Correction Appended
A team led by Dartmouth biology professor Kevin Peterson may have just resolved a long-standing debate in the scientific community over the evolution of vertebrate animals.
Peterson along with University of Bristol professor Philip Donoghue, graduate students Alysha Heimberg and Richard Cowper Sal Lari and University of Lyon professor Marie Semon published a study that refutes the popular view that the lamprey, a jawless vertebrate, was the most closely related creature to subsequent vertebrate species. The study, "MicroRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate," was published in the Oct. 19 edition of the Proceedings of the National Academy of Sciences.
The team determined that lampreys are most closely related to another jawless vertebrate the hagfish Peterson said. The research shows that the two creatures are monophyletic, meaning they evolved from an common ancestor that is not shared by jawed vertebrates.
"It was previously thought that the hagfish was simpler because it is more primitive," Peterson said.
Instead, he said, the research has shown that the hagfish is simpler because it is degenerate, meaning that the species has regressed to a simpler form and lost evolutionary characteristics it had in the past.
Peterson's research shows a widened evolutionary gap between vertebrates and invertebrates and suggests that the ancestral vertebrate was much more complicated than scientists originally thought, he said.
"It removes hagfish from representing the intermediate step, and makes the jump from invertebrates to vertebrates all the more formidable," Donoghue said in an interview with Wired Magazine. "All of a sudden, you realize that you haven't got the faintest idea to sketch a last common ancestor."
In an attempt to arrange hagfish and lampreys in the tree of life, scientists have argued for years over the evolution of these jawless vertebrates, according to The Scientist, a life science magazine.
In the 1970s, morphologists who analyzed the physical features of hagfish and lampreys concluded that hagfish were more primitive, according to The Scientist. This view remained dominant until the early 1990s, when molecular evidence showed that hagfish and lampreys were most closely related to each other. Even as molecular evidence piled up, morphologists continued to maintain their original viewpoint.
To study this question, members of Peterson's lab turned to a technique they developed as a team: the molecular technique of comparing microRNA, or molecules that regulate the expression of genes, Peterson said.
The research team discovered that 46 microRNAs found in both hagfish and lampreys are shared with jawed fishes, and four microRNAs were found that are unique to both creatures, supporting the idea that they are indeed each other's closest relatives, Peterson said. The team also noticed an enormous jump in the number of microRNAs that evolved during the transition from the invertebrates to vertebrates.
"Our research shows that nearly 50 new microRNAs evolved within that time," Peterson said. "This is an order of magnitude higher than expected, and we are at a loss to explain mechanistically how this happened."
Peterson's lab is now looking into how these microRNAs evolved and contributed to the vertebrate body plan, Heimberg, a molecular and cellular biology graduate student, said.
"What we're doing is picking the microRNA genes that evolved in vertebrates, and looking at them in vertebrate specific tissues," she said.
The team is specifically looking at microRNA-122, which is known to play a role in the development of the liver, Heimberg said.
The lab developed the microRNA comparison technique after linking the development of vertebrates to the evolution of microRNAs. The lab recently discovered that microRNAs are enormously diverse in all vertebrate animals, but are found rarely in invertebrate animals.
