Herbaceous peonies are perennial flowers belonging to section Paeonia of the genus Paeonia in the Paeoniaceae. Herbaceous peonies usually blossom from April to June and have a variety of flower patterns and flower colors, making them famous traditional flowers in China and offering them an extremely important position in the global cut-flower market (Yang et al., 2015; Yu et al., 2011). According to a new classification, section Paeonia has 25 species and more than 1000 cultivars around the world (Hong et al., 2010; Sang et al., 2004). The American Peony Society has divided herbaceous peony into three groups: lactiflora, hybrid, and Itoh. The LG uses Paeonia lactiflora as the parent (2n = 2x = 10) (Hong et al., 2010), and the hybrid group (HG) uses P. lactiflora, P. officinalis, P. macrophylla, and other species as the parents, whereas members of the Itoh group (IG) are the product of distant hybridization in which peony (section Paeonia) served as the female parent and species or cultivars of tree peony (section Moutan) served as the male parent (Qin, 2004; Yang and Yu, 2016).
Herbaceous peonies have rich germplasm diversity in terms of external morphology and chromosome ploidy (Hao et al., 2016; Ji et al., 2014). Species and cultivars have been used in the hybridization of different cultivar groups resulting in complex genetic backgrounds. Members of the LG are all diploid (Hong et al., 2010). The HG consists of a variety of species involved in hybridization. There are many natural tetraploids such as P. officinalis (2n = 4x = 20) and P. macrophylla (2n = 4x = 20). Cultivars of the HG usually have either of these species as their parents, so most cultivars in this group are triploid or tetraploid (Sang et al., 2004). Cultivars of the IG have characters that are intermediate to herbaceous and tree peony (Wu et al., 2011).
Much cytological work has been carried out on Paeonia. Dark (1936) found that the basic chromosome number was x = 5. Ding and Liu (1991), Hong et al. (1988), La Cour (1952), and Yang and Zhu (1989) studied the ploidy of P. mairei and P. obovata, finding that diploid (2n = 2x = 10) and tetraploid (2n = 4x = 20) species exist in China. Hong et al. (1988) and Sang et al. (2004) found that P. veitchii and P. anomala are both diploid (2n = 2x = 10). According to Zhang and Shao (2000), P. lactiflora and P. obovata are diploid (2n = 2x = 10), and Li and Xu (1990) found that ‘Huang Jin Lun’ and ‘He Bao’, two cultivars of the LG, are diploid (2n = 2x = 10). Few researchers have focused on the study of different peony cultivar groups. Only Ma and Yu (2013) reported two triploid (2n = 3x = 15) cultivars, Buckeye Belle and Red Red Rose, which belong to the HG.
There is some research on the karyotype of peony and tree peony species or cultivars. When the arm ratio is between 0.1 and 0.5, using the ratio of the longest chromosome to the shortest one, the karyotype of a cultivar can be judged. When the ratio is less than 2:1, it is referred to as 2A, but when the ratio is between 2:1 and 4:1, it is referred to as 2B (Stebbins, 1971). The karyotype of peony is 2n = 2x = 6m + 2sm + 2st; i.e., most peonies belong to 2A (Sang et al., 2004), whereas only Hong et al. (1988) and Liu (2016) found 2B among a population of P. obovata and a hybrid of ‘Zhu Sha Pan’ and ‘Cream Delight’, respectively. All tree peony species are diploid (Zhao, 2014), but Li and Zhang (1982) found that one cultivar, Shou’An Hong, is triploid (2n = 3x = 15). The karyotype of tree peony is usually 2n = 2x = 10 = 6m + 2sm + 2st, 2n = 2x = 10 = 8m + 2st, 2n = 2x = 10 = 8m + 2sm, or 2n = 2x = 10 = 5m + 2sm + 3st but all are 2A (Hou et al., 2006).
Karyotype analysis, which is the comparison of chromosome measurements, is a traditional cytogenetic step to compare genomes among related species. In this study, a karyotypic analysis of the chromosomes of 21 non-Chinese cultivars of herbaceous peony is reported. The characteristics and variation in chromosome number and karyotype have also been analyzed. Cytological data of these cultivars will allow better guidance of future breeding work.
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