The genomics of linkage drag in inbred lines of sunflower

Crop wild relatives represent valuable sources of alleles for crop improvement, including adaptation to climate change and emerging diseases. However, introgressions from wild relatives might have deleterious effects on desirable traits, including yield, due to linkage drag. Here, we analyzed the genomic and phenotypic impacts of wild introgressions in inbred lines of cultivated sunflower to estimate the impacts of linkage drag. First, we generated reference sequences for seven cultivated and one wild sunflower genotype, as well as improved assemblies for two additional cultivars. Next, relying on previously generated sequences from wild donor species, we identified introgressions in the cultivated reference sequences, as well as the sequence and structural variants they contain. We then used a ridge-regression best linear unbiased prediction (BLUP) model to test the effects of the introgressions on phenotypic traits in the cultivated sunflower association mapping population. We found that introgression has introduced substantial sequence and structural variation into the cultivated sunflower gene pool, including >3,000 new genes. While introgressions reduced genetic load at protein-coding sequences, they mostly had negative impacts on yield and quality traits. Introgressions found at high frequency in the cultivated gene pool had larger effects than low-frequency introgressions, suggesting that the former likely were targeted by artificial selection. Also, introgressions from more distantly related species were more likely to be maladaptive than those from the wild progenitor of cultivated sunflower. Thus, breeding efforts should focus, as far as possible, on closely related and fully compatible wild relatives.

Whole-genome landscape of Medicago truncatula symbiotic genes

Advances in deciphering the functional architecture of eukaryotic genomes have been facilitated by recent breakthroughs in sequencing technologies, enabling a more comprehensive representation of genes and repeat elements in genome sequence assemblies, as well as more sensitive and tissue-specific analyses of gene expression. Here we show that PacBio sequencing has led to a substantially improved genome assembly of Medicago truncatula A17, a legume model species notable for endosymbiosis studies¹, and has enabled the identification of genome rearrangements between genotypes at a near-base-pair resolution. Annotation of the new M. truncatula genome sequence has allowed for a thorough analysis of transposable elements and their dynamics, as well as the identification of new players involved in symbiotic nodule development, in particular 1,037 upregulated long non-coding RNAs (lncRNAs). We have also discovered that a substantial proportion (~35% and 38%, respectively) of the genes upregulated in nodules or expressed in the nodule differentiation zone colocalize in genomic clusters (270 and 211, respectively), here termed symbiotic islands. These islands contain numerous expressed lncRNA genes and display differentially both DNA methylation and histone marks. Epigenetic regulations and lncRNAs are therefore attractive candidate elements for the orchestration of symbiotic gene expression in the M. truncatula genome.

Evolutionary transitions in the Asteraceae coincide with marked shifts in transposable element abundance

BACKGROUND

The transposable element (TE) content of the genomes of plant species varies from near zero in the genome of Utricularia gibba to more than 80% in many species. It is not well understood whether this variation in genome composition results from common mechanisms or stochastic variation. The major obstacles to investigating mechanisms of TE evolution have been a lack of comparative genomic data sets and efficient computational methods for measuring differences in TE composition between species. In this study, we describe patterns of TE evolution in 14 species in the flowering plant family Asteraceae and 1 outgroup species in the Calyceraceae to investigate phylogenetic patterns of TE dynamics in this important group of plants.

RESULTS

Our findings indicate that TE families in the Asteraceae exhibit distinct patterns of non-neutral evolution, and that there has been a directional increase in copy number of Gypsy retrotransposons since the origin of the Asteraceae. Specifically, there is marked increase in Gypsy abundance at the origin of the Asteraceae and at the base of the tribe Heliantheae. This latter shift in genome composition has had a significant impact on the diversity and abundance distribution of TEs in a lineage-specific manner.

CONCLUSIONS

We show that the TE-driven expansion of plant genomes can be facilitated by just a few TE families, and is likely accompanied by the modification and/or replacement of the TE community. Importantly, large shifts in TE composition may be correlated with major of phylogenetic transitions.

Transposome: a toolkit for annotation of transposable element families from unassembled sequence reads

MOTIVATION

Transposable elements (TEs) can be found in virtually all eukaryotic genomes and have the potential to produce evolutionary novelty. Despite the broad taxonomic distribution of TEs, the evolutionary history of these sequences is largely unknown for many taxa due to a lack of genomic resources and identification methods. Given that most TE annotation methods are designed to work on genome assemblies, we sought to develop a method to provide a fine-grained classification of TEs from DNA sequence reads. Here, we present a toolkit for the efficient annotation of TE families from low-coverage whole-genome shotgun (WGS) data, enabling the rapid identification of TEs in a large number of taxa. We compared our software, Transposome, with other approaches for annotating repeats from WGS data, and we show that it offers significant improvements in run time and produces more precise estimates of genomic repeat abundance. Transposome may also be used as a general toolkit for working with Next Generation Sequencing (NGS) data, and for constructing custom genome analysis pipelines.

