To investigate the phylogenetic relationships and geographical structure among brown trout S. trutta L. Populations from the South Adriatic-Ionian and Aegean sea basins, mitochondrial DNA sequence comparisons were used. A 310-base-pair (bp) segment of the control region (D-loop), and an additional 280-bp segment of the cytochrome beta gene were sequenced from representatives of 13 brown trout populations. Phylogenetic analyses, conducted after combining the data presented with published data from other Eurasian brown trout, revealed four major phylogenetic groups, three of which were found widely distributed within the southern Balkan region. The phylogeographical patterns revealed by mtDNA represent one of the few cases where phylogenetic discontinuity in a gene tree exists without obvious geographical localization within a species' range and has most likely resulted from the differentiation of the major mtDNA clades during Messinian or early Pleistocene times. Finally, the genetic relationships among the populations suggested by mtDNA were generally not in accordance with either allozyme or morphological data.

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The genetic differentiation and phylogenetic relationships of 13 populations (11 from Greece, one from Albania and one from France) of brown trout (Salmo trutta L.) were investigated at the mtDNA level. RFLP analysis of four segments of mitochondrial DNA (D-loop, cytochrome b and ND-5!6) amplified by PCR was used. Seven of 14 restriction endonucleases were found to detect variability in the ND-516 regions, whereas four and two out of 17 and 16 were polymorphic in the D-loop and cytochrome b, respectively. Eleven different haplotypes were observed. The observed interpopulation diversity was very high (mean = 1.65 per cent), whereas the intrapopulation diversity was low in most cases (mean =0.063 per cent). Five phylogenetic assemblages were identified. The results demonstrate that Greece is one of the regions where brown trout display very high levels of genetic diversity. Most populations were genetically very distinct, possessing private mtDNA genotypes. Therefore, they represent unique gene pools which may warrant individual recognition for conservation and management. The genetic relationships among populations suggested by mtDNA data were not in accordance with allozyme data. This study illustrates the importance of considering the historical biogeography of a species in order to understand its population genetic structure. It also reinforces the view that mtDNA analysis represents a powerful tool to study past and present demographic phenomena.

Among vertebrates, teleost fish exhibit a considerably wide range of sex determination patterns that may be influenced by extrinsic parameters. However even for model fish species like the zebrafish Danio rerio the precise mechanisms involved in primary sex determination have not been studied extensively. The zebrafish, a gonochoristic species, is lacking discernible sex chromosomes and the sex of juvenile fish is difficult to determine. Sequential protandrous hermaphrodite species provide distinct determination of the gender and allow studying the sex determination process by looking at the mechanism of sex reversal. This is the first attempt to understand the genetic basis of phenotypic variation for sex determination and body weight in a sequential protandrous hermaphrodite species, the gilthead sea bream (Sparus aurata). This work demonstrates a fast and efficient strategy for Quantitative Trait Loci (QTL) detection in the gilthead sea bream, a non-model but target hermaphrodite fish species. Therefore a comparative mapping approach was performed to query syntenies against two other Perciformes, the European sea bass (Dicentrarchus labrax), a gonochoristic species and the Asian sea bass (Lates calcarifer) a protandrous hermaphrodite. In this manner two significant QTLs, one QTL affecting both body weight and sex and one QTL affecting sex, were detected on the same linkage group. The co-segregation of the two QTLs provides a genomic base to the observed genetic correlation between these two traits in sea bream as well as in other teleosts. The identification of QTLs linked to sex reversal and growth, will contribute significantly to a better understanding of the complex nature of sex determination in S. aurata where most individuals reverse to the female sex at the age of two years through development and maturation of the ovarian portion of the gonad and regression of the testicular area. [Genomic sequences reported in this manuscript have been submitted to GenBank under accession numbers HQ021443–HQ021749.]

Morphometric variation in 15 characters of the red mullet Mullus barbatus samples from sevenGreek localities was examined using multivariate analysis. ANOVA, stepwise discriminant analysis and cluster analysis revealed a rather high morphological variability between the populations studied. The discriminant analysis revealed that about 80% of the examined fish could be correctly classified into the seven areas. This variability could be explained in terms of genetic structuring of the populations and/or environmental conditions prevailing in each geographic area in combination with fish migration and egg/larvae transportation from one area to another. The results suggest that multivariate analysis when combined with other important biological parameters of red mullet may have important implications for the management of the Greek demersal fish resources. 1998 The Fisheries Society of the British Isles

1. The present study was designed to characterize the genetic structure of brown trout (Salmo trutta) populations from the southern Balkans and to assess the spread of nonnative strains and their introgression into native trout gene pools. We analysed polymorphism at nine microsatellite loci in seven supposedly non-admixed and three stocked brown trout populations. 2. The analyses confirmed the absence of immigration and extraordinarily strong genetic differentiation among the seven non-introgressed populations in parallel with low levels of intrapopulation genetic variability. In contrast, analyses of the stocked populations revealed that the genetic integrity of the local populations had been substantially changed, and the populations must be characterized as hybrid swarms. The pattern of population differentiation observed at microsatellites contrasted to that depicted previously by mtDNA variation. However, the close relationships between populations from the Danube and Axios river systems proposed solely by microsatellites could be explained by palaeogeographic events. 3. Our research showed that most of the populations examined represent unique gene pools, whose existence is critically compromised. Therefore, appropriate management and conservation strategies should be developed urgently in order to protect the subspecific biodiversity and to reverse currently negative trends.

In order to clarify the genetic structure and the phylogenetic relationships among brown trout (Satmo trutta) populations from Greece and other European countries starch gel electrophoresis was used. The populations come from various rivers from Greece and other European countries, flowing to the Atlantic or to the Mediterranean. Eleven enzymic systems were investigated. These correspond to 26 putative loci. A high degree of genetic polymorphism was found. The percentage of polymorphic loci ranged from 3.8 to 34.6 and the degree of expected heterozygosity from 0.016 to 0.1. F-statistics and clustering analyses indicated the existence of a high degree of differentiation. This differentiation is mainly between the Atlantic and the Mediterranean populations. Furthermore the Mediterranean populations seem to be divided into two groups. One includes the western Mediterranean populations and populations from western Greece and the other north-eastern Greek populations. The latter seem to be related to other Balkan populations and probably to Danubian or Black Sea populations. These results support the idea of two different lineages of Mediterranean brown trout populations, one of which is probably of aboriginal origin.

Gilthead sea bream (Sparus aurata L.) is an important marine fish in Mediterranean aquaculture. Sex determination by age and/or body weight is a critical life-history trait, the genetic basis for which is largely unknown in this sequential hermaphrodite species. Herein, we performed a partial genome scan to map quantitative trait loci (QTL) affecting body weight and sex using 74 informative microsatellite markers from 10 paternal half-sib families to construct nine linkage groups (LG). In total, four growth-related QTL (two chromosome-wide and two genome-wide) and six QTL related to sex determination (three pairs in three different LGs) were detected (two chromosome-wide and one genome-wide). The proportion of phenotypic variation explained by the body-weight QTL ranged from 9.3% to 17.2%, showing their potential for use in marker-assisted selection. The results obtained offer solid ground to investigate the structure and function of the genomic regions involved in the mechanisms of sex reversal.