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After dinosaurs, mammals rise but their genomes get smaller
Date:7/27/2009

s come in a range of sizes and ages, and it is the age distribution of LTRs that interested Lynch and his colleagues.

"This study started out as independent observations in the literature," Lynch said. "The data we saw suggested a bulge in age distribution of transposable elements in humans and mouse."

Left enough time, Lynch says, transposable elements are eventually lost from the genome, sometimes by accident and sometimes, perhaps, as the result of natural selection against excess DNA. An LTR is far more likely to survive a few years of cell divisions -- and the chance of obliteration via a DNA replication error -- than 10 million years of cell divisions. Plotting the full range of 17 species' LTRs, young and old, Lynch and his colleagues usually saw a descending curve with lots of new transposable elements and a dramatic drop-off in the number of older elements.

But not in most mammals. In humans, macaques, cows, dogs and mouse, Lynch's group observed a hill-shaped curve, with a peak of middle-aged LTRs and drop-offs both in the number of older and younger LTRs. The shape of the curve is consistent with previously published data for other types of so-called "junk" DNA elements.

The depressed numbers of very young LTRs, Lynch says, strongly suggests a contraction in overall genome sizes of the lineages of the mammals the scientists studied. That could come about in one of two ways, he says. One possibility is an increase in the efficiency of natural selection that accompanies population growth.

"We think that's the most likely explanation," Lynch said. "Another possibility is that natural selection was just stronger, but we doubt it. For that to be the case, natural selection would have to act in the same way on several lineages around the globe simultaneously."


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Contact: David Bricker
brickerd@indiana.edu
812-856-9035
Indiana University
Source:Eurekalert

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