A microsatellite genetic linkage map of half smooth tongue sole (Cynoglossus semilaevis)

Comparative piRNAs Profiles Give a Clue to Transgenerational Inheritance of Sex-Biased piRNAs in Cynoglossus semilaevis

Piwi interacting RNAs (piRNAs) are involved in the epigenetic and post-transcriptional gene silencing of retrotransposons in germ line cells, especially in spermatogenesis. There are many related reports on model organisms, such as flies and mice. In fish, however, there are few studies on piRNAs. Cynoglossus semilaevis, a benthic warm water flatfish, with remarkable sexual dimorphism, especially the "pseudo males" with sex reversal, mating with normal females to produce viable offspring, is an ideal material for the study of sex development. Here, sperm piwi-interacting RNAs profiles of Cynoglossus semilaevis were characterized, comparing between male and pseudomale groups. Differential piRNAs were identified with their predicted and annotated targets. Attention was then focused on candidate piRNAs associated with sex development and methylation. We continued to compare the expression levels of 10 candidates differentially expressed piRNAs in F1 spermatozoa. Quantitative RT-PCR demonstrated that five of the ten piRNAs showed sex bias consistent with parental sequencing results, with four significantly higher expression level in sperm of five males offspring than that of pseudomales, while one piRNAs showed the opposite expression profile. The five signature piRNAs (piR-mmu-49600337, piR-mmu-95849, piR-xtr-7474223, piR-xtr-1790334, and piR-mmu-4491546) could be employed as male-specific molecular biomarkers for C. semilaevis. Besides, this study also implied the possibility of transgenerational inheritance of sex-biased piRNAs exiting in sperm of Cynoglossus semilaevis.

A High-Density Genetic Linkage Map and QTL Mapping for Sex and Growth-Related Traits of Large-Scale Loach (Paramisgurnus dabryanus)

Large-scale loach (Paramisgurnus dabryanus) is a commercially important species in East Asia; however, the cultured population that exhibited degradation of germplasm resource cannot meet the market needs, and the genome resources for P. dabryanus are still lacking. In this study, the first high-density genetic map of P. dabryanus was constructed using 15,830 SNP markers based on high-throughput sequencing with an improved SLAF-seq strategy. The quantitative trait locus (QTL) mapping for sex, growth, and morphology traits was performed for the first time. The genetic map spanned 4,657.64 cM in length with an average inter-marker distance of 0.30 cM. QTL mapping and association analysis identified eight QTLs of growth traits, nine QTLs of morphology traits, and five QTLs of sex-related traits, respectively. Interestingly, the most significant QTLs for almost all the traits were concentrated on the same linkage group LG11. Seven candidate markers and 12 potentially key genes, which were associated with sex determination and growth, were identified within the overlapped QTL regions on LG11. Further, the first genome survey analysis of P. dabryanus was performed which represents the first step toward fully decoding the P. dabryanus genome. The genome scaffolds were anchored to the high-density linkage map, spanning 960.27 Mb of P. dabryanus reference genome. The collinearity analysis revealed a high level of collinearity between the genetic map and the reference genome of P. dabryanus. Moreover, a certain degree of homology was observed between large-scale loach and zebrafish using comparative genomic analysis. The constructed high-density genetic map was an important basis for QTL fine mapping, genome assembly, and genome comparison. The present study will provide a valuable resource for future marker-assisted breeding, and further genetic and genomic researches in P. dabryanus.

Identification of Quantitative Trait Loci for Resistance to RSIVD in Red Sea Bream (Pagrus major)

Red sea bream iridoviral disease (RSIVD) is a major viral disease in red sea bream farming in Japan. Previously, we identified one candidate male individual of red sea bream that was significantly associated with convalescent individuals after RSIVD. The purpose of this study is to identify the quantitative trait loci (QTL) linked to the RSIVD-resistant trait for future marker-assisted selection (MAS). Two test families were developed using the candidate male in 2014 (Fam-2014) and 2015 (Fam-2015). These test families were challenged with RSIV, and phenotypes were evaluated. Then, de novo genome sequences of red sea bream were obtained through next-generation sequencing, and microsatellite markers were searched and selected for linkage map construction. One immune-related gene, MHC class IIβ, was also used for linkage map construction. Of the microsatellite markers searched, 148 and 197 were mapped on 23 and 27 linkage groups in the female and male linkage maps, respectively, covering approximately 65% of genomes in both sexes. One QTL linked to an RSIVD-resistant trait was found in linkage group 2 of the candidate male in Fam-2014, and the phenotypic variance of the QTL was 31.1%. The QTL was closely linked to MHC class IIβ. Moreover, the QTL observed in Fam-2014 was also significantly linked to an RSIVD-resistant trait in the candidate male of Fam-2015. Our results suggest that the RSIVD-resistant trait in the candidate male was controlled by one major QTL closely linked to the MHC class IIβ gene and could be useful for MAS of red sea bream.

Integrating genomic resources of flatfish (Pleuronectiformes) to boost aquaculture production

Flatfish have a high market acceptance thus representing a profitable aquaculture production. The main farmed species is the turbot (Scophthalmus maximus) followed by Japanese flounder (Paralichthys olivaceous) and tongue sole (Cynoglossus semilaevis), but other species like Atlantic halibut (Hippoglossus hippoglossus), Senegalese sole (Solea senegalensis) and common sole (Solea solea) also register an important production and are very promising for farming. Important genomic resources are available for most of these species including whole genome sequencing projects, genetic maps and transcriptomes. In this work, we integrate all available genomic information of these species within a common framework, taking as reference the whole assembled genomes of turbot and tongue sole (>210× coverage). New insights related to the genetic basis of productive traits and new data useful to understand the evolutionary origin and diversification of this group were obtained. Despite a general 1:1 chromosome syntenic relationship between species, the comparison of turbot and tongue sole genomes showed huge intrachromosomic reorganizations. The integration of available mapping information supported specific chromosome fusions along flatfish evolution and facilitated the comparison between species of previously reported genetic associations for productive traits. When comparing transcriptomic resources of the six species, a common set of ~2500 othologues and ~150 common miRNAs were identified, and specific sets of putative missing genes were detected in flatfish transcriptomes, likely reflecting their evolutionary diversification.

Preliminary genetic linkage map of Indian major carp, Labeo rohita (Hamilton 1822) based on microsatellite markers

Linkage map with wide marker coverage is an essential resource for genetic improvement study for any species. Sex-averaged genetic linkage map of Labeo rohita, popularly known as 'rohu', widely cultured in the Indian subcontinent, was developed by placing 68 microsatellite markers generated by a simplified method. The parents and their F1 progeny (92 individuals) were used as segregating populations. The genetic linkage map spans a sex-averaged total length of 1462.2 cM, in 25 linkage groups. The genome length of rohu was estimated to be 3087.9 cM. This genetic linkage map may facilitate systematic searches of the genome to identify genes associated with commercially important characters and marker-assisted selection programmes of this species.