The ELT-3 GATA Factor Specifies Endoderm inCaenorhabditis angariain an ancestral gene network

Endoderm specification in the nematode,C. elegans, occurs through a well-characterized pathway that is initiated by maternally provided SKN-1/Nrf, and with additional input from POP-1/TCF, which activates the GATA factor cascade MED-1,2 → END-1,3 → ELT-2,7. Orthologues of the MED and END factors, and ELT-7, are found only among nematodes of the Elegans Supergroup consisting of species closely related toC. elegans, which raises the question of how gut is specified in their absence. In this work, we investigate gut specification outside the Elegans Supergroup. We find that theC. angariaandC. portoensisorthologues of theelt-3GATA factor gene are expressed in the early E lineage, just before theirelt-2orthologues. InC. angaria, bothCan-pop-1(RNAi)andCan-elt-3(RNAi)result in a penetrant ‘gutless’ phenotype.Can-pop-1is necessary forCan-elt-3activation, showing that it acts upstream. When introduced intoC. elegansas transgenes, overexpressedCan-elt-3is sufficient to specify gut, whileCan-elt-2can rescue gut differentiation under the control of its own promoter. Our results demonstrate an ancestral mechanism for gut specification and differentiation inCaenorhabditisinvolving a simplified gene network consisting of POP-1 → ELT-3 → ELT-2.