Identical Skin Toxins by Convergent Molecular Adaptation in Frogs
Kim Roelants, Bryan G. Fry, Janette A. Norman, Elke Clynen, Liliane Schoofs, and Franky Bossuyt
Current Biology (2010), doi:10.1016/j.cub.2009.11.015
The Tree of Life is rife with adaptive convergences at all scales and biological levels of complexity. However, natural selection is not likely to result in the independent evolution of identical gene products. Here we report such a striking example of evolutionary convergence in the toxic skin secre- tions of two distantly related frog lineages. Caeruleins are important decapeptides in pharmacological and clinical research and are commonly believed to represent a single evolutionary class of peptides. Instead, our phylogenetic analyses combining transcriptome and genome data reveal that independently evolved precursor genes encode identical caeruleins in Xenopus and Litoria frogs. The former arose by duplication from the cholecystokinin (cck) gene, whereas the latter was derived from the gastrin gene. These hormone genes that are involved in many physiological processes diverged early in vertebrate evolution, after a segmental duplication during the Cambrian period. Besides implicating convergent mutations of the peptide-encoding sequence, recurrent caerulein origins entail parallel shifts of expression from the gut-brain axis to skin secretory glands. These results highlight extreme structural conver- gence in anciently diverged genes as an evolutionary mech- anism through which recurrent adaptation is attained across large phylogenetic distances.
le dernier paragraphe de la discussion, pour éviter aux neuneux (Salut Jean) de sauter à des conclusions hâtives.
A crucial precondition for the parallel origins of caerulein was the conservation of the C-terminal bioactive domain. First, this domain allowed the recurrent evolution of a new biological function (from metabolic regulation to passive antipredator defense) while preserving a similar underlying biochemistry (as an agonist of CCK receptors ). Second, as a result of its widespread evolutionary conservation, the domain allows the peptides to be effective against a wide range of vertebrate predators. This broad-scale effectiveness could explain why identical toxins arose in frogs that inhabit different geographic realms and ecological habitats, where they are likely to face different types of predators (African pipids are strictly aquatic, whereas Australian/Papuan Litoria frogs have a terrestrial and/or arboreal lifestyle). Third, it determined the necessary preexisting structure (the peptide-encoding region and flanking residues) on which subsequent adaptive mutations could act. This situation is analogous to observations of deep homology in gene regulatory networks underlying the development of complex morphological adaptations such as eyes and limbs that evolved independently in distant animal phyla . Our findings suggest that the same evolutionary patterns scale down to single-gene molecular adaptations as well.