Ecological factors influence balancing selection on leaf chemical profiles of a wildflower.

Balancing selection is frequently invoked as a mechanism that maintains variation within and across populations. However, there are few examples of balancing selection operating on loci underpinning complex traits, which frequently display high levels of variation. We investigated mechanisms that may maintain variation in a focal polymorphism-leaf chemical profiles of a perennial wildflower (Boechera stricta, Brassicaceae)-explicitly interrogating multiple ecological and genetic processes including spatial variation in selection, antagonistic pleiotropy and frequency-dependent selection. A suite of common garden and greenhouse experiments showed that the alleles underlying variation in chemical profile have contrasting fitness effects across environments, implicating two ecological drivers of selection on chemical profile: herbivory and drought. Phenotype-environment associations and molecular genetic analyses revealed additional evidence of past selection by these drivers. Together, these data are consistent with balancing selection on chemical profile, probably caused by pleiotropic effects of secondary chemical biosynthesis genes on herbivore defence and drought response.

Elaboration of the Corticosteroid Synthesis Pathway in Primates through a Multistep Enzyme.

Metabolic networks are complex cellular systems dependent on the interactions among, and regulation of, the enzymes in the network. Although there is great diversity of types of enzymes that make up metabolic networks, the models meant to understand the possible evolutionary outcomes following duplication neglect specifics about the enzyme, pathway context, and cellular constraints. To illuminate the mechanisms that shape the evolution of biochemical pathways, I functionally characterize the consequences of gene duplication of an enzyme family that performs multiple subsequent enzymatic reactions (a multistep enzyme) in the corticosteroid pathway in primates. The products of the corticosteroid pathway (aldosterone and cortisol) are steroid hormones that regulate metabolism and stress response in tetrapods. These steroid hormones are synthesized by a multistep enzyme Cytochrome P450 11B (CYP11B) that performs subsequent steps on different carbon atoms of the steroid derivatives. Through ancestral state reconstruction and in vitro characterization, I find that the primate ancestor of the CYP11B1 and CYP11B2 paralogs had moderate ability to synthesize both cortisol and aldosterone. Following duplication in Old World primates, the CYP11B1 homolog specialized on the production of cortisol, whereas its paralog, CYP11B2, maintained its ability to perform multiple subsequent steps as in the ancestral pathway. Unlike CYP11B1, CYP11B2 could not specialize on the production of aldosterone because it is constrained to perform earlier steps in the corticosteroid synthesis pathway to achieve the final product aldosterone. These results suggest that enzyme function, pathway context, along with tissue-specific regulation, both play a role in shaping potential outcomes of metabolic network elaboration.

Behaviour before beauty: signal weighting during mate selection in the butterfly Papilio polytes.

Mating displays often contain multiple signals. Different combinations of these signals may be equally successful at attracting a mate, as environment and signal combination may influence relative signal weighting by choosy individuals. This variation in signal weighting among choosy individuals may facilitate the maintenance of polymorphic displays and signalling behaviour. One group of animals known for their polymorphic patterning are Batesian mimetic butterflies, where the interaction of sexual selection and predation pressures are hypothesized to influence the maintenance of polymorphic wing patterning and behaviour. Males in the female-limited polymorphic Batesian mimetic butterfly Papilio polytes use female wing pattern and female activity levels when determining whom to court. They court stationary females with mimetic wing patterns more often than stationary females with non-mimetic, male-like wing patterns, and active females more often than inactive females. It is unclear whether females modify their behaviour to increase (or decrease) their likelihood of receiving male courtship, or whether non-mimetic females spend more time in cryptic environments than mimetic females, to compensate for their lack of mimicry-driven predation protection (at the cost of decreased visibility to males). In addition, relative signal weighting of female wing pattern and activity to male mate selection is unknown. To address these questions, we conducted a series of observational studies of a polymorphic P. polytes population in a large butterfly enclosure. We found that males exclusively courted active females, irrespective of female wing pattern. However, males did court active non-mimetic females significantly more often than expected given their relative abundance in the population. Females exhibited similar activity levels, and selected similar resting environments, irrespective of wing pattern. Our results suggest that male preference for non-mimetic females may play an active role in the maintenance of the non-mimetic female form in natural populations, where males are likely to be in the presence of active, as well as inactive, mimetic and non-mimetic females.

Flux Control in a Defense Pathway in Arabidopsis thaliana Is Robust to Environmental Perturbations and Controls Variation in Adaptive Traits.

The connections leading from genotype to fitness are not well understood, yet they are crucial for a diverse set of disciplines. Uncovering the general properties of biochemical pathways that influence ecologically important traits is an effective way to understand these connections. Enzyme flux control (or, control over pathway output) is one such pathway property. The flux-controlling enzyme in the antiherbivory aliphatic glucosinolate pathway of Arabidopsis thaliana has majority flux control under benign greenhouse conditions and has evidence of nonneutral evolution. However, it is unknown how patterns of flux control may change in different environments, or if insect herbivores respond to differences in pathway flux. We test this, first through genetic manipulation of the loci that code for the aliphatic glucosinolate pathway enzymes under a variety of environments (reduced water, reduced soil nutrients, leaf wounding and methyl jasmonate treatments), and find that flux control is consistently in the first enzyme of the pathway. We also find that a generalist herbivore, Trichoplusia ni, modifies its feeding behavior depending on the flux through the glucosinolate pathway. The influence over herbivore behavior combined with the consistency of flux control suggests that genes controlling flux might be repeatedly targeted by natural selection in diverse environments and species.

Evolution of flux control in the glucosinolate pathway in Arabidopsis thaliana.

Network characteristics of biochemical pathways are believed to influence the rate of evolutionary change in constituent enzymes. One characteristic that may affect rate heterogeneity is control of the amount of product produced by a biochemical pathway or flux control. In particular, theoretical analyses suggest that adaptive substitutions should be concentrated in the enzyme(s) that exert the greatest control over flux. Although a handful of studies have found a correlation between position in a pathway and evolutionary rate, these investigations have not examined the relationship between evolutionary rate and flux control. Given that genes with greater control will experience stronger selection and that the probability of fixation is proportional to the selective advantage, we ask the following: 1) do upstream enzymes have majority flux control, 2) do enzymes with majority flux control accumulate adaptive substitutions, and 3) are upstream enzymes under higher selective constraint? First, by perturbing the enzymes in the aliphatic glucosinolate pathway in Arabidopsis thaliana with gene insertion lines, we show that flux control is focused in the first enzyme in the pathway. Next, by analyzing several sequence signatures of selection, we also show that this enzyme is the only one in the pathway that shows convincing evidence of selection. Our results support the hypothesis that natural selection preferentially acts on enzymes with high flux control.

Adaptive evolution: evaluating empirical support for theoretical predictions.

Adaptive evolution is shaped by the interaction of population genetics, natural selection and underlying network and biochemical constraints. Variation created by mutation, the raw material for evolutionary change, is translated into phenotypes by flux through metabolic pathways and by the topography and dynamics of molecular networks. Finally, the retention of genetic variation and the efficacy of selection depend on population genetics and demographic history. Emergent high-throughput experimental methods and sequencing technologies allow us to gather more evidence and to move beyond the theory in different systems and populations. Here we review the extent to which recent evidence supports long-established theoretical principles of adaptation.