Constructing a High-Density Genetic Linkage Map for Large Yellow Croaker (Larimichthys crocea) and Mapping Resistance Trait Against Ciliate Parasite Cryptocaryon irritans.

The large yellow croaker (Larimichthys crocea) is the most economically important marine cage-farming fish in China in the past decade. However, the sustainable development of large yellow croaker aquaculture has been severely hampered by several diseases, of which, the white spot disease caused by ciliate protozoan parasite Cryptocaryon irritans ranks the most damaging disease in large yellow croaker cage farms. To better understand the genetic basis of parasite infection and disease resistance to C. irritans, it is vital to map the traits and localize the underlying candidate genes in L. crocea genome. Here, we constructed a high-density genetic linkage map using double-digest restriction-site associated DNA (ddRAD)-based high-throughput SNP genotyping data of a F1 mapping family, which had been challenged with C. irritans for resistant trait measure. A total of 5261 SNPs was grouped and oriented into 24 linkage groups (LGs), representing 24 chromosomes of L. crocea. The total genetic map length was 1885.67 cM with an average inter-locus distance of 0.36 cM. Quantitative trait loci (QTL) mapping identified seven significant QTLs in four LGs linked to C. irritans disease resistance. Candidate genes underlying disease resistance were identified from the reference genome, including ifnar1, ifngr2, ikbke, and CD112. Comparative genomic analysis between large yellow croaker and the four closely related species revealed high evolutionary conservation of chromosomes, though inter-chromosomal rearrangements do exist. Especially, the croaker genome structure was closer to the medaka genome than stickleback, indicating that the croaker genome might retain the teleost ancestral genome structure. The high-density genetic linkage map provides an important tool and resource for fine mapping, comparative genome analysis, and molecular selective breeding of large yellow croaker.