Comparative transcriptomes and genetic diversity assessment of two Sichuan peppers (Zanthoxylum bungeanum and Zanthoxylum armatum)

Abstract

Background: Species of Zanthoxylum (Sichuan pepper) are diverse and valuable economic trees that have been cultivated across Asia for thousands of years for their aromatic and medicinal properties. The current lack of transcriptome and EST-SSR data for these species represents a constraint with regards to identifying increasingly more diverse cultivars in China. This study was conducted to compare the transcriptome profiles of two Zanthoxylum species (Z. bungeanum and Z. armatum) and develop the EST-SSR markers to assess the genetic diversity in Sichuan pepper. Results: A total of 36.76 G high-quality clean data were screened for subsequent analysis. For Z. bungeanum and Z. armatum, 64,944 and 75,669 unigenes were obtained, respectively. After comparing different databases, we found that the highest number of unigenes (43,3198 and 49,638 for Z. bungeanum and Z. armatum, respectively) were annotated using the NR database. In addition, six pairs of EST-SSR markers were selected to determine the genetic diversity of 125 samples of three Zanthoxylum species. A population clustering dendrogram indicated that there was an average of four samples per population, with the number of allelic genes (Na) ranging from 1 to 1.4510 (average of 1.1462). The number of effective allelic genes (Ne) ranged from 1 to 1.2744 (average of 1.0971), whereas genetic diversity (Nei) ranged from 0 to 0.1486 (average of 0.05512) and Shannon genetic diversity (I) ranged from 0 to 0.2177 (average 0.08123). In individual clustering dendrograms, the 125 samples were divided into five cohorts. Cohort I contained Z. bungeanum and its varieties, cohort II included Z. armatum and its varieties, cohort III contained mostly Z. piperitum and two other Z. bungeanum samples, and cohorts IV and V each contained only two samples, with genetic distances of 0 to 1.14 between them. Conclusion: In this study, we analyzed the comparative transcriptome data of two Zanthoxylum species, for which we assessed genetic diversity using EST-SSR primers derived from their EST information. The genetic diversity of 125 samples obtained from the three Zanthoxylum species Z. bungeanum, Z. armatum and Z. piperitum was relatively low, which can probably be attributed to apomixis.