Three-spined Stickleback Gasterosteus Aculeatus Research Articles

Does the match between individual and group behavior matter in shoaling sticklebacks?

In animals living in groups, the social environment is fundamental to shaping the behaviors and life histories of an individual. A mismatch between individual and group behavior patterns may have disadvantages if the individual is incapable of flexibly changing its state in response to the social environment that influences its energy gain and expenditure. We used different social groups of juvenile three‐spined sticklebacks (Gasterosteus aculeatus) with experimentally manipulated compositions of individual sociability to study the feedback between individual and group behaviors and to test how the social environment shapes behavior, metabolic rate, and growth. Experimentally created unsociable groups, containing a high proportion of less sociable fish, showed bolder collective behaviors during feeding than did corresponding sociable groups. Fish within groups where the majority of members had a level of sociability similar to their own gained more mass than did those within mismatched groups. Less sociable individuals within sociable groups tended to have a relatively low mass but a high standard metabolic rate. A mismatch between the sociability of an individual and that of the majority of the group in which it is living confers a growth disadvantage probably due to the expression of nonadaptive behaviors that increase energetic costs.

Temperature-Dependent Reproductive Success of Stickleback Lateral Plate Morphs: Implications for Population Polymorphism and Range Shifts Under Ocean Warming

Changing environments associated with rapid climate change can shape direct measures of fitness such as reproductive success by altering mating behavior, fecundity and offspring development. Using a polymorphic oceanic population of threespine stickleback (Gasterosteus aculeatus), we investigated whether a 4°C increase in sea surface temperature influenced clutch siring success, reproductive output, and offspring growth among lateral plate morphs. Since low plated morphs are thought to have a selective advantage in warmer environments, we predicted that low plated males should have higher clutch siring success in +4°C environments, and that thermal plasticity of traits (e.g., egg size, offspring growth) should reflect different trait optima in different environments among plate morphs. Parentage analysis of egg clutches revealed temperature-specific clutch siring success, in that low plated males sired more clutches in +4°C environments and completely plated males sired more clutches at ambient (seasonal) temperature. Both completely and low plated females laid larger eggs when acclimated to +4°C, but only completely plated females had smaller clutches at +4°C. Offspring of low and partially plated females grew much less at +4°C compared to those of completely plated females. Taken together, our results demonstrate that ocean warming could impact reproductive success at various levels, with differential effects depending on phenotype, in this case, lateral plate morph. Some traits (clutch siring success, egg size) showed better performance for low plated fish at +4°C, whereas others (e.g., growth) did not. Higher clutch siring success of low plated males at elevated temperature might indicate a future shift in plate morph composition for polymorphic stickleback populations, with potential implications for colonization ability during range shifts under climate change.

The effects of eutrophication and browning on prey availability and body growth of the three-spined stickleback (Gasterosteus aculeatus)

Shallow coastal areas often have high productivity and diversity, in part due to the high availability of light and nutrients. At the same time, they are exposed to multiple environmental pressures, such as browning and eutrophication. Browning is mainly caused by runoff bringing coloured dissolved organic matter (CDOM), reducing light availability in waters, whereas eutrophication is caused by high nutrient loading, leading to eutrophication symptoms such as algal blooms. Existing variation and further change in light and nutrients of coastal areas could have large implications for aquatic food webs, including fish. For instance, reduced light might alter food availability and reduce foraging abilities, whereas increased nutrient supply might, depending on the extent, increase food availability. In this study, we performed a mesocosm experiment, including benthic and pelagic communities, together with young-of-the-year three-spined stickleback (Gasterosteus aculeatus) as predators. The three-spined stickleback is not only a common model organism but also an increasingly common and important mesopredator in the Baltic Sea. We examined the extent to which browning and nutrient-enrichment, alone and in combination, influenced the density, biomass, and composition of stickleback prey, and diet choice, body growth and condition of the stickleback. Stickleback body growth was positively affected by nutrient-enrichment, probably because of a positive bottom-up effect with increased primary production, as evident in the much higher chlorophyll-a concentrations in the pelagic habitat, and increased food availability. In contrast, there was a marginal negative effect of browning on stickleback body growth and condition, most likely due to negative effects of reduced visibility on feeding rates. We also found that prey availability increased with nutrient-enrichment but not with browning. Interestingly, nutrient-enrichment counteracted the negative effects of browning when combined. Our findings add novel understandings about the potential for both eutrophication and browning to affect coastal food webs and fish body growth in the Baltic Sea.

