Gene regulatory networks (GRNs) that drive specification of the endoderm are well-studied in Caenorhabditis elegans. Here, a number of GATA factors are crucial to endoderm and subsequent gut formation, including ELT-2, ELT-7, END-1 and END-3, which specify C. elegans gut progenitor E cells that give rise to intestinal cell fates. However, orthologues of ELT-7, END-1 and END-3 are not found in distantly related nematodes, such as C. angaria. Therefore, Morris Maduro and colleagues aimed to uncover the GATA factors and associated GRNs that specify C. angaria endoderm. Here, the authors characterise a simpler pathway, consisting of only one specification factor, ELT-3, in place of the three found in C. elegans. Using single-molecule fluorescence in situ hybridisation, they find that ELT-3 is expressed in both endodermal and hypodermal regions, but this expression is attenuated by knockdown of POP-1, a GATA factor that acts upstream in the C. elegans GRN. An additional conserved feature they describe is ELT-2, which acts downstream in both nematodes and is necessary for gut formation. Together, these findings suggest that the role of ELT-3 in C. angaria endoderm specification is likely to be ancestral, but the absence of GATA factors found in C. elegans suggests a divergent evolution.
