National News

This Worm Evolved Self-Fertilization and Lost a Quarter of Its DNA

Posted January 8, 2018 3:56 p.m. EST

Inspecting the tiny roundworms Caenorhabditis briggsae and Caenorhabditis nigoni through a microscope, you’d have trouble telling them apart. Both are about a millimeter long and transparent. On the evolutionary tree, they’re closer together than horses and donkeys.

The key distinction between the two nematodes is their sex lives. Sex in C. nigoni takes place between a male and female. But only a small minority of C. briggsae are males. The rest are hermaphroditic females that reproduce by self-fertilizing, or selfing. They have evolved the ability to produce sperm that merge with their own eggs.

This sexual switch may have caused profound changes at the genetic level for C. briggsae. In a study published last week in Science, biologists reported that C. briggsae lost thousands of genes — a staggering quarter of its genome — since it diverged from C. nigoni a million years ago.

“Many of these genes had been around, and were presumably needed, for tens of millions of years or longer,” said Eric Haag, a biology professor at the University of Maryland, College Park, and an author of the paper. “In the blink of an eye, they disappeared.” He and his co-authors believe that a large portion of the genes shed are related to male reproduction.

In their study, the biologists compared C. briggsae and C. nigoni, and discovered that C. briggsae has about 7,000 fewer genes. A disproportionately high number of shed genes, they found, were more heavily expressed in male than female C. nigoni.

“That tells us that the stuff being lost in Caenorhabditis briggsae is disproportionately involved in male biology,” said Erich Schwarz, an assistant research professor at Cornell University who led the sequencing efforts for the study.

Digging into a specific example of what C. briggsae lost when it dumped all those genes, the researchers studied male secreted short (or MSS) genes, which have been found in all studied Caenorhabditis species except those with selfing hermaphrodites. “We thought this gene family was maybe emblematic of a larger phenomenon,” Haag said.

Da Yin, a graduate student in Haag’s lab and lead author of the study, showed in lab experiments that C. briggsae fathered more offspring when those genes were added to their genomes, suggesting that these genes contributed to a reproductive edge in males.

This bolsters the scientists’ hypothesis that C. briggsae’s dramatic reduction in DNA had to do with its change in sexual strategy. Over time, the species possibly gave up many genes facilitating male-dependent procreation because a mostly hermaphroditic lifestyle offered an advantage, Yin said.

C. briggsae are opportunistic creatures that often colonize isolated oases — say a rotting apple — as lone individuals. If the wrigglers always had to rely on mates to multiply, it would be harder for them to kick-start these new colonies. As a result, it might be more efficient for the species to skew heavily toward hermaphrodites — having more males may “put a brake on population growth,” Haag said.

Being able to examine the genetic underpinnings of C. briggsae, particularly in relation to such a close relative, C. nigoni, has been a gift, he added.

“We have this natural experiment where a species has given up the way it used to reproduce,” he said. “It shows us how much of the genome is involved in the subtleties of mating and reproduction — and it’s startling just how much of it is.”