How Single-Cell Transcriptomics of Hydractinia Is Informing the Evolution of Cnidarian Sensory Systems

For over a century, the colonial cnidarian Hydractinia has been employed as a research organism to study stem cells, germ cells, regeneration, and coloniality. For the last 70 years, it has also been used in allorecognition research within the field of comparative immunology. More recently, other aspects of Hydractinia biology, including sensory biology, have been explored. Hydractinia colonies are composed of a limited number of repeating structural units: polyps and the stolon tissue that connects polyps. Polyps are divided into three major types used for feeding, reproduction, or defense. Clonal lines grown in the lab provide unlimited material from a single genetic unit. Colonies have separate sexes and spawn regularly with exposure to light. Recently, genomic and transcriptomic resources have been released for two species of Hydractinia: H. symbiolongicarpus and H. echinata. Tools for gene expression manipulation have been developed for this organism, including CRISPR/Cas9 knockout, shRNA knockdown, and overexpression via synthetic RNA. Fluorescent transgenic reporter lines have been created via random integration of circular DNA plasmids and CRISPR/Cas9-mediated gene knockin. We recently constructed an updated single-cell transcriptomic atlas of adult Hydractinia colonies to explore the cellular biology and cell-type expression profiles of the animal. We are investigating known and novel cell types and validating spatial expression patterns of cell-type specific markers to enable further understanding of the animal's cellular biology. This includes gaining a deeper understanding of the genetic control of cell differentiation of specific cell types from progenitor populations and uncovering the diversity of transcriptional subtypes that may be relevant to specific functions. Since Hydractinia is a model for whole-body regeneration, the identification and validation of new cell type and cell state markers will now allow for the elucidation of potential pathways involved in regenerating specific cell types, including testing alternative pathways for regeneration that include dedifferentiation and transdifferentiation. Hydractinia is poised to become a model for sensory biology research, as we can now fully explore their sensory cell types, including cnidocytes and neurons, and the expression and evolution of their gene complement with modern approaches and tools.