The Strait of Gibraltar has been proposed to be the divide between two marine biogeographical regions, the Mediterranean Sea and the Northeast Atlantic. Intraspecific studies have shown, for several of the examined species, a reduction of gene flow between the two basins. The present study examines genetic variation at nuclear and mitochondrial loci in five marine teleost species belonging to the family Sparidae. Four samples for each species were analysed spanning the Northeast Atlantic and the Mediterranean. For all individuals 17 allozyme loci were scored and a combined single strand conformation polymorphism‐sequencing approach was used to survey approximately 190 bp of the mitochondrial DNA (mtDNA) D‐loop region. All five species share similar biological features. For three species, namely Lithognathus mormyrus, Spondyliosoma cantharus, and Dentex dentex, large mtDNA divergence was observed between Atlantic and Mediterranean samples. Little or no mtDNA differentiation was found in the other two species, Pagrus pagrus and Pagellus bogaraveo. Allozyme data revealed strong differentiation when comparing Atlantic and Mediterranean samples of L. mormyrus and D. dentex, moderate for P. pagrus, and no differentiation for P. bogaraveo and S. cantharus. These results provide evidence for a sharp phylogeographical break (sensu Avise) between the Atlantic and the Mediterranean for two (or possibly three) sparid species of the five investigated. At the same time, the obtained results for the other two species raise the question on which ecological/historical factors might have caused the observed discrepancy in the geographical distribution of genetic variation among otherwise biologically similar species.

The use of the BrdU-FPG methodology in the scorpion fish (Scorpaena porcus) has allowed us to estimate the mean cell cycle duration in kidney cells in vivo as being approximately 32.5 h. On the basis of this datum, replication protocols from 30 to 10 h of BrdU incorporation were employed revealing a precise order in the replication of the chromosomes. Treatments of 18–20 h proved to be the most useful in getting optimum replication bands and were used to elaborate the RBG-banding pattern karyotypes of S. porcus and S. notata. Their comparison confirms the high karyotypic distance between these morphologically similar species.

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The karyotypes of the Mediterranean blenniid species: Blennius gattorugine, B. ponticus incognitus, B. pavo, B. sanguinolentus, B. galerfta, B. trigloides and B. pholis were analyzed by means of conventional, silver staining and C-banding techniques. All have shown 2n=48, except B. pholis with 2n=46, while the NF varies from 54 to 72. Only two NORs, in terminal position on the short arms of the carrier pair, were detected in all species. These regions have proved to be structurally polymorphic in four species. Two markedly different C-banding patterns lead us to separate the species into two different karyoevolutionary categories, whose taxonomic and specialization implications are discussed.

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Chromosome preparations from 46 specimens of C. paganellus were studied, revealing seven different karyomorphs: D2 (2n =46, NF46), D1 (2n =47, NF=47), B (2n =48, NF=48), F1 (2n =47, NF=48), F2 (2n =46, NF=48), D1F1 (2n =46, NF = 47), I (2n = 48, NF = 49). It is proposed that the mechanisms involved are deletions, fusions and inversions. The meiotic results suggest that the same chromosome pair was engaged in both the fusion and the deletion rearrangements. Intra-individual polymorphism was found in only one specimen. This and the range of karyomorphs suggest that this variation is regularly inherited.

A karyotype analysis was carried out in nine specimens of the Sparid species Diplodus bellottii using conventional staining, as well as C-banding and Ag-NOR banding techniques, showing, respectively, 2n = 46 and fundamental number (FN) = 54, and scarce heterochromatic areas irregularly distributed and up to four NOR active regions that were C positive. When compared with the karyotypes of other related species, one centric fusion giving rise to a large metacentric pair and several pericentric inversions seem to have been involved in the karyotype evolution. An intra-individual polymorphism was detected in one specimen, resulting in two karyotypic forms in roughly identical proportion, owing to a larger C-band by the NOR regions, appearing either in a terminal position of the short arms of pair 2 or in telomeric position of pair 3. These findings suggest that the extra heterochromatic segment responsible for the heteromorphism apparently only involves associated heterochromatin and not the NORs themselves. This C-positive block seems to have eventually been transferred between heterologous NOR chromosomes by a somatic event, facilitated by the physical proximity of NOR pairs in the nucleolus.

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