667 J. AMER. SOC. HORT. SCI. 128(5):667–671. 2003. J. AMER. SOC. HORT. SCI. 128(5):667–671. 2003. Relationships among Peach, Almond, and Related Species as Detected by Simple Sequence Repeat Markers P. Martínez-Gómez, S. Arulsekar, D. Potter, and T.M. Gradziel 1 Department of Pomology, University of California, Davis, CA 95616 ADDITIONAL INDEX WORDS. Prunus spp., cluster analysis, phylogenetic relations, breeding ABSTRACT. The genetic relationships among peach [Prunus persica (L.) Batsch], almond [P. dulcis (Mill.) D.A. Webb or P. amygdalus (L.) Batsch] and 10 related Prunus species within the subgenus Amygdalus were investigated using simple sequence repeat (SSR) markers. P. glandulosa Pall. was included as an outgroup. Polymorphic alleles were scored as present or absent for each accession. The number of alleles revealed by the SSR analysis in peach and almond cultivars ranged from one to three whereas related Prunus species showed a range of one to 10 alleles. Results demonstrated an extensive genetic variability within this readily intercrossed germplasm as well as the value of SSR markers developed in one species of Prunus for the characterization of related species. Mean character difference distances were calculated for all pairwise comparisons and were used to construct an unrooted dendogram depicting the phenetic relationships among species. Four main groups were distinguished. Peach cultivars clustered with accessions of P. davidiana (Carr.) Franch. and P. mira Koehne. The second group contained almond cultivars. A third group included accessions of P. argentea (Lam) Rehd., P. bucharica Korschinsky, P. kuramica Korschinsky, P. pedunculata Pall, P. petunikowii Lits., P. tangutica (Spach) Batal., and P. webbii (Spach) Vieh.. P. glandulosa and P. scoparia Batal. were included in a fourth group. Peach (Prunus persica) and almond (P. dulcis) syn. (P. amyg- dalus) are two species of genus Prunus subgenus Amygdalus (Rosaceae, subfamily Prunoideae) that are commercially grown worldwide. These species originated in Southeastern and Central Asia respectively, and represent divergent evolution under two distinct environments, being warmer and more humid in the case of peach, and colder and xerophytic for almond (Watkins, 1976). Related Prunus species are found growing wild from eastern China to the mountainous areas and deserts of western China, Kurdistan, Turkestan, Afghanistan and Iran (Browick and Zohary, 1996; Faust and Timon, 1995; Grasselly, 1976; Hesse, 1975; Kester and Gradziel, 1996; Kester et al., 1991; Scorza and Sherman, 1996). The direct use of these related Prunus species as a rootstock for peach and almond, mainly under non-irrigated native conditions, has been reported by several authors (Denisov, 1988; Grasselly, 1976; Hesse, 1975). Interspecific crosses (peach x almond, peach x P. davidiana, and P. webbii x peach) have also been used as peach and almond rootstocks (Berhnard, 1949; Brooks and Olmo, 1982; Kester and Hansen, 1966). Hesse (1975) and Scorza and Sherman (1996) suggested the value of closely related Prunus species in peach breeding. Related species have also been reported as having potential in almond breeding to improve the quality of kernels and as sources of self-compatibility (Gradziel and Kester, 1998; Gradziel et al., 2001; Kester and Gradziel, 1996; Kester et al., 1991). Studies of germplasm diversity and genetic relationships can be used to assess the value of these species in cultivar develop- ment. Early studies of Prunus involved isozymes (Mowrey and Werner, 1990) and restriction fragment analysis of chloroplast DNA (Badenes and Parfitt, 1995; Uematsu et al., 1991). Recent studies analysed variation in DNA sequences of internal transcribed spacers (ITS) in nuclear ribosomal DNA (Lee and Wen, 2001) and chloroplast DNA (Bortiri et al., 2001). Analysis was from a taxonomic perspective, assaying a wide range of species. Received for publication 12 Aug. 2002. Accepted for publication 18 Mar. 2003. 1 Corresponding author; e-mail tmgradziel@ucdavis.edu. Simple sequence repeat (SSR) markers (microsatellites) are characterized by high polymorphism and abundance, with co- dominant inheritance, and are often transferable across closely related species (Gupta et al., 1996). These molecular markers are ideal for assessing genetic variability in related species and understanding the genetic relationships among them (Westman and Kresovich, 1997). Recently, SSR primers generated in dif- ferent Prunus species have been reported (Aranzana et al., 2002; Cantini et al., 2001; Cipriani et al., 1999; Dirlewanger et al., 2002; Downey and Iezzoni, 2000; Joobeur et al., 2000; Sosinski et al., 2000; Wang et al., 2002 ). These include highly informative markers that are required for accurate estimation of the amount and nature of genetic variability. The objective of this research was to establish the genetic re- lationships among peach, almond and 10 related Prunus species using SSR markers, possibly facilitating their use in interspecific introgression and cultivar improvement. Materials and Methods PLANT MATERIAL AND DNA ISOLATION. Evaluated germplasm included six peach cultivars (Chinese Cling, Bailey, Fay Elberta, Halford, Siberian C, and Tzim Pee Tao) and five almond cultivars (Ferragnes, Garden Prince, Mission, Ne Plus Ultra and Nonpareil), which are representative of the diversity in these species (Table 1). Two accession of each of 10 related Prunus species within the subgenus Amygdalus (P. argentea, P. bucharica, P. davidiana, P. kuramica, P. mira, P. pedunculata, P. petunikowii, P. scoparia, P. tangutica, and P. webbii) were also included. An accession of P. glandulosa was included as an outgroup for statistical analysis (Table 2). Total genomic DNA was isolated using the procedure described by Gepts and Clegg (1989). DNA was quantified using a fluorometer DyNAQuant 200 (Amershan-Pharmacia, Piscataway, N.J.). SSR ANALYSIS. Eighteen SSR markers developed in sweet cherry or peach (Table 3) were screened for DNA polymorphism. The primers were synthesized by Gibco-BRL (Gibco BRL, Carlsbad,