Caenorhabditis monodelphis sp. n.: defining the stem morphology and genomics of the genus Caenorhabditis

Background: The genus Caenorhabditis has been central to our understanding of metazoan biology. The best-known species, Caenorhabditis elegans, is but one member of a genus with around 50 known species, and knowledge of these species will place the singular example of C. elegans in a rich phylogenetic context. How did the model come to be as it is today, and what are the dynamics of change in the genus? Results: As part of this effort to “put C. elegans in its place”, we here describe the morphology and genome of Caenorhabditis monodelphis sp. n., previously known as Caenorhabditis sp. 1. Like many other Caenorhabditis, C. monodelphis sp. n. has a phoretic association with a transport host, in this case with the fungivorous beetle Cis castaneus. Using genomic data, we place C. monodelphis sp. n. as sister to all other Caenorhabditis for which genome data are available. Using this genome phylogeny, we reconstruct the stemspecies morphological pattern of Caenorhabditis. Conclusions: With the morphological and genomic description of C. monodelphis sp. n., another key species for evolutionary and developmental studies within Caenorhabditis becomes available. The most important characters are its early diverging position, unique morphology for the genus and its similarities with the hypothetical ancestor of Caenorhabditis.