A thyroid hormone-mediated opsin switch initiates metamorphosis in a proto-vertebrate.

Abstract

Thyroid hormones (THs) initiate metamorphosis in vertebrates, although the evolutionary origins of this process are uncertain. Here, we show that most TH processing genes are present in the proto-vertebrate model <i>Ciona</i> and play an instructive role in initiating metamorphosis, whereby swimming tadpoles with a chordate body plan are transformed into sessile filter feeders. Exogenous thyroxine (T4) accelerates metamorphosis, whereas TH inhibitors delay onset. Most notably, we present evidence that TH activates <i>Opsin2</i> (<i>Opsin1/2b</i>) in a subset of photoreceptor cells in the simple brain of swimming tadpoles to initiate attachment, the first step in metamorphosis. Our findings suggest a deep evolutionary origin of TH-driven visual plasticity in vertebrates. We highlight the parallels between attachment of <i>Ciona</i> tadpoles and smoltification, whereby young salmonid fishes switch from UV to blue opsins for their transition from fresh to saltwater.