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Unicellular Genome Provides Clues to Multicellular Evolution

NEW YORK (GenomeWeb News) – The genome of a unicellular marine organism is revealing a lot about its relationship to multicellular animals, as well as how these animals may have evolved, researchers reported today.
 
An international research team looking for the molecular mechanisms behind multicellular animal evolution sequenced and analyzed the genome of a single-celled organism, Monosiga brevicollis, which has interesting similarities to animal genomes. Although it is quite small, the genome contains an inordinate number of intron-rich genes as well as genes coding for proteins related to cell-cell communication and adhesion. The paper appeared in the online version of Nature today.
 
M. brevicollis, a unicellular organism living in marine environments, belongs to a group of organisms called choanoflagellates. Although these creatures live independently, they’re uncannily similar to the feeding cells, called choanocytes, that some sponges use to nab food. These cells line the inside wall of some sponges, beating their flagella to move water and food through the organism. Consequently, choanoflagellates have long been presumed to be animals’ closest living unicellular relative.
 
Previous nuclear and mitochondrial analyses and comparative genomics also have provided intriguing links between choanoflagellates and multicellular animals, suggesting the organisms preceded the metazoans.
 
“There are many other studies that, I think, demonstrate that choanoflagellates are the closest relatives of animals,” lead author Nicole King, a molecular and cell biologist at the University of California, Berkeley, told GenomeWeb Daily News. This work, she noted, gives scientists the opportunity to start reconstructing the genome of their closest common unicellular ancestor.
 
Using approximately 8.5-fold redundant, paired-end whole-genome shotgun sequencing, King and her colleagues sequenced M. brevicollis replicate libraries. They then used the whole-genome shotgun assembler JAZZ to assemble six whole-genome shotgun libraries. During their subsequent analysis, the group predicted and annotated M. brevicollis genes using the Joint Genome Annotation Pipeline and aligned 473 conserved M. brevicollis genes with similar eukaryotic genes, identifying nearly 2,000 intron splice sites.
 
The number of intron-containing genes was unexpectedly high given the organism and the size of its genome, roughly 41.6 megabases coding for about 9,200 genes. “I think the intron finding was one of the most surprising, particularly given the size of the genome” King said. 
 
In addition, even though the organism is unicellular, the M. brevicollis genome contained regions similar to those coding for cell adhesion proteins such as C-type lectin, cadherin EC, and immunoglobulin and extracellular matrix proteins including fibronectin type 3 and laminins G and N.
 
On the other hand, the authors noted that other genomic features that are present in metazoans are missing in M. brevicollis. They speculate that these may have either evolved in metazoans or been lost from M. brevicollis after the groups split from their last shared ancestor. In the future, the group plans to continue sequencing other choanoflagellate species, including one that forms colonies to try to pin down the particulars of this evolution.
 
“The M. brevicollis genome provides unprecedented insight into the early evolution of metazoan genes,” the authors wrote. “Because genomic features shared by M. brevicollis and metazoans were probably present in their last common ancestor, this study extends the evolutionary history of a cohort of important protein domains to the premetazoan era.”
 
The M. brevicollis genome was deposited into ATCC.org under accession number PRA-258.
 

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