Genome assembly of a tropical maize inbred line provides insights into structural variation and crop improvement

Background Transposable elements are abundant in eukaryotic genomes and it is believed that they have a significant impact on the evolution of gene and chromosome structure. While there are several completed eukaryotic genome projects, there are only few high quality genome wide annotations of transposable elements. Therefore, there is a considerable demand for computational identification of transposable elements. LTR retrotransposons, an important subclass of transposable elements, are well suited for computational identification, as they contain long terminal repeats (LTRs). Results We have developed a software tool LTRharvest for the de novo detection of full length LTR retrotransposons in large sequence sets. LTRharvest efficiently delivers high quality annotations based on known LTR transposon features like length, distance, and sequence motifs. A quality validation of LTRharvest against a gold standard annotation for Saccharomyces cerevisae and Drosophila melanogaster shows a sensitivity of up to 90% and 97% and specificity of 100% and 72%, respectively. This is comparable or slightly better than annotations for previous software tools. The main advantage of LTRharvest over previous tools is (a) its ability to efficiently handle large datasets from finished or unfinished genome projects, (b) its flexibility in incorporating known sequence features into the prediction, and (c) its availability as an open source software. Conclusion LTRharvest is an efficient software tool delivering high quality annotation of LTR retrotransposons. It can, for example, process the largest human chromosome in approx. 8 minutes on a Linux PC with 4 GB of memory. Its flexibility and small space and run-time requirements makes LTRharvest a very competitive candidate for future LTR retrotransposon annotation projects. Moreover, the structured design and implementation and the availability as open source provides an excellent base for incorporating novel concepts to further improve prediction of LTR retrotransposons.

There exists extraordinary morphological and genetic diversity among the maize landraces that have been developed by pre-Columbian cultivators. To explain this high level of diversity in maize, several authors have proposed that maize landraces were the products of multiple independent domestications from their wild relative (teosinte). We present phylogenetic analyses based on 264 individual plants, each genotyped at 99 microsatellites, that challenge the multiple-origins hypothesis. Instead, our results indicate that all maize arose from a single domestication in southern Mexico about 9,000 years ago. Our analyses also indicate that the oldest surviving maize types are those of the Mexican highlands with maize spreading from this region over the Americas along two major paths. Our phylogenetic work is consistent with a model based on the archaeological record suggesting that maize diversified in the highlands of Mexico before spreading to the lowlands. We also found only modest evidence for postdomestication gene flow from teosinte into maize.

Abstract Assembling a plant genome is challenging due to the abundance of repetitive sequences, yet no standard is available to evaluate the assembly of repeat space. LTR retrotransposons (LTR-RTs) are the predominant interspersed repeat that is poorly assembled in draft genomes. Here, we propose a reference-free genome metric called LTR Assembly Index (LAI) that evaluates assembly continuity using LTR-RTs. After correcting for LTR-RT amplification dynamics, we show that LAI is independent of genome size, genomic LTR-RT content, and gene space evaluation metrics (i.e., BUSCO and CEGMA). By comparing genomic sequences produced by various sequencing techniques, we reveal the significant gain of assembly continuity by using long-read-based techniques over short-read-based methods. Moreover, LAI can facilitate iterative assembly improvement with assembler selection and identify low-quality genomic regions. To apply LAI, intact LTR-RTs and total LTR-RTs should contribute at least 0.1% and 5% to the genome size, respectively. The LAI program is freely available on GitHub: https://github.com/oushujun/LTR_retriever.