The body plan for most animals is set at birth. For humans, we get two arms, two legs and no more. It doesn't matter if you lose one, or if you really want another hand to hold your coffee, our genetic code decides how many appendages we have. But research led by Aissam Ikmi from the European Molecular Biology Unit in Germany has recently shown that sea anemones can sprout extra limbs when they are given lots of food. The team, which also included researchers from the University of Kansas and Stowers Institute for Medical Research, USA, wanted to explore how these squishy little animals can be so flexible with their body plans and what molecular machinery was causing these huge changes.

Ikmi's team focused on the starlet sea anemone (Nematostella vectensis), fascinating animals because they can branch out to grow new body parts across their lives – kind of like plants. Starlet sea anemones have quite simple body plans, with a bulbous, oblong body and some squirmy tentacles (or arms) at one end that help them capture prey. Researchers have long admired the flexible lives that these little creatures lead, but it was unclear what triggered the anemones to sprout more arms. Ikmi suspected that food might fuel this growth. To test the idea, the researchers fed some anemones daily brine-shrimp snacks while others were left empty handed. The fed anemones all started out with four arms, doubled their size and then budded new pairs of arms after about 5 days, some eventually grew up to 16 arms.

But, how does a meal get turned into new arms? The research group looked at two key components of the mechanism that manages growth to answer this. They first measured the target of rapamycin protein (TOR), because it regulates cell growth based on the level of nutrients and energy available. Only the fed anemones had a lot of TOR activation and the TOR activation shifted to places where new arms would soon pop out with each passing day of brine-shrimp munching.

The team next investigated fibroblast growth factor receptor (FGFR) as a potential second cog in the arm-growing machine and they found the gene for this receptor was only expressed where a new arm would unfurl. When the researchers chemically blocked the receptor or rendered the gene functionless, the anemones would still grow in size, but they couldn't sprout any new arms. This suggested that feeding tells TOR to first make the anemones grow, which then triggers FGFR to be expressed at new arm sites. TOR activity then becomes more localized, and voilà, new arms shoot out.

Ikmi analysed over 1000 sea anemones to show that TOR and FGFR must volley back and forth to transform a feeding frenzy into new appendages. Their elegant series of experiments traverses many scales of biological organization, from genes to environmental cues, and these sea anemones certainly give a whole new meaning to the consequences of over-eating.

References

Ikmi
,
A.
,
Steenbergen
,
P.
,
Anzo
,
M.
,
McMullen
,
M.
,
Stokkermans
,
A.
,
Ellington
,
L.
and
Gibson
,
M.
(
2020
).
Feeding-dependent tentacle development in the sea anemone Nematostella vectensis
.
Nature Commun.
11
,
4399
.