The Scientist
Volume 25 | Issue 1 | Page 38
Date: 2011-01-01
By Jef Akst
From Simple To Complex
The switch from single-celled organisms to ones made up of many cells has evolved independently more than two dozen times. What can this transition teach us about the origin of complex organisms such as animals and plants?
Sean Mccabe
Given the complexity of most organisms—sophisticated embryogenesis, differentiation of multiple tissue types, intricate coordination among millions of cells—the emergence of multicellularity was ostensibly a major evolutionary leap. Indeed, most biologists consider it one of the most significant transitions in the evolutionary history of Earth’s inhabitants. But single-celled organisms have stuck together or assembled to spawn multicellular descendants more than two dozen times, suggesting that maybe it’s not such a big leap after all.
“The transition from unicellularity to multicellularity is critical for explaining the diversity of life on Earth,” says evolutionary biologist Casey Dunn of Brown University. “We tend to think of it as quite special, but maybe it’s not. Maybe this is an easier transition than we think.”
To understand how and why it happened, scientists are utilizing the recent explosion in genomics data to assemble more accurate phylogenies and piece together each step in the transition to multicellular life. Despite their efforts, however, the origins of this intriguing phenomenon remain shrouded in mystery. Evolution and extinction over hundreds of millions of years have blurred the details of the transition, and the answers provided by genome sequencing only lead to more questions.
“New studies are always pushing the envelope on our thinking,” says evolutionary biologist Mansi Srivastava of the Whitehead Institute for Biomedical Research in Massachusetts. Since scientists began studying a much wider array of animals, far afield from the classic model systems of fruit flies and mice, “our thinking about what having certain kinds of genes means to being an animal has shifted.”
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. This suggests that the appearance of new genes cannot fully explain the appearance of new traits that are key to multicellularity. Sponges are commonly considered the most basal of all the metazoan (animal) lineages, yet a recently published sponge genome revealed genes known to be involved in the development of a neuromuscular system, which sponges lack.1 “These genes that we previously thought were associated with complex multicellular animals really have to do with basic multicellular functions—to get the simplest multicellular animals, you have to have these genes present,” says Srivastava, who coauthored the analysis.
As some of the most ancient animals, sponges can provide information regarding the evolution of the metazoan lineage, but for true insights about the origin of multicellularity, scientists must look even further back on the evolutionary tree. 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.2
“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.3
“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.”
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