Imagine a fish with legs, not just fins. A creature that strolls along the ocean floor, hunting for prey with an almost eerie precision. This is the remarkable reality of the sea robin, an unusual fish with a host of dramatic adaptations suited for life on the sea floor. These animals are now providing insights into the evolutionary processes that gave rise to limbs and novel organs.
How did life transition from water to land? How did our ancient ancestors acquire limbs that allowed them to explore terrestrial environments? To explore these questions, a team of researchers led by Nicholas Bellono, MCB Professor, including those from David Kingsley’s lab at Stanford, established the sea robin as a model organism to probe the evolution of biological novelty and natural behavior.
This fascinating fish has evolved leg-like appendages that it uses to “walk” along the seafloor, searching for buried prey. By studying these creatures, the team hopes to uncover the genetic and developmental blueprints that drive the formation of new organs and behaviors.
In two new studies published in Current Biology, the team explores the genetic programs that specify novel limb formation in the sea robin, identifies legs as sensory organs, and defines how these adaptations drive species-specific behaviors.
Ancient Genes, New Limbs
(PDF)
The first study, “Ancient developmental genes underlie evolutionary novelties in walking fish,” examines the genetics of organ formation. By generating high-quality genomes for two distinct sea robin species and utilizing transcriptomics and CRISPR-Cas9 genome editing, the team identified the gene tbx3a as a critical driver of leg formation. Intriguingly, tbx3a is important for the formation of limbs in other animals. This suggests fish employ similar genetic programs that we use to make limbs to turn their fins into legs.
Next, the team assessed the functional consequences of tbx3a disruption to identify the evolutionary mechanisms of gene regulation across two sea robin species separated by ~18 million years of evolution that use legs for unique behaviors.
Sensory Legs: A Novel Adaptation
(PDF)
In the second study, “Evolution of novel sensory organs in fish with legs,” the researchers explore the sensory capabilities of sea robin legs. These appendages are not just for walking; they have evolved into sophisticated chemosensory organs that help the fish locate buried prey. Through a combination of physical modeling, genetic profiling, molecular and neurophysiology, and behavioral analyses, the team identified novel chemoreceptors that drive this unique predatory behavior.
The study also characterized the development and function of leg sensory specialization and conducted comparative evolutionary analyses across sea robin species from around the world to define how novel trait gain mediates new animal behavior.
This research not only sheds light on the evolutionary origins of limbs but also provides a new lens into how integrative genetic, developmental, and physiological adaptations mediate the gain of novel traits.
Looking ahead, Corey Allard, postdoctoral fellow in the Bellono lab and co-first author of the Current Biology papers, will soon establish his own lab at Harvard Medical School. There, he will continue studying the mechanisms by which sea robins develop sensory legs to understand where limbs come from and how they specialize for various forms of sensation.
