Major changes in the core developmental pathways of nema- todes: Romanomermis culicivorax reveals the derived status of the Caenorhabditis elegans model. Romanomermis culicivorax

Despite its status as a model organism, the development of Caenorhabditis elegans is not necessarily archetypical for nematodes. The phylum Nematoda is divided into the Chromadorea, which indcludes C. elegans, and the Enoplea (which includes subclasses Enoplia and Dorylaimia). Compared to C. elegans, enoplean nematodes have very different patterns of cell division and determination from early embryogenesis onward.Embryogenesis of the dorylaim Romanomermis culicivorax has been studied in great detail, but the genetic circuitry underpinning development in this species is unknown. We sequenced and assembled a draft genome of R. culicivorax and compared its developmental gene content with those of C. elegans, the dorylaim nematode Trichinella spiralis, and the arthropod Tribolium castaneum. The ∼300 Mb genome contains many repetitive elements, but its non-repetitive part is about twice as large as that of C. elegans and about three times that of T. spiralis. R. culicivorax has retained components of the conserved metazoan developmental toolkit that have been lost in C. elegans. T. spiralis, despite its close phylogenetic affinity to R. culicivorax, has independently lost even more of the toolkit than C. elegans. However, the C. eleganstoolkit is not simply depauperate, as many genes essential for embryogenesis in C. elegans are missing, or extremely diverged, in R. culicivorax and T. spiralis. These data imply fundamental differences in the genetic programmes for early cell specification, inductive interactions, vulva formation, sex determination and light sensing between these nematodes. The considerable molecular differences in developmental pathways between nematode species, despite their apparent morphological conservatism, indicate that important changes in developmental programmes have evolved. R. culicivorax serves as a contrasting model to C. elegans for understanding how very different genomic and thus regulatory backgrounds can generate an apparently conserved phenotype. The availability of the draft genome will promote use of R. culicivorax as a research model.