Almost all plants in the genus Populus are dioecious (i.e. trees are either male or female), but it is unknown whether dioecy evolved in a common ancestor or independently in different subgenera. Here, we sequence the small peritelomeric X- and Y-linked regions of P. deltoides chromosome XIX. Two genes are present only in the Y-linked region. One is a duplication of a non-Y-linked, female-specifically expressed response regulator, which produces siRNAs that block this gene’s expression, repressing femaleness. The other is an LTR/Gypsy transposable element family member, which generates long non-coding RNAs. Overexpression of this gene in A. thaliana promotes androecium development. We also find both genes in the sex-determining region of P. simonii, a different poplar subgenus, which suggests that they are both stable components of poplar sex-determining systems. By contrast, only the duplicated response regulator gene is present in the sex-linked regions of P. davidiana and P. tremula. Therefore, findings in our study suggest dioecy may have evolved independently in different poplar subgenera.

Although organellar genomes (including chloroplast and mitochondrial genomes) are smaller than nuclear genomes in size and gene number, organellar genomes are very important for the investigation of plant evolution and molecular ecology mechanisms. Few studies have focused on the organellar genomes of horticultural plants. Approximately 1193 chloroplast genomes and 199 mitochondrial genomes of land plants are available in the National Center for Biotechnology Information (NCBI), of which only 39 are from horticultural plants. In this paper, we report an innovative and efficient method for high-quality horticultural organellar genome assembly from next-generation sequencing (NGS) data. Sequencing reads were first assembled by Newbler, Amos, and Minimus software with default parameters. The remaining gaps were then filled through BLASTN search and PCR. The complete DNA sequence was corrected based on Illumina sequencing data using BWA (Burrows–Wheeler Alignment tool) software. The advantage of this approach is that there is no need to isolate organellar DNA from total DNA during sample preparation. Using this procedure, the complete mitochondrial and chloroplast genomes of an ornamental plant, Salix suchowensis, and a fruit tree, Ziziphus jujuba, were identified. This study shows that horticultural plants have similar mitochondrial and chloroplast sequence organization to other seed plants. Most horticultural plants demonstrate a slight bias toward A+T rich features in the mitochondrial genome. In addition, a phylogenetic analysis of 39 horticultural plants based on 15 protein-coding genes showed that some mitochondrial genes are horizontally transferred from chloroplast DNA. Our study will provide an important reference for organellar genome assembly in other horticultural plants. Furthermore, phylogenetic analysis of the organellar genomes of horticultural plants could accurately clarify the unanticipated relationships among these plants.

Background. In order to understand the colocalization of genetic loci amongst species, synteny and collinearity analysis is a frequent task in comparative genomics research. However many analysis software packages are not effective in visualizing results. Problems include lack of graphic visualization, simple representation, or inextensible format of outputs. Moreover, higher throughput sequencing technology requires higher resolution image output. Implementation. To fill this gap, this paper publishes VGSC, the Vector Graph toolkit of genome Synteny and Collinearity, and its online service, to visualize the synteny and collinearity in the common graphical format, including both raster (JPEG, Bitmap, and PNG) and vector graphic (SVG, EPS, and PDF). Result. Users can upload sequence alignments from blast and collinearity relationship from the synteny analysis tools. The website can generate the vector or raster graphical results automatically. We also provide a java-based bytecode binary to enable the command-line execution.

Salix suchowensis is an early-flowering shrub willow that provides a desirable system for studies on the basic biology of woody plants. The current reference genome of S. suchowensis was assembled with 454 sequencing reads. Here, we report a chromosome-scale assembly of S. suchowensis generated by combining PacBio sequencing with Hi-C technologies. The obtained genome assemblies covered a total length of 356 Mb. The contig N50 of these assemblies was 263,908 bp, which was~65-fold higher than that reported previously. The contiguity and completeness of the genome were significantly improved. By applying Hi-C data, 339.67 Mb (95.29%) of the assembled sequences were allocated to the 19 chromosomes of haploid willow. With the chromosome-scale assembly, we revealed a series of major chromosomal fissions and fusions that explain the genome divergence between the sister genera of Salix and Populus. The more complete and accurate willow reference genome obtained in this study provides a fundamental resource for studying many genetic and genomic characteristics of woody plants.

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