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Unicellular Organism's Genome Provides More Fodder for Studies on Multicellular Evolution

NEW YORK (GenomeWeb News) – A newly published unicellular genome is providing insights into the ancestry of a group of signaling and adhesion proteins found in multicellular animals, researchers reported today.
Just yesterday, a team of researchers from eight institutions reported in Nature online that they had sequenced and analyzed the genome of a unicellular organism called Monosiga brevicollis. Today, new details are emerging about a specific group of M. brevicollis genes: those encoding proteins called cadherins. University of California, Berkeley, researchers Monika Abedin and Nicole King reported these findings in the online version of Science today.
In a telephone interview with GenomeWeb Daily News, senior author King called the paper “a very focused treatment on, I think, one of the most interesting classes of proteins encoded by choanoflagellates.”
In general, choanoflagellates are a group of organisms found in aquatic environments that appear to be closely related to multicellular animals or metazoans. Although they are all unicellular, the choanoflagellates do form colonies and share many physical similarities with feeding cells in sponges.
Now that one of these choanoflagellates, M. brevicollis, has been fully sequenced, researchers are starting to examine their genetic relationship to other organisms, which may give them more clues about a common ancestor for choanoflagellates and metazoans as well as the evolutionary process leading from unicellular to multicellular animals.
The genome — sequenced by the Department of Energy's Joint Genome Institute — revealed a number of M. brevicollis genes that are similar to those in multicellular animals. This includes 23 genes coding for cadherins, proteins that mediate cell-cell adhesion and signaling in multicellular animals but not, it seems, in other multicellular organisms such as plants.
“It was shocking to find any cadherins in a choanoflagellate,” King said. “Not only were there cadherin domains, but there were a huge number.” During their analysis, Abedin and King compared the M. brevicollis cadherins with those of the starlet sea anemone, sea squirt, fruit fly, and mouse. Even though the organism is not multicellular, King noted, they contain roughly as many cadherin genes as have been found in fruit flies or mice.
Despite the number of cadherin genes, though, it’s still unclear what these proteins do in the organism. Because it’s unicellular, M. brevicollis doesn’t exhibit the same sort of cell-cell interactions observed in multicellular animals.
But, King said, the cadherin proteins might help the organism capture its prey. This is a particularly intriguing hypothesis given the location of two cadherins at the feeding collar of M. brevicollis cells and near the cell’s base, where it attaches to surfaces. The findings also suggest metazoan cadherins may have evolved from proteins with other sensory functions.
“[C]adherin-mediated cell adhesion in metazoans may reflect the co-option of a class of proteins whose earliest function was to interpret and respond to cues from the extracellular milieu,” the authors wrote. “Indeed, the transition to multicellularity likely rested on the co-option of diverse transmembrane and secreted proteins of new functions in intercellular signaling and adhesion.”

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