Testing hypotheses without considering predictions

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.

Reconciling an archaeal origin of eukaryotes with engulfment: a biologically plausible update of the Eocyte hypothesis

An archaeal origin of eukaryotes is often equated with the engulfment of the bacterial ancestor of mitochondria by an archaeon. Such an event is problematic in that it is not supported by archaeal cell biology. We show that placing phylogenetic results within a stem-and-crown framework eliminates such incompatibilities, and that an archaeal origin for eukaryotes (as suggested from recent phylogenies) can be uncontroversially reconciled with phagocytosis as the mechanism for engulfment of the mitochondrial ancestor. This is significant because it eliminates a perceived problem with eukaryote origins: that an archaeal origin of eukaryotes (as under the Eocyte hypothesis) cannot be reconciled with existing cell biological mechanisms through which bacteria may take up residence inside eukaryote cells.

The origin of eukaryotes and their relationship with the Archaea: are we at a phylogenomic impasse?

The origin of eukaryotes and their evolutionary relationship with the Archaea is a major biological question and the subject of intense debate. In the context of the classical view of the universal tree of life, the Archaea and the Eukarya have a common ancestor, the nature of which remains undetermined. Alternative views propose instead that the Eukarya evolved directly from a bona fide archaeal lineage. Several recent large-scale phylogenomic studies using an array of approaches are divided in supporting either one or the other scenario, despite analysing largely overlapping data sets of universal genes. We examine the reasons for such a lack of consensus and consider how alternative approaches may enable progress in answering this fascinating and as-yet-unresolved question.

The first eukaryote cell: an unfinished history of contestation

The eukaryote cell is one of the most radical innovations in the history of life, and the circumstances of its emergence are still deeply contested. This paper will outline the recent history of attempts to reveal these origins, with special attention to the argumentative strategies used to support claims about the first eukaryote cell. I will focus on two general models of eukaryogenesis: the phagotrophy model and the syntrophy model. As their labels indicate, they are based on claims about metabolic relationships. The first foregrounds the ability to consume other organisms; the second the ability to enter into symbiotic metabolic arrangements. More importantly, however, the first model argues for the autogenous or self-generated origins of the eukaryote cell, and the second for its exogenous or externally generated origins. Framing cell evolution this way leads each model to assert different priorities in regard to cell-biological versus molecular evidence, cellular versus environmental influences, plausibility versus evolutionary probability, and irreducibility versus the continuity of cell types. My examination of these issues will conclude with broader reflections on the implications of eukaryogenesis studies for a philosophical understanding of scientific contestation.

Getting a better picture of microbial evolution en route to a network of genomes

Most current thinking about evolution is couched in the concept of trees. The notion of a tree with recursively bifurcating branches representing recurrent divergence events is a plausible metaphor to describe the evolution of multicellular organisms like vertebrates or land plants. But if we try to force the tree metaphor onto the whole of the evolutionary process, things go badly awry, because the more closely we inspect microbial genomes through the looking glass of gene and genome sequence comparisons, the smaller the amount of the data that fits the concept of a bifurcating tree becomes. That is mainly because among microbes, endosymbiosis and lateral gene transfer are important, two mechanisms of natural variation that differ from the kind of natural variation that Darwin had in mind. For such reasons, when it comes to discussing the relationships among all living things, that is, including the microbes and all of their genes rather than just one or a select few, many biologists are now beginning to talk about networks rather than trees in the context of evolutionary relationships among microbial chromosomes. But talk is not enough. If we were to actually construct networks instead of trees to describe the evolutionary process, what would they look like? Here we consider endosymbiosis and an example of a network of genomes involving 181 sequenced prokaryotes and how that squares off with some ideas about early cell evolution.

