A Briefly Argued Case That Asgard Archaea Are Part of the Eukaryote Tree - PubMed (original) (raw)

A Briefly Argued Case That Asgard Archaea Are Part of the Eukaryote Tree

Gregory P Fournier et al. Front Microbiol. 2018.

Abstract

The recent discovery of the Lokiarchaeota and other members of the Asgard superphylum suggests that closer analysis of the cell biology and evolution of these groups may help shed light on the origin of the eukaryote cell. Asgard lineages often appear in molecular phylogenies as closely related to eukaryotes, and possess "Eukaryote Signature Proteins" coded by genes previously thought to be unique to eukaryotes. This phylogenetic affinity to eukaryotes has been widely interpreted as indicating that Asgard lineages are "eukaryote-like archaea," with eukaryotes evolving from within a paraphyletic Archaea. Guided by the established principles of systematics, we examine the potential implications of the monophyly of Asgard lineages and Eukarya. We show that a helpful parallel case is that of Synapsida, a group that includes modern mammals and their more "reptile-like" ancestors, united by shared derived characters that evolved in their common ancestor. While this group contains extinct members that share many similarities with modern reptiles and their extinct relatives, they are evolutionarily distinct from Sauropsida, the group which includes modern birds, reptiles, and all other amniotes. Similarly, Asgard lineages and eukaryotes are united by shared derived characters to the exclusion of all other groups. Consequently, the Asgard group is not only highly informative for our understanding of eukaryogenesis, but may be better understood as being early diverging members of a broader group including eukaryotes, for which we propose the name "Eukaryomorpha." Significantly, this means that the relationship between Eukarya and Asgard lineages cannot, on its own, resolve the debate over 2 vs. 3 Domains of life; instead, resolving this debate depends upon identifying the root of Archaea with respect to Bacteria.

Keywords: Archaea; Asgard; Domains; Eukarya; cladistics; eukaryogenesis; synapomorphy; systematics.

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Figures

FIGURE 1

FIGURE 1

Phylogeny of selected major groups of amniotes. Selected synapomorphies are indicated by gray bars. Two basal extinct lineages Parareptilia and Archaeothyris (representing Ophiacodontidae) are included to indicate the retention of shared ancestral characters (large gray box). Sauropsida includes all extant reptiles and their extinct relatives. Extant anapsids (turtles) are omitted due to their uncertain placement within Sauropsida. Synapsida and Sauropsida are clades of equal taxonomic rank, with a last common ancestor congruent with the ancestor of all crown group amniotes (Amniota). Archosauria is a clade nested within Sauropsida. As such, it has a lower taxonomic rank than either Sauropsida or Synapsida, even if shared derived characters of this group distinguish them from other amniotes. Representative taxon images were downloaded from PhyloPic (

http://phylopic.org/

). All images are uncopyrighted except parareptilia (

http://phylopic.org/image/00b96cf3-1802-4bda-a6cc-76aea0f6f05e/

) which is owned by Nobu Tamura (vectorized by T. Michael Keesey), under the following creative commons license (

https://creativecommons.org/licenses/by-sa/3.0/legalcode

).

FIGURE 2

FIGURE 2

Hypothetical rooted archaeal tree including Asgard and eukaryotes, consistent with a “3-Domain” topology. ESPs (black dots) are synapomorphies uniting Asgard and eukaryotes as a clade (gray bar). Additional archaeal groups (e.g., DPANN) are omitted for clarity. Archaeal and Asgard groups share many “archaeal-like” ancestral characters (large gray box). Archaea are depicted as rooted between Euryarchaeota and TACK, although this hypothetical 3-Domain topology is also valid under all other rootings for this group. Despite these ancestral characters, with this rooting, Asgard + Eukarya together constitute a clade of equal taxonomic rank to that of Archaea. Here, we propose the name “Eukaryomorpha,” reflecting the shared characters uniting this group and distinguishing them from the archaeal outgroup.

FIGURE 3

FIGURE 3

Hypothetical rooted archaeal tree including Asgard and eukaryotes, consistent with a “2-Domain” topology. With this rooting, the Asgard + Eukarya clade is nested within Archaea, i.e., the last common ancestor of all Archaea (excluding Asgard) is also the last common ancestor of Eukarya and Asgard. As such, Asgard + Eukarya constitute a clade of unequal taxonomic rank to Archaea. Note that the scenarios in Figures 2, 3 only differ in the placement of the root, not in the topology of the tree or mapping of the characters.

FIGURE 4

FIGURE 4

Cladogram of Asgard and eukaryote evolution as related to different definitions of Eukarya. Eukaryal-specific characters (ESPs, gray boxes) were acquired in the ancestor of Asgard + Eukarya, and continue to be acquired in the eukaryal stem lineage. An apomorphy-based definition of Eukarya requires the identification of a specific defining character for Eukarya, which would include some eukaryal stem groups, but exclude others. The selection of a defining character for an apomorphy-based definition is therefore inherently subjective. The Asgard group is depicted as an evolutionary grade, although a monophyletic Asgard group is also consistent with this model.

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