Genetic diversity and adaptability of two species of Mugilidae (Teleostei: Perciformes) of the Po river delta coastal lagoons

Molecular polymorphisms (allozyme and amplified fragment length polymorphisms (AFLP)) were investigated in the mullet specie's Liza ramada and Liza saliens to determine levels of genetic variability and assess possible correlations between the population genetic structure and the specie's adaptability to shallow water environments. The thinlipped (L. ramada) and the sharpnose (L. saliens) mullets are diadromous species that enter coastal lagoons during early life stages to complete growth previous to sexual maturation. Bi-monthly samples (n = 30–50) were collected in the period 2000–2002 in different sites of the Sacca di Goro (Po river delta) and a nearby aquaculture lagoon (Valle Bertuzzi). Biochemical analyses were carried out by means of starch gel electrophoresis (SGE) on 24 genetic loci corresponding to 15 gene–enzyme systems. In addition, molecular species-specific AFLP (15 primer combinations) were obtained by means of capillary electrophoresis. The degree of allozyme variability determined at three polymorphic loci, GPI-A, GPI-B and G3PDH in L. saliens and L. ramada (polymorphic loci P = 0.08; mean heterozygosity H = 0.010–0.022) and AFLP diversity (P = 0.110–0.430), seems lower than the one reported in the literature for ecologically similar teleost species. The identification of diagnostic alleles at GPI-A, G3PDH, AK-2, CK-A, CK-B, AAT, IDH, PGM-1, PGM-2 loci in the two mullet species was used for the taxonomic classification of L. saliens and L. ramada. Molecular taxonomy (allozyme and AFLP) demonstrated that the presence of small individuals inside the lagoons previously identified according to morphological characters has often been misinterpreted. The results are discussed in relation to the adaptive role of genetic variation and the migratory characteristics of Mugilidae.