Probabilistic Reconstruction of Ancestral Gene Orders with Insertions and Deletions

AGO, a Framework for the Reconstruction of Ancestral Syntenies and Gene Orders

Reconstructing ancestral gene orders from the genome data of extant species is an important problem in comparative and evolutionary genomics. In a phylogenomics setting that accounts for gene family evolution through gene duplication and gene loss, the reconstruction of ancestral gene orders involves several steps, including multiple sequence alignment, the inference of reconciled gene trees, and the inference of ancestral syntenies and gene adjacencies. For each of the steps of such a process, several methods can be used and implemented using a growing corpus of, often parameterized, tools; in practice, interfacing such tools into an ancestral gene order reconstruction pipeline is far from trivial. This chapter introduces AGO, a Python-based framework aimed at creating ancestral gene order reconstruction pipelines allowing to interface and parameterize different bioinformatics tools. The authors illustrate the features of AGO by reconstructing ancestral gene orders for the X chromosome of three ancestral Anopheles species using three different pipelines. AGO is freely available at https://github.com/cchauve/AGO-pipeline .

Plastome structure, evolution, and phylogeny of Selaginella

As one of the earliest land plant lineages, Selaginella is important for studying land plant evolution. It is the largest genus of lycophytes containing 700-800 species. Some unique characters of Selaginella plastomes have been reported, but based only on 20 species. There have been no plastome phylogenies of Selaginella based on a relatively large sampling, and no efforts have been made to resolve the phylogeny of the enigmatic Sinensis group whose relationships have been unclear based on small datasets. Here we investigated the structures of 59 plastomes representing 51 species covering all six subgenera and 18 sections of Selaginella except two sections and including the intriguing Sinensis group for the first time. Our major results include: (1) the plastome size of Selaginella ranges tremendously from 78,492 bp to 187,632 bp; (2) there are numerous gene losses in Selaginella comparing with other lycophytes, Isoëtaceae and Lycopodiaceae; (3) the gene contents and plastome structures in Selaginella vary lineage-specifically and all infrageneric taxa are well supported in the plastome phylogeny; (4) the ndh gene family tends to lose or pseudogenize in those species with DR structure and without other short or medium repeats; (5) the short and medium repeat regions in SC mediate many conformations causing diverse and complex plastome structures, and six new conformations are discovered; (6) forty-eight species sampled have high GC content (>50%) but three species in the Sinensis group have ∼30% GC content in plastomes, similar to most vascular plants; (7) the Sinensis group is monophyletic, includes at least two subgroups, and has the smallest plastomes in land plants except some parasitic plants, and their plastomes do not contain any tRNAs; (8) the younger lineages in Selaginella tend to have higher GC content, whereas the older lineages tend to have lower GC content; and (9) because of incomplete genomic data and abnormal structures or some unknown reasons, even the concatenated plastomes could not well resolve the phylogenetic relationships in Selaginella with confidence, highlighting the difficulty in resolving the phylogeny and evolution of this particularly important land plant lineage.

Analysis of local genome rearrangement improves resolution of ancestral genomic maps in plants

BACKGROUND

Computationally inferred ancestral genomes play an important role in many areas of genome research. We present an improved workflow for the reconstruction from highly diverged genomes such as those of plants.

RESULTS

Our work relies on an established workflow in the reconstruction of ancestral plants, but improves several steps of this process. Instead of using gene annotations for inferring the genome content of the ancestral sequence, we identify genomic markers through a process called genome segmentation. This enables us to reconstruct the ancestral genome from hundreds of thousands of markers rather than the tens of thousands of annotated genes. We also introduce the concept of local genome rearrangement, through which we refine syntenic blocks before they are used in the reconstruction of contiguous ancestral regions. With the enhanced workflow at hand, we reconstruct the ancestral genome of eudicots, a major sub-clade of flowering plants, using whole genome sequences of five modern plants.

