Review of genetic diversity studies in almond (Prunus dulcis)

Almond [ Prunus dulcis (Mill.) D.A. Webb.] is cultivated for its nuts and is one of the oldest domesticated plants. Due to the genetically controlled self-incompatibility system that operates in this species, almond is characterized by great genetic diversity, the estimation of which has benefited from a range of marker techniques, including morphological characteristics, isoenzyme detection and molecular markers. Among the DNA-based molecular markers, simple sequence repeats (SSR) have been used most widely, although analyses have ranged from restriction fragment length polymorphism to the most recent single nucleotide polymorphism detection methods. Molecular markers have also been used to trace specific agronomic traits, e.g. self-(in)compatibility or kernel bitterness. Genetic diversity studies in almond have not revealed a direct relationship between the level of diversity and the origin of the germplasm. This might be explained by the relatively recent occurrence of self-compatibility in almond, which has not yet caused a serious loss of genetic diversity. The markers reviewed will be useful in monitoring and maintaining genetic diversity in almond breeding programmes, while others may permit marker-assisted selection for favourable agronomic traits. The cultivation, breeding and conservation of wild-growing almonds may equally benefit from the genetic diversity studies (especially those applying molecular markers).