Retrotransposons are the main components of eukaryotic genomes, representing up to 80% of some large plant genomes. These mobile elements transpose via a “copy and paste” mechanism, thus increasing their copy number while active. Their accumulation is now accepted as the main factor of genome size increase in higher eukaryotes, besides polyploidy. However, the dynamics of this process are poorly understood. In this study, we show that Oryza australiensis, a wild relative of the Asian cultivated rice O. sativa, has undergone recent bursts of three LTR-retrotransposon families. This genome has accumulated more than 90,000 retrotransposon copies during the last three million years, leading to a rapid twofold increase of its size. In addition, phenetic analyses of these retrotransposons clearly confirm that the genomic bursts occurred posterior to the radiation of the species. This provides direct evidence of retrotransposon-mediated variation of genome size within a plant genus.

While prokaryotic pan-genomes have been shown to contain many more genes than any individual organism, the prevalence and functional significance of differentially present genes in eukaryotes remains poorly understood. Whole-genome de novo assembly and annotation of 54 lines of the grass Brachypodium distachyon yield a pan-genome containing nearly twice the number of genes found in any individual genome. Genes present in all lines are enriched for essential biological functions, while genes present in only some lines are enriched for conditionally beneficial functions (e.g., defense and development), display faster evolutionary rates, lie closer to transposable elements and are less likely to be syntenic with orthologous genes in other grasses. Our data suggest that differentially present genes contribute substantially to phenotypic variation within a eukaryote species, these genes have a major influence in population genetics, and transposable elements play a key role in pan-genome evolution.

These authors contributed equally to this work. SUMMARYPolyploidy is generally not tolerated in animals, but is widespread in plant genomes and may result in extensive genetic redundancy. The fate of duplicated genes is poorly understood, both functionally and evolutionarily. Soybean (Glycine max L.) has undergone two separate polyploidy events (13 and 59 million years ago) that have resulted in 75% of its genes being present in multiple copies. It therefore constitutes a good model to study the impact of whole-genome duplication on gene expression. Using RNA-seq, we tested the functional fate of a set of approximately 18 000 duplicated genes. Across seven tissues tested, approximately 50% of paralogs were differentially expressed and thus had undergone expression subfunctionalization. Based on gene ontology and expression data, our analysis also revealed that only a small proportion of the duplicated genes have been neo-functionalized or non-functionalized. In addition, duplicated genes were often found in collinear blocks, and several blocks of duplicated genes were co-regulated, suggesting some type of epigenetic or positional regulation. We also found that transcription factors and ribosomal protein genes were differentially expressed in many tissues, suggesting that the main consequence of polyploidy in soybean may be at the regulatory level.

Transposable element (TE) activity has emerged as a major cause of variation in genome size and structure among species. To what extent TEs contribute to genetic variation and divergence within species, however, is much less clear, mainly because population genomic data have so far only been available for the classical model organisms. In this study, we use the annual Mediterranean grass Brachypodium distachyon to investigate TE dynamics in natural populations. Using whole-genome sequencing data for 53 natural accessions, we identified more than 5,400 TE polymorphisms across the studied genomes. We found, first, that while population bottlenecks and expansions have shaped genetic diversity in B. distachyon, these events did not lead to lineage-specific activations of TE families, as observed in other species. Instead, the same families have been active across the species range and TE activity is homogeneous across populations, indicating the presence of conserved regulatory mechanisms. Second, almost half of the TE insertion polymorphisms are accession-specific, most likely because of recent activity in expanding populations and the action of purifying selection. And finally, although TE insertion polymorphisms are underrepresented in and around genes, more than 1,000 of them occur in genic regions and could thus contribute to functional divergence. Our study shows that while TEs in B. distachyon are “well-behaved” compared with TEs in other species with larger genomes, they are an abundant source of lineage-specific genetic variation and may play an important role in population divergence and adaptation.