The pou-iv (pow-four) gene is present in the tentacles of the starlet sea anemone (Nematostella vectensis), which is important for the animal's touch sensation.
Cnidaria, the phylum in which sea anemones are classified, is the most closely related to Bilateria, which includes animals with bilateral symmetry such as humans, having diverged from their last common ancestor 748 to 604 million years ago.
Hair cells are the sensory receptors of the auditory system in humans and other vertebrates. These cells have bundles of finger-like organelles called stereocilia that detect mechanical stimuli, specifically the vibrations we perceive as sound. Pou-iv is required for the development of hair cells in mammals; we know this because mice with pou-iv knocked out are deaf.
The tentacles of the starlet sea anemone include comparable mechanosensory hair cells that detect movement. However, nothing was known about the pou-iv gene in anemones and what, if any, role it performed in sensory development.
A team of researchers led by University of Arkansas biologist Ethan Ozment wanted to figure out what the gene was doing. The best way to do this is to use the CRISPR-Cas9 gene-editing tool to disable the gene and then observe what happens. So that's what the group did.
They injected a Cas9 protein cocktail into fertilized starlet sea anemone eggs to remove the pou-iv gene and then studied the developing embryos as well as the grown, mutated anemones.
"This study is exciting because it not only opens a new field of research into how mechanosensation develops and functions in a sea anemone... but it also tells us that the building blocks of our sense of hearing have ancient evolutionary roots dating back hundreds of millions of years into the Precambrian," said Nagayasu Nakanishi, a biologist at the University of Arkansas.
The mutant anemones had aberrant tentacular hair cell growth and no sensitivity to touch as compared to wild-type control anemones. The anemones were unable to feel mechanical stimuli via their hair cells without pou-iv.
Furthermore, knocking down pou-iv in anemones greatly inhibited a gene that is extremely similar to the one that produces polycystin invertebrates, which is necessary for fluid flow sensing in kidneys. Although sea anemones do not have kidneys, the capacity to detect fluid movement might be beneficial to marine species.
"How early the significance of pou-iv in mechanoreceptor differentiation originated in animal evolution is uncertain," the researchers stated in their report, "and requires comparative data from placozoans and sponges, which are lacking."
The findings suggest that pou-iv played a role in the development of mechanosensory cells in the common ancestor of Cnidaria and Bilateria, according the researchers. To trace the gene back even further, data from other phyla with earlier divergence points will be needed.
