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Discussion

ISSR markers show a high leve! of genetic variabi lity throughout the study area,
indicating the presence of three major genetic discontinuities. The uppermost
hierarchical structure, detected by STRUCTURE analyses, identified a genetic cluster
corresponding to samples from the SAS group and two genetic clusters in the SCR
group (Fig. 2a). Remarkably, grouping samples into the three clusters maximised the
portion of genetic differentiation among groups with respect to alternative grouping
schemes, which, although significant, showed lower fixation indices (Table 3).

The genetic break observed between the two main geographic areas considered
in this study (the SAS group and the SCR group) is probably linked to the presence of a
barrier to gene flow represented by the Sardinian Channel. No data are available
regarding the actual level of larva! dispersa! for Patella ferruginea; however, a stretch
of approximately 180 km of open sea, which separates North Africa from Sardinia, is
likely to be the most serious hindrance to gene flow between the two regions. Indeed, a
low potential for dispersa! may enhance the perception of a species to barriers (Baus et
al. 2005), particularly in organisms living in fragmented habitats (see Mejri et al. 2009
and references therein). Furthermore, the clear North-South geographic separation of
these two groups coincides w ith some sub-basins ofthe Mediterranean that have
different biologica! and ecological conditions (Bianchi 2007). Interestingly, the genetic
structuring found between the SCR and the SAS groups fits with the surface isotherms
recorded in the Mediterranean during February: populations from the SAS group lie
between the isotherms of 15° C and 14° C, whereas the SCR group lie between the
isotherms of 14° C and 13° C (Bianchi 2007). The population history of P.ferruginea
from SCR and SAS might thus include fragmentation due to physical-chemical

24

Gìan Luca Dedola: ANALISI DELLA VARIABILITÀ GENETICA DI PATELLA FéRRUGINEA, PATEL.LA ULl'SSIPONENSIS (MOLLUSCA: GASTROPODA) E PINNA
NOBILIS (MOLLUSCA: BIVALVIA): lLCONTRJBUTO DEl DATI MOLECOLARI ALLA CONSERVAZIONE DI SPECIE MINACCIATE - Tesi di douorato in Scienze
della natura e delle sue risorse • indiri7.zo BK>logi.a Ambientale, Università degli Studi di Sassarl