A million years ago, a species of tiny worms called Caenorhabditis briggsae evolved the ability to breed via selfing. As a result, most C. briggsae are hermaphrodites with both male and female sex organs. Haag’s group, which focuses on the evolution of sex, has long studied C. briggsae because of their unusual reproductive behavior.
To study how selfing shaped the evolution of C. briggsae, Erich Schwarz, an assistant research professor of molecular biology and genetics at Cornell University and co-corresponding author of the study, sequenced the genome of Caenorhabditis nigoni, the closest relative of C. briggsae. C. nigoni always reproduce by mating with other individuals, or outcrossing. By comparing the genomes of the two species, the researchers found that the selfing C. briggsae worms had 7,000 fewer genes than C. nigoni. Over time, C. briggsae lost approximately a quarter of its genome.
The selfing species of worms may have lost the mss genes because having competitive male sperm is harmful, according to Haag. During the study, the researchers discovered that having more competitive male sperm changed the species’ sex ratio towards greater production of males. This shift could put the worms’ survival at risk because having too many males slows population growth, and in the wild the worms must reproduce as fast as possible to survive. Paper. (paywall) – Da Yin, Erich M. Schwarz, Cristel G. Thomas, Rebecca L. Felde, Ian F. Korf, Asher D. Cutter, Caitlin M. Schartner, Edward J. Ralston, Barbara J. Meyer, Eric S. Haag. Rapid genome shrinkage in a self-fertile nematode reveals sperm competition proteins. Science, 05 Jan 2018 DOI: 10.1126/science.aao0827 More.
Now, the idea that “having too many males slows population growth, and in the wild the worms must reproduce as fast as possible to survive” sounds like a guess if similar worms haven’t followed suit. But consider this: From Steph Yin at the New York Times:
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,” Dr. 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. More.
But then are C. briggsae unusually opportunistic loners, relative to closely related species? In any event, this is clearly an instance of devolution, the loss of a complex, possibly costly trait not needed for survival.
It will be interesting to see whether, as with the blind cave fish, the trait can in fact be regained:
Losses can be reversed. Blind Mexican cavefish are considered an excellent model for studying evolution, with revealing results. In the lab, researchers have mated blind cave fish from separate and distant underwater caves and produced sighted offspring. Apparently, separate mutations had produced the blindness, and some hybrid offspring inherited a mix that includes enough genes for functioning sight. So no irrevocable devolution had taken place after all. More.
Note: From the McDonnell Institute:
When inspected side-by-side, C. briggsae and C. elegans are almost indistinquishable, even though they diverged from their last common ancestor approximately 100 million years ago. C. briggsae shares many of the same features that have made C. elegans a model organism for the study of genetics, as developmental biology, neurobiology, cell biology and behavior: it is easy to maintain in the laboratory, has a short generation time, an invariant anatomy and transparent body. Again, like C. elegans, the C. briggsae embryo goes through four larval stages before becoming a mature adult. Each larval stage looks similar to the adult, only smaller. C. briggsae can adapt an alternative life form, call a dauer larva, in response to overcrowding or the absence of adequate food supply.
See also: Giant shipworm found alive is example of devolution
Devolution: Getting back to the simple life
A male and a hermaphroditic C. briggsae mating, or breeding via outcrossing. The hermaphrodite is the larger of the pair. Video credit: Da Yin, University of Maryland: