Setting a trap to study the divergence of two homologous organs

Abstract

The tracheal and endocrine primordia form at homologous positions and can be transformed into one another by misexpression of Hox genes. Besides, we recently showed that both organs are specified by similar Wnt, Hh and JAK/STAT upstream inputs. These observations led us to hypothesize that these organs are serial homologs that formed by the divergent evolution of their developmental gene networks. Among the main differences, the endocrine primordia induce the snail (sna) gene through a sna-rg enhancer, which drives an epithelial to mesenchymal transition (EMT), whereas in the tracheal primordia sna is not expressed and no EMT is induced. Therefore, elucidating which genes are differentially expressed between these two organs may give us an idea of how these homologous organs have diverged during evolution. To try to answer this question, we carried the Translating Ribosome Affinity Purification (TRAP) technique to isolate actively translated mRNA on both organs. We used a trh and a sna enhancer to express a tagged ribosomal protein in tracheal and endocrine cells respectively. Using this approach, we were able to enrich for genes expressed in the tracheal system, although enrichment for gland genes seems to be less evident. One explanation for this might be given by the low levels of unspecific expression activated by the sna driver. Additionally, the low number of cells present in the CA and PG primordia, only 48-54 cells per embryo, can have an impact on the performance of the technique. We will summarize our current results and discuss the efficiency of the technique on both organs.