SALT LAKE CITY - Evolution is often thought of as a gradual remodeling of the 基因组, the genetic blueprints for building an organism. In some instances it might be more appropriate to call it an overhaul. Over the past 100 million years, the human lineage has lost one-fifth of its DNA, while an even greater amount was added, 科学家们在 University of Utah School of Medicine. Until now, the extent to which our 基因组 has expanded and contracted had been underappreciated.
Humans aren't the only ones with elastic 基因组s. A new look at a virtual zoo-full of animals, from 蜂鸟 to bats to elephants, suggests that many vertebrate 基因组s have the same accordion-like properties.
"I didn't expect this at all," says the study's senior author castric Feschotte博士.D., professor of human genetics. "The dynamic nature of these 基因组s had remained hidden because of the remarkable balance between gain and loss."
Previous research had shown that 基因组 sizes vary widely across different species of insects or plants, a telltale sign of fluctuation. This survey is the first to compare a diverse array of warm-blooded vertebrates, 10 mammals and 24 birds altogether. The study appears online in Proceedings of the National Academies of Sciences (PNAS) 在2月8日的那一周. 6.
修整起飞
When 进化 repeats itself, there's usually a good reason. For most vertebrates it's not immediately apparent why 基因组 deletions and add-ons typically go hand-in-hand. For flying animals, however, there is a clue.
Feschotte's foray into the field began five years ago after his research had turned up a paradox. His group and others had found that the 基因组s of bats were littered with small pieces of DNA, 叫转座子, that had invaded and copied themselves throughout the flying mammals' genetic material. 特别是, this massive transposon amplification had expanded the 基因组 of a species called the microbat by 460 megabases, more genetic material than there is in a pufferfish. Yet the overall size of the bat's 基因组 had remained relatively small in comparison to other mammals, suggesting that while transposons added new DNA, old DNA must have been removed somehow.
"These data begged the question: where did the old DNA go?Feschotte说. In order to keep their 基因组s trim, 他推断, these animals must have been good at jettisoning DNA.
In order to test the hypothesis, his team needed to quantify something that wasn't there, the amount of DNA lost over many millenia. Feschotte and the study's lead author Aurélie Kapusta, Ph.D., a research associate in human genetics, developed methods to extrapolate the amount of DNA that vanished by comparing 基因组 sizes from present day animals to that of their common ancestors.
正如他们所怀疑的, the microbat lost more DNA over time – three times as much - than it had gained since its divergence from a mammalian ancestor. 这是蝙蝠的表亲, 的megabat, slimmed down its 基因组 even more, losing eight times more than had been added.
The findings were a first clue that mammalian 基因组s were more dynamic than previously thought. 更重要的是, the data fit in nicely with an idea that scientists had been bantering around for a while. Animals that fly have smaller 基因组s. Feschotte says that one reason could be that the metabolic cost of powered flight imposes a constraint on 基因组 size.
事实上, expanding the survey to include the bats' compatriots of the skies: woodpeckers, 白鹭, 蜂鸟, 还有其他鸟类, showed that the 基因组 dynamcis of the two flying mammal species was more like that of the birds than the land-bound mammals. While most mammals trended toward an equilibrium between the amounts of DNA gained and lost over deep 进化ary time, the bats skewed toward shedding DNA over the same time frame.
The biological factors underlying the differences in 基因组 dynamics observed across species are likely to be complex and remains to be explored. But whether streamlining 基因组 content may have allowed flying animals to get off the ground is an intriguing proposal worth investigating, Feschotte说.
"If you look at small parts of the 基因组, 或者只有一个时间点, you don't see how the whole 基因组 landscape has changed over time,卡普斯塔说. ‘You can see so much more when you step back and look at the fuller picture."
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The work was supported by the National Institutes of Health, and will publish as "Dynamics of 基因组 size 进化 in birds and mammals" in PNAS 2月. 6, 2017. In addition to Kapusta and Feschotte, Alexander Suh from Uppsala University is a co-author.