Phenotypic plasticity drives a depth gradient in male conspicuousness in threespine stickleback,Gasterosteus aculeatus

Evolutionary gain and loss of a pathological immune response to parasitism

Parasites impose fitness costs on their hosts. Biologists often assume that natural selection favors infection-resistant hosts. Yet, when the immune response itself is costly, theory suggests that selection may sometimes favor loss of resistance, which may result in alternative stable states where some populations are resistant and others are tolerant. Intraspecific variation in immune costs is rarely surveyed in a manner that tests evolutionary patterns, and there are few examples of adaptive loss of resistance. Here, we show that when marine threespine stickleback colonized freshwater lakes, they gained resistance to the freshwater-associated cestode Schistocephalus solidus. Extensive peritoneal fibrosis and inflammation are a commonly observed phenotype that contributes to suppression of cestode growth and viability but also imposes a substantial cost on fecundity. Combining genetic mapping and population genomics, we find that opposing selection generates immune system differences between tolerant and resistant populations, consistent with divergent optimization.

Visual Background Choice and Light Environment Affect Male Guppy Visual Contrast

Male guppies (Poecilia reticulata) have multiple colored spots and perform courtship displays near the edges of streams in Trinidad in shallow water flowing through rainforest. Depending upon the orientation of the pair, the female sees the male displays against gravel or other stream bed substrates or against the spacelight—the roughly uniform light coming from the water column away from the bank. We observed courting pairs in two adjacent natural streams and noted the directions of each male display. We found that the female sees the male more often against spacelight than against gravel when females either faced the spacelight from the opposite bank or from downstream, or both. Visual modelling using natural substrate reflectances and field light measurements showed higher chromatic contrast of males against spacelight than against substrates independent of the two ambient light environments used during displays, but achromatic contrast depended upon the ambient light habitat. This suggests that courtship involves both chromatic and achromatic contrast. We conclude that the orientation of courting pairs and the ambient light spectrum should be accounted for in studies of mate choice, because the visual background and light affect visibility, and these differ with orientation.

Polyphenisms and polymorphisms: Genetic variation in plasticity and color variation within and among bluefin killifish populations

The presence of stable color polymorphisms within populations begs the question of how genetic variation is maintained. Consistent variation among populations in coloration, especially when correlated with environmental variation, raises questions about whether environmental conditions affect either the fulcrum of those balanced polymorphisms, the plastic expression of coloration, or both. Color patterns in male bluefin killifish provoke both types of questions. Red and yellow morphs are common in all populations. Blue males are more common in tannin-stained swamps relative to clear springs. Here, we combined crosses with a manipulation of light to explore how genetic variation and phenotypic plasticity shape these patterns. We found that the variation in coloration is attributable mainly to two axes of variation: (1) a red-yellow axis with yellow being dominant to red, and (2) a blue axis that can override red-yellow and is controlled by genetics, phenotypic plasticity, and genetic variation for phenotypic plasticity. The variation among populations in plasticity suggests it is adaptive in some populations but not others. The variation among sires in plasticity within the swamp population suggests balancing selection may be acting not only on the red-yellow polymorphism but also on plasticity for blue coloration.

Genetic divergence among threespine stickleback that differ in nuptial coloration

Sexual signals are shaped by their intended and unintended receivers as well as the signalling environment. This interplay between sexual and natural selection can lead to divergence in signals in heterogeneous environments. Yet, the extent to which gene flow is restricted when signalling phenotypes vary across environments and over what spatial scales remains an outstanding question. In this study, we quantify gene flow between two colour morphs, red and black, of freshwater threespine stickleback fish (Gasterosteus aculeatus). We capitalize on the very recent divergence of signalling phenotypes in this system to characterize within-species and among-morph genetic variation and to test for levels of gene flow between colour morphs in Oregon and Washington. Despite limited evidence for assortative mating between allopatric red and black populations, we found that black populations are genetically distinct from nearby red populations and that the black morph appears to have evolved independently at least twice in Oregon and Washington. Surprisingly, we uncovered a group of stickleback in one small coastal stream, Connor Creek, which is genetically and morphologically distinct from the red and black colour morphs and from marine stickleback. Historically, both colour morphs have coexisted in this location and sometimes hybridized, raising new questions about the origins and history of these fish, which were first described as anadromous-black hybrids >50 years ago. Understanding how genetic variation is currently partitioned within and among populations and colour morphs in this system should prompt future studies to assess the relative roles of habitat, ecological and pre- and post-reproductive barriers in the genetic divergence and phenotypic patterns we observe in nature.