CONCLUSIONS

Our reconstructed genome is highly detailed, yet its layout agrees well with that reported in Badouin et al. (2017). Using local genome rearrangement, not only the marker-based, but also the gene-based reconstruction of the eudicot ancestor exhibited increased genome content, evidencing the power of this novel concept.

Ancestral Reconstruction and Investigations of Genomic Recombination on some Pentapetalae Chloroplasts

In this article, we propose a semi-automated method to rebuild genome ancestors of chloroplasts by taking into account gene duplication. Two methods have been used in order to achieve this work: a naked eye investigation using homemade scripts, whose results are considered as a basis of knowledge, and a dynamic programming based approach similar to Needleman-Wunsch. The latter fundamentally uses the Gestalt pattern matching method of sequence matcher to evaluate the occurrences probability of each gene in the last common ancestor of two given genomes. The two approaches have been applied on chloroplastic genomes from Apiales, Asterales, and Fabids orders, the latter belonging to Pentapetalae group. We found that Apiales species do not undergo indels, while they occur in the Asterales and Fabids orders. A series of experiments was then carried out to extensively verify our findings by comparing the obtained ancestral reconstruction results with the latest released approach called MLGO (Maximum Likelihood for Gene-Order analysis).

High and Variable Rates of Repeat-Mediated Mitochondrial Genome Rearrangement in a Genus of Plants

For 30 years, it has been clear that angiosperm mitochondrial genomes evolve rapidly in sequence arrangement (i.e., synteny), yet absolute rates of rearrangement have not been measured in any plant group, nor is it known how much these rates vary. To investigate these issues, we sequenced and reconstructed the rearrangement history of seven mitochondrial genomes in Monsonia (Geraniaceae). We show that rearrangements (occurring mostly as inversions) not only take place at generally high rates in these genomes but also uncover significant variation in rearrangement rates. For example, the hyperactive mitochondrial genome of Monsonia ciliata has accumulated at least 30 rearrangements over the last million years, whereas the branch leading to M. ciliata and its sister species has sustained rearrangement at a rate that is at least ten times lower. Furthermore, our analysis of published data shows that rates of mitochondrial genome rearrangement in seed plants vary by at least 600-fold. We find that sites of rearrangement are highly preferentially located in very close proximity to repeated sequences in Monsonia. This provides strong support for the hypothesis that rearrangement in angiosperm mitochondrial genomes occurs largely through repeat-mediated recombination. Because there is little variation in the amount of repeat sequence among Monsonia genomes, the variable rates of rearrangement in Monsonia probably reflect variable rates of mitochondrial recombination itself. Finally, we show that mitochondrial synonymous substitutions occur in a clock-like manner in Monsonia; rates of mitochondrial substitutions and rearrangements are therefore highly uncoupled in this group.

On the reconstruction of the ancestral bacterial genomes in genus Mycobacterium and Brucella

BACKGROUND

To reconstruct the evolution history of DNA sequences, novel models of increasing complexity regarding the number of free parameters taken into account in the sequence evolution, as well as faster and more accurate algorithms, and statistical and computational methods, are needed. More particularly, as the principal forces that have led to major structural changes are genome rearrangements (such as translocations, fusions, and so on), understanding their underlying mechanisms, among other things via the ancestral genome reconstruction, are essential. In this problem, since finding the ancestral genomes that minimize the number of rearrangements in a phylogenetic tree is known to be NP-hard for three or more genomes, heuristics are commonly chosen to obtain approximations of the exact solution. The aim of this work is to show that another path is possible.

RESULTS

Various algorithms and software already deal with the difficult nature of the problem of reconstruction of the ancestral genome, but they do not function with precision, in particular when indels or single nucleotide polymorphisms fall into repeated sequences. In this article, and despite the theoretical NP-hardness of the ancestral reconstruction problem, we show that an exact solution can be found in practice in various cases, encompassing organelles and some bacteria. A practical example proves that an accurate reconstruction, which also allows to highlight homoplasic events, can be obtained. This is illustrated by the reconstruction of ancestral genomes of two bacterial pathogens, belonging in Mycobacterium and Brucella genera.

