Natures wonders: glue your predator to defend yourself (Introduction)

by David Turell , Friday, September 20, 2024, 21:13 (32 days ago) @ David Turell

Very common in frogs:

https://phys.org/news/2024-09-independent-frog-role-predator-evasion.html

"Skin-secreted adhesives, or glues, are highly effective defense adaptations that have evolved recurrently in a small number of amphibians. From an ecological standpoint, this rapidly solidifying material—essentially, a sticky slime—encumbers the predator long enough for its would-be prey to escape.

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"Skin-secreted chemicals constitute the most widespread antipredator adaptation among amphibians. In a small number of amphibians, this mechanism occurs in the form of glue. When stressed, the amphibian discharges a viscous fluid from its back that quickly solidifies into a sticky mass (i.e., glue).

"This glue functions as an effective defense weapon, incapacitating the attacking predator—often a snake—by clogging its mouth and making the act of swallowing impossible. The energetic cost of overcoming this stickiness eventually becomes too high, forcing the predator to give up and release the amphibian.

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"Although frog glue has been discovered throughout the world, its absence in most species (and especially close relatives) is conspicuous. For instance, a glue-producing frog in Madagascar might not share the island with glue-producing amphibians from other lineages.

"Instead, similar sticky secretions may be found in frogs with distributions restricted to Australia or South America, for example.

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"To this end, I identified two proteins that demonstrably interact within the glue milieu to sustain its adhesive and cohesive strength. One is a large glycoprotein (cleverly assigned the acronym PRIT, courtesy of my supervisor) with a presumed glue-specific role, and contains duplicate copies of an evolutionarily conserved domain that is also present in many extracellular metazoan proteins.

"The second, much smaller protein is a glycan-binding member of an ubiquitous protein family known as galectin. These findings are consistent with previous reports on the importance of both glycoproteins and glycan-binding proteins in other animal glues, although their interactions and probable mechanism of action were unresolved until recently.

"Structural models predicted that while the conserved domains within PRIT are well defined, their intervening regions are structurally heterogenous. This is in contrast to most (nonadhesive) proteins, which have rigid and defined structures, whereas the structural dynamism of PRIT renders it highly flexible.

"In practical terms, this means that frog glue can conformationally adapt to any surface it comes into contact with—for example, the oral epithelia of a snake. The transition from a viscous but fluid slime into a tough, fast-acting adhesive occurs once pressure is applied, such as the force exerted by a predator's bite.

"Reverting to our earlier question: What do the humble sticky tape and frog glue have in common? They're both pressure sensitive, meaning that compressive force is required to fully "activate" their sticking power.

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"The specific architecture of the gene encoding PRIT, however, involves a deviation that evolved in an early amphibian ancestor. In other words, glue genes evolved before the glue itself. (my bold)

"Intriguingly, a second glue-producing species (the Mozambique rain frog, Breviceps mossambicus) also encodes a PRIT gene. Dyscophus and Breviceps diverged about 100 million years ago and belong to distinct radiations of frogs (Microhylidae and Afrobatrachia, respectively).

"Other members of these lineages produce nonadhesive toxins that are known to have originated early in frog evolution, thus leaving little doubt that: (1) Dyscophus and Breviceps both descended from a poisonous ancestor; and (2) their skin secretions evolved into glues independently.

"Alongside structural changes, shifting gene expression was identified as a decisive factor in the recurrent evolution of glue: PRITs and galectins exhibit the same pattern of elevated expression in both glue-producing species, from which we can surmise that regulatory changes also contributed to the parallel evolution of frog glue.

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"Frog glue is therefore a culmination of evolutionary processes that came before it, with recurrent structural and regulatory changes acting on an ancient and near-universal template." (my bold)

Comment: note my bolds. This study is an advertisement for design! The genes preceded the glue!