Resolving the genomic basis underlying phenotypic variations is a question of great importance in evolutionary biology. However, understanding how genotypes determine the phenotypes is still challenging. Centuries of artificial selective breeding for beauty and aggression resulted in a plethora of colors, long fin varieties, and hyper-aggressive behavior in the air-breathing Siamese fighting fish (Betta splendens), supplying an excellent system for studying the genomic basis of phenotypic variations. Combining whole genome sequencing, QTL mapping, genome-wide association studies and genome editing, we investigated the genomic basis of huge morphological variation in fins and striking differences in coloration in the fighting fish. Results revealed that the double tail, elephant ear, albino and fin spot mutants each were determined by single major-effect loci. The elephant ear phenotype was likely related to differential expression of a potassium ion channel gene, kcnh8. The albinotic phenotype was likely linked to a cis-regulatory element acting on the mitfa gene and the double tail mutant was suggested to be caused by a deletion in a zic1/zic4 co-enhancer. Our data highlight that major loci and cis-regulatory elements play important roles in bringing about phenotypic innovations and establish Bettas as new powerful model to study the genomic basis of evolved changes.

In the aquaculture industry, selective breeding has played an important role in increasing aquaculture production significantly. To meet the global growing demand for high quality proteins, it is essential to apply novel technologies to accelerate breeding to facilitate the increase in aquaculture production. Gene and genome editing technologies, including ZFNs, TALLENS and Crispr/Cas9, are promising tools to speed up genetic improvement. Among all the genome editing approaches, the CRISPR/Cas9 system is faster, cheaper and precise in editing genes/genomes. Therefore, the application of CRISPR/Cas9 technology in the editing of genomes in aquaculture species is emerging rapidly. It has been applied to precisely edit genes to identify gene functions and generate the preferred traits in over 20 aquaculture species. This review summarises the genome editing technologies and their applications in the rapid improvement of economic traits in the aquaculture industry. Challenges and future directions of genome editing are also discussed.

Background Fishes are the one of the most diverse groups of animals with respect to their modes of sex determination, providing unique models for uncovering the evolutionary and molecular mechanisms underlying sex determination and reversal. Here, we have investigated how sex is determined in a species of both commercial and ecological importance, the Siamese fighting fish Betta splendens. Results We conducted association mapping on four commercial and two wild populations of B. splendens. In three of the four commercial populations, the master sex determining (MSD) locus was found to be located in a region of ~ 80 kb on LG2 which harbours five protein coding genes, including dmrt1, a gene involved in male sex determination in different animal taxa. In these fish, dmrt1 shows a male-biased gonadal expression from undifferentiated stages to adult organs and the knockout of this gene resulted in ovarian development in XY genotypes. Genome sequencing of XX and YY genotypes identified a transposon, drbx1, inserted into the fourth intron of the X-linked dmrt1 allele. Methylation assays revealed that epigenetic changes induced by drbx1 spread out to the promoter region of dmrt1. In addition, drbx1 being inserted between two closely linked cis-regulatory elements reduced their enhancer activities. Thus, epigenetic changes, induced by drbx1, contribute to the reduced expression of the X-linked dmrt1 allele, leading to female development. This represents a previously undescribed solution in animals relying on dmrt1 function for sex determination. Differentiation between the X and Y chromosomes is limited to a small region of ~ 200 kb surrounding the MSD gene. Recombination suppression spread slightly out of the SD locus. However, this mechanism was not found in the fourth commercial stock we studied, or in the two wild populations analysed, suggesting that it originated recently during domestication. Conclusions Taken together, our data provide novel insights into the role of epigenetic regulation of dmrt1 in sex determination and turnover of SD systems and suggest that fighting fish are a suitable model to study the initial stages of sex chromosome evolution.

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