CONCLUSIONS

By putting together automatically reconstructed ancestral regions with handmade ones for problematic cases, we show that an accurate reconstruction of ancestors of the Brucella genus and of the Mycobacterium tuberculosis complex is possible. By doing so, we are able to investigate the evolutionary history of each pathogen by computing their common ancestors. They can be investigated extensively, by studying the gene content evolution over time, the resistance acquisition, and the impacts of mobile elements on genome plasticity.

Three new Diplozoidae mitogenomes expose unusual compositional biases within the Monogenea class: implications for phylogenetic studies

BACKGROUND

As the topologies produced by previous molecular and morphological studies were contradictory and unstable (polytomy), evolutionary relationships within the Diplozoidae family and the Monogenea class (controversial relationships among the Discocotylinea, Microcotylinea and Gastrocotylinea suborders) remain unresolved. Complete mitogenomes carry a relatively large amount of information, sufficient to provide a much higher phylogenetic resolution than traditionally used morphological traits and/or single molecular markers. However, their implementation is hampered by the scarcity of available monogenean mitogenomes. Therefore, we sequenced and characterized mitogenomes belonging to three Diplozoidae family species, and conducted comparative genomic and phylogenomic analyses for the entire Monogenea class.

RESULTS

Taxonomic identification was inconclusive, so two of the species were identified merely to the genus level. The complete mitogenomes of Sindiplozoon sp. and Eudiplozoon sp. are 14,334 bp and 15,239 bp in size, respectively. Paradiplozoon opsariichthydis (15,385 bp) is incomplete: an approximately 2000 bp-long gap within a non-coding region could not be sequenced. Each genome contains the standard 36 genes (atp8 is missing). G + T content and the degree of GC- and AT-skews of these three mitogenome (and their individual elements) were higher than in other monogeneans. nad2, atp6 and nad6 were the most variable PCGs, whereas cox1, nad1 and cytb were the most conserved. Mitochondrial phylogenomics analysis, conducted using concatenated amino acid sequences of all PCGs, indicates that evolutionary relationships of the three genera are: (Eudiplozoon, (Paradiplozoon, Sindiplozoon)); and of the three suborders: (Discocotylinea, (Microcotylinea, Gastrocotylinea)). These intergeneric relationships were also supported by the skewness and principal component analyses.

CONCLUSIONS

Our results show that nad2, atp6 and nad6 (fast-evolving) would be better candidates than cox1 (slow-evolving) for species identification and population genetics studies in Diplozoidae. Nucleotide bias and codon and amino acid usage patterns of the three diplozoid mitogenomes are more similar to cestodes and trematodes than to other monogenean flatworms. This unusual mutational bias was reflected in disproportionately long branches in the phylogram. Our study offsets the scarcity of molecular data for the subclass Polyopisthocotylea to some extent, and might provide important new insights into the evolutionary history of the three genera and three suborders.

Comparative Methods for Reconstructing Ancient Genome Organization

Comparative genomics considers the detection of similarities and differences between extant genomes, and, based on more or less formalized hypotheses regarding the involved evolutionary processes, inferring ancestral states explaining the similarities and an evolutionary history explaining the differences. In this chapter, we focus on the reconstruction of the organization of ancient genomes into chromosomes. We review different methodological approaches and software, applied to a wide range of datasets from different kingdoms of life and at different evolutionary depths. We discuss relations with genome assembly, and potential approaches to validate computational predictions on ancient genomes that are almost always only accessible through these predictions.

Reconstructing evolutionary trees in parallel for massive sequences

BACKGROUND

Building the evolutionary trees for massive unaligned DNA sequences is challenging and crucial. However, reconstructing evolutionary tree for ultra-large sequences is hard. Massive multiple sequence alignment is also challenging and time/space consuming. Hadoop and Spark are developed recently, which bring spring light for the classical computational biology problems. In this paper, we tried to solve the multiple sequence alignment and evolutionary reconstruction in parallel.

