Daniel Vaulot and Adriana Lopes dos Santos
Origin of Eukaryotes
2023-06-19
Early Life Evolution
How many domains ?
The endosymbiotic theory
Results suggest a nonhyperthermophilic LUCA and bacterial ancestor, with hyperthermophily emerging early in the evolution of the archaeal and bacterial domains.
Molecular features shared only between archaea and eukaryotes appeared consistent with 3 domains hypothesis
In 2010, a 2m long gravity core (GC14) was retrieved from the Arctic Mid-Ocean Ridge during summer, approximately 15 km northwest of the active venting site Loki’s Castle (between Greenland and Norway), at 3283m below sea level.
Loki is a god in Norse Mythology. He creates trouble. He is elusive and ambiguous.
In 2016 and 2017 Ettema’s team and other researchers announced the discovery of 3 new archaeal phyla related to the Lokis: Thorarchaeota, Odinarchaeota, Heimdallarchaeota.
The entire grouping of new superphylum was named Asgard after the realm of the Norse gods.
Phylogenomic analyses strongly indicated that the new archaea lineages representing the Asgard superphylum form a monophyletic group with eukaryotes.
Members of Asgard were found to carry genes encoding proteins that were assumed to be specific to eukaryotes (e.g. actins).
Took long time …. “Through subsequent transfers, we were able (…) to obtain a pure co-culture of the target archaeon MK-D1 and Methanogenium after a 12-year study—from bioreactor-based pre-enrichment of deep-sea sediments to a final 7 years of in vitro enrichment.”
Doubling time was estimated to be approximately 14–25 days
The genome encodes proteins that are only found in Eukarya (eukaryotic signature proteins): membrane trafficking, vesicle formation and/or transportation, ubiquitin and cytoskeleton formation
It catabolizes ten amino acids and peptides through syntrophic growth with Halodesulfovibrio (sulfate-reducing bacterium) and Methanogenium (methanogenic Archaea)
Etymology:
The cells produce long branching (g) and straight (h) membrane protrusions
Cytoplasmatic membrane contains “biphytane”(j) (probably monolayer)
Most early models envisaged that eukaryotic cells evolved from prokaryotic cells (bacteria) by a process of complexification.
In 1967, Lynn Margulis championed the idea of an endosymbiotic origin of organelles and further hypothesized that not only mitochondria, chloroplast have symbiotic origin but also cell flagella derived from symbiotic spirochetes (this one she got wrong!).
She presented a comprehensive symbiotic view of eukaryotic cell evolution based on cytological, biochemical, and paleontological evidence in her seminal work “On the origin of mitosing cells”.
Part of Margulis ideas started to become credible with the advent of molecular phylogeny that allowed demonstrating that conserved genes in chloroplast and mitochondrial genomes clustered with, respectively, cyanobacterial and alphaproteobacterial genes in phylogenetic trees.
Cryptophytes and Chlorarachniophytes retained a nucleomorph which is a remnant of the nucleus of the primitive photosynthetic cell. This is a further evidence of the endosymbiotic theory.
Apicomplexa are non-photosynthetic successful group of endoparasites of animals, from corals to humans (e.g. Plasmodium causing malaria).
Evolved from free living algae.
Contain an apicoplast which is a plastid that lost its photosynthetic ability.
Bound by four membrane. Own genome and expresses a small number of genes.
Origin of eukaryotes is key question
Discovery of Asgard induce change of paradigm
Endosymbiosis is key to understand evolution of eukaryotes