The genomic signature of ecological divergence along the benthic-limnetic axis in allopatric and sympatric threespine stickleback

The repeated occurrence of similar phenotypes in independent lineages (i.e., parallel evolution) in response to similar ecological conditions can provide compelling insights into the process of adaptive evolution. An intriguing question is to what extent repeated phenotypic changes are underlain by repeated changes at the genomic level and whether patterns of genomic divergence differ with the geographic context in which populations evolve. Here, we combined genomic, morphological and ecological data sets to investigate the genomic signatures of divergence across populations of threespine stickleback (Gasterosteus aculeatus) that adapted to contrasting ecological niches (benthic or limnetic) in either sympatry or allopatry. We found that genome-wide differentiation (F_ST ) was an order of magnitude higher and substantially more repeatable for sympatric benthic and limnetic specialists compared to allopatric populations with similar levels of ecological divergence. We identified genomic regions consistently differentiated between sympatric ecotypes that were also differentiated between or associated with benthic vs. limnetic niche in allopatric populations. These candidate regions were enriched on three chromosomes known to be involved in the benthic-limnetic divergence of threespine stickleback. Some candidate regions overlapped with QTL for body shape and trophic traits such as gill raker number, traits that strongly differ between benthic and limnetic ecotypes. In summary, our study shows that magnitude and repeatability of genomic signatures of ecological divergence in threespine stickleback highly depend on the geographic context. The identified candidate regions provide starting points to identify functionally important genes for the adaptation to benthic and limnetic niches.

Phenotypic flexibility in background-mediated color change in sticklebacks

Phenotypic flexibility may incur a selective advantage in changing and heterogeneous environments, and is increasingly recognized as an integral aspect of organismal adaptation. Despite the widespread occurrence and potential importance of rapid and reversible background-mediated color change for predator avoidance, knowledge gaps remain regarding its adaptive value, repeatability within individuals, phenotypic correlates, and whether its expression is context dependent. We used manipulative experiments to investigate these issues in two fish species, the three-spined stickleback (Gasterosteus aculeatus) and nine-spined stickleback (Pungitius pungitius). We sequentially exposed individuals to dark and light visual background treatments, quantified color change from video recordings, and examined associations of color change with phenotypic dimensions that can influence the outcome of predator-prey interactions. G. aculeatus expressed a greater degree of color change compared to P. pungitius. In G. aculeatus, the color change response was repeatable within individuals. Moreover, the color change response was independent of body size but affected by sex and boldness, with males and bolder individuals changing less. Infection by the parasite Schistocephalus solidus did not affect the degree of color change, but it did modulate its association with sex and boldness. G. aculeatus adjusted the expression of color change in response to predation risk, with enhanced color change expression in individuals exposed to either simulated attacks, or olfactory cues from a natural predator. These results provide novel evidence on repeatability, correlated traits, and context dependence in the color change response and highlight how a suite of factors can contribute to individual variation in phenotypic flexibility.

Microhabitat contributes to microgeographic divergence in threespine stickleback

Since the New Synthesis, most migration-selection balance theory has predicted that there should be negligible differentiation over small spatial scales (relative to dispersal), because gene flow should erode any effect of divergent selection. Nevertheless, there are classic examples of microgeographic divergence, which theory suggests can arise under specific conditions: exceptionally strong selection, phenotypic plasticity in philopatric individuals, or nonrandom dispersal. Here, we present evidence of microgeographic morphological variation within lake and stream populations of threespine stickleback (Gasterosteus aculeatus). It seems reasonable to assume that a given lake or stream population of fish is well-mixed. However, we found this assumption to be untenable. We examined trap-to-trap variation in 34 morphological traits measured on stickleback from 16 lakes and 16 streams. Most traits varied appreciably among traps within populations. Both between-trap distance and microhabitat characteristics such as depth and substrate explained some of the within-population morphological variance. Microhabitat was also associated with genotype at particular loci but there was no genetic isolation by distance, implying that heritable habitat preferences may contribute to microgeographic variation. Our study adds to growing evidence that microgeographic divergence can occur across small spatial scales within individuals' daily dispersal neighborhood where gene flow is expected to be strong.

