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Sweet potato [Ipomoea batatas (L.) Lam] ranks among the top seven most important food crops cultivated worldwide and is hexaploid plant (2n=6x=90) in the Convolvulaceae family with a genome size between 2,200 to 3,000 Mb. The genomic resources for this crop are deficient due to its complicated genetic structure. Here, we report the complete nucleotide sequence of the chloroplast (cp) genome of sweet potato, which is a circular molecule of 161,303 bp in the typical quadripartite structure with large (LSC) and small (SSC) single-copy regions separated by a pair of inverted repeats (IRs). The chloroplast DNA contains a total of 145 genes, including 94 protein-encoding genes of which there are 72 single-copy and 11 double-copy genes. The organization and structure of the chloroplast genome (gene content and order, IR expansion/contraction, random repeating sequences, structural rearrangement) of sweet potato were compared with those of Ipomoea (L.) species and some basal important angiosperms, respectively. Some boundary gene-flow and gene gain-and-loss events were identified at intra- and inter-species levels. In addition, by comparing with the transcriptome sequences of sweet potato, the RNA editing events and differential expressions of the chloroplast functional-genes were detected. Moreover, phylogenetic analysis was conducted based on 77 protein-coding genes from 33 taxa and the result may contribute to a better understanding of the evolution progress of the genus Ipomoea (L.), including phylogenetic relationships, intraspecific differentiation and interspecific introgression.

BACKGROUND: Transposable elements (TEs) are the most abundant genomic components in eukaryotes and affect the genome by their replications and movements to generate genetic plasticity. Sweet potato performs asexual reproduction generally and the TEs may be an important genetic factor for genome reorganization. Complete identification of TEs is essential for the study of genome evolution. However, the TEs of sweet potato are still poorly understood because of its complex hexaploid genome and difficulty in genome sequencing. The recent availability of the sweet potato transcriptome databases provides an opportunity for discovering and characterizing the expressed TEs. METHODOLOGY/PRINCIPAL FINDINGS: We first established the integrated-transcriptome database by de novo assembling four published sweet potato transcriptome databases from three cultivars in China. Using sequence-similarity search and analysis, a total of 1,405 TEs including 883 retrotransposons and 522 DNA transposons were predicted and categorized. Depending on mapping sets of RNA-Seq raw short reads to the predicted TEs, we compared the quantities, classifications and expression activities of TEs inter- and intra-cultivars. Moreover, the differential expressions of TEs in seven tissues of Xushu 18 cultivar were analyzed by using Illumina digital gene expression (DGE) tag profiling. It was found that 417 TEs were expressed in one or more tissues and 107 in all seven tissues. Furthermore, the copy number of 11 transposase genes was determined to be 1-3 copies in the genome of sweet potato by Real-time PCR-based absolute quantification. CONCLUSIONS/SIGNIFICANCE: Our result provides a new method for TE searching on species with transcriptome sequences while lacking genome information. The searching, identification and expression analysis of TEs will provide useful TE information in sweet potato, which are valuable for the further studies of TE-mediated gene mutation and optimization in asexual reproduction. It contributes to elucidating the roles of TEs in genome evolution.

Soybean isoflavone synthase (IFS) and flavanone 3-hydroxylase (F3H) are two key enzymes catalyzing the biosynthesis of isoflavonoids and flavonoids, both of which play diverse roles in stress responses. However, little is known about the evolutionary pattern of these genes in cultivated soybean and its wild progenitors. Herein, we investigated the nucleotide polymorphisms in Isoflavone synthase (IFS1, IFS2) and Flavanone 3-hydroxylase (F3H2) genes from 33 soybean accessions, including 17 cultivars (Glycine max) and 16 their wild progenitors (Glycine soja). Our data showed that the target genes shared the levels of nucleotide polymorphism with three reference genes involved in plant-microbe interactions, but possessed a much higher nucleotide polymorphism than other reference genes. Moreover, no significant genetic differentiation was found between cultivated soybean and its wild relatives in three target genes, despite of considering bottleneck and founder effect during domestication. These results indicate that IFS and F3H genes could have experienced gene introgressions or diversifying selection events during domestication process. Especially, F3H2 gene appears to evolve under positive selection and enjoy a faster evolutionary rate than IFS1 and IFS2 genes.