RESULTS

HPTree, which is developed in this paper, can deal with big DNA sequence files quickly. It works well on the >1GB files, and gets better performance than other evolutionary reconstruction tools. Users could use HPTree for reonstructing evolutioanry trees on the computer clusters or cloud platform (eg. Amazon Cloud). HPTree could help on population evolution research and metagenomics analysis.

CONCLUSIONS

In this paper, we employ the Hadoop and Spark platform and design an evolutionary tree reconstruction software tool for unaligned massive DNA sequences. Clustering and multiple sequence alignment are done in parallel. Neighbour-joining model was employed for the evolutionary tree building. We opened our software together with source codes via http://lab.malab.cn/soft/HPtree/ .

Reconstructing Yeasts Phylogenies and Ancestors from Whole Genome Data

Phylogenetic studies aim to discover evolutionary relationships and histories. These studies are based on similarities of morphological characters and molecular sequences. Currently, widely accepted phylogenetic approaches are based on multiple sequence alignments, which analyze shared gene datasets and concatenate/coalesce these results to a final phylogeny with maximum support. However, these approaches still have limitations, and often have conflicting results with each other. Reconstructing ancestral genomes helps us understand mechanisms and corresponding consequences of evolution. Most existing genome level phylogeny and ancestor reconstruction methods can only process simplified real genome datasets or simulated datasets with identical genome content, unique genome markers, and limited types of evolutionary events. Here, we provide an alternative way to resolve phylogenetic problems based on analyses of real genome data. We use phylogenetic signals from all types of genome level evolutionary events, and overcome the conflicting issues existing in traditional phylogenetic approaches. Further, we build an automated computational pipeline to reconstruct phylogenies and ancestral genomes for two high-resolution real yeast genome datasets. Comparison results with recent studies and publications show that we reconstruct very accurate and robust phylogenies and ancestors. Finally, we identify and analyze the conserved syntenic blocks among reconstructed ancestral genomes and present yeast species.

Phylogeny analysis from gene-order data with massive duplications

BACKGROUND

Gene order changes, under rearrangements, insertions, deletions and duplications, have been used as a new type of data source for phylogenetic reconstruction. Because these changes are rare compared to sequence mutations, they allow the inference of phylogeny further back in evolutionary time. There exist many computational methods for the reconstruction of gene-order phylogenies, including widely used maximum parsimonious methods and maximum likelihood methods. However, both methods face challenges in handling large genomes with many duplicated genes, especially in the presence of whole genome duplication.

METHODS

In this paper, we present three simple yet powerful methods based on maximum-likelihood (ML) approaches that encode multiplicities of both gene adjacency and gene content information for phylogenetic reconstruction.

RESULTS

Extensive experiments on simulated data sets show that our new method achieves the most accurate phylogenies compared to existing approaches. We also evaluate our method on real whole-genome data from eleven mammals. The package is publicly accessible at http://www.geneorder.org .

CONCLUSIONS

Our new encoding schemes successfully incorporate the multiplicity information of gene adjacencies and gene content into an ML framework, and show promising results in reconstruct phylogenies for whole-genome data in the presence of massive duplications.

Comparative genomics meets topology: a novel view on genome median and halving problems

BACKGROUND

Genome median and genome halving are combinatorial optimization problems that aim at reconstruction of ancestral genomes by minimizing the number of evolutionary events between them and genomes of the extant species. While these problems have been widely studied in past decades, their solutions are often either not efficient or not biologically adequate. These shortcomings have been recently addressed by restricting the problems solution space.

RESULTS

We show that the restricted variants of genome median and halving problems are, in fact, closely related. We demonstrate that these problems have a neat topological interpretation in terms of embedded graphs and polygon gluings. We illustrate how such interpretation can lead to solutions to these problems in particular cases.

CONCLUSIONS

This study provides an unexpected link between comparative genomics and topology, and demonstrates advantages of solving genome median and halving problems within the topological framework.

