An emerging paradigm for the origin and evolution of shelled amoebae, integrating advances from molecular phylogenetics, morphology and paleontology.

Daniel Jg Lahr

doi:10.1590/0074-02760200620

Abstract

The phylogenetic paradigm of eukaryotic evolution has changed dramatically over the past two decades, with profound reflections on the understanding of life on earth. Arcellinida testate (shelled) amoebae lineages represent some of the oldest fossils of eukaryotes, and the elucidation of their phylogenetic relationships opened a window to the distant past, with important implications for understanding the evolution of life on earth. This four-part essay summarises advances made in the past 20 years regarding: (i) the phylogenetic relationships among amoebae with shells evolving in concert with the advances made in the phylogeny of eukaryotes; (ii) paleobiological studies unraveling the biological affinities of Neoproterozoic vase-shaped microfossils (VSMs); (iii) the interwoven interpretation of these different sets of data concluding that the Neoproterozoic contains a surprising diversity of organisms, in turn demanding a reinterpretation of the most profound events we know in the history of eukaryotes, and; (iv) a synthesis of the current knowledge about the evolution of Arcellinida, together with the possibilities and pitfalls of their interpretation.

Highlights

The evolutionary affinities of shelled amoebae in contemporary eukaryotic systematics The largest group of amoebae with shells, the Arcellinida, are single-celled, sexual eukaryotic microorganisms that originated at least 730 million years ago (Fig. 1)
This work was promoted by the Taxonomy Commission formed by the International Society of Protistologists (ISoP, at the time still named Society of Protozoologists), being widely considered as one of the most authoritative and comprehensive consensus on eukaryotic microorganisms
Eukaryotic microorganisms are currently classified into larger lineages popularly and informally called “Supergroups”: within Amorphea, Amoebozoa gathering most of the organisms capable of producing lobed pseudopods and Opisthokonta gathering Metazoa and Fungi, in addition to other residual lineages; SARs comprising Stramenopila, Alveolata, and Rhizaria; the Excavata bringing together a large part of the flagellated heterotrophs, in addition to the Euglenozoa and several parasitic lineages (e.g., Giardia); and the Archaeplastida gathering a large part of the photosynthetic organisms including red, green algae and plants

Summary

Daniel JG Lahr

Universidade de São Paulo, Instituto de Biociências, Departamento de Zoologia, São Paulo, SP, Brasil. Arcellinida testate (shelled) amoebae lineages represent some of the oldest fossils of eukaryotes, and the elucidation of their phylogenetic relationships opened a window to the distant past, with important implications for understanding the evolution of life on earth. Reconstructions using SSU-rDNA and cytoskeletal genes (actin and tubulin) consistently resulted in reconstructions where Heleopera sphagni appeared quite distant from other Arcellinida, usually nested within other naked amoebae At this point, the understanding emerged that the amount of genes used was not sufficient to reconstruct the deepest lines of Arcellinida; the main issues would need to be revealed using large-scale, multigene analysis techniques, such as high-throughput sequencing, within the context of phylogenomics

Microbial eukaryotes enter the Phylogenomic Era

Were there shelled amoebae in the Neoproterozoic oceans?

Findings

The moment and environment of origin of the modern Arcellinida