Genetic flow among olive populations within the Mediterranean basin

Background: The olive tree is a typical crop of the Mediterranean basin where it
shows a wide diversity, accounting for more than 2,600 cultivars. The ability to
discriminate olive cultivars and determine their genetic variability is pivotal for an
optimal exploitation of olive genetic resources.
Methods:We investigated the genetic diversity within 128 olive accessions belonging
to four countries in the Mediterranean Basin (Italy, Algeria, Syria, and Malta), with
the purpose of better understanding the origin and spread of the olive genotypes
across Mediterranean Basin countries. Eleven highly polymorphic simple sequence
repeat (SSR) markers were used and proved to be very informative, producing a total
of 179 alleles.
Results: Cluster analysis distinguished three main groups according to their
geographical origin, with the current sample of Maltese accessions included in the
Italian group. Phylogenetic analysis further differentiated Italian and Maltese olive
accessions, clarifying the intermediate position of Maltese accessions along the
x/y-axes of principal coordinate analysis (PCoA). Model-based and neighbor
clustering, PCoA, and migration analysis suggested the existence of two different
gene pools (Algerian and Syrian) and that the genetic exchange occurred between the
Syrian, Italian and Maltese populations.
Discussion: The close relationship between Syrian and Italian andMaltese olives was
consistent with the historical domestication and migration of olive tree from the
North Levant to eastern Mediterranean basin. This study lays the foundations for a
better understanding of olive genetic diversity in the Mediterranean basin and
represents a step toward an optimal conservation and exploitation of olive genetic
resources.