Fast ancestral gene order reconstruction of genomes with unequal gene content

Background

During evolution, genomes are modified by large scale structural events, such as rearrangements, deletions or insertions of large blocks of DNA. Of particular interest, in order to better understand how this type of genomic evolution happens, is the reconstruction of ancestral genomes, given a phylogenetic tree with extant genomes at its leaves. One way of solving this problem is to assume a rearrangement model, such as Double Cut and Join (DCJ), and find a set of ancestral genomes that minimizes the number of events on the input tree. Since this problem is NP-hard for most rearrangement models, exact solutions are practical only for small instances, and heuristics have to be used for larger datasets. This type of approach can be called event-based. Another common approach is based on finding conserved structures between the input genomes, such as adjacencies between genes, possibly also assigning weights that indicate a measure of confidence or probability that this particular structure is present on each ancestral genome, and then finding a set of non conflicting adjacencies that optimize some given function, usually trying to maximize total weight and minimizing character changes in the tree. We call this type of methods homology-based.

Results

In previous work, we proposed an ancestral reconstruction method that combines homology- and event-based ideas, using the concept of intermediate genomes, that arise in DCJ rearrangement scenarios. This method showed better rate of correctly reconstructed adjacencies than other methods, while also being faster, since the use of intermediate genomes greatly reduces the search space. Here, we generalize the intermediate genome concept to genomes with unequal gene content, extending our method to account for gene insertions and deletions of any length. In many of the simulated datasets, our proposed method had better results than MLGO and MGRA, two state-of-the-art algorithms for ancestral reconstruction with unequal gene content, while running much faster, making it more scalable to larger datasets.

Conclusion

Studing ancestral reconstruction problems under a new light, using the concept of intermediate genomes, allows the design of very fast algorithms by greatly reducing the solution search space, while also giving very good results. The algorithms introduced in this paper were implemented in an open-source software called RINGO (ancestral Reconstruction with INtermediate GenOmes), available at https://github.com/pedrofeijao/RINGO.

Analysis of gene copy number changes in tumor phylogenetics

BACKGOUND

Evolution of cancer cells is characterized by large scale and rapid changes in the chromosomal  landscape. The fluorescence in situ hybridization (FISH) technique provides a way to measure the copy numbers of preselected genes in a group of cells and has been found to be a reliable source of data to model the evolution of tumor cells. Chowdhury et al. (Bioinformatics 29(13):189-98, 23; PLoS Comput Biol 10(7):1003740, 24) recently develop a computational model for tumor progression driven by gains and losses in cell count patterns obtained by FISH probes. Their model aims to find the rectilinear Steiner minimum tree (RSMT) (Chowdhury et al. in Bioinformatics 29(13):189-98, 23) and the duplication Steiner minimum tree (DSMT) (Chowdhury et al. in PLoS Comput Biol 10(7):1003740, 24) that describe the progression of FISH cell count patterns over its branches in a parsimonious manner. Both the RSMT and DSMT problems are NP-hard and heuristics are required to solve the problems efficiently.

METHODS

In this paper we propose two approaches to solve the RSMT problem, one inspired by iterative methods to address the "small phylogeny" problem (Sankoff et al. in J Mol Evol 7(2):133-49, 27; Blanchette et al. in Genome Inform 8:25-34, 28), and the other based on maximum parsimony phylogeny inference. We further show how to extend these heuristics to obtain solutions to the DSMT problem, that models large scale duplication events.

RESULTS

Experimental results from both simulated and real tumor data show that our methods outperform previous heuristics (Chowdhury et al. in Bioinformatics 29(13):189-98, 23; Chowdhury et al. in PLoS Comput Biol 10(7):1003740, 24) in obtaining solutions to both RSMT and DSMT problems.

CONCLUSION

The methods introduced here are able to provide more parsimony phylogenies compared to earlier ones which are consider better choices.

