Placozoan diversity and taxonomy


If I didn’t study Volvox, I would probably study placozoa. Placozoa are animals, but you wouldn’t know it to look at them. They look and behave very much like giant amoebae, big enough to be visible to the naked eye.

Trichoplax adhaerens

Trichoplax adhaerens. By Bernd Schierwater – Eitel M, Osigus H-J, DeSalle R, Schierwater B (2013) Global Diversity of the Placozoa. PLoS ONE 8(4): e57131. doi:10.1371/journal.pone.0057131, CC BY 4.0, Link

Isn’t it gorgeous? The placozoa seem ready-made for studying the evolution of multicellularity in animals: they have a simple body plan with just a few differentiated cell types (sort of like Volvox, come to think of it). They are also vastly understudied. There’s just one described species, even though we know they are genetically diverse:

Figure 4 from Eitel et al. 2013. Unexpected diversity in the phylum Placozoa. Shown is the 16S Bayesian inference phylogram of different placozoan haplotypes. Presently seven genetically highly different clades (I–VII) have been identified. Current knowledge on placozoan biodiversity is still limited and more samples are urgently needed. Numbers beside nodes are from left to right: Bayesian posterior probabilities, Maximum likelihood and Maximum Parsimony bootstrap support. Values below 70% are marked with ‘-’.

We’re pretty sure they have sex, but no one has successfully bred them. Usually they reproduce asexually, either by binary fission or budding. They have a worldwide distribution in tropical and temperate latitudes:

Eitel 2013 Fig. 5

Figure 5 from Eitel et al. 2013. Worldwide distribution of genetically characterized placozoan specimens.

Bernd Schierwater’s World Placozoa Database has a nice summary of what we know about placozoa. Professor Scheirwater is one of my scientific heroes for getting a paper through peer review with the title “My favorite animal, Trichoplax adhaerens.” I do wish he’d quit calling them ‘basal’, though. Extant taxa cannot be basal. The database website includes this gem:

This is the smallest of all animal databases yet; it contains a single nominal species, Trichoplax adhaerens.

Well, it’s about to get bigger. Michael Eitel and colleagues have posted a preprint to bioRxiv describing not only a new species, but a new genus of placozoa: “A ​taxogenomics approach uncovers a new genus in the phylum Placozoa.” The new taxon was called Hoilungia hongkongensis in the original preprint, but this was withdrawn in the revised version because,

According to the International Code of Zoological Nomenclature preprint publication of taxonomic names is discouraged. Consequently, the Xxxxxxxxx yyyyyyyyyyyyy / X. yyyyyyyyyyyyy given here is a dummy only. The valid name will be available upon formal journal publication.

Whole genome comparisons between Trichoplax adhaerens and the new taxon showed large-scale genomic rearrangements, comparable to those between mice and humans:

Figure 2 from Eitel et al. 2017. Synteny between Xxxxxxxxx yyyyyyyyyyyyy and Trichoplax adhaerens. Scaled schematic drawings of the ten longest T. adhaerens scaffolds on the left (ta1-ta10) and matching X. yyyyyyyyyyyyy contigs (hh) on the right. Each line represents one gene. While a general macro-synteny between the two placozoan species is present, 25% of the genes are translocated or inverted relative to the order of the respective T. adhaerens scaffold (orange and blue lines, respectively; illustrated for ta1 only). Often, entire blocks are translocated (different colors in boxed X. yyyyyyyyyyyyy contigs). Black stretches mark genomic regions not matching any of the ten T. adhaerens scaffolds while white stretches mark gaps in the T. adhaerens scaffolds.

Genetic distances between Trichoplax adhaerens and the new taxon, based on homologous proteins, are comparable to those between genera in other animal groups:

Eitel 2017 Fig. 3A

Figure 3A from Eitel et al. 2017. Calculated uncorrected pairwise genetic distances for 212 concatenated nuclear-encoded proteins. Mean group distances for different taxonomic ranks in the non-bilaterian phyla Cnidaria, Ctenophora and Porifera. The interspecific protein distance of 11% between Xxxxxxxxx yyyyyyyyyyyyy and Trichoplax adhaerens (right) is comparable to mean group distances between genera within families in the Porifera and Ctenophora, respectively. With respect to the Cnidaria, the placozoan distance is even comparable to the mean group distance between families within orders.

The new taxon corresponds to H13 in Figure 4 above, and the sequenced haplotype of Trichoplax adhaerens to H1. As the authors point out, these are two of the most distantly related haplotypes. Still, the depth of the divergences within ‘Group A’, at least based on 16S sequences, suggests that there are likely to be several other placozoan taxa yet to be described.

Just as in the volvocine algae, placozoan taxonomy has until recently been based on morphology. And just as in the volvocine algae, morphology is clearly insufficient to parse deep divergences within this group, though for different reasons. With cheap whole genome sequencing, I expect we’ll see more species, and maybe even higher taxa, described in the next few years.

There is an immense amount we don’t know about the placozoa: what are their relationships to other animals, how do they have sex, what is their basic ecology…The landscape of placozoan biology is largely unexplored, and there are likely to be wonders as yet undiscovered.

 

Stable links:

Eitel M, Francis WR, Osigus H-J, Krebs S, Vargas S, Blum H, Williams GA, Schierwater B, Wörheide G. 2017. A ​taxogenomics approach uncovers a new genus in the phylum Placozoa. bioRxiv , 0–47. doi: 10.1101/202119

Eitel M, Osigus H-J, Desalle R, Schierwater B. 2013. Global Diversity of the Placozoa. PLoS One 8, 1–12. doi: 10.1371/journal.pone.0057131

Schierwater B. 2005. My favorite animal, Trichoplax adhaerens. BioEssays 27, 1294–1302. doi: 10.1002/bies.20320

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