Polyploids are common among higher plants, especially flowering plants, and polyploidy is considered one of the most important mechanisms in plants for adapting to the changing environment, and stimulating the emergence of plant diversity (Ramsey & Schemske 1998; Fox et al. 2020). Polyploidization can not only produce new germplasm directly and rapidly but also overcome incompatibility among parents and the sterility of F1 progeny (Van Tuyl & Lim 2003). Cytological mechanisms of polyploid formation have been described in some plants (Session et al. 2016; Ramsey & Schemske 1998). The triploid bridge pathway, i.e., sexual polyploidization associated with unreduced gamete formation in diploid populations, is regarded as a major mechanism of plant polyploidization and speciation (Bretagnolle & Thompson 1995; Rieseberg & Willis 2007). A recent study in Brassica showed that allotriploids serve as a bridge in polyploid formation by gradually increasing chromosome numbers, with aneuploids as intermediates (Cao et al. 2023). However, the mechanism underlying polyploid formation has remained elusive, although different pathways have been proposed.
Chinese chive is a perennial herbaceous plant species belonging to the Allium genus, Liliaceae family, which has high nutritional and economic value because its leaves and inflorescences are used as edible vegetables and its seeds and leaves can be used as medicine (Tong et al. 2021). This crop is cultivated mainly in China and other Asian countries as well as many European countries (Liu et al. 2021; Jiang et al. 2021; Sharma & Gohil 2013; Xie et al. 2022). In general, Chinese chive refers to the Sect. Rhiziridium group of the Allium genus, including A. tuberosum Rottl. ex Spreng., A. ramosum L. and A. hookeri Thwaiter. Enum. and so on. However, Chinese chive in the strict sense refers to only A. tuberosum Roottl. ex Spreng. (Jiao et al. 2022), which is easily confused with A. ramosum due to the resemblance in gross morphological features including the linear leaves, rhizomatous cylindrical bulbs, shorter filaments than tepals and hemispherical white inflorescences (Sharma & Gohil 2013).
To date, A. tuberosum has mostly been considered an autotetraploid based on cytological studies (Li et al. 2020; Mathur & Tandon 1965; Yamashita et al. 2012). Other materials with different ploidy levels are not easily discovered in nature. A Triploid of A. tuberosum was discovered in Yunnan Province, China (1986). Some diploids and triploids of A. tuberosum were screened among the tetraploids’ offspring (Kojima & Nagato 1997; Yamashita et al. 2012), which have not been further examined. To date, few studies have focused on the evolutionary relationships among Chinese chive germplasms with different ploidy levels.
Karyotype analysis is an important method for determining the heredity of cells and has been used in plant germplasm exploration, classification and species evolution research. Karyotype analysis has mainly considered chromosome structure, chromosome morphology and chromosome function (Peruzzi & Eroğlu 2013). For Chinese chive, Yang and Chen (1994) reported that karyotype formula of diploid A. tuberosum is 2n = 2x = 14m + 2sm. Yang et al. (1998) discovered that the karyotype formula of diploid A. tuberosum is 2n = 2x = 14m + 2sm (2SAT), with satellites on the No. 8 chromosome pairs. However, Li and Shang (1982) reported A. ramosum as having the 2n = 2x = 14m + 2sm (2SAT) karyotype formula. Therefore, there is controversy about the karyotypes of A. tuberosum and A. ramosum based on recent studies. A triploid was discovered only in Yunnan by Yan (1986), which had the 2n = 3x = 21m + 3st (SAT) karyotype. The tetraploid of A. tuberosum was considered an autotetraploid with a karyotype formula of 2n = 4x = 28m + 4st (2SAT) (Li & Shang 1982; Yang et al. 1998). To date, few studies have focused on the evolutionary relationships among Chinese chive germplasms with different ploidies. The aim of this study was to explore the evolutionary relationships among diploid, triploid, tetraploid Chinese chive germplasms using karyotype analysis and microsporogenesis observations. This study will help us to further understand polyploidization in Chinese chive. This study will also lay foundation for the exploitation and utilization of Chinese chive resources with different ploidy levels.