Initial Mutations Direct Alternative Pathways of Protein Evolution

Whether evolution is erratic due to random historical details, or is repeatedly directed along similar paths by certain constraints, remains unclear. Epistasis (i.e. non-additive interaction between mutations that affect fitness) is a mechanism that can contribute to both scenarios. Epistasis can constrain the type and order of selected mutations, but it can also make adaptive trajectories contingent upon the first random substitution. This effect is particularly strong under sign epistasis, when the sign of the fitness effects of a mutation depends on its genetic background. In the current study, we examine how epistatic interactions between mutations determine alternative evolutionary pathways, using in vitro evolution of the antibiotic resistance enzyme TEM-1 β-lactamase. First, we describe the diversity of adaptive pathways among replicate lines during evolution for resistance to a novel antibiotic (cefotaxime). Consistent with the prediction of epistatic constraints, most lines increased resistance by acquiring three mutations in a fixed order. However, a few lines deviated from this pattern. Next, to test whether negative interactions between alternative initial substitutions drive this divergence, alleles containing initial substitutions from the deviating lines were evolved under identical conditions. Indeed, these alternative initial substitutions consistently led to lower adaptive peaks, involving more and other substitutions than those observed in the common pathway. We found that a combination of decreased enzymatic activity and lower folding cooperativity underlies negative sign epistasis in the clash between key mutations in the common and deviating lines (Gly238Ser and Arg164Ser, respectively). Our results demonstrate that epistasis contributes to contingency in protein evolution by amplifying the selective consequences of random mutations

Colour and decay resistance and its relationships in Eperua grandiflora

$\bullet$ Eperua grandiflora, which is widely distributed in the French Guiana forest region, shows high variability in decay resistance. Further information concerning this wood quality parameter is necessary, but standard testing methods are complex and time-consuming. We assessed the use of colorimetry to determine durability in heartwood samples from a range of trees. $\bullet$ Eperua grandiflora colour parameters were measured using a CIELAB system, revealing that the tree effect was greater than the radial position and height effects. $\bullet$ The wood samples were exposed to Coriolus versicolor and Antrodia sp. according to two European standards (En 350-1 and XP CEN TS 15083-1). Eperua grandiflora is more susceptible to brown rot. These two standards did not give the same durability classes. The high variation in natural durability was due to the tree effect. $\bullet$ These two properties were found to be correlated and the assessment also distinguished the extreme durability classes but they are not sufficient to classify the class of durability of this species.Étude de la variabilité de la couleur, de la durabilité naturelle et recherche de corrélations chez Eperua grandiflora. $\bullet$ Eperua grandiflora, essence largement répandue dans les forêts de Guyane soufre d'un défaut majeur : une grande variabilité de sa durabilité naturelle à l'égard des champignons lignivores. Dans un premier temps, nous avons étudié la variabilité de la mesure de la couleur selon le système CIElab afin de vérifier si la colorimétrie peut être utilisée comme un indicateur de la durabilité naturelle. $\bullet$ Nous avons étudié la résistance de cette essence à l'échelle inter, et intra-arbres à l'égard de Coriolus versicolor et Antrodia sp., conformément aux normes européennes en vigueur : EN 350-1 et XP TS 15083-1. Les résultats ont révélé qu'Eperua grandiflora est plus sensible au champignon de pourriture brune. $\bullet$ De même, l'utilisation des normes a montré que l'on n'obtenait pas les mêmes classes de durabilité. La variabilité de la durabilité et de la couleur est plus importante à l'échelle inter-arbres qu'à l'échelle intra-arbre. $\bullet$ Enfin, ces deux propriétés sont corrélées mais nous ne pouvons envisager d'utiliser la colorimétrie comme indicateur de la durabilité naturelle, car elle ne permet de différencier que les classes extrêmes