Microbiology: Mind the gaps in cellular evolution

Archaeal Histone Contributions to the Origin of Eukaryotes

The eukaryotic lineage arose from bacterial and archaeal cells that underwent a symbiotic merger. At the origin of the eukaryote lineage, the bacterial partner contributed genes, metabolic energy, and the building blocks of the endomembrane system. What did the archaeal partner donate that made the eukaryotic experiment a success? The archaeal partner provided the potential for complex information processing. Archaeal histones were crucial in that regard by providing the basic functional unit with which eukaryotes organize DNA into nucleosomes, exert epigenetic control of gene expression, transcribe genes with CCAAT-box promoters, and a manifest cell cycle with condensed chromosomes. While mitochondrial energy lifted energetic constraints on eukaryotic protein production, histone-based chromatin organization paved the path to eukaryotic genome complexity, a critical hurdle en route to the evolution of complex cells.

The role of public goods in planetary evolution

Biological public goods are broadly shared within an ecosystem and readily available. They appear to be widespread and may have played important roles in the history of life on Earth. Of particular importance to events in the early history of life are the roles of public goods in the merging of genomes, protein domains and even cells. We suggest that public goods facilitated the origin of the eukaryotic cell, a classic major evolutionary transition. The recognition of genomic public goods challenges advocates of a direct graph view of phylogeny, and those who deny that any useful phylogenetic signal persists in modern genomes. Ecological spillovers generate public goods that provide new ecological opportunities.This article is part of the themed issue 'Reconceptualizing the origins of life'.

The Physiology of Phagocytosis in the Context of Mitochondrial Origin

How mitochondria came to reside within the cytosol of their host has been debated for 50 years. Though current data indicate that the last eukaryote common ancestor possessed mitochondria and was a complex cell, whether mitochondria or complexity came first in eukaryotic evolution is still discussed. In autogenous models (complexity first), the origin of phagocytosis poses the limiting step at eukaryote origin, with mitochondria coming late as an undigested growth substrate. In symbiosis-based models (mitochondria first), the host was an archaeon, and the origin of mitochondria was the limiting step at eukaryote origin, with mitochondria providing bacterial genes, ATP synthesis on internalized bioenergetic membranes, and mitochondrion-derived vesicles as the seed of the eukaryote endomembrane system. Metagenomic studies are uncovering new host-related archaeal lineages that are reported as complex or phagocytosing, although images of such cells are lacking. Here we review the physiology and components of phagocytosis in eukaryotes, critically inspecting the concept of a phagotrophic host. From ATP supply and demand, a mitochondrion-lacking phagotrophic archaeal fermenter would have to ingest about 34 times its body weight in prokaryotic prey to obtain enough ATP to support one cell division. It would lack chemiosmotic ATP synthesis at the plasma membrane, because phagocytosis and chemiosmosis in the same membrane are incompatible. It would have lived from amino acid fermentations, because prokaryotes are mainly protein. Its ATP yield would have been impaired relative to typical archaeal amino acid fermentations, which involve chemiosmosis. In contrast, phagocytosis would have had great physiological benefit for a mitochondrion-bearing cell.