Isolation and characterization of polymorphic microsatellite loci in the endangered plant Dipteronia sinensis (Sapindaceae)

Transcriptome Sequencing and Development of Genic SSR Markers of an Endangered Chinese Endemic Genus Dipteronia Oliver (Aceraceae)

Dipteronia Oliver (Aceraceae) is an endangered Chinese endemic genus consisting of two living species, Dipteronia sinensis and Dipteronia dyeriana. However, studies on the population genetics and evolutionary analyses of Dipteronia have been hindered by limited genomic resources and genetic markers. Here, the generation, de novo assembly and annotation of transcriptome datasets, and a large set of microsatellite or simple sequence repeat (SSR) markers derived from Dipteronia have been described. After Illumina pair-end sequencing, approximately 93.2 million reads were generated and assembled to yield a total of 99,358 unigenes. A majority of these unigenes (53%, 52,789) had at least one blast hit against the public protein databases. Further, 12,377 SSR loci were detected and 4179 primer pairs were designed for experimental validation. Of these 4179 primer pairs, 435 primer pairs were randomly selected to test polymorphism. Our results show that products from 132 primer pairs were polymorphic, in which 97 polymorphic SSR markers were further selected to analyze the genetic diversity of 10 natural populations of Dipteronia. The identification of SSR markers during our research will provide the much valuable data for population genetic analyses and evolutionary studies in Dipteronia.

Comparative Transcriptome and Chloroplast Genome Analyses of Two Related Dipteronia Species

Dipteronia (order Sapindales) is an endangered genus endemic to China and has two living species, D.sinensis and D. dyeriana. The plants are closely related to the genus Acer, which is also classified in the order Sapindales. Evolutionary studies on Dipteronia have been hindered by the paucity of information on their genomes and plastids. Here, we used next generation sequencing to characterize the transcriptomes and complete chloroplast genomes of both Dipteronia species. A comparison of the transcriptomes of both species identified a total of 7814 orthologs. Estimation of selection pressures using Ka/Ks ratios showed that only 30 of 5435 orthologous pairs had a ratio significantly >1, i.e., showing positive selection. However, 4041 orthologs had a Ka/Ks < 0.5 (p < 0.05), suggesting that most genes had likely undergone purifying selection. Based on orthologous unigenes, 314 single copy nuclear genes (SCNGs) were identified. Through a combination of de novo and reference guided assembly, plastid genomes were obtained; that of D. sinensis was 157,080 bp and that of D. dyeriana was 157,071 bp. Both plastid genomes encoded 87 protein coding genes, 40 tRNAs, and 8 rRNAs; no significant differences were detected in the size, gene content, and organization of the two plastomes. We used the whole chloroplast genomes to determine the phylogeny of D. sinensis and D. dyeriana and confirmed that the two species were highly divergent. Overall, our study provides comprehensive transcriptomic and chloroplast genomic resources, which will be valuable for future evolutionary studies of Dipteronia.