Asymmetric Distribution of Gene Trees Can Arise under Purifying Selection If Differences in Population Size Exist.

Incomplete lineage sorting (ILS) is an important factor that causes gene tree discordance. For gene trees of three species, under neutrality, random mating, and the absence of interspecific gene flow, ILS creates a symmetric distribution of gene trees: the gene tree that accords with the species tree has the highest frequency, and the two discordant trees are equally frequent. If the neutral condition is violated, the impact of ILS may change, altering the gene tree distribution. Here, we show that under purifying selection, even assuming that the fitness effect of mutations is constant throughout the species tree, if differences in population size exist among species, asymmetric distributions of gene trees will arise, which is different from the expectation under neutrality. In extremes, one of the discordant trees rather than the concordant tree becomes the most frequent gene tree. In addition, we found that in a real case, the position of Scandentia relative to Primate and Glires, the symmetry in the gene tree distribution can be influenced by the strength of purifying selection. In current phylogenetic inference, the impact of purifying selection on the gene tree distribution is rarely considered by researchers. This study highlights the necessity of considering this impact.

The gene tree delusion.

Higher-level relationships among placental mammals are mostly resolved, but several polytomies remain contentious. Song et al. (2012) claimed to have resolved three of these using shortcut coalescence methods (MP-EST, STAR) and further concluded that these methods, which assume no within-locus recombination, are required to unravel deep-level phylogenetic problems that have stymied concatenation. Here, we reanalyze Song et al.'s (2012) data and leverage these re-analyses to explore key issues in systematics including the recombination ratchet, gene tree stoichiometry, the proportion of gene tree incongruence that results from deep coalescence versus other factors, and simulations that compare the performance of coalescence and concatenation methods in species tree estimation. Song et al. (2012) reported an average locus length of 3.1 kb for the 447 protein-coding genes in their phylogenomic dataset, but the true mean length of these loci (start codon to stop codon) is 139.6 kb. Empirical estimates of recombination breakpoints in primates, coupled with consideration of the recombination ratchet, suggest that individual coalescence genes (c-genes) approach ∼12 bp or less for Song et al.'s (2012) dataset, three to four orders of magnitude shorter than the c-genes reported by these authors. This result has general implications for the application of coalescence methods in species tree estimation. We contend that it is illogical to apply coalescence methods to complete protein-coding sequences. Such analyses amalgamate c-genes with different evolutionary histories (i.e., exons separated by >100,000 bp), distort true gene tree stoichiometry that is required for accurate species tree inference, and contradict the central rationale for applying coalescence methods to difficult phylogenetic problems. In addition, Song et al.'s (2012) dataset of 447 genes includes 21 loci with switched taxonomic names, eight duplicated loci, 26 loci with non-homologous sequences that are grossly misaligned, and numerous loci with >50% missing data for taxa that are misplaced in their gene trees. These problems were compounded by inadequate tree searches with nearest neighbor interchange branch swapping and inadvertent application of substitution models that did not account for among-site rate heterogeneity. Sixty-six gene trees imply unrealistic deep coalescences that exceed 100 million years (MY). Gene trees that were obtained with better justified models and search parameters show large increases in both likelihood scores and congruence. Coalescence analyses based on a curated set of 413 improved gene trees and a superior coalescence method (ASTRAL) support a Scandentia (treeshrews)+Glires (rabbits, rodents) clade, contradicting one of the three primary systematic conclusions of Song et al. (2012). Robust support for a Perissodactyla+Carnivora clade within Laurasiatheria is also lost, contradicting a second major conclusion of this study. Song et al.'s (2012) MP-EST species tree provided the basis for circular simulations that led these authors to conclude that the multispecies coalescent accounts for 77% of the gene tree conflicts in their dataset, but many internal branches of their MP-EST tree are stunted by an order of magnitude or more due to wholesale gene tree reconstruction errors. An independent assessment of branch lengths suggests the multispecies coalescent accounts for ⩽ 15% of the conflicts among Song et al.'s (2012) 447 gene trees. Unfortunately, Song et al.'s (2012) flawed phylogenomic dataset has been used as a model for additional simulation work that suggests the superiority of shortcut coalescence methods relative to concatenation. Investigator error was passed on to the subsequent simulation studies, which also incorporated further logical errors that should be avoided in future simulation studies. Illegitimate branch length switches in the simulation routines unfairly protected coalescence methods from their Achilles' heel, high gene tree reconstruction error at short internodes. These simulations therefore provide no evidence that shortcut coalescence methods out-compete concatenation at deep timescales. In summary, the long c-genes that are required for accurate reconstruction of species trees using shortcut coalescence methods do not exist and are a delusion. Coalescence approaches based on SNPs that are widely spaced in the genome avoid problems with the recombination ratchet and merit further pursuit in both empirical systematic research and simulations.

