Opinion: The Key Steps in the Origin of Life to the Formation of the Eukaryotic Cell.

The path from life's origin to the emergence of the eukaryotic cell was long and complex, and as such it is rarely treated in one publication. Here, we offer a sketch of this path, recognizing that there are points of disagreement and that many transitions are still shrouded in mystery. We assume life developed within microchambers of an alkaline hydrothermal vent system. Initial simple reactions were built into more sophisticated reflexively autocatalytic food-generated networks (RAFs), laying the foundation for life's anastomosing metabolism, and eventually for the origin of RNA, which functioned as a genetic repository and as a catalyst (ribozymes). Eventually, protein synthesis developed, leading to life's biology becoming dominated by enzymes and not ribozymes. Subsequent enzymatic innovation included ATP synthase, which generates ATP, fueled by the proton gradient between the alkaline vent flux and the acidic sea. This gradient was later internalized via the evolution of the electron transport chain, a preadaptation for the subsequent emergence of the vent creatures from their microchamber cradles. Differences between bacteria and archaea suggests cellularization evolved at least twice. Later, the bacterial development of oxidative phosphorylation and the archaeal development of proteins to stabilize its DNA laid the foundation for the merger that led to the formation of eukaryotic cells.

'Whole Organism', Systems Biology, and Top-Down Criteria for Evaluating Scenarios for the Origin of Life.

While most advances in the study of the origin of life on Earth (OoLoE) are piecemeal, tested against the laws of chemistry and physics, ultimately the goal is to develop an overall scenario for life's origin(s). However, the dimensionality of non-equilibrium chemical systems, from the range of possible boundary conditions and chemical interactions, renders the application of chemical and physical laws difficult. Here we outline a set of simple criteria for evaluating OoLoE scenarios. These include the need for containment, steady energy and material flows, and structured spatial heterogeneity from the outset. The Principle of Continuity, the fact that all life today was derived from first life, suggests favoring scenarios with fewer non-analog (not seen in life today) to analog (seen in life today) transitions in the inferred first biochemical pathways. Top-down data also indicate that a complex metabolism predated ribozymes and enzymes, and that full cellular autonomy and motility occurred post-LUCA. Using these criteria, we find the alkaline hydrothermal vent microchamber complex scenario with a late evolving exploitation of the natural occurring pH (or Na+ gradient) by ATP synthase the most compelling. However, there are as yet so many unknowns, we also advocate for the continued development of as many plausible scenarios as possible.

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.

Cohesion of Clonal Life History, Senescence and Rejuvenation Induced by Autogamy of the Histophagous Ciliate Tetrahymena rostrata.

The histophagous ciliate Tetrahymena rostrata was found as a parasite in the renal organs of the land snails Zonitoides nitidus and Cochlicopa lubrica. A starvation medium induced encystment, meiosis, autogamy, and development of new macronuclei. The cell division rate declined linearly with number of divisions from the last autogamy until senescence. The senescing strains were rejuvenated by further encystment-induced autogamy. It is expected, that these processes contribute to genetic variability among the local, small, and isolated T. rostrata populations. Consistent with this expectation, small divergences in the cox-1 sequences appeared even among these strains, which had been isolated from different specimens of the same host species at the same site. The divergences in this gene between our T. rostrata strains from Z. nitidus and other strains from C. lubrica, Helix aspersa, and Deroceras reticulatum in Spain (Segade et al. 2009, Parasitology 136:771-782), were beyond the limits of intra-species variability within the genus Tetrahymena. However there is the lack of inter-strain differences in the life history and cytology among our, the Spanish, and those T.rostrata strains, that are not available for "barcoding" anymore. Therefore, variability in the life history and morphology within T .rostrata is constrained by natural selection.

Complete sequence of the mitochondrial genome of Tetrahymena thermophila and comparative methods for identifying highly divergent genes.

The complete sequence of the mitochondrial genome of Tetrahymena thermophila has been determined and compared with the mitochondrial genome of Tetrahymena pyriformis. The sequence similarity clearly indicates homology of the entire T.thermophila and T.pyriformis mitochondrial genomes. The T.thermophila genome is very compact, most of the intergenic regions are short (only three are longer than 63 bp) and comprise only 3.8% of the genome. The nad9 gene is tandemly duplicated in T.thermophila. Long terminal inverted repeats and the nad9 genes are undergoing concerted evolution. There are 55 putative genes: three ribosomal RNA genes, eight transfer RNA genes, 22 proteins with putatively assigned functions and 22 additional open reading frames of unknown function. In order to extend indications of homology beyond amino acid sequence similarity we have examined a number of physico-chemical properties of the mitochondrial proteins, including theoretical pI, molecular weight and particularly the predicted transmembrane spanning regions. This approach has allowed us to identify homologs to ymf58 (nad4L), ymf62 (nad6) and ymf60 (rpl6).

Analysis of specific bacteria from environmental samples using a quantitative polymerase chain reaction.

This article describes the use of quantitative PCR for measuring bacterial abundance in environmental samples. The two approaches discussed are: 1) The use of an internal PCR standard constructed to be the same size and have the same sequence as the primary amplification target, but differing from the primary target by 2-3 bases, corresponding to a unique restriction site. This allows the amount of target amplicon to be compared with the internal standard and circumvents the problem of differential amplification efficiencies when using dissimilar targets and standard amplicons. 2) The use of Taqman technology (Applied Biosystems, Foster City, California) with a dual labeled oligonucleotide probe which binds internal to the PCR primers. The detection of Bacteroides is used as an example for both approaches.