The humble worm known as C. elegans is famous among biology labs as an experimental workhorse. New findings in this week’s Proceedings of the National Academy of Sciences now boost its obscure relative, C. brenneri, into the limelight, by showing it may not just be the most genetically diverse animal now known, but the most genetically diverse of the eukaryotes — those organisms whose cells have nuclei.
The bacteria-eating, 1-millimeter-long worm C. brenneri is named after Nobel laureate Sydney Brenner, who won the prize for establishing C. elegans as an experimental model organism. Research with C. elegans over the years has helped revolutionize our understanding of the role genes play in developmental biology.
In the last decade, researchers began to suspect C. brenneri was far more genetically diverse than its renowned sibling. For one thing, a C. brenneri worm reproduces by having sex with others, which boosts genetic diversity, as opposed to a C. elegans worm, which has sex with itself to procreate. The bacteria-eater also lives across the tropics, landscapes marked by variety that often spur biological diversity, and the small size of C. brenneri and its abundant food — bacteria in rotting fruit and vegetation — can support vast populations, which increase the chances of novel mutations emerging and persisting.
Evolutionary biologist Asher Cutter at the University of Toronto and his colleagues analyzed C. brenneri from South Asia and northern South America. They found specimens from different continents nevertheless could interbreed to give birth to fully viable offspring, confirming they were one species and not several.
The researchers found C. brenneri was the most genetically diverse eukaryote discovered so far.
Eukaryotes include animals, plants, and fungi, among others.
“We’re used to hearing about biodiversity of rainforests, for example, as cradles of huge numbers of species,” Cutter says. “But our research drives home that biological diversity even within a single species can also be really impressive. The copies of DNA that one of these worms gets from its mother and father can be even more distinct from each other as the DNA we might look at between humans and macaques.”
For instance, comparisons of any randomly chosen analogous sequence of DNA from two different specimens of C. brenneri would find these sequences would differ from each other by some 14.1 percent, meaning they would differ on nearly every sixth base pair on average. By comparison, C. elegans has about 100-fold less genetic variation, and humans have more than 150 times less variation.
“It’s simply amazing that we found an animal that has just as much as, or more, genetic diversity than many kinds of bacteria,” Cutter said.
In recent years, scientists have discovered that much of the so-called “junk” DNA found in genomes that do not encode sequences for proteins actually do serve vital functions — for instance, encoding RNA sequences that regulate genetic activity. The hyperdiversity seen in C. brenneri could help identify such functional noncoding sequences — natural selection would promote the spread of useful mutations, so any noncoding sequences that stay relatively similar across the species likely have some function and would stick out from the variability otherwise seen in the worm’s genome. Such research could shed light on C. elegans, which given its crucial role in modern genetic research could in turn yield insight on human biology.
Theories predict the complexity of a species’ genome can depend on its population size. Past research found such predictions were true in bacteria, and now C. brenneri could help test if this also holds true in eukaryotes.
“A big question is whether C. brenneri is just the tip of the iceberg in terms of how common it is to find animal species with such incredible diversity,” Cutter says. “I think that the DNA sequencing revolution that is underway will help us find many more examples of organisms that have been neglected by scientific study that are similarly hyperdiverse.”