The
let-7 gene encodes a highly conserved microRNA with critical functions integral to cell fate specification and developmental progression in diverse animals. In Caenorhabditis elegans,
let-7 is a component of the heterochronic (developmental timing) gene regulatory network, and loss-of-function mutations of
let-7 result in lethality during the larval to adult transition due to misregulation of the conserved
let-7 target,
lin-41. To date, no bilaterian animal lacking
let-7 has been characterized. In this study, we identify a cohort of nematode species within the genus Caenorhabditis, closely related to C. elegans, that lack the
let-7 microRNA, owing to absence of the
let-7 gene. Using Caenorhabditis sulstoni as a representative
let-7-lacking species to characterize normal larval development in the absence of
let-7, we demonstrate that, except for the lack of
let-7, the heterochronic gene network is otherwise functionally conserved. We also report that species lacking
let-7 contain a group of divergent
let-7 paralogs-also known as the
let-7-family of microRNAs-that have apparently assumed the role of targeting the LIN-41 mRNA.