How Single-Cell Organisms Evolve Into Multicellular Ones
Show Me The Science Month Day 18
The transition from one-celled microbes to multicellularity was a huge step in the evolution of life on this planet, but as daunting as this evolutionary step seems, it didn't happen just once. Today's plants, fungi, animals, and various types of algae are all descendants of separate transitions to multicellular life.
All of these transitions from a single-cell lifestyle to multicellularity occurred in the very distant past, so how can we learn anything about them? It turns out that it is not hard to find living, modern examples that closely parallel the momentous evolutionary transitions that led to animals, plants, and fungi. Right now on earth there are primitive multicellular organisms that, in many ways, resemble the first multicellular creatures that existed a billion years ago. Researchers are using these organisms to understand what kinds of genetic changes are needed to turn a single-celled organism into a multicellular one.
A group at the University of Arizona has published a study of of one group the these amazing organisms, the volvocine green algae. What's amazing about this group of algae is that you can find a range of multicellular sophistication in closely relate algae species. There are species that form simple sets of four identical cells stuck together, other that form balls of 32-64 not quite identical cells with some specialized functions, up to full-blown multicellular organisms with 50,000 highly specialized cells, including reproductive germ cells. The evolution of multicellularity is not an irrecoverable event from an unimaginably distant past; it is something we can observe, manipulate, and test in the lab today.
With the availability of so many different types of green algae at varying levels of multicellular sophistication, the U. of Arizona researchers were able to put a timeline on the evolution of specific features of multicellular algae. They did this by calibrating DNA differences between species with absolutely dated fossils: DNA provides a relative time scale, since the more DNA differences there are between species, the longer it's been since their lineages diverged; and this relative time scale can be matched up against dated fossils that show when new major types of multicellular algae began to appear.
Here is part of the time line the researchers came up with:
1) ~223 million years ago, a species of single-celled green algae began forming aggregates of cells stuck together by a glue of secreted proteins and sugars (and we can see species which do this today).
2) Also ~200 million years ago, the rate of cell division began to be controlled genetically. Unlike single-celled organisms, which reproduce whenever the surrounding environment is right, the new multicellular algae began controlling exactly how many daughter cells they produce. This is a critical step towards establishing a multi-cellular body-plan with genetically controlled dimensions.
3) Roughly 10 million years later, the cells of some multicellular algae species began to orient their whip-like flagella in the same direction, so that all of the flagella would work together to control the swimming direction of the organism.
4) By ~100 million years ago, some of the algae species had established separate reproductive germ cells, and ever since then, various volvocine algae species have developed more cells with highly specialized functions.
One feature of this time scale is that the major innovations occur sporadically. The researchers suggested that these major events coincided with the inventions of new ways for resolving conflicts among individual cells in the organism: in other words, formerly independent cells had to learn how to be civilized. Single-celled microbes function very well as individuals. Some of that individuality has to be given up for the greater good when cells hitch their evolutionary fates together as one multicellular organism. A key example of conflict resolution is the evolution of genetic limits on cell division: to have a coherent, multicellular body plan, individual cells can't just divide with abandon, the way bacteria do. When cells escape these genetic controls on division in humans, you get cancer.
The evolution of multicellular organisms is a major evolutionary step. In our history (the history of animals), how that step happened is lost somewhere in deep history. Nevertheless, the evolution of multicellularity has happened over and over again, and in the case of the volvocine algae, we can study this key evolutionary step in the lab.
Join me tomorrow, here at Adaptive Complexity, for day 19 of 30 Days of Evolution Blogging Evolution as a science is alive and well. Each day I will blog about a paper related to evolution published in 2009.
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Front Page image of Volvox aureus by Dr. Ralf Wagner, courtesy the Wikimedia Commons, published under the GNU Free Documentation License.