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Background: Adaptation to different ecological environments is thought to drive ecological speciation. This phenomenon culminates in the radiations of cichlid fishes in the African Great Lakes. Multiple characteristic traits of cichlids, targeted by natural or sexual selection, are considered among the driving factors of these radiations. Parasites and pathogens have been suggested to initiate or accelerate speciation by triggering both natural and sexual selection. Three prerequisites for parasite-driven speciation can be inferred from ecological speciation theory. The first prerequisite is that different populations experience divergent infection levels. The second prerequisite is that these infection levels cause divergent selection and facilitate adaptive divergence. The third prerequisite is that parasite-driven adaptive divergence facilitates the evolution of reproductive isolation. Here we investigate the first and the second prerequisite in allopatric chromatically differentiated lineages of the rock-dwelling cichlid Tropheus spp. from southern Lake Tanganyika (Central Africa). Macroparasite communities were screened in eight populations belonging to five different colour morphs., Results: Parasite communities were mainly composed of acanthocephalans, nematodes, monogeneans, copepods, branchiurans, and digeneans. In two consecutive years (2011 and 2012), we observed significant variation across populations for infection with acanthocephalans, nematodes, monogeneans of the genera Gyrodactylus and Cichlidogyrus, and the copepod Ergasilus spp. Overall, parasite community composition differed significantly between populations of different colour morphs. Differences in parasite community composition were stable in time. The genetic structure of Tropheus populations was strong and showed a significant isolation-by-distance pattern, confirming that spatial isolation is limiting host dispersal. Correlations between parasite community composition and Tropheus genetic differentiation were not significant, suggesting that host dispersal does not influence parasite community diversification., Conclusions: Subject to alternating episodes of isolation and secondary contact because of lake level fluctuations, Tropheus colour morphs are believed to accumulate and maintain genetic differentiation through a combination of vicariance, philopatric behaviour and mate discrimination. Provided that the observed contrasts in parasitism facilitate adaptive divergence among populations in allopatry (which is the current situation), and promote the evolution of reproductive isolation during episodes of sympatry, parasites might facilitate speciation in this genus.

Background: In fish, the most studied production traits in terms of heritability are body weight or growth, stress or disease resistance, while heritability of cortisol levels, widely used as a measure of response to stress, is less studied. In this study, we have estimated heritabilities of two growth traits (body weight and length) and of cortisol response to confinement stress in the European sea bass., Findings: The F1 progeny analysed (n = 922) belonged to a small effective breeding population with contributions from an unbalanced family structure of just 10 males and 2 females. Heritability values ranged from 0.54 (± 0.21) for body weight to 0.65 (± 0.22) for standard body length and were low for cortisol response i.e. 0.08 (± 0.06). Genetic correlations were positive (0.94) between standard body length and body weight and negative between cortisol and body weight and between cortisol and standard body length (-0.60 and -0.55, respectively)., Conclusion: This study confirms that in European sea bass, heritability of growth-related traits is high and that selection on such traits has potential. However, heritability of cortisol response to stress is low in European sea bass and since it is known to vary greatly among species, further studies are necessary to understand the reasons for these differences.

Background: Food supply from the ocean is constrained by the shortage of domesticated and selected fish. Development of genomic models of economically important fishes should assist with the removal of this bottleneck. European sea bass Dicentrarchus labrax L. (Moronidae, Perciformes, Teleostei) is one of the most important fishes in European marine aquaculture; growing genomic resources put it on its way to serve as an economic model., Results: End sequencing of a sea bass genomic BAC-library enabled the comparative mapping of the sea bass genome using the three-spined stickleback Gasterosteus aculeatus genome as a reference. BAC-end sequences (102,690) were aligned to the stickleback genome. The number of mappable BACs was improved using a two-fold coverage WGS dataset of sea bass resulting in a comparative BAC-map covering 87% of stickleback chromosomes with 588 BAC-contigs. The minimum size of 83 contigs covering 50% of the reference was 1.2 Mbp; the largest BAC-contig comprised 8.86 Mbp. More than 22,000 BAC-clones aligned with both ends to the reference genome. Intra-chromosomal rearrangements between sea bass and stickleback were identified. Size distributions of mapped BACs were used to calculate that the genome of sea bass may be only 1.3 fold larger than the 460 Mbp stickleback genome., Conclusions: The BAC map is used for sequencing single BACs or BAC-pools covering defined genomic entities by second generation sequencing technologies. Together with the WGS dataset it initiates a sea bass genome sequencing project. This will allow the quantification of polymorphisms through resequencing, which is important for selecting highly performing domesticated fish.