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1.  A late origin of the extant eukaryotic diversity: divergence time estimates using rare genomic changes 
Biology Direct  2011;6:26.
Accurate estimation of the divergence time of the extant eukaryotes is a fundamentally important but extremely difficult problem owing primarily to gross violations of the molecular clock at long evolutionary distances and the lack of appropriate calibration points close to the date of interest. These difficulties are intrinsic to the dating of ancient divergence events and are reflected in the large discrepancies between estimates obtained with different approaches. Estimates of the age of Last Eukaryotic Common Ancestor (LECA) vary approximately twofold, from ~1,100 million years ago (Mya) to ~2,300 Mya.
We applied the genome-wide analysis of rare genomic changes associated with conserved amino acids (RGC_CAs) and used several independent techniques to obtain date estimates for the divergence of the major lineages of eukaryotes with calibration intervals for insects, land plants and vertebrates. The results suggest an early divergence of monocot and dicot plants, approximately 340 Mya, raising the possibility of plant-insect coevolution. The divergence of bilaterian animal phyla is estimated at ~400-700 Mya, a range of dates that is consistent with cladogenesis immediately preceding the Cambrian explosion. The origin of opisthokonts (the supergroup of eukaryotes that includes metazoa and fungi) is estimated at ~700-1,000 Mya, and the age of LECA at ~1,000-1,300 Mya. We separately analyzed the red algal calibration interval which is based on single fossil. This analysis produced time estimates that were systematically older compared to the other estimates. Nevertheless, the majority of the estimates for the age of the LECA using the red algal data fell within the 1,200-1,400 Mya interval.
The inference of a "young LECA" is compatible with the latest of previously estimated dates and has substantial biological implications. If these estimates are valid, the approximately 1 to 1.4 billion years of evolution of eukaryotes that is open to comparative-genomic study probably was preceded by hundreds of millions years of evolution that might have included extinct diversity inaccessible to comparative approaches.
This article was reviewed by William Martin, Herve Philippe (nominated by I. King Jordan), and Romain Derelle.
PMCID: PMC3125394  PMID: 21595937
bilateria; opisthokonts; angiosperms; last eukaryotic common ancestor; molecular dating
The deep phylogeny of eukaryotes is an important but extremely difficult problem of evolutionary biology. Five eukaryotic supergroups are relatively well established but the relationship between these supergroups remains elusive, and their divergence seems to best fit a “Big Bang” model. Attempts were made to root the tree of eukaryotes by using potential derived shared characters such as unique fusions of conserved genes. One popular model of eukaryotic evolution that emerged from this type of analysis is the unikont–bikont phylogeny: The unikont branch consists of Metazoa, Choanozoa, Fungi, and Amoebozoa, whereas bikonts include the rest of eukaryotes, namely, Plantae (green plants, Chlorophyta, and Rhodophyta), Chromalveolata, excavates, and Rhizaria. We reexamine the relationships between the eukaryotic supergroups using a genome-wide analysis of rare genomic changes (RGCs) associated with multiple, conserved amino acids (RGC_CAMs and RGC_CAs), to resolve trifurcations of major eukaryotic lineages. The results do not support the basal position of Chromalveolata with respect to Plantae and unikonts or the monophyly of the bikont group and appear to be best compatible with the monophyly of unikonts and Chromalveolata. Chromalveolata show a distinct, additional signal of affinity with Plantae, conceivably, owing to genes transferred from the secondary, red algal symbiont. Excavates are derived forms, with extremely long branches that complicate phylogenetic inference; nevertheless, the RGC analysis suggests that they are significantly more likely to cluster with the unikont–Chromalveolata assemblage than with the Plantae. Thus, the first split in eukaryotic evolution might lie between photosynthetic and nonphotosynthetic forms and so could have been triggered by the endosymbiosis between an ancestral unicellular eukaryote and a cyanobacterium that gave rise to the chloroplast.
PMCID: PMC2817406  PMID: 20333181
eukaryotic phylogeny; rare genomic changes; parsimony; substitutions; insertions; deletions
3.  Homoplasy in genome-wide analysis of rare amino acid replacements: the molecular-evolutionary basis for Vavilov's law of homologous series 
Biology Direct  2008;3:7.
Rare genomic changes (RGCs) that are thought to comprise derived shared characters of individual clades are becoming an increasingly important class of markers in genome-wide phylogenetic studies. Recently, we proposed a new type of RGCs designated RGC_CAMs (after Conserved Amino acids-Multiple substitutions) that were inferred using genome-wide identification of amino acid replacements that were: i) located in unambiguously aligned regions of orthologous genes, ii) shared by two or more taxa in positions that contain a different, conserved amino acid in a much broader range of taxa, and iii) require two or three nucleotide substitutions. When applied to animal phylogeny, the RGC_CAM approach supported the coelomate clade that unites deuterostomes with arthropods as opposed to the ecdysozoan (molting animals) clade. However, a non-negligible level of homoplasy was detected.
We provide a direct estimate of the level of homoplasy caused by parallel changes and reversals among the RGC_CAMs using 462 alignments of orthologous genes from 19 eukaryotic species. It is shown that the impact of parallel changes and reversals on the results of phylogenetic inference using RGC_CAMs cannot explain the observed support for the Coelomata clade. In contrast, the evidence in support of the Ecdysozoa clade, in large part, can be attributed to parallel changes. It is demonstrated that parallel changes are significantly more common in internal branches of different subtrees that are separated from the respective common ancestor by relatively short times than in terminal branches separated by longer time intervals. A similar but much weaker trend was detected for reversals. The observed evolutionary trend of parallel changes is explained in terms of the covarion model of molecular evolution. As the overlap between the covarion sets in orthologous genes from different lineages decreases with time after divergence, the likelihood of parallel changes decreases as well.
The level of homoplasy observed here appears to be low enough to justify the utility of RGC_CAMs and other types of RGCs for resolution of hard problems in phylogeny. Parallel changes, one of the major classes of events leading to homoplasy, occur much more often in relatively recently diverged lineages than in those separated from their last common ancestor by longer time intervals of time. This pattern seems to provide the molecular-evolutionary underpinning of Vavilov's law of homologous series and is readily interpreted within the framework of the covarion model of molecular evolution.
This article was reviewed by Alex Kondrashov, Nicolas Galtier, and Maximilian Telford and Robert Lanfear (nominated by Laurence Hurst).
PMCID: PMC2292158  PMID: 18346278

Results 1-3 (3)