Simple organisms not so simple, after all


Living blobs without left-right symmetry revealed to be more diverse than thought. Andrew Masterson reports.


A placozoan: not as simple as it appears.
A placozoan: not as simple as it appears.
Ana Signorovitch / Yale University

An organism long held to be the simplest on Earth has turned out to be rather more complex than thought.

Placozoans are tiny marine animals that superficially resemble amoebas. The creatures possess two identifiable cell layers, but beyond that have no internal structures, neural network, or even left-right symmetry.

Externally, at least, they are as close as it is possible to get to being simply a little living blob. They were first discovered, weirdly enough, in an Austrian aquarium in 1883, and ever since then have been classified as comprising a single species, named Trichoplax adhaerens.

Now, however, researchers led by Michael Eitel and Gert Wörheide of Ludwig-Maximilians-Universität in Munich, Germany, have discovered that the overwhelming external uniformity of placozoans hides considerable genetic diversity.

In a paper published in the journal PLOS Biology, Eitel and colleagues report the results of sequencing the genome of a placozoan lineage known as H13, and its comparison to a previously sequenced population, known as H1.

The comparison looked at the physical location of genes within the chromosomes, as well as the venalysis of duplicated genes and gene sets. The results indicate that both lineages had undergone different gene duplication events – perhaps indicative of individual environmental adaptation challenges.

The distance between the two genomes turns out to be so great that the researchers propose that the H13 population be reclassified, not just as a different species, but belonging to a separate genus. They suggest it should be called Hoilungia hongkongensis.

“The lack of classical morphological differences in the placozoans and other small animals has led to an underappreciation of their taxonomic diversity at the genomic level,” Wörheide says.

The results of the sequencing will come as welcome confirmation but little surprise to the small band of scientists who study the placozoans.

As early as 2007, researchers Vicki Buchsbaum Pearse and Oliver Voigt suggested that future work would reveal that placozoan diversity was “certainly underrepresented by its single named species”.

  1. http://animaldiversity.org/accounts/Placozoa/
  2. http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2005359
  3. https://academic.oup.com/icb/article/47/5/677/604122
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