Abstract
BackgroundTilapia is the common name for a group of cichlid fishes and is one of the most important aquacultured freshwater food fish. Mozambique tilapia and its hybrids, including red tilapia are main representatives of salt tolerant tilapias. A linkage map is an essential framework for mapping QTL for important traits, positional cloning of genes and understanding of genome evolution.ResultsWe constructed a consensus linkage map of Mozambique tilapia and red tilapia using 95 individuals from two F1 families and 401 microsatellites including 282 EST-derived markers. In addition, we conducted comparative mapping and searched for sex-determining loci on the whole genome. These 401 microsatellites were assigned to 22 linkage groups. The map spanned 1067.6 cM with an average inter-marker distance of 3.3 cM. Comparative mapping between tilapia and stickleback, medaka, pufferfish and zebrafish revealed clear homologous relationships between chromosomes from different species. We found evidence for the fusion of two sets of two independent chromosomes forming two new chromosome pairs, leading to a reduction of 24 chromosome pairs in their ancestor to 22 pairs in tilapias. The XY sex determination locus in Mozambique tilapia was mapped on LG1, and verified in five families containing 549 individuals. The major XY sex determination locus in red tilapia was located on LG22, and verified in two families containing 275 individuals.ConclusionsA first-generation linkage map of salt tolerant tilapia was constructed using 401 microsatellites. Two separate fusions of two sets of two independent chromosomes may lead to a reduction of 24 chromosome pairs in their ancestor to 22 pairs in tilapias. The XY sex-determining loci from Mozambique tilapia and red tilapia were mapped on LG1 and LG22, respectively. This map provides a useful resource for QTL mapping for important traits and comparative genome studies. The DNA markers linked to the sex-determining loci could be used in the selection of YY males for breeding all-male populations of salt tolerant tilapia, as well as in studies on mechanisms of sex determination in fish.
Highlights
Tilapia is the common name for a group of cichlid fishes and is one of the most important aquacultured freshwater food fish
When we verified the sex-determining loci in seven families, all families showed significant correlations between sex and genotypes of markers from LG1 or LG22. These results indicate the locus on LG1 was the only sexdetermining locus in Mozambique tilapia and hybrid tilapia produced by Mozambique tilapia males, and the locus on LG22 was the main sex-determining locus in hybrid tilapia produced by red tilapia males
The consensus and linkage maps of Mozambique tilapia and red tilapia, all consisted of 22 linkage groups. 282 mapped markers were derived from ESTs, and 125 of them were annotated by known genes, including genes related to immunity and growth
Summary
Tilapia is the common name for a group of cichlid fishes and is one of the most important aquacultured freshwater food fish. Mozambique tilapia and its hybrids, including red tilapia are main representatives of salt tolerant tilapias. The annual yield of farmed tilapia and captured tilapia reached 3.10 and 0.79 million tons, respectively, in 2009 [1], making this fish one of the most important food fishes in the world. Though tilapias normally live in freshwater, a few species of them show high salt tolerance and could be raised in brackish water or sea water [2,3]. Mozambique tilapia and its hybrids, including red tilapia are the major representatives of these euryhaline cichlids in aquaculture [4,5]. Based on the growth performance in salt water, Mozambique tilapia and red tilapia are competent candidates for breeding saline tilapia strains [5,6]
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