Behavioural responses of threespine stickleback with lateral line asymmetries to experimental mechanosensory stimuli.

Behavioural asymmetry, typically referred to as laterality, is widespread among bilaterians and is often associated with asymmetry in brain structure. However, the influence of sensory receptor asymmetry on laterality has undergone limited investigation. Here we used threespine stickleback (Gasterosteus aculeatus) to investigate the influence of lateral line asymmetry on laterality during lab simulations of three mechanosensation-dependent behaviours: predator evasion, prey localization and rheotaxis. We recorded the response of stickleback to impacts at the water surface and water flow in photic conditions and low-frequency oscillations in the dark, across four repeat trials. We then compared individuals' laterality with asymmetry in the number of neuromasts on either side of their body. Stickleback hovered with their right side against the arena wall 57% of the time (P<0.001) in illuminated surface impact trials and 56% of the time (P=0.085) in dark low-frequency stimulation trials. Light regime modulated the effect of neuromast count on laterality, as fish with more neuromasts were more likely to hover with the wall on their right during illumination (P=0.007) but were less likely to do so in darkness (P=0.025). Population-level laterality diminished in later trials across multiple behaviours and individuals did not show a consistent side bias in any behaviours. Our results demonstrate a complex relationship between sensory structure asymmetry and laterality, suggesting that laterality is modulated by multiple sensory modalities and is temporally dynamic.

Changes in Red Color Sensitivity over the Spawning Cycle of Female Three-spined Stickleback (Gasterosteus aculeatus).

Male red nuptial coloration is a primary mating signal for three-spined sticklebacks (Gasterosteus aculeatus), and the retinae of both sexes are especially sensitive to red during the breeding season. Red sensitivity is an important aspect of female mate choice in this species, but only when they are ready to spawn and not over the entire breeding period. Here, we aimed to determine if the red sensitivity of female sticklebacks change over their repeat spawning cycle. To this end, we assessed retinal opsin mRNA levels and behavioral red sensitivity in females over this cycle. Both methods indicated that females were more sensitive to red during spawning than in the inter-spawning intervals. Relative expression levels of red color opsin genes (lws) and optical motor sensitivity were high during spawning, decreased after the spawning period, and then increased again 72-96 h later when they were ready to spawn again. Thus, female sticklebacks altered their color sensitivity according to need, but the underlying mechanism remains unclear.

Comparing genome scans among species of the stickleback order reveals three different patterns of genetic diversity.

Comparing genome scans among species is a powerful approach for investigating the patterns left by evolutionary processes. In particular, this offers a way to detect candidate genes that drive convergent evolution. We compared genome scan results to investigate if patterns of genetic diversity and divergence are shared among divergent species within the stickleback order (Gasterosteiformes): the threespine stickleback (Gasterosteus aculeatus), ninespine stickleback (Pungitius pungitus), and tubesnout (Aulorhynchus flavidus). Populations were sampled from the southern and northern edges of each species’ range, to identify patterns associated with latitudinal changes in genetic diversity. Weak correlations in genetic diversity (F ST and expected heterozygosity) and three different patterns in the genomic landscape were found among these species. Additionally, no candidate genes for convergent evolution were detected. This is a counterexample to the growing number of studies that have shown overlapping genetic patterns, demonstrating that genome scan comparisons can be noisy due to the effects of several interacting evolutionary forces.

Marine parasite in a freshwater wetland: new host and geographical records of Progrillotia dasyatidis (Cestoda: Trypanorhyncha) from Gasterosteus aculeatus (Actinopterygii: Gasterosteidae) in Bulgaria, with comments on its life-cycle.

Plerocerci of the trypanorhynch cestode Progrillotia dasyatidis Beveridge, Neifar & Euzet, (Progrillotiidae) were isolated from the gallbladder of the three-spined stickleback Gasterosteus aculeatus from a small freshwater marsh in the Lake Atanasovsko Wetlands, a coastal area adjacent to the Black Sea coast, Bulgaria. The parasite was recorded in five out of 134 fish individuals studied (prevalence 3.73%, intensity 1-7, mean intensity 2.40 ± 1.17 and mean abundance 0.09 ± 0.06). A description of the plerocerci is presented, expanding data on intraspecific variation. The present report is the first record of P. dasyatidis from G. aculeatus (new host record) and from Bulgaria (new geographical record). Recording only plerocerci with evaginated scoleces in the sticklebacks is consistent with the hypothesis that teleosts are paratenic hosts and not intermediate hosts of P. dasyatidis.

Sex-biased dynamics of three-spined stickleback (Gasterosteus aculeatus) gene expression patterns

The study of the differences between sexes presents an excellent model to unravel how phenotypic variation is achieved from a similar genetic background. Sticklebacks are of particular interest since evidence of a heteromorphic chromosome pair has not always been detected. The present study investigated sex-biased mRNA and small non-coding RNA (sncRNA) expression patterns in the brain, adipose tissues, and gonads of the three-spined stickleback. The sncRNA analysis indicated that regulatory functions occurred mainly in the gonads. Alleged miRNA-mRNA interactions were established and a mapping bias of differential expressed transcripts towards chromosome 19 was observed. Key players previously shown to control sex determination and differentiation in other fish species but also genes like gapdh were among the transcripts identified. This is the first report in the three-spined stickleback demonstrating tissue-specific expression comprising both mRNA and sncRNA between sexes, emphasizing the importance of mRNA-miRNA interactions as well as new presumed genes not yet identified to have gender-specific roles.

Chromosomal Fusions Facilitate Adaptation to Divergent Environments in Threespine Stickleback.

Chromosomal fusions are hypothesized to facilitate adaptation to divergent environments, both by bringing together previously unlinked adaptive alleles and by creating regions of low recombination that facilitate the linkage of adaptive alleles; but, there is little empirical evidence to support this hypothesis. Here, we address this knowledge gap by studying threespine stickleback (Gasterosteus aculeatus), in which ancestral marine fish have repeatedly adapted to freshwater across the northern hemisphere. By comparing the threespine and ninespine stickleback (Pungitius pungitius) genomes to a de novo assembly of the fourspine stickleback (Apeltes quadracus) and an outgroup species, we find two chromosomal fusion events involving the same chromosomes have occurred independently in the threespine and ninespine stickleback lineages. On the fused chromosomes in threespine stickleback, we find an enrichment of quantitative trait loci underlying traits that contribute to marine versus freshwater adaptation. By comparing whole-genome sequences of freshwater and marine threespine stickleback populations, we also find an enrichment of regions under divergent selection on these two fused chromosomes. There is elevated genetic diversity within regions under selection in the freshwater population, consistent with a simulation study showing that gene flow can increase diversity in genomic regions associated with local adaptation and our demographic models showing gene flow between the marine and freshwater populations. Integrating our results with previous studies, we propose that these fusions created regions of low recombination that enabled the formation of adaptative clusters, thereby facilitating freshwater adaptation in the face of recurrent gene flow between marine and freshwater threespine sticklebacks.

Shedding light on the circadian clock of the threespine stickleback.

ABSTRACTThe circadian clock is an internal timekeeping system shared by most organisms, and knowledge about its functional importance and evolution in natural environments is still needed. Here, we investigated the circadian clock of wild-caught threespine sticklebacks (Gasterosteus aculeatus) at the behavioural and molecular levels. Although their behaviour, ecology and evolution are well studied, information on their circadian rhythms are scarce. We quantified the daily locomotor activity rhythm under a light:dark cycle (LD) and under constant darkness (DD). Under LD, all fish exhibited significant daily rhythmicity, while under DD, only 18% of individuals remained rhythmic. This interindividual variation suggests that the circadian clock controls activity only in certain individuals. Moreover, under LD, some fish were almost exclusively nocturnal, while others were active around the clock. Furthermore, the most nocturnal fish were also the least active. These results suggest that light masks activity (i.e. suppresses activity without entraining the internal clock) more strongly in some individuals than others. Finally, we quantified the expression of five clock genes in the brain of sticklebacks under DD using qPCR. We did not detect circadian rhythmicity, which could indicate either that the clock molecular oscillator is highly light-dependent, or that there was an oscillation but that we were unable to detect it. Overall, our study suggests that a strong circadian control on behavioural rhythms may not necessarily be advantageous in a natural population of sticklebacks and that the daily phase of activity varies greatly between individuals because of a differential masking effect of light.

