Photo credit: Da-Yin/University of Maryland

How is it possible to lose thousands of genes? Well, one answer lies in self-fertilisation. 

Once the roundworm, Caenorhabditis briggsae became capable of this ability it triggered that exact reaction. A recent study carried out by biologists, published in Science, made the claim after it diverged from C. nigoni a million years ago. 

Eric Haag, a biology professor at the University of Maryland, College Park, and an author of the paper, said, “Many of these genes had been around, and were presumably needed, for tens of millions of years or longer. In the blink of an eye, they disappeared.”

He, alongside his co-authors believe that a large portion of the genes shed are related to male reproduction. 

In this particular 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, that they found, were more heavily expressed in male than female C. nigoni. 

Assistant researcher professor at Cornell University, who led the sequencing efforts for the study, Erich Schwarz, added, “That tells us that the stuff being list in Caenorhabditis briggsae is disproportionately involved in male biology.”

In order to focus on a specific example of what C. briggsae lost when it dumped all of those genes, the researchers turned their attention to male secreted short (or m.s.s.) 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,” said Dr Haag. 

Da Yin, a graduate student in Dr Haag’s lab and lead author of the study, managed to successfully prove 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. 

Such work supports 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 have up many genes facilitating male-dependent procreation because a mostly hermaphrodite lifestyle offered an advantage, Yin explained. 

Dr Haag went on to explain how 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 concluded. “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.”

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