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Conventional thought on evolutionary change has led researchers to believe that genetic innovations underlie the transition. Advances in genomics research, however, are revealing that more and more of the genes associated with complex processes also exist in simpler animals and even in their unicellular cousins.
Choanoflagellates, unicellular organisms that look remarkably similar to the feeding structures of sponges, are the closest living relatives of metazoans. It turns out that they also share a number of genes once thought to be unique to multicellular animals. Tyrosine kinases (TK), for example, enzymes that function in cell-cell interactions and regulation of development in animals, were identified in the choanoflagellates in the early part of this decade, and the first sequenced choanoflagellate genome, published in 2008, revealed that they have more TK genes than any animal—and many other components of the TK signaling pathway as well.
“So this gene family that was thought to be essentially a trigger that unleashed animal origins, we can now say with great confidence evolved long before the origin of animals,” says evolutionary biologist Nicole King of the University of California, Berkeley, who has been studying choanoflagellate biology for over 10 years.
Scientists have also identified choanoflagellate homologs of cadherins, known to be involved in cell-cell adhesion and signaling in animals. And more recently, a widespread search for genes associated with integrin-mediated adhesion and signaling pathways revealed that the integrin adhesion complex originated much earlier than even the choanoflagellates, dating back to the common ancestor of animals and fungi.
“It’s pretty surprising to find these adhesion genes in far-flung species,” says Srivastava. “We would have thought that integrin signaling has to do with cells sticking together, but it goes much further back in time than our most recent unicellular cousins.”
The genomic exploration of the evolution of multicellularity is really just beginning, but already, a trend is emerging. “Almost every month now we are seeing genes that were supposed to be exclusive to metazoans that are already present in their single-cell relatives,” says evolutionary biologist Iñaki Ruiz-Trillo of the University of Barcelona. “I think that means co-option of ancestral genes into new functions is important for evolutionary innovations like the origin of multicellularity.”
“Probably the more data we collect, the fewer and fewer animal-specific genes there are going to be,” agrees Dunn. “And we’re going to have to explain the origins of multicellularity in terms of changes in the way these gene products interact with each other.”
Read more: From Simple To Complex – The Scientist – Magazine of the Life Sciences http://www.the-scientist.com/article/display/57883/#ixzz1JuiPZdUx