After dino extinction, the human genome undergoes shrinking.

Byline: ANI

Washington, July 28 (ANI): Evidence buried in the chromosomes of animals and plants strongly suggests that only mammals, including humans, have seen their genomes shrink after the dinosaurs' extinction, with that trend continuing today as well.

Indiana University (IU) Bloomington scientists found the evidence.

The scientists' finding might seem counter-intuitive; given that the last 65 million years have seen mammals expand in diversity and number, not to mention dominance in a wide variety of ecological roles.

But, it is precisely their success in numbers that could have led to the contraction of their genomes.

"Larger population sizes make natural selection more efficient," said IU Bloomington evolutionary biologist Michael Lynch, who led the study.

"If we are correct, we have shown how to bring ancient genomic information together with the paleontological record to learn more about the past," he added.

According to Lynch, the data he and his colleagues analyzed suggest human genomes are still undergoing a contraction - though you shouldn't expect to see noticeable changes in our chromosomes for a few million years yet.

Lynch's group examined the genomes of seven mammals, eight non-mammalian animals and three plants, specifically with regard for the long terminal repeat (LTR) sequences of transposable elements, a curious sort of "jumping" genetic sequence initially dropped into genomes by viruses.

Transposable elements often lose their functionality soon after insertion but nevertheless are disturbingly common.

LTRs 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," he added.

According to Lynch, left enough time, 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 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 depressed numbers of very young LTRs strongly suggests a contraction in overall genome sizes of the lineages of the mammals the scientists studied. (ANI)

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