Cast away in the Adriatic: Low degree of parallel genetic differentiation in three-spined sticklebacks.

The three-spined stickleback (Gasterosteus aculeatus) has repeatedly and independently adapted to freshwater habitats from standing genetic variation (SGV) following colonization from the sea. However, in the Mediterranean Sea G.aculeatus is believed to have gone extinct, and thus the spread of locally adapted alleles between different freshwater populations via the sea since then has been highly unlikely. This is expected to limit parallel evolution, that is the extent to which phylogenetically related alleles can be shared among independently colonized freshwater populations. Using whole genome and 2b-RAD sequencing data, we compared levels of genetic differentiation and genetic parallelism of 15 Adriatic stickleback populations to 19 Pacific, Atlantic and Caspian populations, where gene flow between freshwater populations across extant marine populations is still possible. Our findings support previous studies suggesting that Adriatic populations are highly differentiated (average FST ≈0.45), of low genetic diversity and connectivity, and likely to stem from multiple independent colonizations during the Pleistocene. Linkage disequilibrium network analyses in combination with linear mixed models nevertheless revealed several parallel marine-freshwater differentiated genomic regions, although still not to the extent observed elsewhere in the world. We hypothesize that current levels of genetic parallelism in the Adriatic lineages are a relic of freshwater adaptation from SGV prior to the extinction of marine sticklebacks in the Mediterranean that has persisted despite substantial genetic drift experienced by the Adriatic stickleback isolates.

Aerobic scope is not maintained at low temperature and is associated with cardiac aerobic capacity in the three-spined stickleback Gasterosteus aculeatus.

Metabolic thermal plasticity is central to the survival of fishes in a changing environment. The eurythermal three-spined stickleback Gasterosteus aculeatus displays thermal plasticity at the cellular level with an increase in the activity of key metabolic enzymes in response to cold acclimation. Nonetheless, it is unknown if these changes are sufficient to completely compensate for the depressive effects of cold temperature on whole organismal metabolic rate (ṀO2 ). The authors hypothesized that as a cold-tolerant, eurythermal fish, absolute aerobic scope (AAS), the difference between the maximum metabolic rate (MMR) and standard metabolic rate (SMR), would be maintained in G. aculeatus following acclimation to a range of temperatures that span its habitat temperatures. To test this hypothesis, G. aculeatus were acclimated to 5, 12 and 20°C for 20-32 weeks, and SMR, MMR and aerobic scope (AS) were quantified at each acclimation temperature. The maximal activity of citrate synthase (CS), a marker enzyme of aerobic metabolism, was also quantified in heart ventricles to determine if cardiac aerobic capacity is associated with AS at these temperatures. SMR increased with acclimation temperature and was significantly different among all three temperature groups. MMR was similar between animals at 5 and 12°C and between animals at 12 and 20°C but was 2.6-fold lower in fish at 5°C compared with those at 20°C, resulting in a lower AAS in fish at 5°C compared with those at 12 and 20°C. Correlated with a higher AAS in animals acclimated to 12 and 20°C was a larger relative ventricular mass and higher CS activity per 100 g body mass compared with animals at 5°C. Together, the results indicate that despite their eurythermal nature, AS is not maintained at low temperature but is associated with cardiac aerobic metabolic capacity.

Got the travel bug?

