Origin of land plants using the multispecies coalescent model

The origin of land plants is a fundamental topic in plant evolutionary biology. Despite the crucial importance for knowing the closest lineages of land plants, this question has not been fully answered yet. Using recently available nuclear sequences from streptophyte algae, the multispecies coalescent model produces a congruent phylogeny that is robust to different data sets, in contrast to the conflicting phylogenies produced by the concatenation method. Using phylogenomic data and the coalescent model, in this opinion article we postulate that the Zygnematales are the closest lineages of land plants. We suggest that the coalescent model can accommodate gene tree heterogeneity in deep-level phylogenies and can be potentially used to resolve other deep species phylogenies.

Robustness to divergence time underestimation when inferring species trees from estimated gene trees

To infer species trees from gene trees estimated from phylogenomic data sets, tractable methods are needed that can handle dozens to hundreds of loci. We examine several computationally efficient approaches-MP-EST, STAR, STEAC, STELLS, and STEM-for inferring species trees from gene trees estimated using maximum likelihood (ML) and Bayesian approaches. Among the methods examined, we found that topology-based methods often performed better using ML gene trees and methods employing coalescent times typically performed better using Bayesian gene trees, with MP-EST, STAR, STEAC, and STELLS outperforming STEM under most conditions. We examine why the STEM tree (also called GLASS or Maximum Tree) is less accurate on estimated gene trees by comparing estimated and true coalescence times, performing species tree inference using simulations, and analyzing a great ape data set keeping track of false positive and false negative rates for inferred clades. We find that although true coalescence times are more ancient than speciation times under the multispecies coalescent model, estimated coalescence times are often more recent than speciation times. This underestimation can lead to increased bias and lack of resolution with increased sampling (either alleles or loci) when gene trees are estimated with ML. The problem appears to be less severe using Bayesian gene-tree estimates.

Effects of missing data on species tree estimation under the coalescent

With recent advances in genomic sequencing, the importance of taking the effects of the processes that can cause discord between the speciation history and the individual gene histories into account has become evident. For multilocus datasets, it is difficult to achieve complete coverage of all sampled loci across all sample specimens, a problem that also arises when combining incompletely overlapping datasets. Here we examine how missing data affects the accuracy of species tree reconstruction. In our study, 10- and 100-locus sequence datasets were simulated under the coalescent model from shallow and deep speciation histories, and species trees were estimated using the maximum likelihood and Bayesian frameworks (with STEM and (*)BEAST, respectively). The accuracy of the estimated species trees was evaluated using the symmetric difference and the SPR distance. We examine the effects of sampling more than one individual per species, as well as the effects of different patterns of missing data (i.e., different amounts of missing data, which is represented among random taxa as opposed to being concentrated in specific taxa, as is often the case for empirical studies). Our general conclusion is that the species tree estimates are remarkably resilient to the effects of missing data. We find that for datasets with more limited numbers of loci, sampling more than one individual per species has the strongest effect on improving species tree accuracy when there is missing data, especially at higher degrees of missing data. For larger multilocus datasets (e.g., 25-100 loci), the amount of missing data has a negligible effect on species tree reconstruction, even at 50% missing data and a single sampled individual per species.

A multilocus phylogenetic analysis of Escallonia (Escalloniaceae): diversification in montane South America

PREMISE OF THE STUDY

The mountains of South America are hotspots of plant diversity. How this diversity originated and evolved, and what roles geographic and environmental factors may have played in the diversification of lineages occurring in these regions, is not well understood. Escallonia, a morphologically and ecologically diverse group of shrubs and trees widely distributed in these mountains, provides an ideal opportunity for studying the historical underpinnings that have shaped the extraordinarily distinctive, diverse, and endangered flora of these regions, and for evaluating the role of abiotic factors in the process of lineage divergence. •

METHODS

I analyzed neutral DNA sequence data from two nuclear loci and one chloroplast locus using maximum parsimony, maximum likelihood, and Bayesian phylogenetic approaches. I used a Bayesian approach to analyze the geographic structure of gene trees, and a phylogenetically controlled decomposition of the variance in bioclimatic variables to analyze the eco-climatic structure of gene trees. •

KEY RESULTS

I found that Escallonia (1) is monophyletic, (2) has a remarkable level of geographic and climatic phylogenetic structure, (3) likely originated in the tropical Andes, and (4) has a widespread absence of species exclusivity. •

CONCLUSIONS

Geography played an important role early in the history of Escallonia by separating populations that later diversify likely in isolation. Although geographic isolation was generally accompanied by changes in climate, it is not clear whether environmental gradients along elevation have influenced more recent diversification events or whether species have evolved broader environmental tolerances.

