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Fat, Fat-like, and Dachsous family cadherins are giant proteins that regulate planar cell polarity (PCP) and cell adhesion in bilaterians. Their evolutionary origin can be traced back to prebilaterian species, but their ancestral function(s) are unknown. We identified Fat-like and Dachsous cadherins in Hydra, a member of phylum Cnidaria a sister group of bilaterian. We found Hydra does not possess a true Fat homolog, but has homologs of Fat-like (HyFatl) and Dachsous (HyDs) that localize at the apical membrane of ectodermal epithelial cells and are planar polarized perpendicular to the oral-aboral axis of the animal. Using a knockdown approach we found that HyFatl is involved in local cell alignment and cell-cell adhesion, and that reduction of HyFatl leads to defects in tissue organization in the body column. Overexpression and knockdown experiments indicate that the intracellular domain (ICD) of HyFatl affects actin organization through proline-rich repeats. Thus, planar polarization of Fat-like and Dachsous cadherins has ancient, prebilaterian origins, and Fat-like cadherins have ancient roles in cell adhesion, spindle orientation, and tissue organization.

Considerable attention has recently been focused on the potential involvement of DNA methylation in regulating gene expression in cnidarians. Much of this work has been centered on corals, in the context of changes in methylation perhaps facilitating adaptation to higher seawater temperatures and other stressful conditions. Although first proposed more than 30 years ago, the possibility that DNA methylation systems function in protecting animal genomes against the harmful effects of transposon activity has largely been ignored since that time. Here, we show that transposons are specifically targeted by the DNA methylation system in cnidarians, and that the youngest transposons (i.e. those most likely to be active) are most highly methylated. Transposons in longer and highly active genes were preferentially methylated and, as transposons aged, methylation levels declined, reducing the potentially harmful side effects of CpG methylation. In Cnidaria and a range of other invertebrates, correlation between the overall extent of methylation and transposon content was strongly supported. Present transposon burden is the dominant factor in determining overall level of genomic methylation in a range of animals that diverged in or before the early Cambrian, suggesting that genome defense represents the ancestral role of CpG methylation. [ABSTRACT FROM AUTHOR]

Understanding the dynamic cellular behaviours driving morphogenesis and regeneration is a long-standing challenge in biology. Live imaging, together with genetically encoded reporters, may provide the necessary tool to address this issue, permitting the in vivo monitoring of the spatial and temporal expression dynamics of a gene of interest during a variety of developmental processes. Canonical Wnt/β-catenin signalling controls a plethora of cellular activities during development, regeneration and adulthood throughout the animal kingdom. Several reporters have been produced in animal models to reveal sites of active Wnt signalling. In order to monitor in vivo Wnt/β-catenin signalling activity in the freshwater polyp Hydra vulgaris, we generated a β-cateGFP transgenic Hydra, in which eGFP is driven by the Hydra β-catenin promoter. We characterized the expression dynamics during budding, regeneration and chemical activation of the Wnt/β-cat signalling pathway using light sheet fluorescence microscopy. Live imaging of the β-cat-eGFP lines recapitulated the previously reported endogenous expression pattern of β-catenin and revealed the dynamic appearance of novel sites of Wnt/β-catenin signalling, that earlier evaded detection by mean of in situ hybridization. By combining the Wnt activity read-out efficiency of the β-catenin promoter with advanced imaging, we have created a novel model system to monitor in real time the activity of Hydra β-cat regulatory sequences in vivo, and open the path to reveal β-catenin modulation in many other physiological contexts. [ABSTRACT FROM AUTHOR]