Practical aliquoting of flowering plant genomes

Genomes as documents of evolutionary history: a probabilistic macrosynteny model for the reconstruction of ancestral genomes

Motivation

It has been argued that whole-genome duplication (WGD) exerted a profound influence on the course of evolution. For the purpose of fully understanding the impact of WGD, several formal algorithms have been developed for reconstructing pre-WGD gene order in yeast and plant. However, to the best of our knowledge, those algorithms have never been successfully applied to WGD events in teleost and vertebrate, impeded by extensive gene shuffling and gene losses.

Results

Here, we present a probabilistic model of macrosynteny (i.e. conserved linkage or chromosome-scale distribution of orthologs), develop a variational Bayes algorithm for inferring the structure of pre-WGD genomes, and study estimation accuracy by simulation. Then, by applying the method to the teleost WGD, we demonstrate effectiveness of the algorithm in a situation where gene-order reconstruction algorithms perform relatively poorly due to a high rate of rearrangement and extensive gene losses. Our high-resolution reconstruction reveals previously overlooked small-scale rearrangements, necessitating a revision to previous views on genome structure evolution in teleost and vertebrate.

Conclusions

We have reconstructed the structure of a pre-WGD genome by employing a variational Bayes approach that was originally developed for inferring topics from millions of text documents. Interestingly, comparison of the macrosynteny and topic model algorithms suggests that macrosynteny can be regarded as documents on ancestral genome structure. From this perspective, the present study would seem to provide a textbook example of the prevalent metaphor that genomes are documents of evolutionary history.

Availability and implementation

The analysis data are available for download at http://www.gen.tcd.ie/molevol/supp_data/MacrosyntenyTGD.zip, and the software written in Java is available upon request.

Supplementary information

Supplementary data are available at Bioinformatics online.

Whole Genome Duplication in Plants: Implications for Evolutionary Analysis

The recurrent cycle of whole genome duplication (WGD) followed by massive duplicate gene loss (fractionation) differentiates plant evolutionary history from that of most other phylogenetic domains, where WGD has occurred relatively rarely, even on an evolutionary time scale. We discuss the mechanism of WGD and its biological consequences. We survey the prevalence of WGD in the flowering plants. We outline some of the major kinds of combinatorial optimization problems arising in computational biology for analyzing WGD. Fractionation and its consequences are the subject of mathematical modeling questions and further combinatorial algorithms. A strong connection is made between WGD in phylogenetic context and the theory of gene trees and species trees. We illustrate the analysis of WGD with studies involving a large number of sequenced plant genomes, including grape, the crucifers and other rosids, the asterid tomato, the eudicot Nelumbo nucifera and pineapple, a monocot.

Fractionation and subfunctionalization following genome duplications: mechanisms that drive gene content and their consequences

A gene's duplication relaxes selection. Loss of duplicate, low-function DNA (fractionation) sometimes follows, mostly by deletion in plants, but mostly via the pseudogene pathway in fish and other clades with smaller population sizes. Subfunctionalization--the founding term of the Xfunctionalization lexicon--while not the general cause of differences in duplicate gene retention, becomes primary as the number of a gene's cis-regulatory sites increases. Balanced gene drive explains retention for the average gene. Both maintenance-of-balance and subfunctionalization drive gene content nonrandomly, and currently fall outside of our accepted Theory of Evolution. The 'typical' mutation encountered by a gene duplicate is not a neutral loss-of-function; dominant mutations (Muller's lexicon; these are not neutral) abound, and confound X functionalization terms like 'neofunctionalization'. Confusion of words may cause confusion of thought. As with many plants, fish tetraploidies provide a higher throughput surrogate-genetic method to infer function from human and other vertebrate ENCODE-like regulatory sites.

Gene expression and fractionation resistance

BACKGROUND

Previous work on whole genome doubling in plants established the importance of gene functional category in provoking or suppressing duplicate gene loss, or fractionation. Other studies, particularly in Paramecium have correlated levels of gene expression with vulnerability or resistance to duplicate loss.

RESULTS

Here we analyze the simultaneous effect of function category and expression in two plant data sets, rosids and asterids.

CONCLUSION

We demonstrate function category and expression level have independent effects, though expression does not play the dominant role it does in Paramecium.

Practical halving; the Nelumbo nucifera evidence on early eudicot evolution

We present a stepwise optimal genome halving algorithm designed for large eukaryote genomes with largely single-copy genes, taking advantage of a signature pattern of paralog distribution in ancient polyploids. This is applied to the genome of Nelumbo nucifera, the sacred lotus, which is the descendant of a duplicated basal eudicot genome. In concert with the reconstructed ancestor of the grape, we investigate early events in eudicot evolution and show that the chromosome number of the common ancestor of lotus and grape was likely between 5 and 7. We show that the duplication of the ancestor of lotus and the triplication of the ancestor of grape were not closely preceded by any additional such event before the divergence of their two lineages.