Photoreceptor Diversification Accompanies the Evolution of Anthozoa.

Anthozoan corals are an ecologically important group of cnidarians, which power the productivity of reef ecosystems. They are sessile, inhabit shallow, tropical oceans and are highly dependent on sun- and moonlight to regulate sexual reproduction, phototaxis, and photosymbiosis. However, their exposure to high levels of sunlight also imposes an increased risk of UV-induced DNA damage. How have these challenging photic environments influenced photoreceptor evolution and function in these animals? To address this question, we initially screened the cnidarian photoreceptor repertoire for Anthozoa-specific signatures by a broad-scale evolutionary analysis. We compared transcriptomic data of more than 36 cnidarian species and revealed a more diverse photoreceptor repertoire in the anthozoan subphylum than in the subphylum Medusozoa. We classified the three principle opsin classes into distinct subtypes and showed that Anthozoa retained all three classes, which diversified into at least six subtypes. In contrast, in Medusozoa, only one class with a single subtype persists. Similarly, in Anthozoa, we documented three photolyase classes and two cryptochrome (CRY) classes, whereas CRYs are entirely absent in Medusozoa. Interestingly, we also identified one anthozoan CRY class, which exhibited unique tandem duplications of the core functional domains. We next explored the functionality of anthozoan photoreceptors in the model species Exaiptasia diaphana (Aiptasia), which recapitulates key photo-behaviors of corals. We show that the diverse opsin genes are differentially expressed in important life stages common to reef-building corals and Aiptasia and that CRY expression is light regulated. We thereby provide important clues linking coral evolution with photoreceptor diversification.

Non-muscle myosin II drives critical steps of nematocyst morphogenesis.

Nematocysts are generated by secretion of proteins into a post-Golgi compartment. They consist of a capsule that elongates into a long tube, which is coiled inside the capsule matrix and expelled during its nano-second discharge deployed for prey capture. The driving force for discharge is an extreme osmotic pressure of 150 bar. The complex processes of tube elongation and invagination under these biomechanical constraints have so far been elusive. Here, we show that a non-muscle myosin II homolog (HyNMII) is essential for nematocyst formation in Hydra. In early nematocysts, HyNMII assembles to a collar around the neck of the protruding tube. HyNMII then facilitates tube outgrowth by compressing it along the longitudinal axis as evidenced by inhibitor treatment and genetic knockdown. In addition, live imaging of a NOWA::NOWA-GFP transgenic line, which re-defined NOWA as a tube component facilitating invagination, allowed us to analyze the impact of HyNMII on tube maturation.

Wnt/ß-catenin signaling induces axial elasticity patterns of Hydra extracellular matrix.

The extracellular matrix (ECM) plays crucial roles in animal development and diseases. Here, we report that Wnt/ß-catenin signaling induces the ECM remodeling during Hydra axis formation. We determined the micro- and nanoscopic arrangement of fibrillar type I collagen along Hydra's body axis using high-resolution microscopy and X-ray scattering. Elasticity mapping of the ECM ex vivo revealed distinctive elasticity patterns along the body axis. A proteomic analysis of the ECM showed that these elasticity patterns correlate with a gradient-like distribution of metalloproteases along the body axis. Activation of the Wnt/ß-catenin pathway in wild-type and transgenic animals alters these patterns toward low ECM elasticity patterns. This suggests a mechanism whereby high protease activity under control of Wnt/ß-catenin signaling causes remodeling and softening of the ECM. This Wnt-dependent spatiotemporal coordination of biochemical and biomechanical cues in ECM formation was likely a central evolutionary innovation for animal tissue morphogenesis.

A small molecule screen identifies novel inhibitors of mechanosensory nematocyst discharge in Hydra.

Cnidarians are characterized by the possession of stinging organelles, called nematocysts, which they use for prey capture and defense. Nematocyst discharge is controlled by a mechanosensory apparatus with analogies to vertebrate hair cells. Members of the transient receptor potential (TRPN) ion channel family are supposed to be involved in the transduction of the mechanical stimulus. A small molecule screen was performed to identify compounds that affect nematocyst discharge in Hydra. We identified several [2.2]paracyclophanes that cause inhibition of nematocyst discharge in the low micro-molar range. Further structure-activity analyses within the compound class of [2.2]paracyclophanes showed common features that are required for the inhibitory activity of the [2.2]paracyclophane core motif. This study demonstrates that Hydra can serve as a model for small molecule screens targeting the mechanosensory apparatus in native tissues.

Extracellular matrix and morphogenesis in cnidarians: a tightly knit relationship.

Cnidarians, members of an early-branching metazoan phylum, possess an extracellular matrix (ECM) between their two epithelial cell layers, called the mesoglea. The cnidarian ECM, which is best studied in Hydra, contains matrix components reflective of both interstitial matrix and basement membrane. The identification of core matrisome components in cnidarian genomes has led to the notion that the basic composition of vertebrate ECM is of highly conserved nature and can be traced back to pre-bilaterians. While in vertebrate classes ECM factors have often diverged and acquired specialized functions in the context of organ development, cnidarians with their simple body plan retained direct links between ECM and morphogenesis. Recent advances in genetic manipulation techniques have provided tools for systematically studying cnidarian ECM function in body axis patterning and regeneration.