Like many of us in 2021, I miss the glee of traveling to new and exciting locations on vacation (and can't wait to get back these adventures post-pandemic). Yet, what I don't miss are the inevitable bugs I pick up along the way. Whether it be a cold or a stomach flu, many of us come down with some sort of infection when we are away from home. Many things could contribute to these illnesses, including travel-induced sleep deprivation, novel foods or the concentration of viruses on mass transit. But, could it be that our bodies are just not primed to defend us against the diseases in foreign locales? Agnes Piecyk from the Max Planck Institute for Evolutionary Biology, Germany, and colleagues from the USA and Germany wanted to investigate this theory to figure out whether hosts and parasites develop specialized relationships depending on their geographic origin.So, they took advantage of a host–parasite system that is found throughout the northern hemisphere: the three-spined stickleback Gasterosteus aculeatus and its tapeworm Schistocephalus solidus. The stickleback picks up this nasty worm in a way commonly found on our own human travels – in its food. One of its favourite foods, the copepod, is commonly infected by this parasite. Once consumed, the parasite burrows through the fish's gut, invading its body and quickly growing to fill every nook and cranny of the fish's insides. The parasite steals much of its host's food, forcing the stickleback to feed constantly to keep up with its own energy needs and the demand of its unwanted guest. This voracious feeding makes it less likely to take shelter to avoid predators because of an overwhelming need for food, attracting the attention of its primary predators – and the next host in the parasite's life cycle – fish-eating birds. The parasite then gladly makes its way to the bird's gut, where it sexually reproduces, starting the process again when the bird deposits the parasite's eggs back into waterways containing the copepod (its first host) through its feces. This incredible chain of events has evolved through an evolutionary arms race among the parasite and its three host species.Using sticklebacks and tapeworms from Europe and Alaska, the researchers set about testing the feasibility of inter-continental infections, exposing Alaskan sticklebacks to European tapeworms and vice versa. Surprisingly, while the Alaskan parasite readily infected the European fish host, the Alaskan stickleback put up a stronger fight against the European tapeworm, with hosts from certain Alaskan regions effectively preventing infection altogether. For those unlucky fish that succumbed to infection, their worms varied greatly in size depending on the host's origin. In addition, even when the Alaskan sticklebacks did pick up an infection, their immune systems succeeded in keeping the worm in check, with much smaller worms relative to their body size than European sticklebacks. While we would typically cheer the hosts that entirely prevented a parasite invasion, those uninfected sticklebacks were skinnier than otherwise healthy sticklebacks, indicating the hidden energetic cost of their unseen battle.While the researchers’ work sheds light on the complex interactions between hosts and parasites on a local scale, we still have much to learn about how these geographic differences develop and whether there are epidemiological implications due to geographic differences in disease susceptibility. And hopefully this knowledge might one day help us to avoid the dreaded vacation bug (once we are finally back out there, galivanting in exciting new places).

An optimized LC-HRMS untargeted metabolomics workflow for multi-matrices investigations in the three-spined stickleback.

Environmental metabolomics has become a growing research field to understand biological and biochemical perturbations of organisms in response to various abiotic or biotic stresses. It focuses on the comprehensive and systematic analysis of a biologic system’s metabolome. This allows the recognition of biochemical pathways impacted by a stressor, and the identification of some metabolites as biomarkers of potential perturbations occurring in a body. In this work, we describe the development and optimization of a complete reliable methodology based on liquid chromatography coupled to high resolution mass spectrometry (LC-HRMS) for untargeted metabolomics studies within a fish model species, the three-spined stickleback (Gasterosteus aculeatus). We evaluated the differences and also the complementarities between four different matrices (brain, gills, liver and whole fish) to obtain metabolome information. To this end, we optimized and compared sample preparation and the analytical method, since the type and number of metabolites detected in any matrix are closely related to these latter. For the sample preparation, a solid-liquid extraction was performed on a low quantity of whole fish, liver, brain, or gills tissues using combinations of methanol/water/heptane. Based on the numbers of features observed in LC-HRMS and on the responses of analytical standards representative of different metabolites groups (amino acids, sugars…), we discuss the influence of the nature, volume, and ratio of extraction solvents, the sample weight, and the reconstitution solvent. Moreover, the analytical conditions (LC columns, pH and additive of mobile phases and ionization modes) were also optimized so as to ensure the maximum metabolome coverages. Thus, two complementary chromatographic procedures were combined in order to cover a broader range of metabolites: a reversed phase separation (RPLC) on a C18 column followed by detection with positive ionization mode (ESI+) and a hydrophilic interaction chromatography (HILIC) on a zwitterionic column followed by detection with negative ionization mode (ESI-). This work provides information on brain, gills, liver, vs the whole body contribution to the stickleback metabolome. These information would help to guide ecotoxicological and biomonitoring studies.

