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

Abstract

Understanding hybridization and introgression between natural plant populations can give important insights into the origins of cultivated species. Recent studies suggest differences in ploidy might 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 L., Chrysanthemum vestitum (Hemsley) Ling and Chrysanthemum vestitum var. latifolium (Zhou & Chen), the most important wild species involved in the origins of cultivated chrysanthemums. We analyzed the 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 could play a role in the different patterns of hybridization among populations.

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

“…Our phylogenetic analyses of cp genomes revealed that all chrysanthemum cultivars formed a strongly supported clade (BS = 100, BI = 1.00), and shared 64 substitutions that were distinct from wild species of the genus Chrysanthemum ( Supplementary Data 1 ). This result is consistent with the suggestion that an extinct or un-sampled wild Chrysanthemum species served as the maternal donor of chrysanthemum cultivars ( Ma et al, 2020 ; Qi et al, 2021 ). This conclusion, however, depends on the assumption that cpDNA variations of chrysanthemum cultivars and wild species have been well represented in phylogenetic analyses.…”

“…In this study, we collected 36 accessions to represent common cultivars of different countries and different flower morphologies. Our sample size is much larger than previous studies (12 accessions; Ma et al, 2020 ; Qi et al, 2021 ), and should comprise most, if not all, of the cpDNA variations in chrysanthemum cultivars. Domesticated chrysanthemum was thought to originate from multiple species, such as C. argyrophyllum , C. indicum , C. lavandulifolium , C. nankingense , C. vestitum , and C. zawadskii ( Chen, 1985 ; Dai et al, 1998 ; Ma et al, 2020 ; Qi et al, 2021 ).…”

“…They found that chrysanthemum cultivars formed a strongly supported clade on cpDNA tree and diverged from all wild species, indicating that the maternally inherited cp genome of modern chrysanthemums might be derived from an extinct progenitor ( Ma et al, 2020 ). By sequencing two cpDNA fragments in multiple populations of wild Chrysanthemum species, Qi et al (2021) found high cpDNA variation within species and suggested that the maternal progenitor of modern chrysanthemums could be an un-sampled population of wild species. Because these studies sampled only a small number of cultivated chrysanthemums ( Ma et al, 2020 ; Qi et al, 2021 ), they may not capture all cpDNA variations in cultivars.…”

“…By sequencing two cpDNA fragments in multiple populations of wild Chrysanthemum species, Qi et al (2021) found high cpDNA variation within species and suggested that the maternal progenitor of modern chrysanthemums could be an un-sampled population of wild species. Because these studies sampled only a small number of cultivated chrysanthemums ( Ma et al, 2020 ; Qi et al, 2021 ), they may not capture all cpDNA variations in cultivars. Therefore, the maternal donor of modern chrysanthemums remains poorly understood.…”

“…Second, C. indicum is widely used for multiple purposes in Central China, where cultivated chrysanthemums most likely originated ( Chen, 1985 , 2012 ). Recently, cpDNA data has been used to investigate the origin of modern chrysanthemums ( Ma et al, 2020 ; Qi et al, 2021 ). For example, Ma et al (2020) investigated the phylogenetic relationships between chrysanthemum cultivars and wild relatives using cp genomes.…”