The genus Prunus L. mainly occurs in the temperate regions of the Northern Hemisphere with some extensions into the Southern Hemisphere in both the Old and New Worlds (Krussman, 1986; Robertson, 1974).  It comprises many species, which are economically important as sources of fruits, nuts, oil, timber, and ornamentals.  The fruit and nut bearing species include almonds (P. dulcis (Miller) D.A. Webb), apricots (P. armeniaca L.), cherries (diploid sweet cherry P. avium L. and tetraploid tart cherry P. cerasus L.), peaches (P. persica (L.) Batsch), and plums (hexaploid P. domestica L. and diploid P. salicina L.).  Cherries and plums are adapted to the cooler temperate regions of the world, while peaches and apricots are grown in warmer temperate, sub-tropical, and tropical highlands, but require adequate winter chilling.  Almonds are adapted to regions with Mediterranean climate with periods of winter chilling for normal production. 

Prunus is a large, diverse genus with a basic chromosome number x = 8, within the subfamily Amygdaloideae (Prunoideae) of the family Rosaceae (Rehder, 1940), and probably originated in Central Asia (Watkins, 1976).  The subfamily Amygdaloideae is unique among the rosaceous subfamilies, in bearing a fleshy fruit called a drupe with a hard endocarp, often called the stone.  The taxonomic classification within the genus Prunus is mainly based on fruit morphology and has been controversial.  The revised classification by Rehder (1940), which describes five subgenera; Amygdalus, Cerasus, Laurocerasus, Padus, and Prunus to accommodate variation within the genus, is a widely accepted taxonomic treatment.  However, from the genetic improvement perspective, the subgenus Amygdalus, to which peaches and almonds belong, and the subgenus Prunus, which includes section Prunophora comprised of diploid Japanese plums and hexaploid European plums and section Armeniaca containing apricots, are considered to be a single gene pool (Watkins, 1976).  The subgenus Cerasus comprising diploid sweet cherry and tetraploid tart cherry constitutes a distinct group distantly related to the other two subgenera, Amygdalus and Prunus, included in the study.  Nevertheless, breeding barriers exist among taxa possessing different ploidy levels, even within the same section, but hybrids are generally successful when both parents have the same ploidy level (Okie and Weinberger, 1996).  The subgenera Padus and Laurocerasus are more isolated within the genus Prunus.

The Plant Genetic Resources Unit in Geneva, New York also houses tetraploid cherry Prunus species.