The Animal That Begins it's Life as a Head without a Body

Direct developers are more often used in research largely for reasons of practicality.

“Terrestrial, direct developing species develop fast, their life cycle is simple and they are easy to rear in the lab,” said Gonzalez, who was lead author of the paper.

By comparison, indirect developers develop slowly, have a long larval stage, and their larvae are difficult to feed and maintain in captivity. The reproductive adults are also challenging to keep in the lab and, as Gonzalez has shown, collecting them can be an arduous process. However, the relative ease of studying direct developers has made for a lack of diversity in what scientists know about evolution and development, Gonzalez said.

“By selecting convenient species, we select a non-random sample of animal diversity, running the risk of missing interesting things,” he said. “That’s what brought me to the Lowe lab. We specialize in asking cool evolutionary questions using developmental biology and molecular genetics, and we’re not afraid to start from scratch and work on animals that no one has worked with before.”
Swimming heads

After spending months perfecting the rearing and breeding techniques needed to study these worms, the researchers were eventually able to sequence the RNA from various stages of the worm’s development. They did this in order to see where specific genes are turned on or off in an embryo.

They found that in the worms, activity of certain genes that would lead to the development of a trunk are delayed. So, during the larval stage, the worms are basically swimming heads.

“When you look at a larva, it’s like you’re looking at an acorn worm that decided to delay development of its trunk, inflate its body to be balloon-shaped and float around in the plankton to feed on delicious algae,” said Gonzalez. “Delayed trunk development is probably very important to evolve a body shape that is different from that of a worm, and more suitable for life in the water column.”

As they continue to grow, the acorn worms eventually undergo a metamorphosis to their adult body plan. At this point, the genes that regulate the development of the trunk activate and the worms begin to develop the long body found in adults, which eventually grows to about 40 cm (15.8 inches) over the span of several years.
Just the beginning

Even with such a fascinating result, this research is only the beginning of the Lowe lab’s examination of indirect developers. These worms will never tell us about human diseases, unlike work with stem cells or mice, but they could reveal the intricacies of how life works for many organisms beyond the model species that we’ve studied so heavily. They may also show us how life in general came to be what it is today.

“Given how pervasive larvae are in the animal world, we understand very little about this critical phase in animal development,” said Lowe. “These are not the kind of species you want to pick if you want deep, mechanistic insights into developmental biology. But, if your goal is to understand how animals have evolved, then you cannot avoid using these species.”

Next, the researchers want to figure out how the acorn worm body development delay happens. They also have begun to sequence the genome of S. californicum.

Taylor Kubota