A High Quality Assembly of the Nile Tilapia (Oreochromis niloticus) Genome Reveals the Structure of Two Sex Determination Regions

Abstract

We report a high-quality assembly of the tilapia genome, a perciform fish important in aquaculture around the world. A homozygous clonal XX female Nile tilapia (Oreochromis niloticus) was sequenced to 44X coverage using Pacific Biosciences (PacBio) SMRT sequencing. Dozens of candidate de novo assemblies were generated and an optimal assembly (contig NG50 of 3.3Mbp) was selected using principal component analysis of likelihood scores calculated from several paired-end sequencing libraries. Comparison of the new assembly to the previous O. niloticus genome assembly reveals that recently duplicated portions of the genome are now well represented. The overall number genes in the new assembly increased by 27.3%, including a 67% increase in pseudogenes. The new tilapia genome assembly correctly represents two recent vasa gene duplication events that have been verified with BAC sequencing. At total of 146Mbp of additional transposable element sequence are now assembled, a large proportion of which are recent insertions. Large centromeric satellite repeats are assembled and annotated in cichlid fish for the first time. Finally, the new assembly identifies the long-range structure of both an ~9Mbp XY sex-determination region on LG1 in O. niloticus, and a ~50Mbp WZ sex-determination region on LG3 in the related species O. aureus. This study highlights the use of long read sequencing to correctly assemble recent duplications and to characterize repeat-filled regions of the genome.