Chrysanthemum (Chrysanthemum morifolium Ramat) is an important floricultural crop and medicinal herb. Modern chrysanthemum cultivars have complex genetic backgrounds because of multiple cycles of hybridization, polyploidization, and prolonged cultivation. Understanding the genetic background and hybrid origin of modern chrysanthemum cultivars can provide pivotal information for chrysanthemum genetic improvement and breeding. By now, the origin of cultivated chrysanthemums remains unclear. In this study, 36 common chrysanthemum cultivars from across the world and multiple wild relatives were studied to identify the maternal donor of modern chrysanthemum. Chloroplast (cp) genomes of chrysanthemum cultivars were assembled and compared with those of the wild relatives. The structure of cp genomes was highly conserved among cultivars and wild relatives. Phylogenetic analyses based on the assembled cp genomes showed that all chrysanthemum cultivars grouped together and shared 64 substitutions that were distinct from those of their wild relatives. These results indicated that a diverged lineage of the genus Chrysanthemum, which was most likely an extinct or un-sampled species/population, provided a maternal source for modern cultivars. These findings provide important insights into the origin of chrysanthemum cultivars, and a source of valuable genetic markers for chrysanthemum breeding programs.

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The Maternal Donor of Chrysanthemum Cultivars Revealed by Comparative Analysis of the Chloroplast Genome

Garden chrysanthemums are a group of chrysanthemums (Chrysanthemum morifolium) that are mostly used in garden landscape collocations. Because most garden chrysanthemum varieties have low plants and poor space effects, they have certain limitations in garden landscape applications. In this study, we selected seven garden chrysanthemum varieties with excellent characteristics for artificial hybridization and intercross parentage to obtain new varieties with good traits we need. The phenotypic characteristics of the F1 offspring in terms of plant height, crown width, crown height ratio, number of main branches, branching intensity, plant form, inflorescence diameter, number of flowers per plant, number of ray florets, flower color, and days from planting period to coloring period were analyzed and systematically evaluated. Subsequently, a comprehensive evaluation system was established using the analytic hierarchy process (AHP) and k-means clustering method. We built a comprehensive analysis model, calculated the weighting of each evaluation factor, and multiplied the weight value by the score of the evaluation index standard of each factor to obtain the comprehensive score of each plant. All F1 plants were divided into four grades: excellent grade accounting for 13%, good grade accounting for 28%, medium grade accounting for 36%, and poor grade accounting for 23%. Then, we analyzed the differences of some quantitative traits between Group E (hybrids in excellent grade) and Group O (hybrids in good, medium, and poor grades). There were significant differences in plant height, crown height ratio, and the number of main branches but no significant difference in crown width. Combining with comprehensive score showed that Group E performed well overall. Finally, we selected five hybrid offspring with the highest overall scores in Group E as excellent variety materials of garden chrysanthemum for hedgerow. They were CH22, YQ73, HY08, CQ80, and HY07, respectively. We also found that plant height, lodging resistance, crown height ratio, plant form, crown width, and the number of main branches could be the main indicators in the AHP, which can be effectively applied to the comprehensive evaluation and breeding of garden chrysanthemums for hedgerows.

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Germplasm Innovation and Establishment of Comprehensive Evaluation System for Hedgerow Garden Chrysanthemum

Abstract Premise Strong postzygotic reproductive isolating barriers are usually expected to limit the extent of natural hybridization between species with contrasting ploidy. However, genomic sequencing has revealed previously overlooked examples of natural cross‐ploidy hybridization in some flowering plant genera, suggesting that the phenomenon may be more common than once thought. We investigated potential cross‐ploidy hybridization in British eyebrights (Euphrasia, Orobanchaceae), a group from which 13 putative cross‐ploidy hybrid combinations have been reported based on morphology. Methods We analyzed a contact zone between diploid Euphrasia rostkoviana and tetraploid E. arctica in Wales. We sequenced part of the internal transcribed spacer (ITS) of nuclear ribosomal DNA and used genotyping by sequencing (GBS) to look for evidence of cross‐ploidy hybridization and introgression. Results Common variant sites in the ITS region were fixed between diploids and tetraploids, indicating a strong barrier to hybridization. Clustering analyses of 356 single‐nucleotide polymorphisms (SNPs) generated using GBS clearly separated samples by ploidy and revealed strong genetic structure (F ST = 0.44). However, the F ST distribution across all SNPs was bimodal, indicating potential differential selection on loci between diploids and tetraploids. Demographic inference suggested potential gene flow, limited to around one or fewer migrants per generation. Conclusions Our results suggest that recent cross‐ploidy hybridization is rare or absent in a site of secondary contact in Euphrasia. While a strong ploidy barrier prevents hybridization over ecological timescales, such hybrids may form in stable populations over evolutionary timescales, potentially allowing cross‐ploidy introgression to take place.

