Genetic diversity and population structure of carob germplasm from Lebanon, Morocco, and Spain reveal potential for innovative breeding programs

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Abstract
The carob tree (Ceratonia siliqua L.) is a drought-tolerant species native to the Mediterranean Basin. It has been cultivated for centuries for its highly nutritious, edible pods. Genetic diversity is one of the key requirements for the effective management and utilization of plant genetic resources. In this study, we evaluated the genetic diversity and population structure of 169 seminatural carob individuals from Lebanon, Spain, and Morocco using nine expressed sequence tag-simple sequence repeat (EST-SSR) markers. A total of eight polymorphic EST-SSR loci produced 43 alleles, with Cesi_187 and Cesi_1187 identified as the most informative markers. Results of AMOVA and SAMOVA revealed that most (>80%) of the genetic variation occurred within populations, while less than 20% was attributed to variation among populations. STRUCTURE analysis indicated three genetic clusters corresponding to the sampled countries; however, some individuals from Morocco and Spain exhibited overlapping genetic structures. The PCoA and discriminant analysis of principal components complemented the STRUCTURE results and provided further insight into the genetic differentiation among countries. Our findings may improve the effectiveness of conservation and management strategies and promote the utilization of these carob genetic resources in breeding and reforestation programs.