Transcriptional feedback targeting Wnt pathway components reveals hidden heterogeneity in C. elegans seam cell lineages.

Asymmetric cell division in the epidermal stem cells of Caenorhabditis elegans, known as seam cells, relies on the Wnt/{beta}-catenin asymmetry pathway to generate daughter cells with distinct fates. However, whether components of this pathway components are transcriptionally regulated during these divisions remains unclear. Here, we employ single molecule fluorescence in situ hybridisation to quantify mRNA distributions of key Wnt pathway components during L2 symmetric and asymmetric seam cell divisions. We find that transcripts encoding the negative regulators pry-1/Axin and apr-1/APC are enriched in posterior daughter cells, while those encoding the positive regulators sys-1/{beta}-catenin, wrm-1/{beta}-catenin, and lit-1/NLK, along with the transcription factor pop-1/TCF, are enriched in anterior daughter cells. Strikingly, molecular asymmetries are already evident following the L2 symmetric division, with anterior and posterior daughters exhibiting distinct levels of Wnt component expression and Wnt pathway activation. These mRNA distributions are surprising considering the established protein localisations that underpin the Wnt asymmetry model and suggest extensive post-divisional transcriptional regulation. We further demonstrate that pop-1 asymmetric expression depends on Wnt signalling activity, supporting a model in which transcriptional feedback reinforces cell fate decisions. Investigation of protein distributions using knock-in reporters for PRY-1 and CAM-1 showed that protein accumulation patterns at L2 are consistent with transcript levels. Our findings uncover pervasive transcriptional feedback within the Wnt pathway that likely contributes to robust fate specification and reveal molecular heterogeneity with potential functional consequences for lineage behaviour.