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Tomasello, Di Maida, Calvo, Pirrotta, Borra & Procaccini
Stagnone inner waters, surely plays an additional role in
determining stressful environmental conditions.
How can the high genetic and genotypic diversity found
within the lagoon be explained considering the genetic
isolation and the absence of recent reproductive events in
the innermost area of the lagoon? First, the genetic diver-
sity of P. oceanica meadows is generally high in the whole
geographic area where the Stagnone is located (i.e. South-
ern Tyrrhenian Sea and Sicily Strait). Values recorded in
our study are comparable to previous results from mead-
ows along the southern Sicilian and Tunisian coasts
(Arnaud-Haond et al. 2007b), where flowering and seeds
washed ashore have been observed more frequently than
in the rest of the basin (Calvo et al. 2006). The presence
of multiple genotypes in each of the atolls and the rela-
tively high genetic diversity in the Atolls area could result
from a high ancestral diversity in the lagoon. Influx of
genotypes into the lagoon, which may have been frequent
in the past, has diminished to almost nothing in recent
times, due to an almost complete closure of the northern
entrance (Agnesi et al. 1993). This hypothesis is also sup-
ported by recent findings of banquette formed by fruits of
P. oceanica, which were probably able to enter the lagoon
in the past from outside its northern opening (Calvo et al.
1995). Reduction in gene flow between the lagoon and the
open sea may have led to genetic drift. The harsher envi-
ronmental conditions may have also favored the selection
of a reduced number of genotypes which were able to
cope with the extreme temperature and salinity conditions
of this area. The excess of heterozygosity present in this
area could support the existence of selection, through the
advantage of heterozygous genotypes, although replicated
genotypes did not have a larger number of heterozygous
loci than the other genotypes.
In light of these results, we can conclude that the
P. oceanica atoll-like structures within the Stagnone di
Marsala lagoon are composed of multiple genotypes.
Although the worst scenario would have been if atolls
were composed of a single genotype, the genetic isolation,
lack of flowering and low growth performances are a cause
for concern, threatening the survival of the species in this
area. The P. oceanica population living in the northern
basin of the lagoon under extreme environmental condi-
tions could serve as an experimental model for what might
happen under predicted environmental changes in the
whole of the Mediterranean as a result of global warming.
Acknowledgements
We thank Luciano Fici, Filippo Luzzu, Alessandro Todaro
and Antonino Scannavino for their support in field activi-
ties, Carmelina Bellavia, Cesare Costantini, Ivan Blanch,
Emauela Dattolo, Marianna Migliaccio and Ilia Serra for
Marine Ecology 30 (2009) 288–300 ª 2009 Blackwell Verlag GmbH 297
Seagrass meadows at the extreme of environmental tolerance
their help in the laboratory. Thanks to Elio Biffali, Elvira
Mauriello and the Molecular Biology Service of SZN for
their assistance during the microsatellite analysis. We
thank Wiebe Kooistra for the constructive comments.
Finally, we would like to thank the anonymous reviewers
who significantly contributed to improve the manuscript.
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