Male sexual signal predicts phenotypic plasticity in offspring: implications for the evolution of plasticity and local adaptation

In a rapidly changing world, understanding the processes that influence a population's ability to respond to natural selection is critical for identifying how to preserve biodiversity. Two such processes are phenotypic plasticity and sexual selection. Whereas plasticity can facilitate local adaptation, sexual selection potentially impedes local adaptation, especially in rapidly changing or variable environments. Here we hypothesize that, when females preferentially choose males that sire plastic offspring, sexual selection can actually facilitate local adaptation to variable or novel environments by promoting the evolution of adaptive plasticity. We tested this hypothesis by evaluating whether male sexual signals could indicate plasticity in their offspring and, concomitantly, their offspring's ability to produce locally adapted phenotypes. Using spadefoot toads ( Spea multiplicata) as our experimental system, we show that a male sexual signal predicts plasticity in his offspring's resource-use morphology. Specifically, faster-calling males (which are preferred by females) produce more plastic offspring; such plasticity, in turn, enables these males' offspring to respond adaptively to the spadefoots' highly variable environment. The association between a preferred male signal and adaptive plasticity in his offspring suggests that female mate choice can favour the evolution and maintenance of phenotypic plasticity and thereby foster adaptation to a variable environment. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

Evolution and assessment of colour patterns in stream-resident and anadromous male threespine stickleback Gasterosteus aculeatus from three regions

We compared the colour patterns of free swimming, reproductively active male threespine stickleback Gasterosteus aculeatus of the anadromous and stream ecotypes from three geographically distinct regions. Consistent with the hypothesis of environmentally mediated selection, our results indicate ecologically replicated differences in G. aculeatus coloration between anadromous and stream-resident populations, and that G. aculeatus probably have the visual acuity to discriminate colour pattern differences between anadromous and stream-resident fish.

Genomic data reject the hypothesis of sympatric ecological speciation in a clade of Desmognathus salamanders

Closely related taxa with dissimilar morphologies are often considered to have diverged via natural selection favoring different phenotypes. However, some studies have found these scenarios to be paired with limited or no genetic differentiation. Desmognathus quadramaculatus and D. marmoratus are sympatric salamander species thought to represent a case of ecological speciation based on distinct morphologies, but the results of previous studies have not resolved corresponding patterns of lineage divergence. Here, we use genome-wide data to test this hypothesis of ecological speciation. Population structure analyses partitioned individuals geographically, but not morphologically, into two adjacent regions of western North Carolina: Pisgah and Nantahala. Phylogenetic analyses confirmed the nominal species are nonmonophyletic and resolved deep divergence between the two geographic clusters. Model-testing overwhelmingly supported the hypothesis that lineage divergence followed geography. Finally, ecological niche modeling showed that Pisgah and Nantahala individuals occupy different climatic niches, and geographic boundaries for the two lineages correspond to differences in precipitation regimes across southern Appalachia. Overall, we reject the previous hypothesis of ecological speciation based on microhabitat partitioning. Instead, our results suggest that there are two cryptic lineages, each containing the same pair of morphotypes.

The pervasive effects of lighting environments on sensory drive in bluefin killifish: an investigation into male/male competition, female choice, and predation

Sensory drive predicts that the conditions under which signaling takes place have large effects on signals, sensory systems, and behavior. The coupling of an ecological genetics approach with sensory drive has been fruitful. An ecological genetics approach compares populations that experience different environments and asks whether population differences are adaptive and are the result of genetic and/or environmental variation. The multi-faceted effects of signaling environments are well-exemplified by the bluefin killifish. In this system, males with blue anal fins are abundant in tannin-stained swamps that lack UV/blue light but are absent in clear springs where UV/blue light is abundant. Past work indicates that lighting environments shape genetic and environmental variation in color patterns, visual systems, and behavior. Less is known about the selective forces creating the across population correlations between UV/blue light and the abundance of blue males. Here, we present three new experiments that investigate the roles of lighting environments on male competition, female mate choice, and predation. We found strong effects of lighting environments on male competition where blue males were more likely to emerge as dominant in tea-stained water than in clear water. Our preliminary study on predation indicated that blue males may be less susceptible to predation in tea-stained water than in clear water. However, there was little evidence for female preferences favoring blue males. The resulting pattern is one where the effects of lighting environments on genetic variation and phenotypic plasticity match the direction of selection and favor the expression of blue males in swamps.

