Kim L. Hoke

Non-adaptive plasticity potentiates rapid adaptive evolution of gene expression in nature.

Phenotypic plasticity is the capacity for an individual genotype to produce different phenotypes in response to environmental variation. Most traits are plastic, but the degree to which plasticity is adaptive or non-adaptive depends on whether environmentally induced phenotypes are closer or further away from the local optimum. Existing theories make conflicting predictions about whether plasticity constrains or facilitates adaptive evolution. Debate persists because few empirical studies have tested the relationship between initial plasticity and subsequent adaptive evolution in natural populations. Here we show that the direction of plasticity in gene expression is generally opposite to the direction of adaptive evolution. We experimentally transplanted Trinidadian guppies (Poecilia reticulata) adapted to living with cichlid predators to cichlid-free streams, and tested for evolutionary divergence in brain gene expression patterns after three to four generations. We find 135 transcripts that evolved parallel changes in expression within the replicated introduction populations. These changes are in the same direction exhibited in a native cichlid-free population, suggesting rapid adaptive evolution. We find 89% of these transcripts exhibited non-adaptive plastic changes in expression when the source population was reared in the absence of predators, as they are in the opposite direction to the evolved changes. By contrast, the remaining transcripts exhibiting adaptive plasticity show reduced population divergence. Furthermore, the most plastic transcripts in the source population evolved reduced plasticity in the introduction populations, suggesting strong selection against non-adaptive plasticity. These results support models predicting that adaptive plasticity constrains evolution, whereas non-adaptive plasticity potentiates evolution by increasing the strength of directional selection. The role of non-adaptive plasticity in evolution has received relatively little attention; however, our results suggest that it may be an important mechanism that predicts evolutionary responses to new environments.Phenotypic plasticity is the capacity for an individual genotype to produce different phenotypes in response to environmental variation. Most traits are plastic, but the degree to which plasticity is adaptive or non-adaptive depends on whether environmentally induced phenotypes are closer or further away from the local optimum. Existing theories make conflicting predictions about whether plasticity constrains or facilitates adaptive evolution. Debate persists because few empirical studies have tested the relationship between initial plasticity and subsequent adaptive evolution in natural populations. Here we show that the direction of plasticity in gene expression is generally opposite to the direction of adaptive evolution. We experimentally transplanted Trinidadian guppies (Poecilia reticulata) adapted to living with cichlid predators to cichlid-free streams, and tested for evolutionary divergence in brain gene expression patterns after three to four generations. We find 135 transcripts that evolved parallel changes in expression within the replicated introduction populations. These changes are in the same direction exhibited in a native cichlid-free population, suggesting rapid adaptive evolution. We find 89% of these transcripts exhibited non-adaptive plastic changes in expression when the source population was reared in the absence of predators, as they are in the opposite direction to the evolved changes. By contrast, the remaining transcripts exhibiting adaptive plasticity show reduced population divergence. Furthermore, the most plastic transcripts in the source population evolved reduced plasticity in the introduction populations, suggesting strong selection against non-adaptive plasticity. These results support models predicting that adaptive plasticity constrains evolution, whereas non-adaptive plasticity potentiates evolution by increasing the strength of directional selection. The role of non-adaptive plasticity in evolution has received relatively little attention; however, our results suggest that it may be an important mechanism that predicts evolutionary responses to new environments.

Functional Mapping of the Auditory Midbrain during Mate Call Reception

We examined patterns of neural activity as assayed by changes in gene expression to localize representation of acoustic mating signals in the auditory midbrain of frogs. We exposed wild-caught male Physalaemus pustulosus to conspecific mating calls that vary in their behavioral salience, nonsalient mating calls, or no sound. We measured expression of the immediate early gene egr-1 (also called ZENK, zif268, NGFI-A, and krox-24) throughout the torus semicircularis, the auditory midbrain homolog of the inferior colliculus. Differential egr-1 induction in response to the acoustic stimuli occurred in the laminar, midline, and principal nuclei of the torus semicircularis, whereas the ventral region did not show significant effects of stimulus. The laminar nucleus differentially responded to conspecific mating calls compared with nonsalient mating calls, whereas the midline and principal nuclei responded preferentially to one of two conspecific calls. These responses were not explained by simple acoustic properties of the stimuli, and they demonstrate a functional heterogeneity of auditory processing of complex biological signals within the frog midbrain. Moreover, using analyses that assess the ability of the torus semicircularis as a whole to discriminate among acoustic stimuli, we found that activity patterns in the four regions together provide more information about biologically relevant acoustic stimuli than activity in any single region.

