Fosmid Library Construction and Initial Analysis of End Sequences in Female Half-smooth Tongue Sole (Cynoglossus semilaevis)

Half-smooth tongue sole (Cynoglossus semilaevis: Pleuronectiformes) is a commercially important cultured marine flatfish in China and forms an important fishery resource, but the research of its genome is underdeveloped. In this study, we constructed a female C. semilaevis fosmid library and analyzed the fosmid end sequences to provide a preliminary assessment of the genome. The library consists of 49,920 clones with an average insert size of about 39 kb, amounting to 3.23 genome equivalents. Fosmid stability assays indicate that female C. semilaevis DNA was stable during propagation in the fosmid system. Library screening with eight microsatellite markers yielded between two and five positive clones, and none of those tested was absent from the library. End-sequencing of both 5′ and 3′ ends of 1,152 individual clones generated 2,247 sequences after trimming, with an average sequence length of 855 bp. BLASTN searches of the nr and EST databases of GenBank and BLASTX searches of the nr database resulted in 259 (11.53%) and 287 (12.77%) significant hits (E < e ⁻⁵), respectively. Repetitive sequences analysis resulted in 5.23% of base pairs masked using both the Fugu and Danio databases, repetitive elements were composed of retroelements, DNA transposons, satellites, simple repeats, and low-complexity sequences. The fosmid library, in conjunction with the fosmid end sequences, will serve as a useful resource for large-scale genome sequencing, physical mapping, and positional cloning, and provide a better understanding of female C. semilaevis genome.