Caenorhabditis elegans is the best-characterized species in the Caenorhabditis genus, or, for that matter, in the nematode phylum of animals.
Although C. elegans belongs to a set of highly similar species (the Elegans group), it has no known sister species (i.e., there exists no known Caenorhabditis species that is more closely related to C. elegans than to C. briggsae et al.). There is a $5000 reward available for the first person to successfully isolate a live, stable culture of a C. elegans sister species; see the methods for species isolation and earlier prize announcement for more details.
Sex Determination: androdioecious
Haploid No. chromosomes: 6 (5 autosomes, XO)