Integration of sensory and motor processing underlying social behaviour in túngara frogs

Social decision making involves the perception and processing of social stimuli, the subsequent evaluation of that information in the context of the individuals internal and external milieus to produce a decision, and then culminates in behavioural output informed by that decision. We examined brain networks in an anuran communication system that relies on acoustic signals to guide simple, stereotyped motor output. We used egr-1 mRNA expression to measure neural activation in male túngara frogs, Physalaemus pustulosus, following exposure to conspecific and heterospecific calls that evoke competitive or aggressive behaviour. We found that acoustically driven activation in auditory brainstem nuclei is transformed into activation related to sensory–motor interactions in the diencephalon, followed by motor-related activation in the telencephalon. Furthermore, under baseline conditions, brain nuclei typically have correlated egr-1 mRNA levels within brain divisions. Hearing conspecific advertisement calls increases correlations between anatomically distant brain divisions; no such effect was observed in response to calls that elicit aggressive behaviour. Neural correlates of social decision making thus take multiple forms: (i) a progressive shift from sensory to motor encoding from lower to higher stages of neural processing and (ii) the emergence of correlated activation patterns among sensory and motor regions in response to behaviourally relevant social cues.

A systems approach to animal communication

Why animal communication displays are so complex and how they have evolved are active foci of research with a long and rich history. Progress towards an evolutionary analysis of signal complexity, however, has been constrained by a lack of hypotheses to explain similarities and/or differences in signalling systems across taxa. To address this, we advocate incorporating a systems approach into studies of animal communication—an approach that includes comprehensive experimental designs and data collection in combination with the implementation of systems concepts and tools. A systems approach evaluates overall display architecture, including how components interact to alter function, and how function varies in different states of the system. We provide a brief overview of the current state of the field, including a focus on select studies that highlight the dynamic nature of animal signalling. We then introduce core concepts from systems biology (redundancy, degeneracy, pluripotentiality, and modularity) and discuss their relationships with system properties (e.g. robustness, flexibility, evolvability). We translate systems concepts into an animal communication framework and accentuate their utility through a case study. Finally, we demonstrate how consideration of the system-level organization of animal communication poses new practical research questions that will aid our understanding of how and why animal displays are so complex.

Predator exposure alters stress physiology in guppies across timescales.

In vertebrates, glucocorticoids mediate a wide-range of responses to stressors. For this reason, they are implicated in adaptation to changes in predation pressure. Trinidadian guppies (Poecilia reticulata) from high-predation environments have repeatedly and independently colonized and adapted to low-predation environments, resulting in parallel changes in life history, morphology, and behavior. We validated methods for non-invasive waterborne hormone sample collection in this species, and used this technique to examine genetic and environmental effects of predation on basal glucocorticoid (cortisol) levels. To examine genetic differences, we compared waterborne cortisol levels in high- and low-predation fish from two distinct population pairs. We found that fish from high-predation localities had lower cortisol levels than their low-predation counterparts. To isolate environmental influences, we compared waterborne cortisol levels in genetically similar fish reared with and without exposure to predator chemical cues. We found that fish reared with predator chemical cues had lower waterborne cortisol levels than those reared without. Comparisons of waterborne and whole-body cortisol levels demonstrated that populations differed in overall cortisol levels in the body, whereas rearing conditions altered the release of cortisol from the body into the water. Thus, evolutionary history with predators and lifetime exposure to predator cues were both associated with lower cortisol release, but depended on distinct physiological mechanisms.