Characterizing the habitat function of bivalve aquaculture using underwater video

Bivalve aquaculture is an expanding coastal industry with the potential to modify the habitat of fish and crab species, affecting their refuge, movement, and feeding. The habitat function of shellfish aquaculture is not yet well understood, in part due to difficulties in data collection using traditional methods. Underwater video was used to observe fish and crab species’ affiliations with cultured Pacific oyster Crassostrea gigas and Manila clam Venerupis philippinarum aquaculture sites in comparison to uncultured reference sediment and eelgrass habitats. Sites were monitored in 9 locations across 3 regions of Puget Sound, Washington, USA, in the summers of 2017 and 2018. Of the 3038 fish and crabs observed, 98% were represented by Embiotocidae (surfperch), crabs, three-spined stickleback Gasterosteus aculeatus, Cottidae (sculpins), and Pleuronectiformes (flatfish). Overall, the affiliations of fish and crabs with bivalve aquaculture varied by species groups, culture type, and regional environmental and habitat conditions. These interactions varied on a scale of approximately 150 km, highlighting variation of aquaculture-ecological interactions at a scale not previously recorded in Puget Sound. Species composition varied between aquaculture and non-aquaculture habitats in 2 of the 3 regions studied. Species diversity and richness in aquaculture habitats varied regionally, relative to reference habitats. Pelagic species were more abundant in aquaculture and reference sites that had vertical structure, but abundances of demersal and benthic species on aquaculture habitat relative to reference sites varied regionally. The availability of habitats within intertidal regions, including varying types of aquaculture, could determine community structure for marine organisms such as fish and crab.

Sexual dimorphism patterns of the White Sea threespine stickleback (&lt;em&gt;Gasterosteus aculeatus&lt;/em&gt;)

Sexual dimorphism (SD) in the threespine stickleback Gasterosteus aculeatus reflects the different roles of the sexes in reproduction and their adaptations to different ecological niches. We quantified SD in one population of marine stickleback from the White Sea, collected during the spawning period from three spawning grounds, each at a distance of 5 km or less from the others. We used a landmark-based approach to quantify variation in 44 morphometric linear traits. In total, 749 females and 693 males were analyzed. In males, anterior body parts are larger — the base of the caudal fin and armor structures such as the first and second dorsal spines and the pelvic spine. Females have larger posterior bodies — the abdomen, pelvic girdle and the third dorsal spine. The SD of caudal body parts exhibits complex patterns. In White Sea threespine stickleback, SD patterns are generally similar to other populations of the species, but more often show male-biased patterns. Female-biased size SD may be associated with the female biased sex ratio of White Sea stickleback.

Sick of eating: Eco-evo-immuno dynamics of predators and their trophically acquired parasites.

When predators consume prey, they risk becoming infected with their prey's parasites, which can then establish the predator as a secondary host. A predator population's diet therefore influences what parasites it is exposed to, as has been repeatedly shown in many species such as threespine stickleback (Gasterosteus aculeatus) (more benthic‐feeding individuals obtain nematodes from oligocheate prey, whereas limnetic‐feeding individuals catch cestodes from copepod prey). These differing parasite encounters, in turn, determine how natural selection acts on the predator's immune system. We might therefore expect that ecoevolutionary dynamics of a predator's diet (as determined by its ecomorphology) should drive correlated evolution of its immune traits. Conversely, the predator's immunity to certain parasites might alter the relative costs and benefits of different prey, driving evolution of its ecomorphology. To evaluate the potential for ecological morphology to drive evolution of immunity, and vice versa, we use a quantitative genetics framework coupled with an ecological model of a predator and two prey species (the diet options). Our analysis reveals fundamental asymmetries in the evolution of ecomorphology and immunity. When ecomorphology rapidly evolves, it determines how immunity evolves, but not vice versa. Weak trade‐offs in ecological morphology select for diet generalists despite strong immunological trade‐offs, but not vice versa. Only weak immunological trade‐offs can explain negative diet‐infection correlations across populations. The analysis also reveals that eco‐evo‐immuno feedbacks destabilize population dynamics when trade‐offs are sufficiently weak and heritability is sufficiently high. Collectively, these results highlight the delicate interplay between multivariate trait evolution and the dynamics of ecological communities.

