In one of the most peculiar ways of reproduction, this small Japanese seaworm actually grows a "mini-me" on its tail. Then it can unfold this "mini-me" and swim far away alone, looking for similar independent tails of the opposite sex to lay eggs. At the same time, the worm will grow another tail so it can mate again.Scientists have been puzzled by how the Japanese green knot insect (Megasyllisnipponica) forms this reproductive unit, called a stolon. Now, a research team led by Toru Miura, a professor at the University of Tokyo, has unraveled the complex gene expression of this aquatic worm in the Japan Sea.
As the marine worm matures, stolons grow from its rear end. When the stolons are fully grown, they bear the gametes (eggs or sperm) and separate from the worm's trunk through the process of stolonization. At this stage, the stolon has developed setae used for swimming as well as eyes and antennae.
The newly independent stolons then seek out stolons of the opposite sex, where they release gametes. In this way, its task is completed. However, the worm itself continues to live, and when mature will regrow another stolon, which unfolds in a similar manner.
Evolutionary biologists believe that worms developed this reproductive ability because stolons keep their primary animals out of harm's way when they go about their perilous job of finding a mate in the wild. It also has the potential to spread the worm's genes further afield, depending on how far the stolon travels to find what it needs.
However, how stolons develop inside the body of the worm has long been a mystery of interest to scientists. New research finally sheds light on this bizarre sexual strategy.
"This shows how normal developmental processes can be altered to adapt to the life history of animals with unique reproductive methods," Miura said.
The team found that the Hox genes responsible for overall body structure are consistent across the entire animal, and in this case, the segmented structure of the green knot bug as well. This surprised the researchers, who had thought the expression might be different at the two ends.
"Interestingly, the expression of Hox genes, which determine the characteristics of each body part, is constant during this process," Miura said. "This suggests that only the head portion is induced at the rear of the body to control egg-laying behavior for reproduction."
Now, the researchers hope to unravel what determines whether a stolon will have eggs or sperm, especially since the worm re-grows this part throughout its life cycle, to shed light on the mechanisms of sex determination and the endocrine regulation of the syllable's reproductive cycle.
The research was published in the journal Scientific Reports.