Placozoans are a promising model system to study fundamental regeneration processes in a morphologically and genetically very simple animal. We here provide a brief introduction to the enigmatic Placozoa and summarize the state of the art of animal handling and experimental manipulation possibilities.

The enigmatic Placozoa part 2: Exploring evolutionary controversies and promising questions on earth and in space.

The placozoan Trichoplax adhaerens has been bridging gaps between research disciplines like no other animal. As outlined in part 1, placozoans have been subject of hot evolutionary debates and placozoans have challenged some fundamental evolutionary concepts. Here in part 2 we discuss the exceptional genetics of the phylum Placozoa and point out some challenging model system applications for the best known species, Trichoplax adhaerens.

Mitochondrial Genome Evolution of Placozoans: Gene Rearrangements and Repeat Expansions.

Placozoans, nonbilaterian animals with the simplest known metazoan bauplan, are currently classified into 20 haplotypes belonging to three genera, Polyplacotoma, Trichoplax, and Hoilungia. The latter two comprise two and five clades, respectively. In Trichoplax and Hoilungia, previous studies on six haplotypes belonging to four different clades have shown that their mtDNAs are circular chromosomes of 32-43 kb in size, which encode 12 protein-coding genes, 24 tRNAs, and two rRNAs.

Genome analyses of a placozoan rickettsial endosymbiont show a combination of mutualistic and parasitic traits.

Symbiotic relationships between eukaryotic hosts and bacteria range from parasitism to mutualism and may deeply influence both partners' fitness. The presence of intracellular bacteria in the metazoan phylum Placozoa has been reported several times, but without any knowledge about the nature of this relationship and possible implications for the placozoan holobiont. This information may be of crucial significance since little is known about placozoan ecology and how different species adapt to different environmental conditions, despite being almost invariable at the morphological level.

Polyplacotoma mediterranea is a new ramified placozoan species.

The enigmatic phylum Placozoa is harboring an unknown number of cryptic species and has become a challenge for modern systematics. Only recently, a second species has been described [1], while the presence of more than a hundred additional species has been suggested [2]. The original placozoan species Trichoplax adhaerens[3], the second species Hoilungia hongkongensis[1] and all yet undescribed species are morphologically indistinguishable (i.

Innate immunity in the simplest animals - placozoans.

Background: Innate immunity provides the core recognition system in animals for preventing infection, but also plays an important role in managing the relationship between an animal host and its symbiont. Most of our knowledge about innate immunity stems from a few animal model systems, but substantial variation between metazoan phyla has been revealed by comparative genomic studies. The exploration of more taxa is still needed to better understand the evolution of immunity related mechanisms.

Trichoplax genomes reveal profound admixture and suggest stable wild populations without bisexual reproduction.

The phylum Placozoa officially consists of only a single described species, Trichoplax adhaerens, although several lineages can be separated by molecular markers, geographical distributions and environmental demands. The placozoan 16S haplotype H2 (Trichoplax sp. H2) is the most robust and cosmopolitan lineage of placozoans found to date.

GC Content of Early Metazoan Genes and Its Impact on Gene Expression Levels in Mammalian Cell Lines.

With the genomes available for many animal clades, including the early-branching metazoans, one can readily study the functional conservation of genes across a diversity of animal lineages. Ectopic expression of an animal protein in, for instance, a mammalian cell line is a generally used strategy in structure-function analysis. However, this might turn out to be problematic in case of distantly related species.

The early evolution of Hox genes: a battle of belief?

For more than a century the origin of metazoan animals and for less than three years the early evolution of Hox genes has been debated. Both discussions are intrinsically tied together. New data from whole genome sequencing and recent progress in phylogeny of basal metazoans allow to provide an answer.

The Trichoplax genome and the nature of placozoans.

As arguably the simplest free-living animals, placozoans may represent a primitive metazoan form, yet their biology is poorly understood. Here we report the sequencing and analysis of the approximately 98 million base pair nuclear genome of the placozoan Trichoplax adhaerens. Whole-genome phylogenetic analysis suggests that placozoans belong to a 'eumetazoan' clade that includes cnidarians and bilaterians, with sponges as the earliest diverging animals.

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.

Ancient complexity of the non-Hox ANTP gene complement in the anthozoan Nematostella vectensis: implications for the evolution of the ANTP superclass.

The origin and evolution of ANTP superclass genes has raised controversial discussions. While recent evidence suggests that a true Hox cluster emerged after the cnidarian bilaterian split, the origin of the ANTP superclass as a whole remains unclear. Based on analyses of bilaterian genomes, it seems very likely that clustering has once been a characteristic of all ANTP homeobox genes and that their ancestors have emerged through several series of cis-duplications from the same genomic region.

Axial patterning and diversification in the cnidaria predate the Hox system.

Across the animal kingdom, Hox genes are organized in clusters whose genomic organization reflects their central roles in patterning along the anterior/posterior (A/P) axis . While a cluster of Hox genes was present in the bilaterian common ancestor, the origins of this system remain unclear (cf. ).