The Complete Mitochondrial Genome of Dendrogale murina (Tupaiidae) and Phylogeny of Scandentia.

In this paper, we report the complete mitochondrial genome of the northern smooth-tailed treeshrew Dendrogale murina, which was sequenced for the first time using the Illumina next-generation sequencing (NGS) technology. The total length of the mitochondrial genome is 16,844-16,850 bp and encodes 37 genes, including two ribosomal RNAs (rRNAs) 12S and 16S, 22 transfer RNAs (tRNAs), 13 protein-coding genes (PCGs), and a D-loop in the characteristic arrangement of family Tupaiidae (Mammalia: Scandentia). The overall base composition of the complete mitochondrial DNA is A (33.5%), C (25.5%), G (13.9%), and T (27.1%). Phylogenetic analysis of Scandentia mitochondrial genomes showed a classic pattern, which was revealed previously while using individual phylogenetic markers. The result of the current study is consistent with one based on the latest morphological studies, with the basal position of Ptilocercus and Dendrogale sister to the rest of the Tupaiidae genera. The divergence time of the Dendrogale genus is estimated as Eocene-Oligocene, with the mean value of 35.8 MYA, and the Ptilocercus genus probably separated at about 46.3 MYA. We observe an increase in the age of all nodes within the Scandentia, except for a decrease in the age of separation of Ptilocercus. This result can be explained both by the addition of new mitochondrial genome data in the analysis and the usage of new calibration points from recently published data.

Molecular cytogenetic studies in strepsirrhine primates, Dermoptera and Scandentia.

Since the first chromosome painting study between human and strepsirrhine primates was performed in 1996, nearly 30 species in Strepsirrhini, Dermoptera and Scandentia have been analyzed by cross-species chromosome painting. Here, the contribution of chromosome painting data to our understanding of primate genome organization, chromosome evolution and the karyotype phylogenetic relationships within strepsirrhine primates, Dermoptera and Scandentia is reviewed. Twenty-six to 43 homologous chromosome segments have been revealed in different species with human chromosome-specific paint probes. Various landmark rearrangements characteristic for each different lineage have been identified, as cytogenetic signatures that potentially unite certain lineages within strepsirrhine primates, Dermoptera and Scandentia.

Molecular phylogeny of treeshrews (Mammalia: Scandentia) and the timescale of diversification in Southeast Asia.

Resolving the phylogeny of treeshrews (Order Scandentia) has historically proven difficult, in large part because of access to specimens and samples from critical taxa. We used "antique" DNA methods with non-destructive sampling of museum specimens to complete taxon sampling for the 20 currently recognized treeshrew species and to estimate their phylogeny and divergence times. Most divergence among extant species is estimated to have taken place within the past 20 million years, with deeper divergences between the two families (Ptilocercidae and Tupaiidae) and between Dendrogale and all other genera within Tupaiidae. All but one of the divergences between currently recognized species had occurred by 4Mya, suggesting that Miocene tectonics, volcanism, and geographic instability drove treeshrew diversification. These geologic processes may be associated with an increase in net diversification rate in the early Miocene. Most evolutionary relationships appear consistent with island-hopping or landbridge colonization between contiguous geographic areas, although there are exceptions in which extinction may play an important part. The single recent divergence is between Tupaia palawanensis and Tupaia moellendorffi, both endemic to the Philippines, and may be due to Pleistocene sea level fluctuations and post-landbridge isolation in allopatry. We provide a time-calibrated phylogenetic framework for answering evolutionary questions about treeshrews and about evolutionary patterns and processes in Euarchonta. We also propose subsuming the monotypic genus Urogale, a Philippine endemic, into Tupaia, thereby reducing the number of extant treeshrew genera from five to four.

