Genetic ‘Junk’ Could Answer Riddle of Vertebrate Evolution
Lampreys, jawless fish closely related to the first vertebrates, possess 41 types of a little-known genetic regulator called microRNA. Some biologists say microRNA answers the mystery of how backbones evolved.
Image: WikiMedia Commons
Why aren’t you a spineless sack of protoplasm?
Because of a little-known molecule called microRNA, say Dartmouth College biologists.
In a study published this month in the Proceedings of the National Academy of Science, researchers found 41 types of microRNA — a molecular “off” signal for genes — that are unique to vertebrates.
The difference may explain the as-yet-unknown origins of creatures with backbones. When pressed for an explanation of the appearance of vertebrates, evolutionary biologists have thus far been able to do little more than say that it happened.
However, the researchers may have replaced one mystery with another: how that microRNA could have accumulated and produced such radical effects.
That mystery’s answer may reside in processes not yet described by evolutionary science.
“Neo-Darwinian evolution couldn’t be wrong,” said study co-author Kevin Peterson, “but it said nothing about things like this. It’s essentially silent on major jumps of morphological complexity.”
Peterson and his colleagues analyzed microRNA — miRNA for short — in lampreys, a jawless, cartilage-spined fish considered to be a direct descendant of whatever organism split from the invertebrate tree some 600 million years ago.
Discovered just 15 years ago, miRNA is produced by sections of the genome once derided as “junk DNA” because of their apparent lack of function. It shuts genes off by interfering with messages sent by other, protein-coding DNA to protein-manufacturing cellular machinery.
Peterson’s lampreys possessed 41 miRNA types that his team subsequently found in mice. But when they looked for those miRNA types in sea squirts and other aquatic animals whose ancestors stayed spineless, the researchers came up empty.
“We’re arguing that in the early ocean history of invertebrates, a stupendous amount of microRNAs evolved out of ‘junk’ DNA. Adding these regulators facilitated new cell types, and the new cell types made possible new structures and new complexity,” said Peterson.
“It’s an intriguing hypothesis,” said Harvard University evolutionary biologist James Hanken, though he cautioned against jumping too quickly from correlation to cause-and-effect.
That turning genes off could set the stage for higher forms of life seems counterintuitive, but Peterson called it a process of “achieving complexity through precision.”
It wouldn’t be the first time miRNA produced an evolutionary jump.
“Jellyfish and sponges don’t have any miRNA at all,” he said. “Triploblasts — animals with guts, brains and organs — are characterized by the first jump in miRNA innovation. We haven’t found a second jump until now, with the evolution of vertebrates.”
Where did that miRNA come from? Nobody knows, said Peterson. Somehow the so-called junk DNA started to produce it; somehow it learned to target active genes; and during that gap, when the miRNA didn’t do anything, it wasn’t discarded by evolution.
And then — most perplexing of all — that scattered miRNA amounted to more than the sum of its parts, interacting to produce the body pattern that gave rise to mammals, amphibians, reptiles and fish.
“It could be explained by neo-Darwinian tenets, but it’s not clear to us how,” said Peterson.
However, according to Harvard University comparative zoologist Andrew Berry, the fossil record’s sparseness could have skewed Peterson’s analysis.
“What looks rapid in the fossil record isn’t very rapid at all to a geneticist accustomed to studying populations of fruit flies in the lab,” he said. “What might appear to be a [leap] in the fossil record shouldn’t be seen as departing from Darwinian orthodoxy.”