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3. Genetic structure and variability

Sicily is in a particular position, in the centre of an acknowledged biogeographic transition sepa-
rating the western and eastern Mediterranean basins [54,55]. It has been considered by different
authors to belong entirely or partially to either the western or eastern Mediterranean basin [55].
The Strait of Sicily, separating Sicily from the Tunisian coast, underwent drastic changes during
and after glaciation [56], affecting levels and patterns of gene flow among populations located
on the two sides of the basin. Moreover, Sicily represents the gate for the westward expansion
of Lessepsian migrants [55], which enter the Mediterranean basin through the Strait of Sicily
or the narrow Strait of Messina. The distribution of the seagrass flora along the Sicilian coasts
and the intraspecific population genetic structure of Mediterranean seagrass species reflect this
peculiarity.

An example of westward expansion of a putative Lessepsian migrant, is given by Halophila stip-
ulacea. The introduction in the Mediterranean basin via the Suez Channel was confirmed by rDNA
sequencing data, which recorded almost complete identity of the ribosomal cistron ITS1-5.8S-
ITS2, between the west Mediterranean and Red Sea genotypes [57]. High variability in random
amplified polymorphic DNA (RAPD) markers was found both in a population located near the
harbour of the island Vulcano (Eolian Archipelago) and in mainland populations located in the
semi-enclosed marine lakes of Oliveri-Tindari (Messina) [22]. Only male flowers were recorded
for this study, as well as for most of the west Mediterranean sites where the species has been
recorded [6,22,29]. Female flowers have been recorded only once on the south-east coast of
Sicily [58]. This seems to suggest that few sexually reproducing populations support the high
level of genotypic diversity found in the recently colonised areas through fragmentation and
re-attachment of mature thalli.

The role of phylogeographic transition zone played by the Strait of Sicily has been demon-
strated for many different marine organisms (e.g. fishes and molluscs) [59,60]. This has been
confirmed recently for the two Mediterranean seagrass species C. nodosa and P. oceanica. In
both species, strong genetic divergence in the allelic distribution of microsatellite markers has
been found between populations growing in the eastern and western basins [54,61]. The only two
C. nodosa meadows sampled in Sicily come from the south-eastern tip of the island (Porto Palo
and Marzamemi) and both fall in an intermediate position between western and eastern population
groups in the Neighbour Joining and Multidimensional Scaling analysis [61]. Although the lack
of other C. nodosa sampling stations in Sicily does not allow us to locate precisely the west/east
transition for this species, recent analysis performed on P. oceanica seems to identify the south-
eastern tip of Sicily as the real west/east transition zone [54,62,63]. The meadow near Marzamemi
(south-eastern coast), in particular, appears to play a central role, defined by its betweenness cen-
trality, in the P. oceanica population’s network built over the whole Mediterranean, and to be a
key population shaping the structure of the network [64].

P. oceanica shows high genetic and genotypic polymorphism in most of the localities analysed
along the coasts of Sicily [23,54,63], which is a strong indication of the good health of the
meadows [65,66]. This is also fits with the frequent record of flowers and beached fruits along the
coasts of the island. Flowering intensity seems to increase in a eastward direction [67], which fits
nicely with the presence of more genetically diverse meadows in the south-east tip of the island
[55]. Although not fully acknowledged in P. oceanica, the contribution of sexual reproduction in
enhancing meadows’ genetic diversity has been shown by Orsini et al. [68]. Nonetheless, very low
genotypic richness has been found in meadows which are either isolated from the main patterns
of local gene flow (Posidonia shoals populations, of the Sicily banks) [63] or at the extremes
of the environmental tolerance of the species (Stagnone di Marsala) [23]. The atoll-like shaped
P. oceanica patches growing within the Stagnone di Marsala, in particular, seem to suffer from
both the extreme salinity and temperatures reached within the lagoon, particularly in summer,