The genome of Diploscapter pachys, a tiny roundworm, has been sequenced for the first time. This little critter is quite special – it reproduces asexually (so its offspring are clones) and belongs to a species that originated 18 million years ago. Miniscule and transparent, it forms one of the oldest known lineages of asexual animals.
The study, carried out by scientists at New York University and Duke University, and published in Current Biology, is extremely interesting as it sheds light on how asexual species can adapt to changes in the environment and survive deleterious mutations in their DNA. More surprisingly, the worm has just a single chromosome pair.
“Scientists have been trying to understand how some animals can survive for millions of years without sex because such strict, long-term abstinence is very rare in the animal world,” said co-author Professor David Fitch, in a statement. “This phenomenon is a significant one in understanding evolutionary genetics because it runs counter to the widely accepted view that sexual reproduction is required to eliminate deleterious mutations and for adaptation to a changing environment.”
The worm's closest living relatives have between five and seven chromosome pairs and reproduce sexually. Having a single chromosome is pretty rare in the animal kingdom and only two other species have this genetic setup – an ant and a parasitic roundworm.
Sequencing D. Pachys' single chromosome proved that this feature is key to its survival. The species has fused its six chromosomes into a single one so its DNA doesn’t recombine during meiosis, guaranteeing high genetic diversity. Meiosis is the process that forms sperm and egg cells.
“Thus, the mystery of its longevity seems largely resolved: D. pachys overcomes the disadvantages of asexual reproduction by maintaining genetic variation, and with it, complementation,” Professor Fitch clarified. “Ironically, this is accomplished by making sure there is no recombination between the gene copies. If there were, the differences between the gene copies might be lost. In fact, D. pachys has gotten rid of several of the genes required to make the recombination machinery that exists in sexual organisms.”
This resilient little animal, just one-third of a millimeter (0.01 inches) in size, shows that as long as you've got good genes to start with, you can endure a lot.