The GATA factor ELT-3 specifies endoderm in Caenorhabditis angaria in an ancestral gene network

Abstract

Endoderm specification in C. elegans occurs through a network in which maternally provided SKN-1/Nrf, with additional input from POP-1/TCF, activates the GATA factor cascade MED-1,2→END-1,3→ELT-2,7. Orthologues of the MED, END, and ELT-7 factors are found only among nematodes closely related to C. elegans, raising the question of how gut is specified in their absence in more distant species in the genus. We find that the C. angaria, C. portoensis and C. monodelphis orthologues of the elt-3 GATA factor gene are expressed in the early E lineage, just before their elt-2 orthologues. In C. angaria, Can-pop-1(RNAi), Can-elt-3(RNAi) and a Can-elt-3 null mutation result in a penetrant 'gutless' phenotype. Can-pop-1 is necessary for Can-elt-3 activation, showing that it acts upstream. Forced early E lineage expression of Can-elt-3 in C. elegans can direct the expression of a Can-elt-2 transgene and rescue an elt-7 end-1 end-3; elt-2 quadruple mutant strain to viability. Our results demonstrate an ancestral mechanism for gut specification and differentiation in Caenorhabditis involving a simpler POP-1→ELT-3→ELT-2 gene network.

Evolutionary Change in Gut Specification in Caenorhabditis Centers on the GATA Factor ELT-3 in an Example of Developmental System Drift

“…This pattern overlaps that of end-3 and end-1 together, suggesting that ELT-3 fulfills the roles of these factors in C. angaria [ 51 , 52 , 74 ]. Consistent with this, RNAi or a null mutation in Can-elt-3 resulted in penetrant loss of gut with a phenotype resembling C. elegans end-1,3 double mutants [ 9 , 16 ].…”

“…Our recent work in the species C. angaria , which lies outside of the Elegans supergroup, revealed that the upstream portion of gut specification in this species consists not of a multifactor GATA cascade but merely of the single GATA factor ELT-3 [ 9 ]. In this study, we showed that a C. angaria elt-2 translational reporter gene is expressed similarly to C. elegans elt-2 when introduced into C. elegans as a transgene.…”

“…Identification of this GATA factor was straightforward, as only ELT-1, ELT-3, and ELT-5, three primarily hypodermal GATA factors in C. elegans , have orthologues in C. angaria in addition to the endodermal ELT-2 factor [ 9 , 48 , 88 , 89 ]. By examining expression using single-molecule inexpensive FISH (smiFISH), we found that all three show similar embryonic expression to the corresponding genes in C. elegans , while Can-elt-3 showed an additional domain of expression in the early E lineage, from the time of its birth until the 4E stage [ 9 , 90 ]. This pattern overlaps that of end-3 and end-1 together, suggesting that ELT-3 fulfills the roles of these factors in C. angaria [ 51 , 52 , 74 ].…”

“…We tested for the putative roles of SKN-1 and POP-1 in C. angaria after identifying their orthologues. Depletion of Can-pop-1 resulted in a similar phenotype to C. briggsae pop-1(RNAi) , namely embryonic arrest and the absence of gut [ 9 , 12 ]. Consistent with a role for Can-POP-1 in the activation of Can-elt-3 , expression of Can-elt-3 was abolished in Can-pop-1(RNAi) embryos.…”

“…Nematodes of the genus Caenorhabditis exhibit examples of DSD [ 7 , 8 ]. We have studied the evolution of an early cell specification event in the nematode C. elegans and its relatives in the genus [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. Here, we will review how the intestinal progenitor, E, is specified through the intersection of maternal factors, an extrinsic Wnt signal, and a gene cascade of GATA-type transcription factors.…”