Some do it horizontally, some do it vertically, some do it sexually, and some asexually. Then there are some organisms that would rather grow a butt that develops into an autonomous appendage equipped with its own antennae, eyes, and brain. This appendage will detach from the main body and swim away, carrying gonads that will merge with those from other disembodied rear ends and give rise to a new generation.

Wait, what in the science fiction B-movie alien star system is this thing?

Megasyllis nipponica really exists on Earth. Otherwise known as the Japanese green syllid worm, it reproduces by a process known as stolonization, which sounds like the brainchild of a sci-fi horror genius but evolved in some annelid (segmented) worms to give future generations the best chance at survival. What was still a mystery (until now) was exactly how that bizarre appendage, or stolon, could form its own head in the middle of the worm’s body. Turns out this is a wonder of gene regulation.

Butt how?

Led by evolutionary biologist and professor Toru Miura of the University of Tokyo, a team of scientists discovered the genetic mechanism behind the formation of the stolon. It starts with Hox genes. These are a set of genes that help determine which segments of an embryo will become the head, thorax, abdomen, and so on. In annelid worms like M. nipponica, different Hox genes regulate the segments that make up the worm’s entire body.

Miura and his colleagues were expecting the activity of Hox genes to be different in the anterior and posterior of a worm. They found out that it is actually not the Hox genes that control the stolon’s segments but gonad development that alters their identity. “These findings suggest that during stolonization, gonad development induces the head formation of a stolon, without up-regulation of anterior Hox genes,” the team said in a study recently published in Scientific Reports.

The anterior part, or stock, of M. nipponica is neither male nor female. The worm has organs called gonad primordia on the underside of its posterior end. When the primordia start maturing into oocytes or testes, head-formation genes (different from the Hox genes), which are also responsible for forming a head in other creatures, become active in the middle of the stock body.

This is when the stolon starts to develop a head. Its head grows a cluster of nerve cells that serve as a brain, along with a central nervous system that extends throughout its body. The stolon’s own eyes, antennae, and swimming bristles also emerge.

Left behind

Before a stolon can take off on its own, it has to develop enough to be fully capable of swimming autonomously and finding its way to another stolon of the opposite sex. The fully developed stolon appears like an alien being attached to the rest of the worm’s body. Besides its own nervous system and something comparable to a brain, it also has two pairs of bulging eyes, two pairs of antennae, and its own digestive tube. Those eyes are enlarged for a reason, as the gonad will often need to navigate in murky waters.

The antennae of the stolon can sense the environment around them, but the researchers suggest that they have a more important function—picking up on pheromones released by the opposite sex. The stolon still isn’t an exact duplication of the stock. It doesn’t have some of the worm’s most sophisticated features, such as a digestive tube with several specialized regions, probably because its purpose is exclusively to spawn. It dies off soon after.

So what could have made stolonization evolve in the first place? Further research needs to be done, but for now, it is thought that this strange capability might have shown up in some annelid worms when genes that develop the head shifted further down the body, but why this shifting of genes evolved to begin with is still unknown.

The worm also regenerates stolons at a high rate, which may also give it the best chance at propagating its species. Hold onto your butts.

Scientific Reports, 2023.  DOI:  10.1038/s41598-023-46358-8