Secondary Simplification of the Tardigrade Gut Patterning Mechanism

The through-gut of nephorozoans is typically patterned by a highly conserved mechanism. This mechanism is predicted to pattern the gut of tardigrades. However, since tardigrades have secondarily evolved a simplified body plan, it is possible that the mechanism that patterns the tardigrade gut is also simplified. This simplification could be identified by the loss of an ancestral gut-patterning gene, the loss of a gene’s function in patterning the gut, or by reduction in regionalized expression patterns of genes during gut development. We investigated the genomes of two species of Tardigrada—Hypsibius exemplarisand Ramazzottius varieornatus. We identified single orthologs of forkhead (fkh), gata456, orthodenticle (otd), caudal (cad), goosecoid, hnf-4, even-skipped, and fgf8/17/18—all genes that play important roles in the gut patterning mechanism in other nephrozoans. However, we were unable to identify orthologs of foxq2, brachyury, nk2.1, or wingless, genes that typically pattern the through-gut of other nephrozoans. This suggests that these genes were lost in the tardigrade lineage. We have investigated the expression patterns of otd, fkh, gata456, and cad in H. exemplaris embryos. We found that He-otd and He-cad were expressed in the anterior and posterior of the developing body axis, as predicted. Furthermore, we found that He-fkh is expressed throughout the developing gut, also as predicted. However, we detected expression of He-gata456 broadly throughout the developing gut, rather than it being restricted to the developing midgut as is typically seen in other nephrozoans. Together, our results provide preliminary evidence of a simplified gut patterning mechanism in Tardigrada.