Is there hybridization between diploid and tetraploid Euphrasia in a secondary contact zone?

GDP-mannose 3, 5-epimerase (GME, EC 5.1.3.18), a key enzyme in the ascorbic acid synthesis pathway, catalyzes the conversion of GDP-D-mannose to GDP-l-galactose in higher plants. Here, a homolog of GME was isolated from Chrysanthemum vestitum. The cDNA sequence of CvGME was 1131 bp and contained a complete open reading frame encoding a protein comprising 376 amino acids. Quantitative real-time PCR analysis revealed that CvGME was most highly expressed in the stems and roots. Phylogenetic analysis showed that CvGME was closely related to LsGME from Lactuca sativa. Subcellular localization studies revealed that CvGME was localized in the nucleus. Heterologous expression of CvGME in transgenic tobacco plants increased the ascorbic acid content in the leaves. In addition, overexpression of CvGME reduced the malondialdehyde content and increased superoxide dismutase and peroxidase activity in tobacco leaves compared to those in the wild-type plants under drought stress conditions, explaining the increased drought tolerance of transgenic tobacco lines. These results suggest that CvGME can effectively enhance the tolerance of plants to drought by increasing the ascorbic acid content, which may help improve the drought tolerance of chrysanthemums through molecular breeding. 

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Molecular cloning and functional analysis of a Chrysanthemum vestitum GME homolog that enhances drought tolerance in transgenic tobacco

Chrysanthemum (Chrysanthemum morifolium Ramat.) is a globally important ornamental plant with great economic, cultural, and symbolic value. However, research on chrysanthemum is challenging due to its complex genetic background. Here, we report a near-complete assembly and annotation for C. morifolium comprising 27 pseudochromosomes (8.15 Gb; scaffold N50 of 303.69 Mb). Comparative and evolutionary analyses reveal a whole-genome triplication (WGT) event shared by Chrysanthemum species approximately 6 million years ago (Mya) and the possible lineage-specific polyploidization of C. morifolium approximately 3 Mya. Multilevel evidence suggests that C. morifolium is likely a segmental allopolyploid. Furthermore, a combination of genomics and transcriptomics approaches demonstrate the C. morifolium genome can be used to identify genes underlying key ornamental traits. Phylogenetic analysis of CmCCD4a traces the flower colour breeding history of cultivated chrysanthemum. Genomic resources generated from this study could help to accelerate chrysanthemum genetic improvement. 

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Analyses of a chromosome-scale genome assembly reveal the origin and evolution of cultivated chrysanthemum

Understanding hybridization and introgression between natural plant populations can give important insights into the origins of cultivated species. Recent studies suggest differences in ploidy may not create such strong reproductive barriers as once thought, and thus studies into cultivated origins should examine all co-occurring taxa, including those with contrasting ploidy levels. Here, we characterized hybridization between Chrysanthemum indicum, Chrysanthemum vestitum and Chrysanthemum vestitum var. latifolium, the most important wild species involved in the origins of cultivated chrysanthemums. We analysed population structure of 317 Chrysanthemum accessions based on 13 microsatellite markers and sequenced chloroplast trnL-trnF for a subset of 103 Chrysanthemum accessions. We identified three distinct genetic clusters, corresponding to the three taxa. We detected 20 hybrids between species of different ploidy levels, of which 19 were between C. indicum (4x) and C. vestitum (6x) and one was between C. indicum and C. vestitum var. latifolium (6x). Fourteen hybrids between C. indicum and C. vestitum were from one of the five study sites. Chrysanthemum vestitum and C. vestitum var. latifolium share only one chloroplast haplotype. The substantially different number of hybrids between hybridizing species was likely due to different levels of reproductive isolation coupled with environmental selection against hybrids. In addition, human activities may play a role in the different patterns of hybridization among populations.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/jse.12810

Natural interploidy hybridization among the key taxa involved in the origin of horticultural chrysanthemums

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Shuai Qi

Papers

Natural interploidy hybridization among the key taxa involved in the origin of horticultural chrysanthemums

Natural interploidy hybridization among the key taxa involved in the origin of horticultural chrysanthemums