Genome annotation of Caenorhabditis briggsae by TEC-RED identifies new exons, paralogs, and conserved and novel operons

Abstract

The nematode Caenorhabditis briggsae is routinely used in comparative and evolutionary studies involving its well-known cousin C. elegans. The C. briggsae genome sequence has accelerated research by facilitating the generation of new resources, tools, and functional studies of genes. While substantial progress has been made in predicting genes and start sites, experimental evidence is still lacking in many cases. Here, we report an improved annotation of the C. briggsae genome using the Trans-spliced Exon Coupled RNA End Determination (TEC-RED) technique. In addition to identifying 5' ends of expressed genes, the technique has enabled the discovery of operons and paralogs. Application of TEC-RED yielded 10,243 unique 5' end sequences with matches in the C. briggsae genome. Of these, 6,395 were found to represent 4,252 unique genes along with 362 paralogs and 52 previously unknown exons. The method also identified 493 operons, including 334 that are fully supported by tags. Additionally, two SL1-type operons were discovered. Comparisons with C. elegans revealed that 40% of operons are conserved. Further, we identified 73 novel operons, including 12 that entirely lack orthologs in C. elegans. Among other results, we found that 14 genes are trans-spliced exclusively in C. briggsae compared with C. elegans. Altogether, the data presented here serves as a rich resource to aid biological studies involving C. briggsae. Additionally, this work demonstrates the use of TEC-RED for the first time in a non-elegans nematode and suggests that it could apply to other organisms with a trans-splicing reaction from spliced leader RNA.