The
med-1 and
med-2 genes encode small highly-similar proteins related to GATA-type transcription factors and have been proposed to be necessary for specification of both C. elegans mesoderm and endoderm. However, we have previously shown that neither maternal nor zygotic expression of the
med-1/2 genes are necessary to specify the C. elegans endoderm: the majority (>80%) of embryos zygotically lacking both the
med-1 gene (removed by a gene specific deletion) and the
med-2 gene (removed by a chromosome deficiency) nonetheless still express standard markers of endoderm specification (e.g. gut granules); standard
med-1/2 RNAi to remove possible
med-1/2 maternal transcripts showed no effect. In the present paper, we re-investigate
med-2(-);
med-1(-) embryos using a
med-2 specific null allele instead of the chromosomal deficiences used previously: again, we show that the large majority (~84%) of
med-2(-);
med-1(-) embryos still express gut granules, in precise agreement with our previous results obtained using the chromosomal deficiency sDf127 to remove the
med-2 gene. A recent report that the
med-1/2 genes show an endoderm-specifying maternal effect was based on the assumption that multiple transgenic copies of the
med-1(+) gene (used to balance the strain) cause cosuppression, removing putative
med-1/2 transcripts from the maternal germline. Here, we re-investigate this proposal by direct
med-1/2 RNAi using the standard protocol known to be effective in ablating maternal transcripts but, as we found previously, we could detect no evidence for a maternal
med-1/2 effect. We do however show that expression of gut-granules in
med-1/2-deficient embryos is exquisitely sensitive to RNAi against the vacuolar ATPase-encoding
unc-32 gene; multiple copies of
unc-32 sequences were present on the same multicopy
med-1(+)-containing transgenic balancer used in support of the maternal
med-1/2 effect. We thus suggest that the experimental evidence for a maternal
med-1/2 effect should be reinterpreted and may instead reflect cosuppression caused by multiple
unc-32 sequences, not by multiple med sequences. Indeed, the
unc-32-mediated cosuppression model provides a simple explanation for several further experimental observations not easily explained by med-based cosuppression. Embryos lacking the
med-1/2 genes are severely deranged and it is clear that the
med-1/2 genes are of great importance for correct embryonic development. Nonetheless, it is also clear that >80% of
med-2(-);
med-1(-) embryos still express markers of endoderm specification (gut granules) and we suggest that recent evidence for a
med-1/2 maternal effect should be re-examined.