developmental biology

we-probably-inherited-our-joints-from…-a-fish

We probably inherited our joints from… a fish

Biology, developmental biology, evolution, joints, paleontology, Science, synovial joints / /

What do we have in common with fish, besides being vertebrates? The types of joints we (and most vertebrates) share most likely originated from the same common ancestor. But it’s not a feature that we share with all vertebrates.

Humans, other land vertebrates, and jawed fish have synovial joints. The lubricated cavity within these joints makes them more mobile and stable because it allows for bones or cartilage to slide against each other without friction, which facilitates movement.

The origin of these joints was uncertain. Now, biologist Neelima Sharma of the University of Chicago and her colleagues have taken a look at which fish form this type of joint. Synovial joints are known to be present in jawed but not jawless fish. This left the question of whether they are just a feature of bony skeletons in general or if they are also found in fish with cartilaginous skeletons, such as sharks and skates (there are no land animals with cartilaginous skeletons).

As Sharma and her team found, cartilaginous fish with jaws, such as the skate embryos they studied, do develop these joints, while jawless fish, such as lampreys and hagfish, lack them.

So what could this mean? If jawed fish have synovial joints in common with all jawed vertebrates, including us, it must have evolved in our shared ancestor.

Something fishy in our past

While the common ancestor of vertebrates with synovial joints is still a mystery, the oldest specimen with evidence of these joints is Bothriolepis canadensis, a fish that lived about 387 to 360 million years ago during the Middle to Late Devonian period.

When using CT scanning to study a Bothriolepis fossil, Sharma observed a joint cavity between the shoulder and pectoral fin. Whether the cavity was filled with synovial fluid or cartilage is impossible to tell, but either way, she thinks it appears to have functioned like a synovial joint would. Fossils of early jawless fish, in contrast, lack any signs of synovial joints.

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worm’s-rear-end-develops-its-own-head,-wanders-off-to-mate

Worm’s rear end develops its own head, wanders off to mate

Biology, developmental biology, Science, worms / /

Butt what? —

The butt even grows its own eyes, antennae, and brain.

–

Three images of worm-like organisms.

Enlarge / From left to right, the head of an actual worm, and the stolon of a male and female.

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

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