For over 160 years, the mollusc's evolutionary tree has remained ambiguous and incomplete. But after three years spent sampling genes, researchers Casey Dunn and Stephen Smith presented a firm new take on the relationship between eight major molluscan groups.
Dunn, assistant professor of biology, and Smith, a postdoctoral research associate, published the results of their study Wednesday in the journal Nature.
Because of the abundance of fossils and the many variations between species, molluscs are one of the best-studied groups of organisms. But despite the large body of research, even within the context of evolution, their evolutionary tree has remained largely unresolved.
"Molluscs are a funny animal, in the sense that when you see a mollusc, you know it's a mollusc — they all share similarities," said Dunn. But most previous evolutionary trees did not group molluscs together, choosing instead to split them apart on different branches. "The relationships between the molluscs were just a mess," he said.
After collecting samples of 14 species from eight major groups, Dunn and Smith were able to use transcriptome data — the sequencing of active genes — in order to better understand the molluscan evolutionary tree, or phylogeny.
"It's like figuring out what your genealogy is," Dunn said. "The way we do it, instead of going through courthouse documents and looking at Ellis Island records, is we go and sample DNA from all the things that we think are related. Then we compare those DNA sequences."
In general, DNA sequences that are more similar come from more closely related species, Dunn said. DNA sequencing has been used before, but not at the scale at which Dunn and Smith were working. Advancements in DNA sequencing technology in the past few years have made this sort of research possible, Dunn said.
While the research reinforced some assertions already put forth by the scientific community, it also unearthed some unexpected relationships between molluscs. One of the biggest surprises for the researchers was determining a close relationship between two mollusc subgroups, cephalopods and monoplacophora.
"Everyone loves cephalopods: octopi, squid, cuttlefish, nautilus," Dunn said. "They're so different from other molluscs. But it turned out that the closest relative to cephalopods is this very poorly known animal called a monoplacophora."
Thought to be extinct until the 1950s, when they were discovered on a cruise ship off the coast of Mexico, monoplacophora are small deep-sea animals with cap-like shells. "It was amazing, like Lazarus," Dunn said. "They just came back from the dead."
Dunn and his researchers were able to collect some samples of the rare species.
The research allows scientists to better understand the significance of shared characteristics among species of molluscs. For example, until recently, it was unclear whether shelled molluscs were closely related to other shelled molluscs, because it could not be determined from the presence of a shell alone. But Dunn and Smith's research resolves the question — all shelled molluscs share a recent common ancestor.
Dunn and his lab will continue to work on refining the phylogeny of molluscs for at least another year. The research also gave Dunn and Smith invaluable experience understanding and interpreting transcriptome data, Dunn said. While they used a variety of tools to sequence and analyze the data, they hope to automate the process in the future so that larger transcriptome analyses will be faster and easier.
