Evolution in invisible life


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(Photo credit: Frank Dazzo, Michigan State University)

Story by Elsa Youngsteadt

Single-celled organisms outnumber humans by a mind-boggling factor. The number of microbes in the gut of a single person is greater than the entire human population, and the number of microbes on the planet is anybody's wild guess.

In an effort to get a handle on how earth's abundant microbial life evolves and specializes, research published in the February 23, 2007 issue of Science magazine suggests that entire families of microbes stick with the same habitat over time, and that microbes evolve at different speeds depending on their environment.

Since the vast majority of single-celled organisms refuse to grow in laboratory cultures, the researchers used an unconventional approach known as metagenomics that is, sequencing random bits of microbial DNA directly from environmental samples. Sequencing DNA from environmental samples rather than cell cultures is not a new idea, but these researchers did it with a twist. Previous methods relied on techniques that would only detect DNA that had a certain level of similarity to DNA of known organisms, whereas this approach, known as shotgun sequencing due to its broad target approach, makes no assumptions about the sequences of the unknown DNA. Therefore, it is likely to include more novel organisms and more give a more accurate idea of the types of microbes present in the sample.

The authors then plugged the DNA sequences of the unknown microbes, sequenced from ocean surface water, soil, mine drainage, and a whale bone, into a family tree of sequences from better-known organisms to find out how microbes from the samples were related and how quickly they evolved.

They found that each environmental sample contained many sequences, but from surprisingly few microbial lineages. In other words, not many microbial families live in extreme environments, such as mine drainage, and once in the environment, they tend to stay there. Different groups of microorganisms specialized on different habitats.

Then they compared the microbes from different environments, and found that rates of evolution varied across the different habitats. Members of the ocean water microbial community, including different microbial lineages, evolved faster that is, accumulated more mutations than those in the soil.

The study highlights the importance of habitat in shaping the evolution and diversity of the most abundant life on earth. Additional studies will investigate what conditions allow certain microbial groups to inhabit different areas, and what factors influence mutation rates. Understanding these factors can help us understand the impact of human driven environmental changes, and perhaps how to work with the environment to clean up areas we have contaminated.


Original research article in February 23, 2007 issue of Science

European Molecular Biology Laboratory news release

Author Bork's website (including additional media links)

Author von Mering's website

Questions for review and discussion

1. What is "metagenomics," how does it differ from conventional methods used to study microbes, and why did the authors use it in this study?

2. What piece of evidence suggested that microbes rarely switch habitats?

3. What piece of evidence suggested that habitat influences rate of evolution? Can you think of any ways that environmental conditions could speed or slow rates of mutation and evolution? How?
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