The ELT-3 GATA Factor Specifies Endoderm in Caenorhabditis angaria in 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 to C. 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 the C. angaria and C. portoensis orthologues of the elt-3 GATA factor gene are expressed in the early E lineage, just before their elt-2 orthologues. In C. angaria, both Can-pop-1(RNAi) and Can-elt-3(RNAi) result in a penetrant gutless phenotype. Can-pop-1 is necessary for Can-elt-3 activation, showing that it acts upstream. When introduced into C. elegans as transgenes, overexpressed Can-elt-3 is sufficient to specify gut, while Can-elt-2 can rescue gut differentiation under the control of its own promoter. Our results demonstrate an ancestral mechanism for gut specification and differentiation in Caenorhabditis involving a simplified gene network consisting of POP-1 [->] ELT-3 [->] ELT-2. Summary statementSpecification of the gut progenitor E in a distant relative of C. elegans uses a different GATA factor, ELT-3, suggesting that the ancestral network was simpler.