The nematode Caenorhabditis elegans has been central to the understanding of metazoan biology. However, C. elegans is but one species among millions and the significance of this important model organism will only be fully revealed if it is placed in a rich evolutionary context. Global sampling efforts have led to the discovery of over 50 putative species from the genus Caenorhabditis , many of which await formal species description. Here, we present species descriptions for 10 new Caenorhabditis species. We also present draft genome sequences for nine of these new species, along with a transcriptome assembly for one. We exploit these whole‐genome data to reconstruct the Caenorhabditis phylogeny and use this phylogenetic tree to dissect the evolution of morphology in the genus. We reveal extensive variation in genome size and investigate the molecular processes that underlie this variation. We show unexpected complexity in the evolutionary history of key developmental pathway genes. These new species and the associated genomic resources will be essential in our attempts to understand the evolutionary origins of the C. elegans model.

SUMMARY In adult mammalian brains, neurogenesis persists in the subventricular zone of the lateral ventricles (SVZ) and the dentate gyrus (DG) of the hippocampus. Although evidence suggest that adult neurogenesis in these two regions is subjected to differential regulation, the underlying mechanism is unclear. Here we show that the RNA-binding protein FXR2 specifically regulates DG neurogenesis by reducing the stability of Noggin mRNA. FXR2 deficiency leads to increased Noggin expression and subsequently reduced BMP signaling, which results in increased proliferation and altered fate specification of neural stem/progenitor cells in DG. In contrast, Noggin is not regulated by FXR2 in the SVZ, because Noggin expression is restricted to the ependymal cells of the lateral ventricles, where FXR2 is not expressed. Differential regulation of SVZ and DG stem cells by FXR2 may be a key component of the mechanism that governs the different neurogenic processes in these two adult germinal zones.