Frequency-Dependent Natural Selection in the Handedness of Scale-Eating Cichlid Fish

Swimming through asymmetry: zebrafish as a model for brain and behavior lateralization

The left and right sides of the brain show anatomical, neurochemical and functional differences. In the past century, brain and behavior lateralization was considered a human peculiarity associated with language and handedness. However, nowadays lateralization is known to occur among all vertebrates, from primates to fish. Fish, especially zebrafish (Danio rerio), have emerged as a crucial model for exploring the evolution and mechanisms of brain asymmetry. This review summarizes recent advances in zebrafish research on brain lateralization, highlighting how genetic tools, imaging, and transgenic methods have been used to investigate left-right asymmetries and their impact on sensory, cognitive, and social behaviors including possible links to neurodevelopmental and neurodegenerative disorders.

Hawks, Doves, and Perissodus microlepis. Undermining the selected effects theory of function

The selected effects theory is supposed to provide a fully naturalistic basis for statements about what biological traits or processes are for without appeal to final causes or intelligent design. On the selected effects theory, biologists are allowed to say, for instance, that hindwing eyespots on butterfly wings serve to deflect predators' attacks away from vital organs because a similar fitness-enhancing effect explains why eyespots themselves were favoured by natural selection and persisted in the population. This is known as the explanatory dimension of the selected effects theory. According to it, appealing to the fitness-enhancing effect of a certain trait or process is sufficient to explain its current presence in a population, namely, why it persisted and still exists in that population. In this paper, however, I will call such a claim into question, and I will do so by discussing a mathematical Hawk-Dove example and a real case scenario taken from evolutionary biology, that of Perissodus microlepis. These are scenarios in which the selective filter does not allow variants with the highest fitness at a certain moment to prevail over their available alternatives. In similar cases, I will argue, citing fitness-enhancing effects does not represent an adequate explanation of what happens in the population, undermining the explanatory dimension of the selected effects theory.

Brain and behavioral asymmetries in nonprimate species

Brain and behavioral asymmetries are widespread across the animal kingdom, suggesting that even simpler nervous systems benefit from such features. In the last 30 years, research conducted on several vertebrate (but also invertebrate) animal models has massively contributed to our understanding of the causation, development, evolution, and function of lateralization. Here, we review some of this research, highlighting the importance of studying this topic in nonprimate species for a deeper understanding of the mechanisms behind cerebral asymmetries. We report evidence of handedness and motor asymmetries as well as the results of research on perceptual and cognitive asymmetries in nonprimate animals, analyzing the contribution of such studies in the research field of cerebral asymmetries.

Morphological Study of Left-Right Head Asymmetry in Doubledaya bucculenta (Coleoptera: Erotylidae: Languriinae)

Left-right asymmetry in paired organs is well documented across various species, including the claws of fiddler crabs and snail-eating snakes' dentition. However, the mechanisms underlying these asymmetries remain largely elusive. This study investigates Doubledaya bucculenta (Coleoptera: Erotylidae), a lizard beetle species known for pronounced left-sided asymmetry in adult female mandible and gena. Given that insect mouthparts comprise multiple functionally significant appendages, we aimed to clarify the degree of asymmetry extending beyond the mandibles and genae. Phenotypic morphology was assessed through trait measurement and asymmetry index calculations. Our detailed morphometric analyses revealed left-longer asymmetry not only in mandibles and genae but also in maxillae and labium. Notably, the degree of asymmetry in other mouthparts was generally less pronounced compared to that in outer mandibles, suggesting a potential influence of left mandible development on other mouthparts. Additionally, male mandibles exhibited region-specific asymmetry, potentially indicative of constrained evolutionary adaptations. This study enhances a comprehensive understanding of adult phenotype morphology and offers insights into the developmental basis of asymmetrical mouthparts.

Preying on cyprinid snout warts (pearl organs) as a novel and peculiar habit in the Lake Malawi cichlid Docimodus evelynae

Cichlid fishes in the African Great Lakes have undergone explosive speciation, acquiring markedly varying ecologies and diets. There are multiple lineages of scale-eating cichlids, and their natural history and evolutionary ecology is only partially understood. We examined the feeding habit of Docimodus evelynae, a known scale eater, in Lake Malawi. The stomach contents of young individuals mainly consisted of unknown 1 mm hard, white warts (> 30%). To clarify the origin of these warts, we conducted an X-ray fluorometer analysis, and found they were rich in sulphur but low in silicon and calcium, suggesting they were epidermal tissues. Histological and morphological analyses revealed they were multicellular and cup-shaped. These characteristics matched only those of the pearl organs of the coexisting cyprinid Labeo cylindricus. DNA was extracted from the warts found in the stomach of five D. evelynae individuals, followed by PCR using primers targeting the partial COI gene of L. cylindricus. The resulting sequences exhibited 98% similarity to those of L. cylindricus. Pearl organs, never reported as a primary food for fish, could offer a substantial nutritional source based on calorific calculations. Understanding how this peculiar diet is foraged is essential for full comprehension of the food-web structure in this lake.