Eight independent nuclear genes support monophyly of the plovers: the role of mutational variance in gene trees

Molecular phylogenies of Charadriiformes based on mtDNA genes and one to three nuclear loci do not support the traditional placement of Pluvialis in the plovers (Charadriidae), assigning it instead to oystercatchers, stilts, and avocets (Haematopodidae and Recurvirostridae). To investigate this hypothesis of plover paraphyly, the relationships among Pluvialis and closely related families were revisited by sequencing two individuals of all taxa except Peltohyas for eight independent single copy nuclear protein-coding loci selected for their informativeness at this phylogenetic depth. The species tree estimated jointly with the gene trees in the coalescent programme (*)BEAST strongly supported plover monophyly, as did Bayesian analysis of the concatenated matrix. The data sets that supported plover paraphyly in Baker et al. (2007) and Fain and Houde (2007) reflect two to four independent gene histories, and thus discordance with the plover monophyly species tree might have arisen by chance through stochastic mutational variance. For the plovers we conclude there is no conclusive evidence of coalescent variance from ancient incomplete lineage sorting across the interior branch leading to Pluvialis in the species tree. Rather, earlier studies seem have been misled by faster evolving mtDNA genes with high mutational variance, and a few nuclear genes that had low resolving power at the Pluvialis sister group level. These findings are of general relevance in avian phylogenetics, as they show that careful attention needs to be paid to the number and the phylogenetic informativeness of genes required to obtain accurate estimates of the species tree, especially where there is mutational heterogeneity in gene trees.

A multi-locus phylogeny reveals a complex pattern of diversification related to climate and habitat heterogeneity in southern African white-eyes

The recent, rapid radiation of Zosteropidae, coupled with their high levels of colonizing ability and phenotypic diversity, makes species delimitation within this family problematic. Given these problems, challenges to establish the mechanisms driving diversity and speciation within this group have arisen. Four morphologically distinct southern African Zosterops taxa, with a contentious taxonomic past, provide such a challenge. Here, supplemented with morphological and environmental analytical techniques, a combination of mitochondrial and nuclear markers were analyzed using Bayesian and Likelihood methods to determine their speciation patterns and to establish the phylogenetic relationships of these four morphologically diverse southern African Zosterops taxa. Nearly all individuals were phenotypically diagnosable, even those individuals collected in areas of contact between taxa. Localities where two or more taxa co-occur appear to possess intermediate environmental characteristics. Initial Bayesian and Likelihood mitochondrial DNA analyses and Bayesian structure analyses of the combined nuclear markers indicated levels of hybridization in areas of sympatry. A combined mtDNA and nuclear DNA analysis and a species tree analysis (with hybrids excluded) placed Z. pallidus as sister to the other southern African taxa, with Z. senegalensis the putative sister taxon to a clade comprising Z. capensis and Z. virens. The grouping of taxon-specific sampling localities and the apparent intermediate nature of birds from areas of sympatry points toward an influence of habitat type and the associated climatic conditions in driving Zosterops diversification in southern Africa.

Molecular phylogeny supports the paraphyletic nature of the genus Trogoderma (Coleoptera: Dermestidae) collected in the Australasian ecozone

To date, a molecular phylogenetic approach has not been used to investigate the evolutionary structure of Trogoderma and closely related genera. Using two mitochondrial genes, Cytochrome Oxidase I and Cytochrome B, and the nuclear gene, 18S, the reported polyphyletic positioning of Trogoderma was examined. Paraphyly in Trogoderma was observed, with one Australian Trogoderma species reconciled as sister to all Dermestidae and the Anthrenocerus genus deeply nested within the Australian Trogoderma clade. In addition, time to most recent common ancestor for a number of Dermestidae was calculated. Based on these estimations, the Dermestidae origin exceeded 175 million years, placing the origins of this family in Pangaea.

