The genus Rosa comprises ≈150 species that are widely distributed in diverse climates and habitats throughout the Northern Hemisphere (Quest-Ritson and Quest-Ritson, 2003; Rehder, 1940) from subtropical to cold-temperate regions. China’s 95 species (65 endemic) account for nearly half of the world’s Rosa species (Ku and Robertson, 2003). Therefore, China is a center of distribution of Rosa as it is with numerous other cultivated plants (Brichet, 2003), and China’s roses are a great genetic resource for rose improvement.
The Xinjiang Uygur autonomous region (hereafter referred to as Xinjiang) lies at the northwestern border of China (Fig. 1). The Tianshan Mountains transverse it centrally, dividing Xinjiang into southern and northern regions, which differ greatly from each other in climate. In regard to phytogeography, Xinjiang spans the junction of the Eurasian Forest subregion, the Eurasian Steppe subregion, and the Central Asiatic Desert subregion. Many of the wild Rosa species in Xinjiang are distributed widely (Fig. 2). Although 11 Rosa species and three botanical varieties were recorded from Xinjiang in Flora of China [Ku and Robertson, 2003 (Table 1)], further research has indicated that more than 20 species occur in Xinjiang with botanical varieties that exhibit all sorts of morphological characteristics (Bao, 1993; Liu, 1993; Liu and Cong, 2000; Ma and Chen, 1990, 1991; Yu et al., 1985, 2011). These botanical varieties have been described by different investigators and are notoriously difficult to distinguish, owing to the presence of intermediate forms (Han, 1995). In addition, nomenclatural synonyms and the selection and breeding of cultivars have added to the confusion (Macphail and Kevan, 2009).
Rosa taxa in Xinjiang, northwestern China, recorded in Flora of China (Ku and Robertson, 2003) and related cytological data.
There is much interest worldwide in the wild Rosa species from China, because they contributed much of the foundation of modern cultivated roses. Wild Rosa species with important traits, such as powdery mildew (Podosphaera pannosa) resistance, large flowers and hips, and cold and drought resistance are regarded as valuable breeding materials. Modern roses are primarily compose of eight to 11 Rosa species and only a few of the species have been involved. Those other species currently are not reported (Ma and Chen, 1992; Zlesak, 2006). Introgressing additional valuable genes from these species into modern rose germplasm is inevitably a long and difficult process (Zlesak et al., 2007), because of the long generation time and the frequent cross-incompatibility and hybrid sterility between parents of different ploidy levels. Therefore, there is interest in the variation of ploidy within and among wild Rosa species. Despite the development of DNA sequence-based technology in recent years, cytological markers remain useful for plant identification, evaluation of biodiversity, and the study of plant evolution. It has long been known that some wild species, mainly diploids, represent potentially valuable genetic resources, especially for disease resistance. The wild Rosa species that have been reported are euploids with chromosome numbers ranging from 2n = 2x = 14 to 2n = 10x = 70 (Crane and Byrne, 2003; Darlington and Wylie, 1955; Jian et al., 2010a, 2012; Ma et al., 1997; Malecka et al., 1990; Malecka and Popek, 1984), and an accurate measurement of ploidy appears to be of enormous significance for cross-breeding. The various ploidy groups are not completely cut off from one another; however, the rose breeder who confines his or her hybridization to one ploidy group of species or botanical varieties will find his or her path smoothed and his or her aims more quickly achieved (Percy, 1964).
Karyotype analysis is a traditional cytogenetic first step in the comparison of genomes among related species (Crane and Byrne, 2003) under the usually correct presumption that karyotypic differences can affect meiotic chromosome pairing and hybrid fertility. Although the karyotypic variation within Rosa implies a rich genetic resource for rose breeding, it also frequently impedes breeding progress because of cross-incompatibility and hybrid sterility. Hence, elucidation of the karyotype and the physical structure of chromosomes can rationalize and accelerate the exploitation of genetic variation from wild relatives of crops (Baenziger et al., 2006). What is more, the results might be helpful in elucidation of some taxonomical problems and relations between particular species within this highly differentiated genus.
Therefore, a 3-year investigation on wild Rosa species in Xinjiang was conducted from 2009 to 2011 with two objectives: 1) to learn more details of the wild Rosa species and to compare species and botanical varieties on the basis of morphological and cytological characters; and 2) to exploit useful traits of wild Rosa species and identify candidate species for breeding strategies. In this article, the chromosome number and karyotype analysis are reported for 13 samples of seven wild Rosa taxa from different subregions of Xinjiang.
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