May 2019    
  Skate key  
  Scientists ask the little skate big questions about limb evolution.  
  The little skate (Leucoraja erinacea) is a cartilaginous fish related to sharks and rays. (Photography by J. Andrew Gillis)  
  How does a fin become a limb? The little skate, with its giant fins and flat head, may hold the answer.  
  Tetsuya Nakamura and José Luis Gómez-Skarmeta, visiting investigators in the Marine Biological Laboratory's Whitman Center, think clues may be hiding in the little skate's DNA--in the genetic instructions that tell the tiny portion of protein-coding genes how to function.  
  Those regulatory elements are what make, for instance, mice look like mice and not humans, though we share nearly 90 percent of the same DNA. We know this because both mice (five species, anyway) and humans have had their genomes sequenced, along with a small but quickly growing number of other animals, plants, and microbes.  
  Nakamura and Gómez-Skarmeta are working to add the little skate to that group by fully mapping its genome and describing its genetic instructions so that it can be studied through the lens of evolutionary developmental biology.  
  Evo-devo, as the field is often called, studies the relationship between evolution and embryonic development, comparing how development is genetically regulated across different species to find how or when features emerge, such as fins evolving into limbs.  
  Skates--along with their shark and ray relatives--are some of the most primitive vertebrates. They don't have legs, but they do use pelvic fins to "walk" across the ocean floor.  
  Watch a juvenile little skate walk with tiny "legs" in this Science magazine video.  
  Skates also have extra wide pectoral fins, a unique trait, notes Nakamura. "In humans, if the hand develops too wide and it has five or six fingers, it's a serious disease. So we thought if we could identify what genes regulate the formation of this super-wide fin in the little skate, it would help us understand [the generation of diversity] in an ancestral vertebrate and also a human disease."  
  Nakamura was a postdoc in the lab of UChicago evolutionary biologist Neil Shubin when they realized they could explore the genetic origins of paired appendages by studying what controls fin formation in the little skate. Shubin sent Nakamura to MBL in 2015 to study skate embryos with Gómez-Skarmeta. There they identified several genes in the little skate that also exist in limbed animals but function differently--a discovery that gave the scientists a leg up in the search for the fin-to-limb transition.  
  Appendage appendix  
In 2004 UChicago paleontologist and evolutionary biologist Neil Shubin, author of Your Inner Fish, unearthed a 375-million-year-old fossil of a fish with limbs. Listen to Shubin discuss his discovery on the Big Brains podcast.
The unique arms of mammals started evolving before dinosaurs roamed the earth.
Did strong tides strand fish in tidal pools 400 million years ago, leading to the evolution of limbs? The moon was 10 percent closer to Earth back then, so…maybe.
Is limb regeneration the norm in nature and simply repressed in humans?
  Toe-tapping gene mapping  
  A Capella Science explains evolutionary development to the tune of Luis Fonsi's song "Despacito."  
  Still not sure what evo-devo is? Learn how genes determine the placement of feet set to a beat.  
  In case you missed it  
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