BackgroundHigh density linkage maps are essential for comparative analysis of synteny, fine mapping of quantitative trait loci (QTL), searching for candidate genes and facilitating genome sequence assembly. However, in most foodfish species, marker density is still low. We previously reported a first generation linkage map with 240 DNA markers and its application to preliminarily map QTL for growth traits in Asian seabass (Lates calcarifer). Here, we report a high-resolution linkage map with 790 microsatellites and SNPs, comparative analysis of synteny, fine-mapping of QTL and the identification of potential candidate genes for growth traits.ResultsA second generation linkage map of Asian seabass was developed with 790 microsatellite and SNP markers. The map spanned a genetic length of 2411.5 cM, with an average intermarker distance of 3.4 cM or 1.1 Mb. This high density map allowed for comparison of the map with Tetraodon nigroviridis genome, which revealed 16 synteny regions between the two species. Moreover, by employing this map we refined QTL to regions of 1.4 and 0.2 cM (or 400 and 50 kb) in linkage groups 2 and 3 in a population containing 380 progeny; potential candidate genes for growth traits in QTL regions were further identified using comparative genome analysis, whose effects on growth traits were investigated. Interestingly, a QTL cluster at Lca371 underlying growth traits of Asian seabass showed similarity to the cathepsin D gene of human, which is related to cancer and Alzheimer's disease.ConclusionsWe constructed a high resolution linkage map, carried out comparative mapping, refined the positions of QTL, identified candidate genes for growth traits and analyzed their effects on growth. Our study developed a framework that will be indispensable for further identification of genes and analysis of molecular variation within the refined QTL to enhance understanding of the molecular basis of growth and speed up genetic improvement of growth performance, and it also provides critical resource for future genome sequence assembly and comparative genomics studies on the evolution of fish genomes.

Barramundi (Lates calcarifer) is an important farmed marine food fish species. Its compact genome ($700 Mb) is among the smallest genomes of food fish species. We established a first-generation genetic linkage map of Barramundi with a mapping panel containing three parents (two males and one female) and 93 progeny. A total of 240 microsatellite markers were mapped into 24 linkage groups. Among these markers, 10 were located in ESTs and known genes. The total lengths of the female and male maps were 873.8 and 414.5 cM with an average marker spacing of 6.20 and 4.70 cM, respectively. Comparing the flanking sequences of the 240 Barramundi microsatellites with the assembled whole-genome sequences of Tetraodon nigrovidiris revealed 55 homologous sequences located in 19 of the 21 chromosomes of T. nigrovidiris. The map will not only enable the mapping of quantitative trait loci, but also provide new resources for understanding the evolution of fish genomes.

No abstract

Background: Body weight and length are economically important traits in foodfish species influenced by quantitative trait loci (QTL) and environmental factors. It is usually difficult to dissect the genetic and environmental effects. Asian seabass (Lates calcarifer) is an important marine foodfish species with a compact genome (~700 Mb). The recent construction of a first generation linkage map of Asian seabass with 240 microsatellites provides a good opportunity to determine the number and position of QTL, and the magnitude of QTL effects with a genome scan.

Jatropha curcas is a potential plant species for biodiesel production. However, its seed yield is too low for profitable production of biodiesel. To improve the productivity, genetic improvement through breeding is essential. A linkage map is an important component in molecular breeding. We established a first-generation linkage map using a mapping panel containing two backcross populations with 93 progeny. We mapped 506 markers (216 microsatellites and 290 SNPs from ESTs) onto 11 linkage groups. The total length of the map was 1440.9 cM with an average marker space of 2.8 cM. Blasting of 222 Jatropha ESTs containing polymorphic SSR or SNP markers against EST-databases revealed that 91.0%, 86.5% and 79.2% of Jatropha ESTs were homologous to counterparts in castor bean, poplar and Arabidopsis respectively. Mapping 192 orthologous markers to the assembled whole genome sequence of Arabidopsis thaliana identified 38 syntenic blocks and revealed that small linkage blocks were well conserved, but often shuffled. The first generation linkage map and the data of comparative mapping could lay a solid foundation for QTL mapping of agronomic traits, marker-assisted breeding and cloning genes responsible for phenotypic variation.

Asian sea bass (Lates calcarifer) is one of the most important marine food ¢sh species in Asia and Australia. To estimate the reproductive success of broodstock and heritabilities of growth-related traits, two independent full-factorial crosses (PI and PII) were created by crossing 10 males and 10 females. At 90 days post hatch (dph), the body weight (BW) and total length (TL) of 804 individuals from PI and 900 individuals from PII were measured and tissues samples of each ¢sh were collected. Parents and o¡spring were genotyped with nine polymorphic microsatellites. Out of 1704 o¡spring from the two crosses, 98.7% were assigned to their parents. In PI, 19 of 20 parents produced o¡spring, while in PII, only ¢ve parents contributed to o¡spring. Low contribution of parents to o¡spring could lead to reduced genetic variation in the next generation. Heritabilities for growth-related traits were estimated using the pedigrees reconstructed using microsatellite genotypes. The estimates of heritability were 0.22 AE 0.16 and 0.25 AE 0.18 for BW, 0.31 AE 0.14 and 0.24 AE 0.21 for TL and 0.22 AE 0.22 and 0.15 AE 0.09 for Fulton's condition factor in the two crosses respectively. Body weights at 90 dph and at harvest (289 dph) were sig-ni¢cantly correlated (r 5 0.601, Po0.01). Therefore, growth-related traits could be improved by exploiting additive genetic e¡ects through selective breeding, and broodstock candidates could be selected early in the production cycle. BW, body weight; TL, total length; K, Fulton's condition factor. ÃÃ Signi¢cance at P 5 0.01.Reproduction and heritability of growth traits in Lates calcarifer C M Wang et al.

Background: Barramundi (Lates calcarifer) is an important farmed marine food fish species. Its first generation linkage map has been applied to map QTL for growth traits. To identify genes located in QTL responsible for specific traits, genomic large insert libraries are of crucial importance. We reported herein a bacterial artificial chromosome (BAC) library and the mapping of BAC clones to the linkage map.

Genetic variation in the genome of a given species is the basis for natural selection and genetic improvement through selective breeding. We applied 29 microsatellites located on 11 linkage groups to study genetic variation in 276 accessions of J. curcas collected from nine locations in five countries in South America, Asia and Africa to initiate a breeding program. To our surprise, we did not detect any genetic diversity at all 29 microsatellites loci. All the 276 accessions were homozygous at all loci and shared the same genotype at each locus, suggesting no microsatellite variation in the genome of Jatropha curcas. This result is quite unusual, and may have a profound influence on the breeding strategies and genome study of this species.