Predator life history and prey ontogeny limit natural selection on the major armour gene,Eda, in threespine stickleback

AbstractNatural selection shapes the evolution of antipredator traits in prey. However, selection in the wild depends on ecological context, including features of predator and prey populations, making field studies of selection critical to understanding how predators shape selection on prey defences. Threespine stickleback (Gasterosteus aculeatus) is a classic system to study the effects of predators on the natural selection of prey. In lakes and rivers, fish predators have been shown to impose selection against low plated adult stickleback phenotypes and genotypes. We directly measured selection by predatory salmonids on theEctodysplasin‐A(Eda) gene in estuary stickleback from California. Despite previous studies showing a positive correlation between predator presence and frequency of theEda“complete” allele in estuary populations, we found thatEda“low” genotypes were not significantly more frequent in salmonid predator diets. Further, we found no evidence of changes inEdagenotype frequencies across generations that would suggest directional selection driven by predators. Prior selection studies have examined the effects of large resident trout on adult stickleback. In contrast, predators in this study were juvenile anadromous salmonids, which only ate juvenile stickleback whose plate phenotypes had not fully developed. Thus, in this case, predator life history and stickleback ontogeny may preclude strong selection on stickleback armour. Our results underscore the importance of selection studies in the wild for understanding the context‐dependent nature of selection in natural populations.

Evolved Bmp6 enhancer alleles drive spatial shifts in gene expression during tooth development in sticklebacks.

Mutations in enhancers have been shown to often underlie natural variation but the evolved differences in enhancer activity can be difficult to identify in vivo. Threespine sticklebacks (Gasterosteus aculeatus) are a robust system for studying enhancer evolution due to abundant natural genetic variation, a diversity of evolved phenotypes between ancestral marine and derived freshwater forms, and the tractability of transgenic techniques. Previous work identified a series of polymorphisms within an intronic enhancer of the Bone morphogenetic protein 6 (Bmp6) gene that are associated with evolved tooth gain, a derived increase in freshwater tooth number that arises late in development. Here, we use a bicistronic reporter construct containing a genetic insulator and a pair of reciprocal two-color transgenic reporter lines to compare enhancer activity of marine and freshwater alleles of this enhancer. In older fish, the two alleles drive partially overlapping expression in both mesenchyme and epithelium of developing teeth, but the freshwater enhancer drives a reduced mesenchymal domain and a larger epithelial domain relative to the marine enhancer. In younger fish, these spatial shifts in enhancer activity are less pronounced. Comparing Bmp6 expression by in situ hybridization in developing teeth of marine and freshwater fish reveals similar evolved spatial shifts in gene expression. Together, these data support a model in which the polymorphisms within this enhancer underlie evolved tooth gain by shifting the spatial expression of Bmp6 during tooth development, and provide a general strategy to identify spatial differences in enhancer activity in vivo.

The measure of spatial position within groups that best predicts predation risk depends on group movement.

Both empirical and theoretical studies show that an individual's spatial position within a group can impact the risk of being targeted by predators. Spatial positions can be quantified in numerous ways, but there are no direct comparisons of different spatial measures in predicting the risk of being targeted by real predators. Here, we assess these spatial measures in groups of stationary and moving virtual prey being attacked by three-spined sticklebacks (Gasterosteus aculeatus). In stationary groups, the limited domain of danger best predicted the likelihood of attack. In moving groups, the number of near neighbours was the best predictor but only over a limited range of distances within which other prey were counted. Otherwise, measures of proximity to the group's edge outperformed measures of local crowding in moving groups. There was no evidence that predators preferentially attacked the front or back of the moving groups. Domains of danger without any limit, as originally used in the selfish herd model, were also a poor predictor of risk. These findings reveal that the collective properties of prey can influence how spatial position affects predation risk, via effects on predators' targeting. Selection may therefore act differently on prey positioning behaviour depending on group movement.