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Genetic Struggles Within Cells May Create New Species

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Carrie Arnold writes in Quanta magazine:

In the complex cells of humans and other organisms, two different genomes collaborate to sustain life. The larger genome, with DNA encoding thousands of genes, resides in the cell nucleus, while copies of the much smaller one sit in all the energy-producing organelles called mitochondria. Normally, they work in quiet alliance.

Over the past five years, however, scientists have begun focusing on the consequences of mismatches between the two. Emerging evidence shows that this “mitonuclear conflict” can drive a wedge between organisms, possibly turning one species into two. It’s too soon to say how frequently mitonuclear conflict acts as a force in speciation, but researchers agree that better understanding of that tension may help to solve mysteries about what barricade separates some apparently similar populations into distinct species.

More than 1.5 billion years ago, an ancient bacterium snuggled inside a fellow simple cell. Instead of digesting the interloper, the larger cell let it stick around for the valuable energy that it produced. In exchange, the invader got refuge and protection from predators, and over thousands of generations evolved into the mitochondrion, which produces energy in the form of a molecule called ATP. Thus began the complex eukaryotic cell, a primordial partnership that has evolved into one of life’s most successful endeavors.

Proof of the mitochondrion’s origins survives in the remnant genome that mitochondria still carry — a small ring of DNA very much like that in bacteria. Over hundreds of millions of years, some of the mitochondrial genes moved into the long, linear genome in the eukaryotic cell’s nucleus, but the mitochondrion hung on to a handful of genes that remained essential for the organelle’s functioning. (Human mitochondria carry just 37 genes.) The cell assembles the protein complexes that help mitochondria produce ATP with building blocks from both mitochondrial and nuclear genes. This requires the nuclear and mitochondrial genomes to cooperate and adapt in tandem.

More and more studies are pointing to that co-adaptation as an essential but mostly overlooked factor in the health and survival of organisms. “And that has big implications for our concept of species and natural selection,” said Geoffrey Hill, an ornithologist and evolutionary biologist at Auburn University.

Incompatible Cousins

For the past 40 years, the marine evolutionary geneticist Ron Burtonhas stalked tide pools along the Pacific Coast, armed with an aquarium fish net in his search for a tiny crustacean named Tigriopus californicus. Populations of this orange copepod live from the Baja California peninsula to Alaska, and Burton has spent his entire career looking at genetic differences among these groups. Not surprisingly, the copepods Burton found outside his lab at the Scripps Institution of Oceanography in San Diego were more closely related to the specimens he scooped out of tide pools in Baja California than those more than 2,000 miles north on the coast of Alaska. Burton wondered what the significance of their genetic differences might be.

To find out, he and his colleagues bred copepods from populations sampled all along the coast. They didn’t just breed copepods from the same population; they also put together males and females of different groups. The first generation of these hybrid offspring — the F1 — appeared normal and healthy when the lab began these experiments in the late 1980s. When Burton then bred the F1 generation with itself, however, problems appeared.

That second generation, the F2, had fewer young and didn’t survive some environmental stresses as well as non-hybrids did. Those results meant that although interbreeding between the geographically separated copepod populations was technically possible, the evolutionary cards were stacked against the long-term survival of hybrid offspring in the wild.

The researchers wanted to know why the second generation did so poorly. For Burton, only mitochondrial problems could possibly explain these difficulties. His previous work had shown that not only did the nuclear genomes of T. californicus vary among populations, so did their mitochondrial genomes. Since proper mitochondrial functioning required the interaction of proteins made by both genomes, Burton hypothesized that a mismatch between mitochondrial and nuclear DNA sat at the heart of the F2’s problems.

“The people thinking about mitochondrial function were not evolutionary biologists, and evolutionary biologists weren’t thinking about mitochondria, so no one was really putting these two ideas together,” Burton said. His copepods and his guess revealed how the forces of natural selection could act on one of life’s central processes.

Evolution by natural selection hinges on the mutability of the genome. If DNA is writ in stone, natural selection has no variation on which to act. Not long after the discovery of the mitochondrial genome in the 1960s, scientists hypothesized that the genes encoded by this DNA were so central to cellular function that they had to resist further shaping by natural selection. The forces of nature had no room to experiment. Or so the theory went.

“I always thought this was a bad idea,” Burton admitted. Instead, evidence is emerging that mitochondrial DNA is far more mutable than researchers thought. Because mitochondrial DNA lacks capabilities for checking DNA for errors and repairing it, in animals it mutates on average 10 times as frequently as its nuclear counterpart does. (The difference varies considerably: In copepods, the mitochondrial DNA mutates 50 times as frequently.) That mutability doesn’t mean anything goes. The conservative evolutionary forces acting on mitochondria are so strong that the wrong changes to their DNA sequence can create problems. Witness the severity of mitochondrial disease, caused by defects in mitochondria, which in humans can cause seizure, stroke, developmental delays or even death.

To evolutionary biologists, this high mutation rate posed an interesting question: . . .

Continue reading.

Written by Leisureguy

28 September 2017 at 9:04 pm

Posted in Evolution, Science

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