Nature's Way of Counting Digits

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The elephant fish, a deep-water cartilaginous cousin to the shark, is often caught and served deep-fried with chips in the Southwestern Pacific whenever the females come up to shallow waters to lay their eggs. But the taste of the fish is not what keeps Andrew Gillis coming back for more. Where local fisherman cast reels for the adult females, Gillis goes on an egg hunt — scouring with SCUBA gear the soft sediment for prickly pouches hidden in the muck that each contain a developing embryo attached to its yolk sac.

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Gillis is tracking evolutionary history and pinning down the process that likely determines how many fins, fingers, toes, or any appendage from the body for that matter, a species has at birth. “Unfortunately, elephant fish like to lay their eggs in cold, muddy, shark-infested bays, so we spent months seeking out sites like this in southeastern Australia and New Zealand. When you finally find a few eggs in the muck, it feels like winning the lottery,” he said in a press release.

Last year, Gillis — while working as a graduate student at the University of Chicago with Neil Shubin, head of the team that discovered the leggy Tiktaalik fossil fish — showed that gills share the same developmental programming as fins.

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The link is a gene that vertebrates and invertebrates alike use during embryonic development. First identified in the fruitfly for causing a break-out of back bristles, the hedgehog gene is the leading culprit behind why some species of lizards have three toes and others five. Turn off one of the vertebrate copies of the gene, the sonic hedgehog gene, in sharks and skates, and the rows of appendages called branchial rays, which spoke outward from frilly gills, disappear or are reduced to nubs. On sharks each row of branchial rays support a flap that protects the delicate gills and corresponds to the number of gill slits seen on the side of the shark, typically five but some species have as many as six or seven gill slits along their sides.

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Knowing that the elephant fish has only one row of branchial rays supporting a single gill flap, Gillis wanted to stain the eggs for the sonic hedgehog gene and see if and when it was active. “The elephant fish have the same number of gills as sharks, just a different way of protecting them. They use one very long flap.” Gillis said. “Independently they have hit on the same way of protecting gills as boney fish,” he told Discovery News.

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Gillis moved to the University of Cambridge to conduct his post-doctoral work on the elephant fish in Clare Baker’s developmental biology lab. Surprisingly, the fish showed a set of five primordial rows of branchial rays during the early stages of the fish’s embryonic development, but not surprisingly the sonic hedgehog gene was turned on for all five rows. Later the signaling for the gene was active, as expected, only on the single row that the fish have as an adult. Gillis and his team published their results in the Jan. 10 issue of the Proceedings of the National Academy of Sciences.

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Image 1: An overhead and side view of the head skeletal anatomy of a skate (left), a shark (center) and an elephant fish (right) showing cartilage that is stained blue. All surrounding tissue has been cleared away. From the PNAS report courtesy of the University of Chicago/University of Cambridge.

Image 2: Inset of an elephant fish embryo on its yolk courtesy of the University of Chicago/University of Cambridge. An elephant egg case in the wild, photo by A. Gillis.

Image 3: Staining of an early stage (top) and late stage (bottom) embryo showing where the sonic hedgehog gene that controls the growth of gill rays is active in the elephant fish. Courtesy of the University of Chicago/University of Cambridge.