Meeting Abstract
Recent analyses of Hox genes have revealed that tardigrades have lost mid-trunk segments. The segments that are missing in tardigrades develop by posterior growth in most panarthropods (Arthropoda, Onychophora, Tardigrada). Posterior growth is thought to be an ancestral mechanism of Nephrozoa. This process is regulated by the canonical Wnt signaling pathway (cWnt) in nephrozoans that have been investigated. Tardigrades lack posterior growth, suggesting that the loss of this process may explain the loss of mid-trunk segments in tardigrades. We analyzed the genomes of two representatives of Eutardigrada, Hypsibius exemplaris and Ramazzottius varieornatus, to identify genomic signatures of the loss of posterior growth in tardigrades. We identified conserved intracellular components of cWnt signaling in the genomes of both species. We identified a full complement of Frizzled genes in both tardigrade genomes, genes that code for the receptor component of the cWnt signaling pathway. However, we were unable to identify several cWnt signaling ligand-coding genes—wnt1, wnt3, wnt6, wnt7, wnt8, and wnt10. Wnt1, wnt7, and wnt8 have been implicated in regulating posterior growth in arthropods. In order to test whether these gene losses are ancestral losses for Tardigrada, we sequenced the genome of Batillipes penakki, a representative of Heterotardigrada, which, with Eutardigrada, spans Tardigrada. We identified an ortholog of wnt1 in the genome of B. penakki. Therefore, our results indicate that the loss of several Wnt ligand-coding genes is correlated with the loss of posterior growth and mid-trunk segments in tardigrades, while the loss of wnt1 occurred specifically in the eutardigrade lineage.
