Page 27 - Dedola2011

that this pattem might reflect, in part, the less-dense geographic coverage of sampling
sites in some areas of the SAS group, we nonetheless hypothesise that a high level of
gene flow occurs in the SAS group. The different degree of genetic structuring may also
depend on the larger population sizes encountered in the SAS group than in the SCR
group (Casu, Rivera-Ingraham, pers. obs.; Espinosa 2009). The lack of strong
population fragmentation may allow a stepping-stone fashioned gene flow (see Kimura
and Weiss 1964) along the coastline, which may be favoured by the surface directional
current. For instance, the influence ofthe Algerian Current, which flows eastward
(Robinson et al. 200 l), may contribute to promote the spread of P. ferruginea larvae.
Indeed, this current has been cited as responsible of the dispersion by rafting of the
vermetid gastropod with direct development Dendropoma petraeum (Calvo et al. 2009)
which shows low genetic structuring in a region encompassing the Northem African
and South-Eastem Spanish coasts from the Atlantic to the Siculo-Tunisian Strait.
Nonetheless, the presence of individuals sharing the same genetic makeup as those
found in the Zembra and the Sicilian islands in Alboran Island suggests that local
currents, gyres and eddies (Send et al. 1999) may also favour an occasionai larva)
dispersa} westward.

Unlike the ISSR analysis, mtDNA sequences show no significant genetic
differentiation throughout the entire P. ferruginea distribution range. Severa} reasons
may be invoked to explain the differences observed between nuclear and mtDNA
markers. First, ISSRs depict pattems of multilocus variations at many independent loci,
whereas mtDNA reflects variations at a single locus or a small number of concatenated
loci. Furthermore, ISSRs may detect higher levels of genetic variability and/or genetic
structuring with respect to mtDNA markers. This may be related to the high potential
for detecting differences using random or semirandom primers that amplify nuclear

27

Gìan Luca Dedola: ANALISI DELLA VARIABILITÀ GENETICA DI PATELLA FéRRUGINEA, PATEL-LA UL l'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