Despite having split from a common ancestor 20 million years ago, scientists have discovered two species of worm that have retained noticeable similarities in gene patterns.
The study, published in Science, used RNA sequencing techniques to compare the gene expression of two separate species of transparent roundworms, Caenorhabditis elegans and Caenorhabditis briggsae.
When fully developed, these worms are about a millimeter long and made up of about 550 cells making them the perfect size to allow the researchers to observe their cell development in real time.
Using a technique called single-cell RNA sequencing, the researcher tracked worm embryos for about 12 hours, observing the worm’s development from an embryo of 28 cells to essentially its final form.
While the goal was to uncover what changes in gene expression had emerged across 20 million years of evolution, the researchers found the worms had almost identical cell types and body plans.
“I was surprised how well everything lined up,” says co-author of the paper, Dr. Robert Waterston, professor of genome sciences at the University of Washington School of Medicine.
“It was just remarkable, with this evolutionary distance, that we should see such coherence in gene expression patterns.”
While most of the gene expression was the same, there were some changes and divergences between the two species.
The researchers found these changes were more likely to be found in cells that had a specialised function.
“If the gene is broadly expressed in many cell types across the organism, it may be difficult to change expression,” says Waterston.
For example, specialised cells like those involved in responding to the worm’s environment were more likely to diverge whereas cells involved in basic functions like the ones found in muscles stayed the same.
“Genes related to neuronal function, for example, seem to diverge more rapidly,” says Christopher Large, the study’s lead author from the University of Pennsylvania.
“Perhaps because changes were needed to adapt to new environments — but for now, that’s speculation.”
The study’s method of comparing the entire genome of two species helps scientists to begin to understand how patterns of gene expression change across evolution, something that up until now remains mostly unknown.
“We’ve been studying the evolution of development since the 1970s,” says Dr. Junhyong Kim, the study’s co-author from Penn Genome Frontiers Institute.
“But this is the first time that we’ve been able to compare development in every single cell of two different organisms.”
The two worm species share roughly 20,000 genes with other organisms like humans which opens the door for future studies.
“It’s hard to say whether any of the differences we observed were due to evolutionary adaptation or simply the result of genetic drift, where changes happen randomly,” says Dr John Isaac Murray, associate professor of genetics at the Perelman School of Medicine in Pennsylvania.
“But this approach will allow us to explore many unanswered questions about evolution.”