The following paragraph is a modified version of the introduction section of Wormbook chapter "Genomics and biology of the nematode Caenorhabditis briggsae" by B. Gupta, R. Johnsen, and N. Chen.
C. briggsae is a natural companion to C. elegans for research because of the similarities in morphology, hermaphroditic life style and ease of cultivation. The two species diverged from a common ancestor roughly 100 million years ago (Stein et al., 2003) yet appear almost identical in morphology and behavior. The phylogenetic relationships of Caenorhabditis nematodes reveals that C. briggsae and C. elegans are not sibling species and that C. remanei, a gonochoristic species, is the closest relative of C. briggsae (see the Wormbook chapter "The phylogenetic relationships of Caenorhabditis and other rhabditids" by Kiontke and Fitch). The seemingly identical morphology of the two nematodes permits straightforward interpretation of results involving conserved genes and pathways. The genome sequence and chromosome assembly of C. briggsae are valuable in reverse genetics and genome-wide comparative studies. The majority of the currently available genetic and reverse genetic tools in C. elegans can be used in C. briggsae with no major modification. These resources make C. briggsae an ideal model for comparative studies.
The whole genome sequencing project (Stein et al., 2003) revealed that the genomes of C. briggsae and C. elegans have much in common. For example, both worms have the same number of chromosomes (six chromosomes each), similar genome size, and similar numbers of protein coding and non-protein coding genes. Further analysis demonstrated that about two-third of the protein coding genes in C. briggsae have orthologs in C. elegans. Nevertheless, many interesting species-specific features including species-specific genes exist, which serve as the foundation for comparative analysis.