Coalescent-based genome analyses resolve the early branches of the euarchontoglires.

Despite numerous large-scale phylogenomic studies, certain parts of the mammalian tree are extraordinarily difficult to resolve. We used the coding regions from 19 completely sequenced genomes to study the relationships within the super-clade Euarchontoglires (Primates, Rodentia, Lagomorpha, Dermoptera and Scandentia) because the placement of Scandentia within this clade is controversial. The difficulty in resolving this issue is due to the short time spans between the early divergences of Euarchontoglires, which may cause incongruent gene trees. The conflict in the data can be depicted by network analyses and the contentious relationships are best reconstructed by coalescent-based analyses. This method is expected to be superior to analyses of concatenated data in reconstructing a species tree from numerous gene trees. The total concatenated dataset used to study the relationships in this group comprises 5,875 protein-coding genes (9,799,170 nucleotides) from all orders except Dermoptera (flying lemurs). Reconstruction of the species tree from 1,006 gene trees using coalescent models placed Scandentia as sister group to the primates, which is in agreement with maximum likelihood analyses of concatenated nucleotide sequence data. Additionally, both analytical approaches favoured the Tarsier to be sister taxon to Anthropoidea, thus belonging to the Haplorrhine clade. When divergence times are short such as in radiations over periods of a few million years, even genome scale analyses struggle to resolve phylogenetic relationships. On these short branches processes such as incomplete lineage sorting and possibly hybridization occur and make it preferable to base phylogenomic analyses on coalescent methods.

Molecular and genomic data identify the closest living relative of primates.

A full understanding of primate morphological and genomic evolution requires the identification of their closest living relative. In order to resolve the ancestral relationships among primates and their closest relatives, we searched multispecies genome alignments for phylogenetically informative rare genomic changes within the superordinal group Euarchonta, which includes the orders Primates, Dermoptera (colugos), and Scandentia (treeshrews). We also constructed phylogenetic trees from 14 kilobases of nuclear genes for representatives from most major primate lineages, both extant colugos, and multiple treeshrews, including the pentail treeshrew, Ptilocercus lowii, the only living member of the family Ptilocercidae. A relaxed molecular clock analysis including Ptilocercus suggests that treeshrews arose approximately 63 million years ago. Our data show that colugos are the closest living relatives of primates and indicate that their divergence occurred in the Cretaceous.

The complete mitochondrial genome of Tupaia belangeri and the phylogenetic affiliation of scandentia to other eutherian orders.

The complete mitochondrial genome of Tupaia belangeri, a representative of the eutherian order Scandentia, was determined and compared with full-length mitochondrial sequences of other eutherian orders described to date. The complete mitochondrial genome is 16, 754 nt in length, with no obvious deviation from the general organization of the mammalian mitochondrial genome. Thus, features such as start codon usage, incomplete stop codons, and overlapping coding regions, as well as the presence of tandem repeats in the control region, are within the range of mammalian mitochondrial (mt) DNA variation. To address the question of a possible close phylogenetic relationship between primates and Tupaia, the evolutionary affinities among primates, Tupaia and bats as representatives of the Archonta superorder, ferungulates, guinea pigs, armadillos, rats, mice, and hedgehogs were examined on the basis of the complete mitochondrial DNA sequences. The opossum sequence was used as an outgroup. The trees, estimated from 12 concatenated genes encoded on the mitochondrial H-strand, add further molecular evidence against an Archonta monophyly. With the new data described in this paper, most of both the mitochondrial and the nuclear data point away from Scandentia as the closest extant relatives to primates. Instead, the complete mitochondrial data support a clustering of Scandentia with Lagomorpha connecting to the branch leading to ferungulates. This closer phylogenetic relationship of Tupaia to rabbits than to primates first received support from several analyses of nuclear and partial mitochondrial DNA data sets. Given that short sequences are of limited use in determining deep mammalian relationships, the partial mitochondrial data available to date support this hypothesis only tentatively. Our complete mitochondrial genome data therefore add considerably more evidence in support of this hypothesis.