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1.  Biased gene transfer and its implications for the concept of lineage 
Biology Direct  2011;6:47.
In the presence of horizontal gene transfer (HGT), the concepts of lineage and genealogy in the microbial world become more ambiguous because chimeric genomes trace their ancestry from a myriad of sources, both living and extinct.
We present the evolutionary histories of three aminoacyl-tRNA synthetases (aaRS) to illustrate that the concept of organismal lineage in the prokaryotic world is defined by both vertical inheritance and reticulations due to HGT. The acquisition of a novel gene from a distantly related taxon can be considered as a shared derived character that demarcates a group of organisms, as in the case of the spirochaete Phenylalanyl-tRNA synthetase (PheRS). On the other hand, when organisms transfer genetic material with their close kin, the similarity and therefore relatedness observed among them is essentially shaped by gene transfer. Studying the distribution patterns of divergent genes with identical functions, referred to as homeoalleles, can reveal preferences for transfer partners. We describe the very ancient origin and the distribution of the archaeal homeoalleles for Threonyl-tRNA synthetases (ThrRS) and Seryl-tRNA synthetases (SerRS).
Patterns created through biased HGT can be undistinguishable from those created through shared organismal ancestry. A re-evaluation of the definition of lineage is necessary to reflect genetic relatedness due to both HGT and vertical inheritance. In most instances, HGT bias will maintain and strengthen similarity within groups. Only in cases where HGT bias is due to other factors, such as shared ecological niche, do patterns emerge from gene phylogenies that are in conflict with those reflecting shared organismal ancestry.
This article was reviewed by W. Ford Doolittle, François-Joseph Lapointe, and Frederic Bouchard.
PMCID: PMC3191353  PMID: 21943000
2.  A Rooted Net of Life 
Biology Direct  2011;6:45.
Phylogenetic reconstruction using DNA and protein sequences has allowed the reconstruction of evolutionary histories encompassing all life. We present and discuss a means to incorporate much of this rich narrative into a single model that acknowledges the discrete evolutionary units that constitute the organism. Briefly, this Rooted Net of Life genome phylogeny is constructed around an initial, well resolved and rooted tree scaffold inferred from a supermatrix of combined ribosomal genes. Extant sampled ribosomes form the leaves of the tree scaffold. These leaves, but not necessarily the deeper parts of the scaffold, can be considered to represent a genome or pan-genome, and to be associated with members of other gene families within that sequenced (pan)genome. Unrooted phylogenies of gene families containing four or more members are reconstructed and superimposed over the scaffold. Initially, reticulations are formed where incongruities between topologies exist. Given sufficient evidence, edges may then be differentiated as those representing vertical lines of inheritance within lineages and those representing horizontal genetic transfers or endosymbioses between lineages.
W. Ford Doolittle, Eric Bapteste and Robert Beiko.
PMCID: PMC3189188  PMID: 21936906
3.  Prokaryotic evolution and the tree of life are two different things 
Biology Direct  2009;4:34.
The concept of a tree of life is prevalent in the evolutionary literature. It stems from attempting to obtain a grand unified natural system that reflects a recurrent process of species and lineage splittings for all forms of life. Traditionally, the discipline of systematics operates in a similar hierarchy of bifurcating (sometimes multifurcating) categories. The assumption of a universal tree of life hinges upon the process of evolution being tree-like throughout all forms of life and all of biological time. In multicellular eukaryotes, the molecular mechanisms and species-level population genetics of variation do indeed mainly cause a tree-like structure over time. In prokaryotes, they do not. Prokaryotic evolution and the tree of life are two different things, and we need to treat them as such, rather than extrapolating from macroscopic life to prokaryotes. In the following we will consider this circumstance from philosophical, scientific, and epistemological perspectives, surmising that phylogeny opted for a single model as a holdover from the Modern Synthesis of evolution.
It was far easier to envision and defend the concept of a universal tree of life before we had data from genomes. But the belief that prokaryotes are related by such a tree has now become stronger than the data to support it. The monistic concept of a single universal tree of life appears, in the face of genome data, increasingly obsolete. This traditional model to describe evolution is no longer the most scientifically productive position to hold, because of the plurality of evolutionary patterns and mechanisms involved. Forcing a single bifurcating scheme onto prokaryotic evolution disregards the non-tree-like nature of natural variation among prokaryotes and accounts for only a minority of observations from genomes.
Prokaryotic evolution and the tree of life are two different things. Hence we will briefly set out alternative models to the tree of life to study their evolution. Ultimately, the plurality of evolutionary patterns and mechanisms involved, such as the discontinuity of the process of evolution across the prokaryote-eukaryote divide, summons forth a pluralistic approach to studying evolution.
This article was reviewed by Ford Doolittle, John Logsdon and Nicolas Galtier.
PMCID: PMC2761302  PMID: 19788731

Results 1-3 (3)