Reconstruction of Ancestral Genomes in Presence of Gene Gain and Loss

Since most dramatic genomic changes are caused by genome rearrangements as well as gene duplications and gain/loss events, it becomes crucial to understand their mechanisms and reconstruct ancestral genomes of the given genomes. This problem was shown to be NP-complete even in the "simplest" case of three genomes, thus calling for heuristic rather than exact algorithmic solutions. At the same time, a larger number of input genomes may actually simplify the problem in practice as it was earlier illustrated with MGRA, a state-of-the-art software tool for reconstruction of ancestral genomes of multiple genomes. One of the key obstacles for MGRA and other similar tools is presence of breakpoint reuses when the same breakpoint region is broken by several different genome rearrangements in the course of evolution. Furthermore, such tools are often limited to genomes composed of the same genes with each gene present in a single copy in every genome. This limitation makes these tools inapplicable for many biological datasets and degrades the resolution of ancestral reconstructions in diverse datasets. We address these deficiencies by extending the MGRA algorithm to genomes with unequal gene contents. The developed next-generation tool MGRA2 can handle gene gain/loss events and shares the ability of MGRA to reconstruct ancestral genomes uniquely in the case of limited breakpoint reuse. Furthermore, MGRA2 employs a number of novel heuristics to cope with higher breakpoint reuse and process datasets inaccessible for MGRA. In practical experiments, MGRA2 shows superior performance for simulated and real genomes as compared to other ancestral genome reconstruction tools.

Probabilistic modeling of the evolution of gene synteny within reconciled phylogenies

BACKGROUND

Most models of genome evolution concern either genetic sequences, gene content or gene order. They sometimes integrate two of the three levels, but rarely the three of them. Probabilistic models of gene order evolution usually have to assume constant gene content or adopt a presence/absence coding of gene neighborhoods which is blind to complex events modifying gene content.

RESULTS

We propose a probabilistic evolutionary model for gene neighborhoods, allowing genes to be inserted, duplicated or lost. It uses reconciled phylogenies, which integrate sequence and gene content evolution. We are then able to optimize parameters such as phylogeny branch lengths, or probabilistic laws depicting the diversity of susceptibility of syntenic regions to rearrangements. We reconstruct a structure for ancestral genomes by optimizing a likelihood, keeping track of all evolutionary events at the level of gene content and gene synteny. Ancestral syntenies are associated with a probability of presence.

Moments of genome evolution by Double Cut-and-Join

We study statistical estimators of the number of genomic events separating two genomes under a Double Cut-and Join (DCJ) rearrangement model, by a method of moment estimation. We first propose an exact, closed, analytically invertible formula for the expected number of breakpoints after a given number of DCJs. This improves over the heuristic, recursive and computationally slower previously proposed one. Then we explore the analogies of genome evolution by DCJ with evolution of binary sequences under substitutions, permutations under transpositions, and random graphs. Each of these are presented in the literature with intuitive justifications, and are used to import results from better known fields. We formalize the relations by proving a correspondence between moments in sequence and genome evolution, provided substitutions appear four by four in the corresponding model. Eventually we prove a bounded error on two estimators of the number of cycles in the breakpoint graph after a given number of rearrangements, by an analogy with cycles in permutations and components in random graphs.

MLGO: phylogeny reconstruction and ancestral inference from gene-order data

BACKGROUND

The rapid accumulation of whole-genome data has renewed interest in the study of using gene-order data for phylogenetic analyses and ancestral reconstruction. Current software and web servers typically do not support duplication and loss events along with rearrangements.

RESULTS

MLGO (Maximum Likelihood for Gene-Order Analysis) is a web tool for the reconstruction of phylogeny and/or ancestral genomes from gene-order data. MLGO is based on likelihood computation and shows advantages over existing methods in terms of accuracy, scalability and flexibility.

CONCLUSIONS

To the best of our knowledge, it is the first web tool for analysis of large-scale genomic changes including not only rearrangements but also gene insertions, deletions and duplications. The web tool is available from http://www.geneorder.org/server.php .