Darwinian evolution in the light of genomics

Comparative genomics and systems biology offer unprecedented opportunities for testing central tenets of evolutionary biology formulated by Darwin in the Origin of Species in 1859 and expanded in the Modern Synthesis 100 years later. Evolutionary-genomic studies show that natural selection is only one of the forces that shape genome evolution and is not quantitatively dominant, whereas non-adaptive processes are much more prominent than previously suspected. Major contributions of horizontal gene transfer and diverse selfish genetic elements to genome evolution undermine the Tree of Life concept. An adequate depiction of evolution requires the more complex concept of a network or ‘forest’ of life. There is no consistent tendency of evolution towards increased genomic complexity, and when complexity increases, this appears to be a non-adaptive consequence of evolution under weak purifying selection rather than an adaptation. Several universals of genome evolution were discovered including the invariant distributions of evolutionary rates among orthologous genes from diverse genomes and of paralogous gene family sizes, and the negative correlation between gene expression level and sequence evolution rate. Simple, non-adaptive models of evolution explain some of these universals, suggesting that a new synthesis of evolutionary biology might become feasible in a not so remote future.

Origin of the nucleus and Ran-dependent transport to safeguard ribosome biogenesis in a chimeric cell

Background

The origin of the nucleus is a central problem about the origin of eukaryotes. The common ancestry of nuclear pore complexes (NPC) and vesicle coating complexes indicates that the nucleus evolved via the modification of a pre-existing endomembrane system. Such an autogenous scenario is cell biologically feasible, but it is not clear what were the selective or neutral mechanisms that had led to the origin of the nuclear compartment.

Results

A key selective force during the autogenous origin of the nucleus could have been the need to segregate ribosome factories from the cytoplasm where ribosomal proteins (RPs) of the protomitochondrium were synthesized. After its uptake by an anuclear cell the protomitochondrium transferred several of its RP genes to the host genome. Alphaproteobacterial RPs and archaebacterial-type host ribosomes were consequently synthesized in the same cytoplasm. This could have led to the formation of chimeric ribosomes. I propose that the nucleus evolved when the host cell compartmentalised its ribosome factories and the tightly linked genome to reduce ribosome chimerism. This was achieved in successive stages by first evolving karyopherin and RanGTP dependent chaperoning of RPs, followed by the evolution of a membrane network to serve as a diffusion barrier, and finally a hydrogel sieve to ensure selective permeability at nuclear pores. Computer simulations show that a gradual segregation of cytoplasm and nucleoplasm via these steps can progressively reduce ribosome chimerism.

Conclusion

Ribosome chimerism can provide a direct link between the selective forces for and the mechanisms of evolving nuclear transport and compartmentalisation. The detailed molecular scenario presented here provides a solution to the gradual evolution of nuclear compartmentalization from an anuclear stage.

Reviewers

This article was reviewed by Eugene V Koonin, Martijn Huynen, Anthony M. Poole and Patrick Forterre.

The deep archaeal roots of eukaryotes

The set of conserved eukaryotic protein-coding genes includes distinct subsets one of which appears to be most closely related to and, by inference, derived from archaea, whereas another one appears to be of bacterial, possibly, endosymbiotic origin. The “archaeal” genes of eukaryotes, primarily, encode components of information-processing systems, whereas the “bacterial” genes are predominantly operational. The precise nature of the archaeo–eukaryotic relationship remains uncertain, and it has been variously argued that eukaryotic informational genes evolved from the homologous genes of Euryarchaeota or Crenarchaeota (the major branches of extant archaea) or that the origin of eukaryotes lies outside the known diversity of archaea. We describe a comprehensive set of 355 eukaryotic genes of apparent archaeal origin identified through ortholog detection and phylogenetic analysis. Phylogenetic hypothesis testing using constrained trees, combined with a systematic search for shared derived characters in the form of homologous inserts in conserved proteins, indicate that, for the majority of these genes, the preferred tree topology is one with the eukaryotic branch placed outside the extant diversity of archaea although small subsets of genes show crenarchaeal and euryarchaeal affinities. Thus, the archaeal genes in eukaryotes appear to descend from a distinct, ancient, and otherwise uncharacterized archaeal lineage that acquired some euryarchaeal and crenarchaeal genes via early horizontal gene transfer.