Rastreando a evolução da visão

sexta-feira, agosto 23, 2019

Chromophore-Independent Roles of Opsin Apoproteins in Drosophila Mechanoreceptors

Radoslaw Katana, Chonglin Guan, Damiano Zanini, Christoph F. Schmidt, Steven G. Britt, Martin C. Göpfert

Published:August 22, 2019 DOI: https://doi.org/10.1016/j.cub.2019.07.036


• Drosophila mechanosensation requires opsins but no retinal chromophore

• Mechanosensory opsin function is independent of the retinal attachment site

• Mechanosensation involves visual chromophore pathway components

• Mechanosensory organs express chromophore pathway genes


Rhodopsins, the major light-detecting molecules of animal visual systems [1], consist of opsin apoproteins that covalently bind a retinal chromophore with a conserved lysine residue [1, 2]. In addition to capturing photons, this chromophore contributes to rhodopsin maturation [3, 4], trafficking [3, 4], and stabilization [5], and defects in chromophore synthesis and recycling can cause dysfunction of the retina and dystrophy [6, 7, 8, 9]. Indications that opsin apoproteins alone might have biological roles have come from archaebacteria and platyhelminths, which present opsin-like proteins that lack the chromophore binding site and are deemed to function independently of light [10, 11]. Light-independent sensory roles have been documented for Drosophila opsins [12, 13, 14, 15], yet also these unconventional opsin functions are thought to require chromophore binding [12, 13, 15]. Unconjugated opsin apoproteins act as phospholipid scramblases in mammalian photoreceptor disks [16], yet chromophore-independent roles of opsin apoproteins outside of eyes have, to the best of our knowledge, hitherto not been described. Drosophila chordotonal mechanoreceptors require opsins [13, 15], and we find that their function remains uncompromised by nutrient carotenoid depletion. Disrupting carotenoid uptake and cleavage also left the mechanoreceptors unaffected, and manipulating the chromophore attachment site of the fly’s major visual opsin Rh1 impaired photoreceptor, but not mechanoreceptor, function. Notwithstanding this chromophore independence, some proteins that process and recycle the chromophore in the retina are also required in mechanoreceptors, including visual cycle components that recycle the chromophore upon its photoisomerization. Our results thus establish biological function for unconjugated opsin apoproteins outside of eyes and, in addition, document chromophore-independent roles for chromophore pathway components.

Keywords chordotonal organ mechanosensation opsin apoprotein phototransduction retinal chromophore sensory cell-type evolution 


A plasticidade fenotípica inicia o viés de desenvolvimento?

quinta-feira, agosto 22, 2019

Does phenotypic plasticity initiate developmental bias?

Kevin J. Parsons Kirsty McWhinnie Natalie Pilakouta Lynsey Walker

First published: 26 July 2019


The generation of variation is paramount for the action of natural selection. Although biologists are now moving beyond the idea that random mutation provides the sole source of variation for adaptive evolution, we still assume that variation occurs randomly. In this review, we discuss an alternative view for how phenotypic plasticity, which has become well accepted as a source of phenotypic variation within evolutionary biology, can generate nonrandom variation. Although phenotypic plasticity is often defined as a property of a genotype, we argue that it needs to be considered more explicitly as a property of developmental systems involving more than the genotype. We provide examples of where plasticity could be initiating developmental bias, either through direct active responses to similar stimuli across populations or as the result of programmed variation within developmental systems. Such biased variation can echo past adaptations that reflect the evolutionary history of a lineage but can also serve to initiate evolution when environments change. Such adaptive programs can remain latent for millions of years and allow development to harbor an array of complex adaptations that can initiate new bouts of evolution. Specifically, we address how ideas such as the flexible stem hypothesis and cryptic genetic variation overlap, how modularity among traits can direct the outcomes of plasticity, and how the structure of developmental signaling pathways is limited to a few outcomes. We highlight key questions throughout and conclude by providing suggestions for future research that can address how plasticity initiates and harbors developmental bias.

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“Although biologists are now moving beyond the idea that random mutation provides the sole source of variation for adaptive evolution, we still assume that variation occurs randomly...Darwin's idea that variation is generated randomly has largely been taken for granted rather than tested, representing a fundamental gap in our understanding of evolution.”