The sand pear (Pyrus pyrifolia Nakai) is one of the most important fruit tree crops in China and is extensively cultivated in central and southwest China. The species occurs naturally in southern and western China, recognized as the center of origin of the genus Pyrus (Rubtsov, 1944). There are very many landraces (local cultivars) of P. pyrifolia owing to nearly 3000 years of cultivation and the complex climatic and geographical variation in China. Many landraces have unique traits. For example, ‘Puguali’, from Zhejiang Province, is a large-fruited cultivar with a mean weight of 553 g and a maximum weight of 950 g. The skin is green when mature and covered with brown russet, which turns reddish brown when fruit are fully ripe. ‘Cangxili’, named after its place of origin, Cangxi county in Sichuan Province, is a traditional landrace with maximum fruit weight of 1850 g (average, 321.3 g), smooth skin, and crisp and tender flesh, which is sweet and juicy and of high quality. The rich genetic resources in sand pear provide great potential for cultivar improvement and enhancement of the sustainability of the pear industry. However, many traditional local cultivars have been threatened with extinction by the changes that have occurred in the modern Chinese fruit industry over the past three decades. There has been large-scale cultivation of a few elite cultivars and top-grafting or replacement of old cultivars or landraces. This genetic loss could lead to serious erosion of the gene pool of the cultivated sand pear. To conserve and manage the diversity of sand pear landraces and cultivars, the Wuhan Sand Pear Germplasm Repository (WSPGR) was established in 1986 as the national repository for sand pears, and since then, an exhaustive collection of local cultivars and landraces of Chinese sand pear has been assembled.
Molecular techniques are useful tools for evaluating genetic diversity and for defining genetic relationships in fruit tree crops. In pear, chloroplast polymerase chain reaction–restriction fragment length polymorphisms (RFLPs) were used to examine relationships between east Asian species (Iketani et al., 1998). Dominant nuclear markers, random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP), and intersimple sequence repeat (ISSR) were used in an investigation of genetic relationships among species and for pear cultivar fingerprinting (Cao et al., 2007; Shen et al., 2006; Teng et al., 2001, 2002; Zhang et al., 2007), and nuclear and chloroplast DNA sequences have been used to identify pear cultivars (Kimura et al., 2003; Lee et al., 2004). Of the DNA marker systems currently available, simple sequence repeats (SSRs) have been considered one of the most useful for assessment of genetic diversity and cultivar fingerprinting at the intraspecific level because of their abundance, hyperpolymorphism, and codominant inheritance (Morgante and Olivieri, 1993; Tautz, 1989). However, the published work to date reported the use of a set of SSRs isolated from apple for verifying the transferability of SSRs between apple and pear (Yamamoto et al., 2001) and the cultivar identification by SSRs in a limited number of cultivars developed in Japan (Kimura et al., 2002). SSR markers have been proved as a robust tool for revealing genetic diversity in sand pear (Cao et al., 2007; Kimura et al., 2002, 2003), red-skin sand pear (Zhang et al., 2007), and some other pears in west China (Fan et al., 2007).
Current germplasm evaluation in China mostly focuses on morphological descriptions and documenting pomological traits. The exchange of plants between repositories or commercial orchards raises problems in that some individual sand pear landraces or cultivars may be known by several different names or the one name may be used for different landraces or cultivars. The information from current evaluation of sand pears was not sufficient. Furthermore, detailed morphological descriptions and comparisons of plants are time-consuming and fruit-related traits cannot be observed until plants are mature to produce fruit. Genetic characterization of the gene pool of the cultivated sand pears in the WSPGR collection has not previously been attempted but is urgently needed for formulating management strategies for the WSPGR and for furnishing useful genetic information for future sand pear breeding efforts. This should provide a better understanding of the genetic diversity that exists in the gene pool of the cultivated sand pears and the diverse sources of useful genes in the germplasm repository. Therefore, the objectives of the present investigation were to determine the genetic diversity of the overall gene pool of sand pear landraces and assess the genetic variation among sand pear landrace groups in relation to their geographical distribution in China.
