Chaetognaths, whose name means "bristle-mandible", can be found all over the world, swimming in brackish estuaries, tropical seas and on the deep dark ocean floor. Also known as arrowworms, creatures have existed since the Cambrian Period, but their precise place in evolutionary history has long been eluded by scientists. Now, researchers at the Graduate Institute of Science and Technology in Okinawa (OIST) have learned where the arrow worms move on the Tree of Life, and their results could reveal important trends in the evolution of bilateral organisms.
The researchers tried to verify the relationship of the predatory arrow worm with other spirals: members of a diverse group of organisms, or clade, believed to share a common ancestor. The spiral clade includes mollusks, segmented worms and flat worms. Surprisingly, the scientists discovered that the arrow worms do not belong to the spirals, but to a new group of animals that form a brother group of the clade. The results, published on January 10, 2019 in. Current biologyChallenge the classic view that complex organisms evolved from simple ancestors by acquiring new traits over time.
"Arrow worms are predators, they have nervous systems, they have developed sensory organs, but the other organisms with which they are grouped are much simpler," said Ferdinand Marlétaz, first author of the study and postdoctoral fellow at the OIST Molecular Genetics Unit. . , directed by Prof. Daniel Rokhsar. "If you put the arrow worms here, it means there's probably a lot of independent simplification, rather than the independent emergence of complexity."
Although they are very different in appearance, arrow worms and their phylogenetic relatives, such as microscopic animals known as rotifers, seem to share a unique jaw structure. Composed of a dense protein matrix and a fibrous substance called chitin, these jaws are located near the mouths of organisms and allow them to grab their prey.
"The arrow worms are grouped with a rather dark collection of small marine animals, they are not animals that most people are familiar with," said Professor Daniel Rokhsar, lead author of the study and principal investigator of the research unit. . "The fact that these different animals had jaws that were probably related to each other was not clear until this document."
Arrow worms from around the world
The approximately 200 species of arrow worms resemble small spears and range from only one millimeter in length to 12 centimeters in length. Most predators delight in small crustaceans called copepods, which use their sharp vibratory sense to hunt prey and swallow them whole. The strange, wormlike creatures actually share many morphological and developmental traits with other organisms, which makes their evolutionary timeline difficult to trace.
"The different animals that share the same early development are often related to each other," Rokhsar said. One reason why researchers have struggled to characterize arrowworms is that their early developmental patterns are ambiguous; they resemble the patterns observed in two large groups of animals. "We really did not have a way to accurately classify arrow worms in one way or another."
The two animal supergroups are known as deuterostomes and protoostomes. Both sets of organisms have a single intestine running through them, from their mouths to the other end. In early development, the intestine of the deuterostome is formed from the bottom up, while the formation of the protostome intestine begins in the mouth. Although arrow worms develop from the bottom up like deuterostomes, they closely resemble proto-tumors both morphologically and genetically.
To clarify this discrepancy, the researchers collected data on 10 arrowworm species and compared them with publicly available data from other animals. They examined the transcriptome species, which serve as a snapshot of all the genes that are expressed in a given cell. Scientists originally drew their sample arrow worms from the Atlantic Ocean, the Gullmar Fjord in Sweden, the Amakusa in Japan and Marseille in France. Dr. Katja Peijnenburg of the University of Amsterdam, Dr. Taichiro Goto of the University of Mie and Professor Noriyuki Satoh of the OIST Unit of Marine Genomics helped Marlétaz to collect and prepare the samples.
The comparison placed the arrow worms solidly in the superclast protostome within a subgroup that includes microscopic organisms known as rotifers, gnatóstomulidos and micrognatozoos. Other established subgroups were considered as an involuntary consequence of this grouping, which means that many relationships between protoostomes are now under scrutiny.
"It surprised me a little," said Marlétaz. "We still do not fully understand this association with rotifers and others, which will be the focus of future research."
Protostomes, past and future
The study improved previous work by collecting new data from the arrow worm and comparing it with a representative sample of other members of the animal kingdom. The researchers also chose to focus on evolutionary lineages that had evolved relatively slowly, rather than rapidly. Rapidly evolving lineages tend to look similar in the analysis, even if they are barely related, so the researchers tried to avoid this bias.
Looking forward, Marlétaz hopes that there will be more morphological studies of arrow worms, to connect the dots between the phylogeny of the animals and their physical characteristics. For example, researchers still do not know which genes give rise to the iconic "bristle jaw" of the Gnathifera clade, or what other specific feature could bind the group.
"We also have to look more closely at the genome of the arrow worm," said Marlétaz. "Arrow worms have very original genomic characteristics, so we want to understand them better in the context of their new phylogenetic position."
The genomes of the worms reveal a link between humans and distant relatives.