and stone fruits). In many fruit crops, fruit color mutations (sports) have become increasingly important, especially in apple, pear, and grapefruit. Many of these mutations are not heritable because they do not occur in the appropriate meristematic layer. B. Interspecific Hybridization and Polyploidization Many of our fruit crops have resulted from interspecific hybridization, polyploidy, or both (Table 4). This is particularly obvious in Actinidia , Citrus, Fragaria, Musa, Prunus, Rubus, and Vaccinium. The evolutionary divergence within these genera into different “species” is often associated with polyploidy. These changes represent the divergence of interbreeding populations that became isolated, known as nominalistic species (Spooner et al. 2003). Domestication within these groups and subsequent transfer by human migration would facilitate intercrosses. This development has been well worked out with bread wheat ( Tritcum vulagare ), a hexaploid amphidiploid of the genomic constitution AABBDD. Genomic analysis has identified the three
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