Allard, R.W. 1988 Genetic changes associated with the evolution of adaptedness in cultivated plants and their wild progenitors J. Hered. 79 225 238
Cao, Y.F., Liu, F.Z., Gao, Y., Jiang, L.J., Wang, K., Ma, Z.Y. & Zhang, K.C. 2007 SSR analysis of genetic diversity of pear cultivars Acta Hort. Sinica 34 305 310
Doyle, J.J. & Doyle, J.L. 1987 A rapid DNA isolation procedure for small quantities of fresh leaf tissue Phytochemical Bulletin 19 11 15
Excoffier, L., Smouse, P.E. & Quattro, J.M. 1992 Analysis of molecular variance inferred from metric distances among DNA haplotypes: Application to human mitochondrial DNA restriction sites Genetics 131 479 491
Fan, T.W., Cai, D.Y., Li, H.X., Wang, F.L., Zhao, C.Z. & Teng, Y.W. 2007 Simple sequence repeat (SSR) analysis for assessment of genetic variation and relationships in pear germplasm native to the middle area of Gansu province J. Fruit Science 24 268 275
Gianfranceschi, L., Seglias, N. & Tarchini, R. 1998 Simple sequence repeats for the genetic analysis of apple Theor. Appl. Genet. 96 1069 1076
Hamrick, J.L., Godt, M.J.W. & Sherman-Broyles, S.L. 1992 Factors influencing levels of genetic diversity in woody plant species New For. 6 95 124
Iketani, H., Manabe, T., Matsuta, N., Akihama, T. & Hayashi, T. 1998 Incongruence between RFLPs of chloroplast DNA and morphological classification in east Asian pear (Pyrus spp.) Gene. Resour. Crop Ev. 45 533 539
Kimura, T., Iketani, H., Kotobuki, K., Matsutan, N., Ban, Y., Hayashi, T. & Yamamoto, T. 2003 Genetic characterization of pear varieties revealed by chloroplast DNA sequences J. Hort. Sci. Biotechnol. 78 241 247
Kimura, T., Yong, Z.S., Shoda, M., Kotobuki, K., Matsuta, N., Hayashi, T., Ban, Y. & Yamamoto, T. 2002 Identification of Asian pear varieties by SSR analysis Breed. Sci. 52 115 121
Kloosterman, A.D., Budowle, B. & Daselaar, P. 1993 PCR amplification and detection of the human D1S80 VNTR locus amplification conditions, population genetics and application in forensic analysis. Intl J. Legal Med. 105 257 264
Lee, G.P., Lee, C.H. & Kim, C.S. 2004 Molecular markers derived from RAPD, SCAR, and the conserved 18S rDNA sequences for classification and identification in Pyrus pyrifolia and P. communis Theor. Appl. Genet. 108 1487 1491
Peakall, R. & Smouse, P.E. 2001 GenAlEx V5.1: Genetic analysis in Excel. Population genetic software for teaching and research Australian National University Canberra, Australia <http://www.anu.edu.au/BoZo/GenAlEx/>.
Sanguinetti, C.J., Dias, N.E. & Simpson, A.J.G. 1994 Rapid silver staining and recovery of PCR products separated on polyacrylamide gels Biotechniques 17 915 919
Shen, Y.Y., Teng, Y.W. & Tanabe, K. 2006 RAPD analysis for genetic assessment of some cultivars of Pyrus pyrifolia derived from China and Japan Acta Hort. Sinica. 33 621 624
Teng, Y.W., Tanabe, K., Tamura, F. & Itai, A. 2001 Genetic relationships of pear cultivars in Xinjiang, China as measured by RAPD markers J. Hort. Sci. Biotechnol. 76 771 779
Teng, Y.W., Tanabe, K., Tamura, F. & Itai, A. 2002 Genetic relationships of Pyrus species and cultivars native to East Asia revealed by randomly amplified polymorphic DNA markers J. Amer. Soc. Hort. Sci. 127 262 270
Yamamoto, T., Kimura, T., Sawamura, Y., Kotobuki, K., Ban, Y., Hayashi, T. & Matsuta, N. 2001 SSRs isolated from apple can identify polymorphism and genetic diversity in pear Theor. Appl. Genet. 102 865 870
Yamamoto, T., Kimura, T., Sawamura, Y., Manabe, T., Kotobuki, K., Hayashi, T., Ban, Y. & Matsuta, N. 2002a Simple sequence repeats for genetic analysis in pear Euphytica 124 129 137
Yamamoto, T., Kimura, T., Shoda, M., Ban, Y., Hayashi, T. & Matsuta, N. 2002b Development of microsatellite markers in the Japanese pear (Pyrus pyrifolia Nakai) Mol. Ecol. Notes 2 14 16
Zhang, D., Shu, Q., Teng, Y.W., Qiu, M.H., Bao, L. & Hu, H.J. 2007 Simple sequence repeat analysis on genetic assessment of Chinese red skinned sand pear cultivars. Acta Hort Sinica 34 47 52