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Steps along the evolutionary trajectories of skeletal muscle voltage-gated sodium channels are guided by biophysical tradeoffs
Authordel Carlo, Robert Eugene
Cell and Molecular Pharmacology and Physiology
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Suspended in an imperfect balance of function and failure, adaptations are often riddled with trade-offs. Although readily observed in simple organisms, characterizing a trade-off in a complex organism requires tracing adaptive benefits of a trait and its concomitant performance deficits through every level of organization. Describing how molecular phenotypes scale up to cellular, physiological, and in turn, organismal phenotypes, compounding or compensating with increasing complexity, is an essential tenet of biology. Here, I examine many factors of such an in-depth investigation in the context of a classic evolutionary interaction between a predator and its toxic prey. The results here compose a robust picture of the dynamic natural arena in which natural selection and the subsequent evolution of species occur.First, I report the biophysical costs of an adaptation scaling up from protein to population, offering explanations for the restricted geographic boundaries of adapted snake populations. These trade-offs are repeatable across two species detailed here, each having taken independent evolutionary trajectories to phenotypic resistance. Observations of sodium channel biophysics, muscular function, organismal performance and the geographic distribution of genotypes and phenotypes comprise a straightforward map from gene sequence to evolutionary consequence. I suggest that performance trade-offs at the molecular level compose the proximate cause of higher-order performance limits, producing locally adapted though broadly uncompetitive organisms.Next, I focus on a single species and physiologically assess the performance all of its extant sodium channel genotypes. The results suggest that each mutation constitutes a step away from a highly competitive channel, implying that the species observed navigated a predictable fitness landscape. In conjunction with the allele frequencies of this species, the physiological data support a parsimonious evolutionary trajectory from ancestral to descendant sodium channel sequences. These results point to a similar phenomenon in another species therefore suggesting convergent evolutionary trajectories.To underscore the importance of these mutations, I pursued hypotheses to explain how deficits at the molecular level directly influence the performance of their resident tissue. Such work challenges assumptions made about the action potential and its role in contractility. However, the results paint a less convincing story that invites further inquiry into the impact of compromised sodium channels on muscular activity and how exactly reduced sodium channel conductance results in reduced muscular activity.To attend to the role of abiotic factors in adaptive evolution, I present a study demonstrating how the physical context of the environment plays an important role in the steps these species take through sequence space. This work points to selective contexts that are critical for predicting evolutionary trajectories by demonstrating that some mutations appear to be available to some populations and not others depending on their physical placement on the landscape and the climatic features of each habitat.Lastly, I present several key points gleaned from this work that should be considered when attempting to investigate stepwise evolutionary change. To this end, I conclude on the importance of accurately predicting evolution as a means to accurately identify those species that will be unable to meet imminent selective demands.