Hidden diversity of Acoelomorpha revealed through metabarcoding

Area of Science:

• Marine Biology
• Evolutionary Biology
• Genomics

Background:

• Bilateral symmetry defines most Metazoa, but the earliest bilaterian ancestor (Urbilateria) remains elusive.
• Xenacoelomorpha is phylogenetically positioned as the sister group to all other Bilateria, making its study critical for evolutionary insights.
• Previous xenacoelomorph research has primarily focused on coastal ecosystems, potentially overlooking significant diversity in other marine habitats.

Purpose of the Study:

• To investigate the diversity and phylogenetic position of xenacoelomorphs in under-sampled marine environments.
• To identify new acoel clades that could illuminate the early evolution of Bilateria.
• To reconstruct the characteristics of the last common bilaterian ancestor.

Main Methods:

• Analysis of 18S ribosomal DNA (rDNA) metabarcoding data from global marine projects.
• Inclusion of data from both benthic and pelagic habitats to ensure comprehensive sampling.
• Phylogenetic analysis to determine the evolutionary relationships of newly identified acoel clades.

Main Results:

• A greater richness of acoels was observed in planktonic environments than previously documented.
• A putative novel acoel clade was discovered in the deep benthos, representing the earliest-branching acoel lineage.
• The findings suggest deep-sea environments harbor key acoel lineages for understanding bilaterian evolution.

Conclusions:

• Deep-sea exploration is vital for uncovering xenacoelomorph diversity and their phylogenetic significance.
• The newly identified deep-benthic acoel clade provides crucial data for reconstructing the evolution of Bilateria.
• Understanding xenacoelomorphs, particularly acoels, is essential for resolving the nature of the Urbilateria.