Genomic evidence of reproductive isolation among the Semisulcospira snails radiated in the ancient Lake Biwa

Determining species boundaries within rapidly evolving species flocks is essential to understanding their evolutionary history but is often difficult to achieve due to the lack of clear diagnostic features. Ancient Lake Biwa harbours endemic snails in the genus Semisulcospira, a species flock with 19 described species. However, their morphological and genetic similarity cast doubt on the validity of their species status and their histories of explosive speciation. To evaluate their species boundaries, we examine patterns of gene flow among the sympatric or parapatric nominal Semisulcospira species in Lake Biwa. The principal component analysis and Bayesian structure analysis based on the genome-wide genotyping dataset demonstrated no gene flow between five pairs of the Semisulcospira species. However, we found the hybrids between the closely related species pair, Semisulcospira decipiens and S. rugosa. Despite the presence of hybrids, these nominal species still formed their own genetic clusters. There are variations in the chromosome numbers among these species, potentially providing an intrinsic barrier to panmictic gene flow. Our study showed complete or partial reproductive isolation among the sympatric or parapatric Semisulcospira species, demonstrating that the Semisulcospira snails are real species assemblages radiated in Lake Biwa. Our study provides significant implications for establishing species boundaries among rapidly evolving freshwater species in ancient lakes.

John M, Chau D, Clair C, Chan H, Holzman R, Martin CH. A multi-peak performance landscape for scale biting in an adaptive radiation of pupfishes

The physical interactions between organisms and their environment ultimately shape diversification rates, but the contributions of biomechanics to evolutionary divergence are frequently overlooked. Here, we estimated a performance landscape for biting in an adaptive radiation of Cyprinodon pupfishes, including scale-biting and molluscivore specialists, and compared performance peaks with previous estimates of the fitness landscape in this system. We used high-speed video to film feeding strikes on gelatin cubes by scale eater, molluscivore, generalist and hybrid pupfishes and measured bite dimensions. We then measured five kinematic variables from 227 strikes using the SLEAP machine-learning model. We found a complex performance landscape with two distinct peaks best predicted gel-biting performance, corresponding to a significant non-linear interaction between peak gape and peak jaw protrusion. Only scale eaters and their hybrids were able to perform strikes within the highest performance peak, characterized by larger peak gapes and greater jaw protrusion. A performance valley separated this peak from a lower performance peak accessible to all species, characterized by smaller peak gapes and less jaw protrusion. However, most individuals exhibited substantial variation in strike kinematics and species could not be reliably distinguished by their strikes, indicating many-to-many mapping of morphology to performance. The two performance peaks observed in the lab were partially consistent with estimates of a two-peak fitness landscape measured in the wild, with the exception of the new performance peak for scale eaters. We thus reveal a new bimodal non-linear biomechanical model that connects morphology to performance to fitness in a sympatric radiation of trophic niche specialists.

Frequency-dependent selection of neoantigens fosters tumor immune escape and predicts immunotherapy response

Cancer is an evolutionary process shaped by selective pressure from the microenvironments. However, recent studies reveal that certain tumors undergo neutral evolution where there is no detectable fitness difference amongst the cells following malignant transformation. Here, through computational modeling, we demonstrate that negative frequency-dependent selection (or NFDS), where the immune response against cancer cells depends on the clonality of neoantigens, can lead to an immunogenic landscape that is highly similar to neutral evolution. Crucially, NFDS promotes high antigenic heterogeneity and early immune evasion in hypermutable tumors, leading to poor responses to immune checkpoint blockade (ICB) therapy. Our model also reveals that NFDS is characterized by a negative association between average clonality and total burden of neoantigens. Indeed, this unique feature of NFDS is common in the whole-exome sequencing (WES) datasets (357 tumor samples from 275 patients) from four melanoma cohorts with ICB therapy and a non-small cell lung cancer (NSCLC) WES dataset (327 tumor samples from 100 patients). Altogether, our study provides quantitative evidence supporting the theory of NFDS in cancer, explaining the high prevalence of neutral-looking tumors. These findings also highlight the critical role of frequency-dependent selection in devising more efficient and predictive immunotherapies.