Multigene phylogenetic analyses to delimit new species in fungal plant pathogens

Supporting the identification of unknown strains or specimens by sequencing a genetic marker commonly used for phylogenetics or DNA barcoding is now standard practice for mycologists and plant pathologists. Does one have a new species when a strain differs by a few base pairs when compared to reference sequences from taxonomically well-characterized species that do not differ morphologically from this new strain? If variation at the intra- and interspecific levels for the locus used for identification is already understood for all the closely related species, it is possible to make a reliable prediction of a new species status, but ultimately this question can only be properly addressed by determining the presence or absence of gene flow among a group of strains of the putative new species and strains of previously delimited species. The Phylogenetic Species Concept (PSC) and its assessment using multigene phylogeny and Genealogical Concordance Phylogenetic Species Recognition (GCPSR) are the basis for this chapter. The theoretical framework and a variety of tools to apply these concepts are explained, to assist in the assessment of whether a species is distinct or new when confronted with some sequence divergence from reference data.

Distinct origin of the Y and St genome in Elymus species: evidence from the analysis of a large sample of St genome species using two nuclear genes

Background

Previous cytological and single copy nuclear genes data suggested the St and Y genome in the StY-genomic Elymus species originated from different donors: the St from a diploid species in Pseudoroegneria and the Y from an unknown diploid species, which are now extinct or undiscovered. However, ITS data suggested that the Y and St genome shared the same progenitor although rather few St genome species were studied. In a recent analysis of many samples of St genome species Pseudoroegneria spicata (Pursh) À. Löve suggested that one accession of P. spicata species was the most likely donor of the Y genome. The present study tested whether intraspecific variation during sampling could affect the outcome of analyses to determining the origin of Y genome in allotetraploid StY species. We also explored the evolutionary dynamics of these species.

Methodology/Principal Findings

Two single copy nuclear genes, the second largest subunit of RNA polymerase II (RPB2) and the translation elongation factor G (EF-G) sequences from 58 accessions of Pseudoroegneria and Elymus species, together with those from Hordeum (H), Agropyron (P), Australopyrum (W), Lophopyrum (E^e), Thinopyrum (E^a), Thinopyrum (E^b), and Dasypyrum (V) were analyzed using maximum parsimony, maximum likelihood and Bayesian methods. Sequence comparisons among all these genomes revealed that the St and Y genomes are relatively dissimilar. Extensive sequence variations have been detected not only between the sequences from St and Y genome, but also among the sequences from diploid St genome species. Phylogenetic analyses separated the Y sequences from the St sequences.

Conclusions/Significance

Our results confirmed that St and Y genome in Elymus species have originated from different donors, and demonstrated that intraspecific variation does not affect the identification of genome origin in polyploids. Moreover, sequence data showed evidence to support the suggestion of the genome convergent evolution in allopolyploid StY genome species.

A species tree for the Australo-Papuan Fairy-wrens and allies (Aves: Maluridae)

We explored the efficacy of species tree methods at the family level in birds, using the Australo-Papuan Fairy-wrens (Passeriformes: Maluridae) as a model system. Fairy-wrens of the genus Malurus are known for high intensities of sexual selection, resulting in some cases in rapid speciation. This history suggests that incomplete lineage sorting (ILS) of neutrally evolving loci could be substantial, a situation that could compromise traditional methods of combining loci in phylogenetic analysis. Using 18 molecular markers (5 anonymous loci, 7 exons, 5 introns, and 1 mitochondrial DNA locus), we show that gene tree monophyly across species could be rejected for 16 of 18 loci, suggesting substantial ILS at the family level in these birds. Using the software Concaterpillar, we also detect three statistically distinct clusters of gene trees among the 18 loci. Despite substantial variation in gene trees, species trees constructed using four different species tree estimation methods (BEST, BUCKy, and STAR) were generally well supported and similar to each other and to the concatenation tree, with a few mild discordances at nodes that could be explained by rapid and recent speciation events. By contrast, minimizing deep coalescences produced a species tree that was topologically more divergent from those of the other methods as measured by multidimensional scaling of trees. Additionally, gene and species trees were topologically more similar in the BEST analysis, presumably because of the species tree prior employed in BEST which appropriately assumes that gene trees are correlated with each other and with the species tree. Among the 18 loci, we also discovered 102 independent indel markers, which also proved phylogenetically informative, primarily among genera, and displayed a ∼4-fold bias towards deletions. As suggested in earlier work, the grasswrens (Amytornis) are sister to the rest of the family and the emu-wrens (Stipiturus) are sister to fairy-wrens (Malurus, Clytomyias). Our study shows that ILS is common at the family level in birds yet, despite this, species tree methods converge on broadly similar results for this family.

