Acoel Module

Acoel worms belong to a lineage (Phylum: Acoelomorpha) that is likely the sister group to all other animals with bilateral symmetry (bilaterians). Acoels are “wormy”, with clear anterior-posterior and dorsal-ventral axes; they have organized subepidermal nervous systems, but lack eyes, excretory organs, and a true gut. Acoels diverged from other bilaterians 550 million years ago, a node on the animal tree that corresponds with the origins of many features, e.g., bilateral symmetry, a centralized nervous system, and mesoderm (the third germ layer) and true muscle. Therefore, the study of acoels is important for many questions about the evolution of development during early animal evolution.

The acoel Hofstenia miamia, commonly known as the three-banded panther worm, is a new laboratory model for molecular studies of development and regeneration.  The animals can be maintained as a sexually reproducing population, producing juvenile worms in substantial numbers to enable large-scale experiments. mRNA and protein localization can be studied via whole-mount in situ hybridization and immunostaining respectively. Hofstenia are amenable to studies of gene function via RNA interference (RNAi) upon soaking or injecting the animals with dsRNA. Genomic resources are also available – the transcriptome can be downloaded from NCBI, and a genome assembly is currently being annotated.

Hofstenia reproduce sexually, producing hundreds of embryos in a single a day. Hofstenia embryos undergo a stereotype cleavage program called “duet cleavage”.  Hofstenia embryos are readily accessible and amenable to experimental procedures such as in situ hybridization and manipulations such as microinjection of dyes, mRNAs, and guide RNAs with Cas9 enzyme.

Regeneration in Hofstenia is similar to regeneration in planarians – both species generate anterior and posterior missing tissues when cut transversely. Acoels utilize similar patterning mechanisms to specify the identity of new tissue along the anterior-posterior (Wnt signaling) and dorsal-ventral axes (Bmp-Admp signaling) as planarians. Furthermore, Hofstenia has a parenchymal population of proliferating cells that are required for regeneration and express piwi, reminiscent of the neoblasts in planarians. Given that acoels and planarians diverged from the last common bilaterian ancestor 550 million years ago, these findings suggest that the genetic mechanisms of regeneration (including those for pluripotent stem cells) may be fundamental features of animal biology that were lost in vertebrates.

Studies in Hofstenia will complement and expand on many of the discoveries made in well- established model systems, and will inform us on the evolution of regenerative and developmetal mechanisms over animal evolution.

Activities in this module include, examining the anatomy of Hofstenia, observing regeneration, observing embryos, live imaging of embryos, whole mount antibody & phalloidin staining of animals, grafting animals together.