John M, Chau D, Clair C, Chan H, Holzman R, Martin CH. Multiple performance peaks for scale-biting in an adaptive radiation of pupfishes

The physical interactions between organisms and their environment ultimately shape their rate of speciation and adaptive radiation, but the contributions of biomechanics to evolutionary divergence are frequently overlooked. Here we investigated an adaptive radiation of Cyprinodon pupfishes to measure the relationship between feeding kinematics and performance during adaptation to a novel trophic niche, lepidophagy, in which a predator removes only the scales, mucus, and sometimes tissue from their prey using scraping and biting attacks. We used high-speed video to film scale-biting strikes on gelatin cubes by scale-eater, molluscivore, generalist, and hybrid pupfishes and subsequently measured the dimensions of each bite. We then trained the SLEAP machine-learning animal tracking model to measure kinematic landmarks and automatically scored over 100,000 frames from 227 recorded strikes. Scale-eaters exhibited increased peak gape and greater bite length; however, substantial within-individual kinematic variation resulted in poor discrimination of strikes by species or strike type. Nonetheless, a complex performance landscape with two distinct peaks best predicted gel-biting performance, corresponding to a significant nonlinear interaction between peak gape and peak jaw protrusion in which scale-eaters and their hybrids occupied a second performance peak requiring larger peak gape and greater jaw protrusion. A bite performance valley separating scale-eaters from other species may have contributed to their rapid evolution and is consistent with multiple estimates of a multi-peak fitness landscape in the wild. We thus present an efficient deep-learning automated pipeline for kinematic analyses of feeding strikes and a new biomechanical model for understanding the performance and rapid evolution of a rare trophic niche.

Camouflage efficiency in a colour-polymorphic predator is dependent on environmental variation and snow presence in the wild

Colour polymorphism can be maintained by colour morph-specific benefits across environmental conditions. Currently, the amount and the duration of snow cover during winter decrease especially in northern latitudes, which can alter the potential for camouflage of animals with light and dark morphs. Tawny owls, Strix aluco, are colour-polymorphic avian predators with dark (brown) and light (grey) colour morphs, where the grey morph is presumed to enjoy camouflage benefits under snowy conditions. We studied the camouflage potential of morphs in two tawny owls potential using passerines' probability to mob in the wild during winter with and without snow. For comparison with other seasons, we also repeated the experiment during spring and autumn. We found that grey tawny owls have a lower probability of being mobbed than the brown tawny owls only during snowy winters. The two colour morphs therefore experience differential benefits across snow conditions, which may help to maintain colour morphs in the population, although further warming of winter climate will reduce the potential for camouflage for grey tawny owls in northern latitudes.

Developmental Process of a Pronounced Laterality in the Scale-eating Cichlid Fish Perissodus microlepis in Lake Tanganyika

Lateral preference in behaviors has been widely documented in many vertebrates and invertebrates. Such preferences are strange, puzzling, and on the surface, not adaptive. However, behavioral laterality may increase an individual's fitness as well as foraging accuracy and speed. There is little experimental evidence regarding the developmental process of laterality, and unsolved questions have perplexed researchers for several decades. Related to these issues, here, I review that the scale-eating cichlid Perissodus microlepis found in Lake Tanganyika is a valuable model to address the developmental mechanism of animal laterality. The scale-eating cichlid has pronounced behavioral laterality and uses its asymmetric mouth during feeding events. Recent studies have shown that behavioral laterality in this fish depends on both genetic factors and past experience. The attack-side preference of scale eaters is an acquired trait in an early developmental stage. Juvenile fish empirically learn which side of the prey is more effective for tearing scales and gradually select the dominant side for attacking. However, the superior kinetics of body flexion during the dominant side attack has innate characteristics. Additionally, left-right differences in scale-eater mandibles also develop during ontogeny. Further progress toward understanding the comprehensive mechanisms of laterality should address the following persistent barriers: (1) the effects of phylogenetic constraints and ecological factors on the level of laterality; and (2) the neuronal and molecular mechanisms that produce left-right behavioral differences.

Does male gonopodial morphology affect male-female mating positioning in the livebearing fish Xenophallus umbratilis?

Xenophallus umbratilis is a freshwater livebearing fish that exhibits unique antisymmetry in the male gonopodium, which terminates in either a dextral or sinistral twist. This asymmetry in the gonopodium suggests that males might exhibit side-biased behavior when interacting with females to mate. We conducted two assays to assess the laterality of male and female mating interactions based on gonopodial morphology. We observed lateralized mating behavior in one test where males with sinistral gonopodial morphology interacted with a single female. However, we did not find lateralized mating behavior in males with dextral gonopodial morphology. We also examined male and female positioning in trials that placed a single female with five males, all with the same morphology. These trials also showed no evidence of lateralized body positioning.

