**6. Genetic diversity in grapefruit and pummelo**

The grapefruit (*C. paradisi* Macf.) was notified as a natural hybrid between pummelo (*Citrus maxima* (Burm.) Merr.) and sweet orange (*C. sinensis* L. Osb). It originates from Barbados in the Caribbean islands and was first named as *Citrus paradisi* Macf. by James Macfedyan in 1837 (Scora et al., 1982; Scora, 1988). Grapefruits are highly polyembryonic, therefore they are of nucellar and mutation origin. Genetic variation among common grapefruit cultivars was reported to be very low due to their mutation origin (Fang & Roose 1997; Corazza-Nunes et al.*,* 2002).

The pummelo is native to tropical and subtropical regions in Asia and has been cultivated China for over 2000 years (Corazza-Nunes et al., 2002; Yong et al., 2006). Pummelo was reported as one of the three true citrus species by Barrett and Rhodes (1976) and most of subsequent studies were in agreement with this statement (Federici et al. 1998; Nicolosi et al., 2000; Barkley et al., 2006; Uzun et al., 2009a). Pummelo has played an important role as a parent of many citrus fruits, such as lemons, oranges and grapefruits.

In recent studies genetic variation in pummelo and grapefruit were investigated. Uzun et al., (2010), was determined genetic diversity among 35 accessions of grapefruits and pummelos using ISSR markers. In that study, grapefruits and pummelos were seperated clearly and similarity value of this two species was 0.79. Besides, all pummelos were distinguished and it might be of thier zygotic origin. Same results obtained from SRAP data (Uzun et al., 2011c). At the same way, Yong et al., (2006) also seperated pummelos using SSR markers. It was concluded that pummelos were monoembryonic and there was a high level of polymorphism in the pummelos (Yong et al., 2006).

In the grapefruit group some accessions such as 'Wheeny', '*Citrus hassaku'*, 'Cocktail' and 'Oroblanco' were clearly separated from pummelos and grapefruits and nested between this two species according to various marker systems (Uzun et al., 2010; Uzun et al., 2011c). 'Wheeny' originated as a chance seedling in Australia and under heat-deficient climatic conditions in Australia and New Zealand it is a summer-maturing variety. While the fruit is grapefruit-like in most respects, the monoembryonic nature of seeds and some ofthe other characters suggest that it is probably a pummelo hybrid (Hodgson 1967). *C. hassaku* was reported as an independent species (*Citrus hassaku* Hort. Ex Tanaka) and originated as a chance seedling in Japan and its characteristics strongly suggest the pummelo-mandarin parentage with pummelo predominant (Hodgson 1967). On the other hand, *C. hassaku* was notified as a pummelo hybrid (Kahn et al., 2001). 'Cocktail' was indicated as a hybrid between 'Frua' mandarin and low acidity pummelo (Kahn et al.,*.*  2001). Another accession 'Oroblanco' was reported as a hybrid between acidless pummelo and grapefruit (Kahn et al.*,* 2001).

High level of similarity was found in grapefruit cultivars in various studies. Low level of polymorphism was detected in grapefruits and some of them were identical (Uzun et al., 2010; Uzun et al., 2011c). Fang and Roose (1997) found very low polymorphism in grapefruits based on ISSR data and notified that all grapefruits were derived from the same ancestral tree by mutation. There was no variation in grapefruits in other previous studies based on izozyme (Roose, 1988) and SSR (Luro et al., 2000) data. At the same way, Corazza-Nunes et al. (2002) detected high level of similarity in grapefruits. Most grapefruits, despite considerable variation in agronomical traits such as, rind and flesh color, fruit size, were nearly identical (Fig. 7). Cultivars with distinct morphological characters (pigmented or yellow flesh colour, seedy and seedless fruits) such as Henderson, Ruby, Duncan showed complete genetic similarity (Uzun et al., 2010).

Fig. 7. Although grapefruits have distinct fruit characters, low level of genetic variation found among them (from left to right; Davis Seedless, Shambar, Red Blush).