GENETIC DIVERSITY AND RELATIONSHIPS IN DIFFERENT POPULATIONS OF SABELLA SPALLANZANII (GMELIN) (POLYCHAETA, SABELLIDAE)
Patti, F. P.1, Gambi, M. C.1, Féral, J-P. 2
1Laboratorio di Ecologia del benthos, Stazione Zoologica "A. Dohrn", Punta S. Pietro, 80077, Ischia, Napoli, Italy.
2Observatoire Océanologique de Banyuls, UMR 7628: Modèles en biologie cellulaire et évolutive. Banyuls-sur-mer, France.
Sabella spallanzanii (Gmelin, 1791) is one of the most common polychaete species in many different coastal Mediterranean habitats, including very polluted ones, and present at various depths. S. spallanzanii is a relatively large-sized species (up to 30-40 cm), a protandric hermaphrodite and a free-spawer, with a typical ect-aquasperm morphology of the mature spermatozoa (rounded nucleus and acrosome), a feature generally associated with external fertilization. The species is characterized by an (as yet undescribed) lecitrotrophic larval stage, probably with a pelagic life-style and a relatively high potential for dispersal. In recent years (i. e. during the early 1990ties), the species was introduced into Australian waters, causing various problems to the local scallop fishery. In order to clarify the taxonomic identity and the genetic relationships within Mediterranean populations, and between these and the Australian populations, a study of population genetics of S. spallanzanii was launched using appropriate DNA markers. The non coding zone ITS2, which is located between the small sub-unit 5.8 S and the larger sub-unit 28 S of the ribosomal DNA gene cluster, has been chosen, as it shows a mutation rate at a level that is known to be acceptable for the detection of genetic differences between populations. PCR amplification of the ribosomal ITS2 for different S. spallanzanii populations provided about 400 base pairs. Sequencing of the ITS2 was performed on 5 populations from the Mediterranean (including 2 localities in italian waters, namely the Gulf of Naples and the Ionian Sea at Taranto), on 3 populations from Australia, and on 1 from the Atlantic (material kindly provided by Dr R. Ward and Dr G. Rouse). The sequence analysis showed that both the three Australian (Port Phillip Bay, Adelaide, Cockburn Sound), and the Atlantic (Roscoff) populations are well separated from the Mediterranean ones. All the Australian specimens show a lower genetic variability, due to the "founder effect" of the introduced population. As to the Mediterranean specimens, those collected in the Ionian Sea (Taranto) are differentiated from all the others, while the population of the Gulf of Naples showed the highest affinity to that of southern Spain (Alicante), compared to those of the French coast (Marseille and Carteau). The structure of the phylogenetic relationships observed between the different populations of S. spallanzanii is very consistent with that reported by R. Ward and obtained by an analysis of the allozymes, both with respect to the taxonomic identity of S. spallanzanii and to the relationships among populations. These first results confirm the efficiency of the molecular marker chosen to discriminate genetic differences between populations.