Fast and accurate methods for phylogenomic analyses

Background

Species phylogenies are not estimated directly, but rather through phylogenetic analyses of different gene datasets. However, true gene trees can differ from the true species tree (and hence from one another) due to biological processes such as horizontal gene transfer, incomplete lineage sorting, and gene duplication and loss, so that no single gene tree is a reliable estimate of the species tree. Several methods have been developed to estimate species trees from estimated gene trees, differing according to the specific algorithmic technique used and the biological model used to explain differences between species and gene trees. Relatively little is known about the relative performance of these methods.

Results

We report on a study evaluating several different methods for estimating species trees from sequence datasets, simulating sequence evolution under a complex model including indels (insertions and deletions), substitutions, and incomplete lineage sorting. The most important finding of our study is that some fast and simple methods are nearly as accurate as the most accurate methods, which employ sophisticated statistical methods and are computationally quite intensive. We also observe that methods that explicitly consider errors in the estimated gene trees produce more accurate trees than methods that assume the estimated gene trees are correct.

Conclusions

Our study shows that highly accurate estimations of species trees are achievable, even when gene trees differ from each other and from the species tree, and that these estimations can be obtained using fairly simple and computationally tractable methods.

Diversification and species boundaries of Rhinebothrium (Cestoda; Rhinebothriidea) in South American freshwater stingrays (Batoidea; Potamotrygonidae)

BACKGROUND

Neotropical freshwater stingrays (Batoidea: Potamotrygonidae) host a diverse parasite fauna, including cestodes. Both cestodes and their stingray hosts are marine-derived, but the taxonomy of this host/parasite system is poorly understood.

METHODOLOGY

Morphological and molecular (Cytochrome oxidase I) data were used to investigate diversity in freshwater lineages of the cestode genus Rhinebothrium Linton, 1890. Results were based on a phylogenetic hypothesis for 74 COI sequences and morphological analysis of over 400 specimens. Cestodes studied were obtained from 888 individual potamotrygonids, representing 14 recognized and 18 potentially undescribed species from most river systems of South America.

RESULTS

Morphological species boundaries were based mainly on microthrix characters observed with scanning electron microscopy, and were supported by COI data. Four species were recognized, including two redescribed (Rhinebothrium copianullum and R. paratrygoni), and two newly described (R. brooksi n. sp. and R. fulbrighti n. sp.). Rhinebothrium paranaensis Menoret & Ivanov, 2009 is considered a junior synonym of R. paratrygoni because the morphological features of the two species overlap substantially. The diagnosis of Rhinebothrium Linton, 1890 is emended to accommodate the presence of marginal longitudinal septa observed in R. copianullum and R. brooksi n. sp. Patterns of host specificity and distribution ranged from use of few host species in few river basins, to use of as many as eight host species in multiple river basins.

SIGNIFICANCE

The level of intra-specific morphological variation observed in features such as total length and number of proglottids is unparalleled among other elasmobranch cestodes. This is attributed to the large representation of host and biogeographical samples. It is unclear whether the intra-specific morphological variation observed is unique to this freshwater system. Nonetheless, caution is urged when using morphological discontinuities to delimit elasmobranch cestode species because the amount of variation encountered is highly dependent on sample size and/or biogeographical representation.