Opsin expression predicts male nuptial color in threespine stickleback

Theoretical models of sexual selection suggest that male courtship signals can evolve through the build-up of genetic correlations between the male signal and female preference. When preference is mediated via increased sensitivity of the signal characteristics, correlations between male signal and perception/sensitivity are expected. When signal expression is limited to males, we would expect to find signal-sensitivity correlations in males. Here, we document such a correlation within a breeding population of threespine stickleback mediated by differences in opsin expression. Males with redder nuptial coloration express more long-wavelength-sensitive (LWS) opsin, making them more sensitive to orange and red. This correlation is not an artifact of shared tuning to the optical microhabitat. Such correlations are an essential feature of many models of sexual selection, and our results highlight the potential importance of opsin expression variation as a substrate for signal-preference evolution. Finally, these results suggest a potential sensory mechanism that could drive negative frequency-dependent selection via male-male competition and thus maintain variation in male nuptial color.

25 Years of sensory drive: the evidence and its watery bias

It has been 25 years since the formalization of the Sensory Drive hypothesis was published in the American Naturalist (1992). Since then, there has been an explosion of research identifying its utility in contributing to our understanding of inter- and intra-specific variation in sensory systems and signaling properties. The main tenet of Sensory Drive is that environmental characteristics will influence the evolutionary trajectory of both sensory (detecting capabilities) and signaling (detectable features and behaviors) traits in predictable directions. We review the accumulating evidence in 154 studies addressing these questions and categorized their approach in terms of testing for environmental influence on sensory tuning, signal characteristics, or both. For the subset of studies that examined sensory tuning, there was greater support for Sensory Drive processes shaping visual than auditory tuning, and it was more prevalent in aquatic than terrestrial habitats. Terrestrial habitats and visual traits were the prevalent habitat and sensory modality in the 104 studies showing support for environmental influence on signaling properties. An additional 19 studies that found no supporting evidence for environmental influence on signaling traits were all based in terrestrial ecosystems and almost exclusively involved auditory signals. Only 29 studies examined the complete coevolutionary process between sensory and signaling traits and were dominated by fish visual communication. We discuss biophysical factors that may contribute to the visual and aquatic bias for Sensory Drive evidence, as well as biotic factors that may contribute to the lack of Sensory Drive processes in terrestrial acoustic signaling systems.

Geographical variation in colour of female threespine stickleback (Gasterosteus aculeatus)

The ecological multifunctionality of colour often results in multiple selective pressures operating on a single trait. Most research on colour evolution focuses on males because they are the most conspicuous sex in most species. This bias can limit inferences about the ecological drivers of colour evolution. For example, little is known about population divergence in colour of female threespine stickleback (Gasterosteus aculeatus), which is among the most intensively-studied model vertebrates in evolution, ecology, and behaviour. In contrast, the evolution and ecology of colour in male stickleback has received considerable attention. One aspect of female colouration that is lacking previous research is non-ornamental body colour. Non-ornamental colour can play defensive and social roles, and indicate other aspects of female stickleback ecology. To remedy this knowledge gap, we measured the colour and brightness of one dorsal and one ventral lateral area on female stickleback from nine lake populations on Vancouver Island. We found that lake populations varied in overall colour brightness and dorso-ventral contrast. In addition, we found that female brightness increased with lake size, indicating potential ecological drivers of these colour differences. Our results demonstrate that there is substantial scope for future research on female colour diversification, which has been overlooked because past researchers focused on dramatic male nuptial colours.

Plasticity contributes to a fine-scale depth gradient in sticklebacks' visual system

The light environment influences an animal's ability to forage, evade predators, and find mates, and consequently is known to drive local adaptation of visual systems. However, the light environment may also vary over fine spatial scales at which genetic adaptation is difficult. For instance, in aquatic systems, the available wavelengths of light change over a few metres depth. Do animals plastically adjust their visual system to such small-scale environmental light variation? Here, we show that in three-spine stickleback (Gasterosteus aculeatus), opsin gene expression (an important determinant of colour vision) changes over a 2-m vertical gradient in nest depth. By experimentally altering the light environment using light filters to cover enclosures in a lake, we found that opsin expression can be adjusted on a short time frame (weeks) in response to the local light environment. This is to our knowledge the smallest spatial scale on which visual adjustments through opsin expression have been recorded in a natural setting along a continuously changing light environment.