Animal taxonomy: Outwardly an identical, but distinct (News)

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IMAGE: That is Hoilungia hongkongensis.
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Credit score: Hans-Jürgen Osigus, Stiftung Tierärztliche Hochschule Hannover

Up till fairly not too long ago, the animal phylum Placozoa loved a singular place in animal systematics. It was the one phylum to which solely a single species had ever been assigned: Trichoplax adhaerens. Now, nonetheless, at staff led by Professor Gert Wörheide of LMU’s Division of Earth and Environmental Sciences and GeoBio-Heart has found that placozoan specimens collected from coastal waters off Hong Kong clearly differ from T. adhaerens of their genetic make-up. Certainly, the variations between the 2 are so putting that the Hong Kong inhabitants not solely represents a brand new species but additionally has been assigned to a brand new genus – though the 2 genera are morphologically indistinguishable. The definition of a brand new species and genus solely on the premise of comparative genomic knowledge constitutes a brand new departure within the systematic classification of animals. The findings seem in PLOS Biology.

Placozoa are among the many easiest recognized multicellular animals, missing each muscle tissue and nerve cells. They’re just a few millimeters lengthy and their cells are organized into two flat layers. They’ve been present in tropical and subtropical waters all over the world. However, no matter locality of origin, all placozoans have the identical gross morphology and identical primary mobile structure. Since standard approaches to the definition and differentiation of animal species depend on variations in total physique plans and detailed morphology, all placozoan specimens to date collected have been attributed to the species T. adhaerens, which was first described in 1883. “Nevertheless, genetic knowledge primarily based on quick DNA signature sequences that serve to tell apart species from each other had already recommended that placozoans exhibit a substantial amount of genetic range. And that in flip signifies that the phylum truly contains many alternative species,” says Wörheide.

For that reason, he and his colleagues determined to sequence the genome of a placozoan line derived from specimens collected in Hong Kong. Their signature sequences indicated that this line was distantly associated to T. adhaerens, whose genome was revealed in 2008. “Primarily based on comparative genomic evaluation, we then developed a novel methodology for the outline of a brand new species primarily based completely on genomic knowledge,” says Michael Eitel, first writer of the brand new research. The researchers seek advice from this strategy as ‘taxogenomics’, which takes under consideration elements equivalent to structural variations between chromosomes, variations within the whole variety of genes, and sequence variations between chosen protein-coding genes.

The genetic and genomic knowledge for the placozoans from Hong Kong revealed such giant variations between them and T. adhaerens that they had been finally assigned not solely to a brand new species, however to a brand new genus, which represents a better rank within the hierarchy of organic taxonomy. “It is a fully new departure. It’s the first time {that a} new genus has been erected purely on the premise of genomic knowledge,” Wörheide explains. The brand new species bears the title Hoilungia hongkongensis. This interprets as ‘Hong Kong Sea Dragon’ – which refers to the truth that, similar to the Dragon King in Chinese language mythology, placozoans can readily alter their shapes.

The authors of the brand new research consider that the placozoans might have undergone a really peculiar mode of evolution, wherein speciation has occurred completely on the genetic stage with out notable morphological diversification. “We now have some indications that time to the operation of adverse choice, so it’s potential that the event of morphological novelties could also be repressed. However we’re nonetheless very a lot in the beginning of the seek for an evidence of this distinctive evolutionary trajectory,” says Eitel.

The authors additionally recommend that the taxogenomic strategy may be used for detailed research of the method of speciation in different animal phyla. This holds specifically for animal teams that include minuscule people, equivalent to nematodes and mites, wherein it’s usually tough to discriminate between species by optical inspection alone.

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PLOS Biology 2018

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