Estimating phylogenetic relationships despite discordant gene trees across loci: the species tree of a diverse species group of feather mites (Acari: Proctophyllodidae)

With the increased availability of multilocus sequence data, the lack of concordance of gene trees estimated for independent loci has focused attention on both the biological processes producing the discord and the methodologies used to estimate phylogenetic relationships. What has emerged is a suite of new analytical tools for phylogenetic inference--species tree approaches. In contrast to traditional phylogenetic methods that are stymied by the idiosyncrasies of gene trees, approaches for estimating species trees explicitly take into account the cause of discord among loci and, in the process, provides a direct estimate of phylogenetic history (i.e. the history of species divergence, not divergence of specific loci). We illustrate the utility of species tree estimates with an analysis of a diverse group of feather mites, the pinnatus species group (genus Proctophyllodes). Discord among four sequenced nuclear loci is consistent with theoretical expectations, given the short time separating speciation events (as evident by short internodes relative to terminal branch lengths in the trees). Nevertheless, many of the relationships are well resolved in a Bayesian estimate of the species tree; the analysis also highlights ambiguous aspects of the phylogeny that require additional loci. The broad utility of species tree approaches is discussed, and specifically, their application to groups with high speciation rates--a history of diversification with particular prevalence in host/parasite systems where species interactions can drive rapid diversification.

Recent trends in molecular phylogenetic analysis: where to next?

The acquisition of large multilocus sequence data is providing researchers with an unprecedented amount of information to resolve difficult phylogenetic problems. With these large quantities of data comes the increasing challenge regarding the best methods of analysis. We review the current trends in molecular phylogenetic analysis, focusing specifically on the topics of multiple sequence alignment and methods of tree reconstruction. We suggest that traditional methods are inadequate for these highly heterogeneous data sets and that researchers employ newer more sophisticated search algorithms in their analyses. If we are to best extract the information present in these data sets, a sound understanding of basic phylogenetic principles combined with modern methodological techniques are necessary.

Species delimitation using a combined coalescent and information-theoretic approach: an example from North American Myotis bats

Coalescent model-based methods for phylogeny estimation force systematists to confront issues related to the identification of species boundaries. Unlike conventional phylogenetic analysis, where species membership can be assessed qualitatively after the phylogeny is estimated, the phylogenies that are estimated under a coalescent model treat aggregates of individuals as the operational taxonomic units and thus require a priori definition of these sets because the models assume that the alleles in a given lineage are sampled from a single panmictic population. Fortunately, the use of coalescent model-based approaches allows systematists to conduct probabilistic tests of species limits by calculating the probability of competing models of lineage composition. Here, we conduct the first exploration of the issues related to applying such tests to a complex empirical system. Sequence data from multiple loci were used to assess species limits and phylogeny in a clade of North American Myotis bats. After estimating gene trees at each locus, the likelihood of models representing all hierarchical permutations of lineage composition was calculated and Akaike information criterion scores were computed. Metrics borrowed from information theory suggest that there is strong support for several models that include multiple evolutionary lineages within the currently described species Myotis lucifugus and M. evotis. Although these results are preliminary, they illustrate the practical importance of coupled species delimitation and phylogeny estimation.

Bayesian inference of species trees from multilocus data

Until recently, it has been common practice for a phylogenetic analysis to use a single gene sequence from a single individual organism as a proxy for an entire species. With technological advances, it is now becoming more common to collect data sets containing multiple gene loci and multiple individuals per species. These data sets often reveal the need to directly model intraspecies polymorphism and incomplete lineage sorting in phylogenetic estimation procedures.

For a single species, coalescent theory is widely used in contemporary population genetics to model intraspecific gene trees. Here, we present a Bayesian Markov chain Monte Carlo method for the multispecies coalescent. Our method coestimates multiple gene trees embedded in a shared species tree along with the effective population size of both extant and ancestral species. The inference is made possible by multilocus data from multiple individuals per species.

Using a multiindividual data set and a series of simulations of rapid species radiations, we demonstrate the efficacy of our new method. These simulations give some insight into the behavior of the method as a function of sampled individuals, sampled loci, and sequence length. Finally, we compare our new method to both an existing method (BEST 2.2) with similar goals and the supermatrix (concatenation) method. We demonstrate that both BEST and our method have much better estimation accuracy for species tree topology than concatenation, and our method outperforms BEST in divergence time and population size estimation.