The genus Corylus belongs to the Betulaceae of the order Fagales. The number of species in the genus has varied depending on the authorities due to recognition of some species as a distinct species or a subspecies, or within a certain species (Rehder, 1947; Kasapligil, 1972; Mehlenbacher, 1991; Thompson et al., 1996). The commercially important European hazelnut (Corylus avellana L.) is native to most of Europe, Turkey, and the Caucasus mountains, and wild populations can be found in these areas today. Ayfer et al. (1986) stated that Turkish cultivars are usually spontaneous hybrids of C. avellana × C. maxima, although the latter was suggested as a distinct species (Mehlenbacher, 1991; Thompson et al., 1996). Centuries ago, native wild hazelnut shrubs were growing in close proximity to the mountains near the Black Sea in northern Turkey. It is believed that some Turkish cultivars originated over many centuries from selection within these local wild populations (Ayfer et al., 1986). Ozkurt (1950) hypothesized that hazelnuts were first cultivated in Turkey in Giresun province, and from there its cultivation spread to adjacent Ordu province. Hazelnuts are now grown along the Black Sea coast, where ecological conditions are favorable, and plantings today cover about 400,000 ha (Ayfer et al., 1986). Turkey is the world's leading producer and exporter of hazelnuts (Yavuz and Ercisli, 2006), accounting for about 70% of total production. Other important hazelnut-producing countries are Italy, the United States, Azerbaijan, Iran, Spain, and Georgia (Food and Agricultural Organization of the United Nations, 2007).
Hazelnuts from Turkey are classified based on the shape of the nut and quality of the kernel. The best-quality hazelnuts have a round shape, a high oil content, and superior aroma and taste. In spite of the crop's economic importance, no extensive studies have been conducted on genetic diversity in Turkish hazelnut germplasm. It appears that there are dozens of clones of each Turkish hazelnut cultivar in production, with a common name used for each group. In the past, cultivar identification studies have been based on morphological characters (Ayfer et al., 1986; Caliskan, 1995; Bostan et al., 1997; Erdogan and Mehlenbacher, 1997; Yao and Mehlenbacher, 2000; Koksal, 2002). In recent years, researchers around the globe have investigated genetic relationships among hazelnut cultivars using several types of molecular markers, including randomly amplified polymorphic DNA (RAPD) (Galderisi et al., 1999; Miaja et al., 2001), amplified fragment length polymorphism (AFLP) (Ferrari et al., 2004), and simple sequence repeat (SSR) (Boccacci et al., 2006; 2008; Gokirmak et al., 2009). Palme and Vendramin (2002) used polymerase chain reaction (PCR)-RFLP to look at chloroplast variation in wild hazelnut populations. The objective of this study was to evaluate RAPD, ISSR, and AFLP markers for identifying 18 hazelnut cultivars that are economically important in Turkey.
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