Abstract
The foam nests of the túngara frog (Engystomops pustulosus) form a biocompatible incubation medium for eggs and sperm while resisting considerable environmental and microbiological assault. We have shown that much of this behaviour can be attributed to a cocktail of six proteins, designated ranaspumins (Rsn-1 to Rsn-6), which predominate in the foam. These fall into two discernable classes based on sequence analysis and biophysical properties. Rsn-2, with an amphiphilic amino acid sequence unlike any hitherto reported, exhibits substantial detergent-like surfactant activity necessary for production of foam, yet is harmless to the membranes of eggs and spermatozoa. A further four (Rsn-3 to Rsn-6) are lectins, three of which are similar to fucolectins found in teleosts but not previously identified in a land vertebrate, though with a carbohydrate binding specificity different from previously described fucolectins. The sixth, Rsn-1, is structurally similar to proteinase inhibitors of the cystatin class, but does not itself appear to exhibit any such activity. The nest foam itself, however, does exhibit potent cystatin activity. Rsn-encoding genes are transcribed in many tissues of the adult frogs, but the full cocktail is present only in oviduct glands. Combinations of lectins and cystatins have known roles in plants and animals for defence against microbial colonization and insect attack. Túngara nest foam displays a novel synergy of selected elements of innate defence plus a specialized surfactant protein, comprising a previously unreported strategy for protection of unattended reproductive stages of animals.
Highlights
The use of foams as incubation environments for eggs or juveniles is widespread among animals, examples including the protective foam enclosures of the nymphs of homopteran bugs, bubble raft nests of fish and the egg chambers of locusts
Kennedy 2004, unpublished observations), such a strategy would seem to be expensive in terms of protein synthesis and the mechanical energy required for nest construction
We have found no evidence for small molecule detergents in tungara foam, so the key to the problem must reside in the particular properties of the surfactant protein itself
Summary
The use of foams as incubation environments for eggs or juveniles is widespread among animals, examples including the protective foam enclosures of the nymphs of homopteran bugs, bubble raft nests of fish and the egg chambers of locusts. I. Fleming et al Foam nest proteins of the tungara frog and sperm suggests that this resistance to microbial assault is not due to the membrane disruption expected of simple detergent activity, but is a more complex property of the foam components.
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More From: Proceedings of the Royal Society B: Biological Sciences
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