In one of the strangest paths to reproduction, this tiny Japanese sea worm actually grows a ‘mini-me’ at its rear end, which it then deploys so it can swim off on its own in search of similar autonomous tail of the opposite sex to spawn with. The worm, meanwhile, will then grow another, to deploy for mating once more.
What’s puzzled scientists is how the Japanese green syllid worm (Megasyllis nipponica) was able to form this reproductive unit, known as the stolon. Now, a team led by University of Tokyo professor Toru Miura has unraveled the complex gene expression of the aquatic worm found in the Sea of Japan.
As the worm matures, it develops a stolon at its rear end in tandem. When fully grown, it’s laden with gametes (eggs or sperm), and through a process of stolonization, detaches from the worm’s trunk. By this stage, it already has fully developed bristles for swimming and a set each of eyes and antennae.
The newly independent stolon then ventures off, in search of a stolon of the opposite sex, in which to release its gametes with. Then its job is done. However, the worm itself, continues living, and will regrow another stolon to deploy in similar fashion when mature.
Evolutionary biologists believe the worms have developed this reproductive ability because it enables the primary animal to stay out of harm’s way while the stolon is out in the open doing the risky mate-seeking work. It also potentially allows for the worm’s genes to be spread further afield, depending on how far the stolon swims to find what it needs.
Just how the stolom develops inside the main body of the worm, however, has been a mystery to scientists. This new research finally sheds light on the curious sexual strategy.
"This shows how normal developmental processes are modified to fit the life history of animals with unique reproductive styles," said Miura.
The team found that the Hox genes, which are responsible for general body plan, and in this case the worm’s segmentation, are consistent throughout the animal. It surprised the researchers, who had thought the expression might have differed at either ends.
"Interestingly, the expressions of Hox genes that determine body-part identity were constant during the process," said Miura. "This indicates that only the head part is induced at the posterior body part to control spawning behavior for reproduction."
Now, the researchers hope to uncover what determines whether the stolon will have eggs or sperm, particularly since the worm regrows this section throughout its life cycle.
"We would like to clarify the sex determination mechanism and the endocrine regulations underlying the reproductive cycles in syllids," added Miura.
The research was published in the journal Scientific Reports.
Source: University of Tokyo