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

Despite its status as a model organism, the development of Caenorhabditis elegans is not archetypical for nematodes. C. elegans is but one species in the diversity of Nematoda, which is divided into the Chromadorea (including C. elegans) and the Enoplea (including Enoplia and Dorylaimia). Enoplean nematodes are known to 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 analysis of genetic circuitry underpinning develiopment in this species is difficult. We sequenced and assembled a draft genome of R. culcivorax and compared its developmental gene content with that of C. elegans, the previously sequenced dorylaim Trichinella spiralis and the beetle 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 were lost in C. elegans. T. spiralis, despite its close phylogenetic affinity to R. culcivorax, has independently lost even more of the toolkit than C. elegans. Many genes essential for embryogenesis in C. elegans are missing, or massively diverged, in R. culicivorax and T. spiralis, suggesting extensive changes in the genetic programmes for early cell specification, inductive interactions, vulva formation, gene regulation, sex determination and light sensing. The considerable molecular differences in developmental pathways between nematode species indicate that Developmental System Drift must have played a predominant role during the evolution of the Nematoda, despite the conservation of their body plan. 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. culcivorax as a research model.