Infographic: Can Archaea Teach Us About the Evolution of Eukaroyotes?

The discovery of copious new archaeal species is shedding light on the tree of life and revealing some unique cellular biology.

Amber Dance
Amber Dance
May 31, 2018

Thanks to a wealth of new genomic sequence data, the family tree of Archaea, which encompassed just two phyla 16 years ago, has exploded in recent years. It now includes more than a dozen phyla, organized into four informal “supergroups,” based mostly on sequence similarities. Scientists have yet to determine precisely how novel archaea should be classified. Also in dispute is how Eukarya fit into the picture—some scientists suggest they’re an offshoot of a branch known as Asgard archaea, while others suspect they diverged from Archaea earlier on. Researchers predict the tree will sprout many more branches in the years to come.

EuryarchaeotaCarl Woese and colleagues divided the Archaea into two “kingdoms,” Euryarchaeota and Crenarchaeota, in 1990.1Includes halophiles and methanogens.
Members of the order Thermoplasmateles
are acidophiles and thermophiles.
TACKScientists proposed the TACK name in 2011 to encompass the phyla Thaumarchaeota, Aigarchaeota, Crenarchaeota,
and Korarchaeota;2 more phyla have been added since.
Includes thermophiles. Thaumarchaeota participate in nitrogen cycling. Some
are also methanogens.
DPANNThe first phylum named was Nanoarchaeota, in 2002, for a tiny deep-sea vent organism that didn’t fit into Euryarchaeota or Crenarchaeota.3 In 2013, researchers proposed linking it with the taxa Diapherotrites, Parvarchaeota, Aenigmarchaeota,
and Nanohaloarchaeota.4 New phyla have been added since.
At least some are small in size, with small genomes lacking genes for key proteins in metabolism and other processes. Some may rely on a symbiont or host organism to survive.
AsgardThe first discovered were Lokiarchaeota, which were initially thought to be members of the TACK superphylum.5 The group now contains a handful of phyla, all named for Norse deities.6Genomes encode several proteins
similar to those found in eukaryotes.
1. PNAS, 87:4576-79, 1990; 2. Trends Microbiol, 19:580-87, 2011; 3. Nature, 417:63-67, 2002; 4. Nature, 499:431-37, 2013; 5. Nature, 521:173-79, 2015; 6. Nature, 541:353-58, 2017

See “Older Sisters

DNA Replication

Bacteria typically possess one chromosome with one origin of replication. Eukaryotes have multiple, paired chromosomes with numerous origins on each. Archaea straddle the divide: while they typically have one main chromosome, it often replicates from multiple origins.

Some archaea also have the unique ability to adopt an alternate version of DNA replication initiation. Across all domains of life, DNA replication starts when initiation proteins bind the origin of replication; deleting the origins typically slows growth or halts cell division entirely. But in the archaeon Haloferax volcanii, deleting the origins causes faster growth. H. volcanii replicates its genome in a way similar to homologous recombination, in which two matching chromosomes swap strands to create a replication fork, though the details of this process are still being worked out (Nature, 503:544-47, 2013).

Genome Organization

Archaea can possess megaplasmids—hundreds of kilobases in size—that contain crucial genes. Some species are haploid like bacteria but many exhibit varying degrees of polyploidy. Many archaea use histones, as eukaryotes do, to organize their genomes, but some rely on alternative Alba proteins.

Modes of Cell Division

Some archaea divide via a mechanism similar to that of bacteria, using the cytoskeleton-like protein FtsZ to form a ring at the eventual division site (left). Others use homologs of eukaryote proteins, such as ESCRTs, to help separate daughter cells (right). Still others lack both of those systems, so they presumably have a distinct, as-yet-unknown mechanism, possibly relying on a form of actin.

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