AVAILABILITY AND IMPLEMENTATION

The source code for Transposome is freely available (http://sestaton.github.io/Transposome), implemented in Perl and is supported on Linux.

A target enrichment method for gathering phylogenetic information from hundreds of loci: An example from the Compositae

PREMISE OF THE STUDY

The Compositae (Asteraceae) are a large and diverse family of plants, and the most comprehensive phylogeny to date is a meta-tree based on 10 chloroplast loci that has several major unresolved nodes. We describe the development of an approach that enables the rapid sequencing of large numbers of orthologous nuclear loci to facilitate efficient phylogenomic analyses. •

METHODS AND RESULTS

We designed a set of sequence capture probes that target conserved orthologous sequences in the Compositae. We also developed a bioinformatic and phylogenetic workflow for processing and analyzing the resulting data. Application of our approach to 15 species from across the Compositae resulted in the production of phylogenetically informative sequence data from 763 loci and the successful reconstruction of known phylogenetic relationships across the family. •

CONCLUSIONS

These methods should be of great use to members of the broader Compositae community, and the general approach should also be of use to researchers studying other families.

A target enrichment method for gathering phylogenetic information from hundreds of loci: An example from the Compositae

PREMISE OF THE STUDY

The Compositae (Asteraceae) are a large and diverse family of plants, and the most comprehensive phylogeny to date is a meta-tree based on 10 chloroplast loci that has several major unresolved nodes. We describe the development of an approach that enables the rapid sequencing of large numbers of orthologous nuclear loci to facilitate efficient phylogenomic analyses. •

METHODS AND RESULTS

We designed a set of sequence capture probes that target conserved orthologous sequences in the Compositae. We also developed a bioinformatic and phylogenetic workflow for processing and analyzing the resulting data. Application of our approach to 15 species from across the Compositae resulted in the production of phylogenetically informative sequence data from 763 loci and the successful reconstruction of known phylogenetic relationships across the family. •

CONCLUSIONS

These methods should be of great use to members of the broader Compositae community, and the general approach should also be of use to researchers studying other families.

The sunflower (Helianthus annuus L.) genome reflects a recent history of biased accumulation of transposable elements

Aside from polyploidy, transposable elements are the major drivers of genome size increases in plants. Thus, understanding the diversity and evolutionary dynamics of transposable elements in sunflower (Helianthus annuus L.), especially given its large genome size (∼3.5 Gb) and the well-documented cases of amplification of certain transposons within the genus, is of considerable importance for understanding the evolutionary history of this emerging model species. By analyzing approximately 25% of the sunflower genome from random sequence reads and assembled bacterial artificial chromosome (BAC) clones, we show that it is composed of over 81% transposable elements, 77% of which are long terminal repeat (LTR) retrotransposons. Moreover, the LTR retrotransposon fraction in BAC clones harboring genes is disproportionately composed of chromodomain-containing Gypsy LTR retrotransposons ('chromoviruses'), and the majority of the intact chromoviruses contain tandem chromodomain duplications. We show that there is a bias in the efficacy of homologous recombination in removing LTR retrotransposon DNA, thereby providing insight into the mechanisms associated with transposable element (TE) composition in the sunflower genome. We also show that the vast majority of observed LTR retrotransposon insertions have likely occurred since the origin of this species, providing further evidence that biased LTR retrotransposon activity has played a major role in shaping the chromatin and DNA landscape of the sunflower genome. Although our findings on LTR retrotransposon age and structure could be influenced by the selection of the BAC clones analyzed, a global analysis of random sequence reads indicates that the evolutionary patterns described herein apply to the sunflower genome as a whole.

The genomic organization of Ty3/gypsy-like retrotransposons in Helianthus (Asteraceae) homoploid hybrid species

The origin of new diploid, or homoploid, hybrid species is associated with rapid genomic restructuring in the hybrid neospecies. This mode of speciation has been best characterized in wild sunflower species in the genus Helianthus, where three homoploid hybrid species (H. anomalus, H. deserticola, and H. paradoxus) have independently arisen via ancient hybridization events between the same two parental species (H. annuus and H. petiolaris). Most previous work examining genomic restructuring in these sunflower hybrid species has focused on chromosomal rearrangements. However, the origin of all three homoploid hybrid sunflower species also is associated with massive proliferation events of Ty3/gypsy-like retrotransposons in the hybrid species' genomes. We compared the genomic organization of these elements in the parent species and two of the homoploid hybrid species using fluorescence in situ hybridization (FISH). We found a significant expansion of Ty3/gypsy-like retrotransposons confined to the pericentromeric regions of two hybrid sunflower species, H. deserticola and H. paradoxus. In contrast, we detected no significant increase in the frequency or extent of dispersed retrotransposon populations in the hybrid species within the resolution limits of our assay. We discuss the potential role that transposable element proliferation and localization plays in the evolution of homoploid hybrid species.