Free The Universal Sense by Seth Horowitz Page A

of developmental research to be carried out on them with greater ease than in a shell-covered chicken or uterus-enclosed mammal. The tadpoles themselves are also transparent, allowing the injection of dyes and tracers to determine cell fate mapping, tracing the course of developmentand migration of individually labeled cells even after they divide. And X. laevis was the first vertebrate species to be cloned. For quite a while it was the de rigueur amphibian for molecular and genetic studies, including hundreds of studies in how ears develop. However, with increasing knowledge comes the potential for the “oops” moment. While Xenopus laevis led the way for some of the most important work in developmental biology, it turns out that its genetics are very odd for a vertebrate—it is an allotetraploid , meaning it has four copies of each gene rather than two copies, as do humans and most other vertebrates, which are diploids. This means that X. laevis can never be selectively manipulated to delete or “knock out” genes or create specific mutations, and some of the genetic work done with the species is now in question. Much of this work that was done in this species is now being replicated in its closely related cousin X. tropicalis , a smaller, shorter-lived, but diploid species.
    But X. laevis is an interesting animal for studying hearing. Although it is an amphibian, it is totally aquatic its whole life, from limbless swimming tadpole through four-legged carnivorous (and sometimes cannibalistic) adult. Like fish, it has a lateral line system, a series of external hair cells organized in interrupted lines called stitches across its head and sides to detect changes in water movement. Young X. laevis tadpoles use this system to determine the direction of the water current and orient themselves toward it to help maintain buoyancy and stability. This behavior, called rheotaxis , is used to help them maintain not only their position within a body of water but also their position relative to other tadpoles in their school and to detect sudden changes in water flow that might indicate the presence of a predator. Adults, which can get to be up to 10 inches long and weigh over a pound, typically lie near the bottom of a murkypond and so have limited access to light. The adult’s lateral line is used to detect the motion of small insects or fish above them, which they then rush up and grab in their clawed fingers and shove into their wide spatulate mouths, their strange upward-looking eyes almost useless until they approach the water’s surface. And like most totally aquatic animals, they have no external ears.
    But the odd, flattened four-legged-fish appearance of these creatures hides the fact that they represent a major step in the evolution of hearing. While they share the fish’s saccule (which may play a role in their hearing, particularly when they are tadpoles), they also have additional inner ear organs, called the amphibian and basilar papillae, small hair-cell-rich structures dedicated to hearing underwater. The amphibian papilla consists of a membrane stretching across the inner ear with hair cells that respond to lower-frequency sounds, from about 50 to 1,000 Hz, arranged in a loosely tonotopic , or frequency-specific, order. The basilar papilla is a smaller cup-shaped organ full of hair cells that respond to higher-frequency sounds, typically up to about 4,000 Hz. And while the saccule is still there, X. laevis , unlike fish, has a middle ear, consisting of internal cartilaginous tympanic disks, homologues of our eardrums, which are different enough in density from the surrounding tissue and water to allow pressure changes from sound to vibrate them and is connected to the inner ear by a piece of cartilage called a stapedium .
    This sounds like X. laevis would be a peculiar species to use if we’re trying to understand anything about humans. But strange and ancient as it is, X. laevis is an amazing model for