Jawless evolution explained
A new genetic analysis tips the scales to one side of a long-term debate on the evolution of jawless vertebrates
Genetic evidence is laying to rest a long-standing argument over the evolution of jawless vertebrates -- hagfish and lampreys -- and providing insights regarding the common ancestor of all vertebrates.
Image: Wikimedia commons
For years, biologists have debated the origins of jawless vertebrates -- molecular biologists have argued that molecular evidence shows they are each other's closest relatives, while morphologists maintained that detailed anatomical features suggest lampreys were more closely related to jawed vertebrates.
In the most recent study, published Monday (18 October) in Proceedings of the National Academy of Sciences (PNAS)
, scientists on opposite sides of the argument looked at microRNA data, and found jawless vertebrates are indeed monophyletic, meaning they evolved from a common ancestor not shared by jawed vertebrates.
"I was staggered by this paper," said linkurl:Philippe Janvier,;http://www.mnhn.fr/museum/foffice/science/science/Enseignement/rubmastere/ssEnsChercheur/ficheEnsChercheurs.xsp?ARTICLE_ARTICLE_ID=1268&idx=65&nav=liste a paleontologist at the Muséum National d'Histoire Naturelle in Paris, France, and a long time supporter of the idea that lampreys were more closely related to jawed vertebrates. "It's very hard for me to recognize that I've been wrong in my assumption," said Janvier, who did not participate in the research, but this paper provides "very, very strong support for the monophyly -- the common origin of lampreys and hagfishes apart from the origin of the jawed vertebrates."
Biologists originally classified lampreys and hagfish (cyclostomes) as part of a single group due to their gross anatomical similarities -- they were both eel-like creatures that lacked jaws. But more detailed morphological analyses in the 1970s suggested that lampreys were actually more closely related to jawed vertebrates, and hagfish were more primitive. This idea dominated the field until the early 1990s, when molecular evidence once again placed lampreys and hagfish together on the evolutionary tree. Over the next two decades, more and more molecular data emerged to support this monophyly hypothesis, but many morphologists continued to argue that lampreys were more similar to jawed vertebrates.
To resolve the debate, paleontologist linkurl:Kevin Peterson;http://www.dartmouth.edu/%7Epeterson/ of Dartmouth College and his colleagues turned to a new source of data -- microRNA. They created small RNA libraries for two different lamprey species, a hagfish, and a catshark (a jawed vertebrate). The team found all but two of the 46 miRNAs found in lampreys and jawed vertebrates in hagfish as well. Furthermore, they found four miRNA families unique to hagfish and lampreys, supporting the idea that the two groups are monophyletic.
"This adds a third type of data, [and] it does come down firmly on the side of cyclostome monophyly," said zoologist linkurl:Sebastian Shimeld;http://www.zoo.ox.ac.uk/staff/academics/shimeld_sm.htm of the University of Oxford, who was not involved in the work.
This was no surprise to Peterson, who was already convinced of this conclusion by the molecular data. But his colleague and coauthor linkurl:Phil Donoghue;http://www.gly.bris.ac.uk/people/pcjd.html of the University of Bristol had been a strong proponent of the opposing view. Upon seeing the miRNA results, he decided to reanalyze the seemingly contradictory morphological data. Compiling all the data previously used to support the paraphyly hypothesis, that lampreys should be lumped with jawed vertebrates, and eliminating obsolete or inaccurate data, the analysis revealed inconclusive results.
"Lo and behold there really is no signal for paraphyly in phylogeny," Peterson said. Between the molecular data, the new miRNA analysis, and "especially with Phil's reanalysis of the morphology, I think you'd be hard pressed to assert strongly that cyclostomes are not monophyletic."
The results could indicate that the common ancestor to vertebrates is more complex than previously believed, Peterson added. If lampreys were more closely related to jawed vertebrates, and hagfish were basal to both groups, one could imagine "a successive building of the vertebrate body plan," Peterson explains -- "you start simple and then you make it more complex." If, on the other hand, lampreys are more closely related to hagfish, the last common ancestor of vertebrates most likely had all the derived characters still found in lampreys and jawed vertebrates, and hagfish simply lost some of these traits after they split from lampreys.
"I think [this study] is the tipping point where the vast majority of new researchers and some of the old morphologists will start to believe" that lampreys and hagfish form a single group, said evolutionary biologist linkurl:Jon Mallatt;http://www.wsu.edu/%7Ejmallatt/ of Washington State University, who did not participate in the research.
A.M. Heimberg, et al., "microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate," PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1010350107, 2010.
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