Candidate neural locus for sex differences in reproductive decisions

Sexual selection and signal detection theories predict that females should be selective in their responses to mating signals in mate choice, while the response of males to signals in male competition should be less selective. The neural processes underlying this behavioural sex difference remain obscure. Differences in behavioural selectivity could result from differences in how sensitive sensory systems are to mating signals, distinct thresholds in motor areas regulating behaviour, or sex differences in selectivity at a gateway relaying sensory information to motor systems. We tested these hypotheses in frogs using the expression of egr-1 to quantify the neural responses of each sex to mating signals. We found that egr-1 expression in a midbrain auditory region was elevated in males in response to both conspecific and heterospecific calls, whereas in females, egr-1 induction occurred only in response to conspecific signals. This differential neural selectivity mirrored the sex differences in behavioural responsiveness to these stimuli. By contrast, egr-1 expression in lower brainstem auditory centres was not different in males and females. Our results support a model in which sex differences in behavioural selectivity arise from sex differences in the neural selectivity in midbrain areas relaying sensory information to the forebrain.

Cell death precedes rod neurogenesis in embryonic teleost retinal development

We measured cell death in the retinas of embryonic and adult teleost fish using TUNEL staining. Following a wave of cell birth during embryogenesis that generates all retinal cell types except rods, cell death occurs in all three nuclear layers. The lack of a corresponding pattern of cell death in the growing adult margin suggests different roles for death during embryogenesis and adult neurogenesis.

Remodeling of the Cone Photoreceptor Mosaic during Metamorphosis of Flounder (Pseudopleuronectes americanus)

The retinal cone mosaic of the winter flounder, Pseudopleuronectes americanus, is extensively remodeled during metamorphosis when its visual system shifts from monochromatic to trichromatic. Here we describe the reorganization and re-specification of existing cone subtypes in which larval cones alter their spatial arrangement, morphology, and opsin expression to determine whether mechanisms controlling cell birth, mosaic position, and opsin selection are coordinated or independent. We labeled dividing cells with tritiated (3H) thymidine prior to mosaic remodeling to determine whether existing cone photoreceptors change phenotype. We also used in situ hybridization to identify mosaic type and opsin expression in transitional retinas to understand the sequence of transformation. Our data indicate that in the winter flounder retina the choice of new opsin species and the cellular rearrangement of the mosaic proceed independently. The production of the precise cone mosaic arrangement is not due to a stereotyped series of sequential cellular inductions, but rather might be the product of a set of distinct, flexible processes that rely on plasticity in cell phenotype.

Sexually dimorphic sensory gating drives behavioral differences in túngara frogs

SUMMARY Males and females can differ both in the social behaviors they perform and in the contexts in which they engage in these behaviors. One possible mechanism of sex differences in behavior is a sexual dimorphism in the relay of sensory information to motor areas, but no studies have examined the role of such a relay in vertebrate sexually dimorphic behaviors. We used egr-1 expression as a marker of neural activation in frogs exposed to conspecific and heterospecific acoustic signals to compare activation patterns throughout the brains of males and females. We determined how the sexes differ in the transformation of social signals into motor responses in the context of social communication. We examined the relationships between egr-1 mRNA levels in the auditory midbrain and forebrain areas, as well as how forebrain expression related to the behavioral responses of the animals. Forebrain network activation patterns and forebrain–behavior relationships were similar in males and females. By contrast, we found a sex difference in the relationship between midbrain and forebrain activation; midbrain auditory responses predicted forebrain responses in females but not in males. This sex difference suggests that sensory inputs differentially regulate motor systems underlying social behaviors in males and females. This sensorimotor transformation may be a common locus for generating sex differences in behavior.

Functional Coupling Between Substantia Nigra and Basal Ganglia Homologues in Amphibians

Neuroanatomical and pharmacological experiments support the existence of a homologue of the mammalian substantia nigra-basal ganglia circuit in the amphibian brain. Demarcation of borders between the striatum and pallidum in frogs, however, has been contentious, and direct evidence of functional coupling between the putative nigral and striatal homologues is lacking. To clarify basal ganglia function in anurans, the authors used expression of immediate-early gene egr-1 as a marker of neural activation in the basal ganglia of túngara frogs (Physalaemus pustulosus). Regional variation in egr-1 mRNA levels distinguished striatal and pallidal portions of the basal ganglia and supported the grouping of the striatopallidal transition zone with the dorsal pallidum. As further evidence for a functional coupling between the dopaminergic cells in the posterior tuberculum (the putative substantia nigra homologue) and the basal ganglia, a positive relationship was demonstrated between the size of the dopaminergic cell population and the neural activation levels within the dorsal pallidum.