Skate Research Opens Window on Human Evolution
Story posted June 14, 2005
The skate is one of the oldest known organisms on the planet, dating back some 500 million years - long before animals walked onto land from the sea. As such, it offers a fascinating window on our evolutionary past.
A cartilaginous fish related to the shark, the skate resembles a ray and shares with its shark cousin a unique feature: Its intestine is a specialized corkscrew-like structure known as a spiral valve.
Little is known about its genetic patterning and evolutionary significance, but Bowdoin Assistant Research Professor of Biology Nicole Theodosiou is hoping to change that.
Through groundbreaking research on the spiral valve, Theodosiou is uncovering data that suggest that the shark/skate's gastrointestinal sytstem is an early, primitive form of the human digestive tract. It may even be an overlooked link in the development of the colon in land-based animals.
"Skates haven't changed in 500 million years, so it's like looking back in time," Theodosiou says. "Every living animal has a digestive tract and they all originate from the same tissue types in the early embryo. Understanding how the function of the structures in the digestive tract have been conserved in evolution - and the genes that control or determine those structures and functions - really gives us an insight into how we evolved."
The structure of the spiral valve reflects the unique environmental demands of the shark, which has no abdominal cavity. Its body is planned around a mouth and a long tail, with scant room to fit the complex organ system it needs to digest meat proteins.
"They have to increase the surface area for absorption of nutrients," explains Theodosiou, "so instead of having three meters of small intestine like we do, they have a spiral valve - and a tiny colon. It's a fantastic structure that you don't see in any other vertebrate class of animals, and we want to know if it is genetically patterned in the same way as our intestines."
To study the spiral valve, Theodosiou turned to skate embryos for tissue sampling. Unlike the shark, which develops inside the mother, skate embryos are relatively easy to procure. The mother lays eggs, which develop over several months inside a translucent, pronged sack. (They resemble seaweed and regularly wash up on Maine beaches.)
Working with lab assistant Andrea Jowdry and student researcher Daniel Hall '05, Theodosiou has sampled tissue from all sections of the skate's digestive tract to analyze its cell types. Thus far, the team has cloned five gene sequences in the skate that also are known to pattern the digestive tract of higher vertebrates, such as chickens, mice, and humans.
Their work took a surprising turn last winter.
During lab work in which student researcher Daniel Hall tallied the percentage of acid mucin cells in various sections of the digestive tract, he came across puzzling results. The concentration of these cells, which help to create the proper lumen, or tube cavity, for water absorption in the colon, was extremely low in the skate's colon - a place where they typically are high.
The percentage of acid mucins increased considerably in samples taken from the final coil of the skate's spiral valve -- before the start of the colon. This finding led Theodosiou to ponder the origin of the vertebrate colon.
"It's not surprising that we found no acid mucins in the colon, since the skate does not need to absorb water from food waste for hydration," says Theodosiou. "What was surprising was the sudden increase in acid mucin cells in the final turn of the spiral valve before entering the colon.
"It could be that this tiny region of acid mucins in the spiral valve expanded during evolution, once vertebrates emerged on land, to become our colon today."
The research project gave Hall a unique window into genome research, he says. "Not a lot of students end up actually sequencing genes that aren't known. It's pretty exciting to get results like these after a semester of work."
Theodosiou's work on the skate digestive tract relates to her other work on understanding the genes that pattern the developing digestive tract in chicken and mouse model-organisms.
Her work on skates is supported by a grant from the Maine IdeA Network for Biomedial Research Excellence (INBRE), a program of the National Institutes of Health that includes summer fellowships for Bowdoin students to engage in faculty-mentored biomedical research on genomes.
Ultimately, their work with Theodosiou on genetic patterning may give medical researchers information to understand underlying causes of gastrointestinal-related cancers and digestive tract malformations.
Theodosiou notes that gastrointestinal abnormalities are more prevalent than many people realize:
"There are over 200,000 new cases of gastrointestinal cancer a year in the U.S. alone," she says, "and one to three of every 1,000 live births have some sort of GI abnormality. If we can understand the genes involved in gastrointestinal development we may be better able to treat, or even prevent, some of these abnormalities."
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