Protein variants reveal early drivers of vertebrate complexity

Researchers have discovered that crucial developmental genes started generating far more protein variants when animals with backbones first emerged.

This greater flexibility changes the way we understand how a minor genetic alteration may have supported the emergence of complex tissues and organs throughout vertebrate life.

The same signalling genes behaved differently in embryos of a sea squirt, a simple invertebrate, a lamprey and a frog.

By comparing these patterns, Professor David Ferrier of the University of St Andrews found that vertebrates produced many more versions from every gene.

The increase was already evident in lampreys, placing the shift near the earliest known evolutionary division of vertebrates.

This timing makes what might seem a technical detail of protein production an indication of how new body parts could first evolve.

Cell messages multiply

Throughout development, cells remain organised through intercellular signalling: the continuous messages they exchange with one another.

These messages ultimately produce proteins that determine which genes are activated or remain silent within each cell. Altering the final protein form means the same external signal can steer a cell towards a different fate.

This is why creating additional versions mattered more than merely increasing the number of gene copies.

One gene, many outcomes

Cells generate this range through alternative splicing, in which a single gene is cut into different RNA messages before protein is made.

Each of these messages, called a transcript, is an RNA copy used for producing proteins and may generate subtly different forms.

A more recent sequencing method enables the team to view complete RNA messages in these animals rather than infer them from fragments.

This mattered because many concealed variants occur in sections that are absent from shorter reads, which can merge them together.

Numbers that stand out

One signalling family illustrated the change clearly: sea squirts possessed one gene and one transcript, whereas frogs had four genes and nine transcripts.

These figures were important because they applied to just one family, rather than representing a broad expansion throughout the genome.

The pattern was immediately striking, as only a small set of genes behaved differently from all the others investigated by the researchers.

The findings transformed a straightforward tally into a potential new account of vertebrate complexity.

Signal-reading genes stood out

Among all other genes measured, the researchers found no across-the-board surge in protein diversity.

Even genes involved in embryonic development more generally remained far more similar between the sea squirt, lamprey and frog.

This makes the signal-reading genes distinctive, since their diversity increased more quickly than the rest of the developmental machinery.

Rather than a genome-wide expansion, the shift appears focused, strengthening the argument for a genuine evolutionary turning point.

More than duplicates

Previous research had shown that vertebrates possessed extra copies of one major signal-reading family, although copy number was only one part of the picture.

The new research introduces a second level, as each copy could also generate several separate protein forms.

Together, this gave vertebrate cells more options for interpreting the same outside signal and selecting a response.

Bodies could therefore fine-tune cell identity more precisely, helping to account for new tissues, organs and body plans.

A sea squirt twist

A finding in sea squirts also made the contrast less straightforward, because one of their signal-reading genes was not completely fixed.

The researchers identified a new gene segment close to transposable elements-pieces of DNA able to move around genomes-which can help generate new protein endings.

This additional version appeared only at a later developmental stage, suggesting that even closely related invertebrates retained some hidden flexibility.

Even so, the vertebrate pattern remained substantially stronger, and the principal trend was still clear.

Implications for the medical field

These genes lie at the endpoints of major pathways involved in shaping embryos, repairing tissues and disease breakdown.

Their proteins function as transcription factors, gene switches within cells that respond once a signal has arrived.

If these final decision-makers change form, the same signal may support healthy growth or contribute to cancer becoming established.

That does not yet make these new protein forms medical targets, but it does make them worth investigating.

Drivers of vertebrate complexity

The greatest challenge now is demonstrating what each form actually does in a living embryo.

Some may activate genes, others may turn them off, while others may function only in particular tissues.

“It will be exciting to determine how these various different protein forms work in distinct ways,” said Ferrier.

Resolving this question will show whether these protein variants were merely passengers in vertebrate history or active creators of complexity.

Considered across embryos and gene families, the evidence suggests that flexible signal-reading proteins were early drivers of vertebrate complexity.

Additional species and direct experiments will examine this idea, but the study already gives the emergence of backbones a clearer genetic definition.