The Asteraceae (Compositae), a large plant family of approximately 24 000-35 000 species, accounts for $10% of all angiosperm species and contributes a lot to plant diversity. The most representative members of the Asteraceae are the economically important chrysanthemums (Chrysanthemum L.) that diversified through reticulate evolution. Biodiversity is typically created by multiple evolutionary mechanisms such as wholegenome duplication (WGD) or polyploidization and locally repetitive genome expansion. However, the lack of genomic data from chrysanthemum species has prevented an in-depth analysis of the evolutionary mechanisms involved in their diversification. Here, we used Oxford Nanopore long-read technology to sequence the diploid Chrysanthemum nankingense genome, which represents one of the progenitor genomes of domesticated chrysanthemums. Our analysis revealed that the evolution of the C. nankingense genome was driven by bursts of repetitive element expansion and WGD events including a recent WGD that distinguishes chrysanthemum from sunflower, which diverged from chrysanthemum approximately 38.8 million years ago. Variations of ornamental and medicinal traits in chrysanthemums are linked to the expansion of candidate gene families by duplication events including paralogous gene duplication. Collectively, our study of the assembled reference genome offers new knowledge and resources to dissect the history and pattern of evolution and diversification of chrysanthemum plants, and also to accelerate their breeding and improvement.

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BackgroundSimple sequence repeats (SSRs) are ubiquitous in eukaryotic genomes. Chrysanthemum is one of the largest genera in the Asteraceae family. Only few Chrysanthemum expressed sequence tag (EST) sequences have been acquired to date, so the number of available EST-SSR markers is very low.Methodology/Principal FindingsIllumina paired-end sequencing technology produced over 53 million sequencing reads from C. nankingense mRNA. The subsequent de novo assembly yielded 70,895 unigenes, of which 45,789 (64.59%) unigenes showed similarity to the sequences in NCBI database. Out of 45,789 sequences, 107 have hits to the Chrysanthemum Nr protein database; 679 and 277 sequences have hits to the database of Helianthus and Lactuca species, respectively. MISA software identified a large number of putative EST-SSRs, allowing 1,788 primer pairs to be designed from the de novo transcriptome sequence and a further 363 from archival EST sequence. Among 100 primer pairs randomly chosen, 81 markers have amplicons and 20 are polymorphic for genotypes analysis in Chrysanthemum. The results showed that most (but not all) of the assays were transferable across species and that they exposed a significant amount of allelic diversity.Conclusions/SignificanceSSR markers acquired by transcriptome sequencing are potentially useful for marker-assisted breeding and genetic analysis in the genus Chrysanthemum and its related genera.

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Quantitative real-time PCR (RT-qPCR) is a reliable method for assessing gene expression, provided that suitable reference genes are included to normalize the data. The stability of expression of eight potential reference genes, namely, tubulin (alpha-2,4 tubulin), actin, EF1 α (elongation factor 1 α), UBC (ubiquitin C), GAPDH (glyceraldehyde-3-phosphate dehydrogenase), psaA (photosynthesis-related plastid gene representing photosystem I), PP2Acs (catalytic subunit of protein phosphatase 2A), and PGK (phosphoglycerate kinase), was assessed in chrysanthemum plants subjected to aphid infestation, heat stress or waterlogging stress using geNorm software. The widely used reference gene EF1 α performed well for aphid infested plants but poorly for waterlogged ones. The catalytic subunit of protein phosphatase 2A (PP2Acs) was the best performing one during heat and waterlogging stress, but was the worst during aphid infestation. The commonly used reference gene actin was generally the least stable of the set. No single gene was suitable for normalization on its own. The choice of reference gene(s) is an important factor in gene expression studies based on RT-qPCR.