Dynamics of Laterality in the Cuttlefish Sepia recurvirostra through Interactions with Prey Prawns

Predator-prey interactions based on laterality have recently been observed between fishes and their prey populations. Maintenance of antisymmetric dimorphism by frequency-dependent selection has been reported in fish, but has not been observed in invertebrates. Over 10 years, we investigated long-term changes in the "ratio of laterality" (frequency of righty morphs in a population) in the cuttlefish Sepia recurvirostra and its potential prey prawns Penaeus semisulcatus and Metapenaeus endeavouri in the Visayan Sea, the Philippines. The morphological laterality of cuttlefish and prey prawns was defined by measuring the asymmetry of the cuttlebone and carapace, respectively. Cuttlefish and prey prawns showed morphological antisymmetry, being composed with righty morphs and lefty morphs. The ratio of laterality of cuttlefish and one prey prawn oscillated significantly, but the oscillation was not strongly synchronized. The ratio of laterality of cuttlefish followed that of the prey prawn, indicating that predation biased to each laterality occurred in relation to their laterality. These results suggest that the lateral dimorphism of cuttlefish is maintained through frequency-dependent selection on lateral morphs of the predator cuttlefish and prey prawns. Our findings provide new insight into the ecological significance and antisymmetry maintenance mechanism in relation to interspecific interactions in marine invertebrates.

Laplace's demon in biology: Models of evolutionary prediction

Our ability to predict natural phenomena can be limited by incomplete information. This issue is exemplified by "Laplace's demon," an imaginary creature proposed in the 18th century, who knew everything about everything, and thus could predict the full nature of the universe forward or backward in time. Quantum mechanics, among other things, has cast doubt on the possibility of Laplace's demon in the full sense, but the idea still serves as a useful metaphor for thinking about the extent to which prediction is limited by incomplete information on deterministic processes versus random factors. Here, we use simple analytical models and computer simulations to illustrate how data limits can be captured in a Bayesian framework, and how they influence our ability to predict evolution. We show how uncertainty in measurements of natural selection, or low predictability of external environmental factors affecting selection, can greatly reduce predictive power, often swamping the influence of intrinsic randomness caused by genetic drift. Thus, more accurate knowledge concerning the causes and action of natural selection is key to improving prediction. Fortunately, our analyses and simulations show quantitatively that reasonable improvements in data quantity and quality can meaningfully increase predictability.

Cerebral Polymorphisms for Lateralisation: Modelling the Genetic and Phenotypic Architectures of Multiple Functional Modules

Recent fMRI and fTCD studies have found that functional modules for aspects of language, praxis, and visuo-spatial functioning, while typically left, left and right hemispheric respectively, frequently show atypical lateralisation. Studies with increasing numbers of modules and participants are finding increasing numbers of module combinations, which here are termed cerebral polymorphisms—qualitatively different lateral organisations of cognitive functions. Polymorphisms are more frequent in left-handers than right-handers, but it is far from the case that right-handers all show the lateral organisation of modules described in introductory textbooks. In computational terms, this paper extends the original, monogenic McManus DC (dextral-chance) model of handedness and language dominance to multiple functional modules, and to a polygenic DC model compatible with the molecular genetics of handedness, and with the biology of visceral asymmetries found in primary ciliary dyskinesia. Distributions of cerebral polymorphisms are calculated for families and twins, and consequences and implications of cerebral polymorphisms are explored for explaining aphasia due to cerebral damage, as well as possible talents and deficits arising from atypical inter- and intra-hemispheric modular connections. The model is set in the broader context of the testing of psychological theories, of issues of laterality measurement, of mutation-selection balance, and the evolution of brain and visceral asymmetries.

A test of frequency‐dependent selection in the evolution of a generalist phenotype

A solitary population of consumers frequently evolves to the middle of a resource gradient and an intermediate mean phenotype compared to a sympatric pair of competing species that diverge to either side via character displacement. The forces governing the distribution of phenotypes in these allopatric populations, however, are little investigated. Theory predicts that the intermediate mean phenotype of the generalist should be maintained by negative frequency‐dependent selection, whereby alternate extreme phenotypes are favored because they experience reduced competition for resources when rare. However, the theory makes assumptions that are not always met, and alternative explanations for an intermediate phenotype are possible. We provide a test of this prediction in a mesocosm experiment using threespine stickleback that are ecologically and phenotypically intermediate between the more specialized stickleback species that occur in pairs. We manipulated the frequency distribution of phenotypes in two treatments and then measured effects on a focal intermediate population. We found a slight frequency‐dependent effect on survival in the predicted direction but not on individual growth rates. This result suggests that frequency‐dependent selection might be a relatively weak force across the range of phenotypes within an intermediate population and we suggest several general reasons why this might be so. We propose that allopatric populations might often be maintained at an intermediate phenotype instead by stabilizing or fluctuating directional selection.

Experience-dependent learning of behavioral laterality in the scale-eating cichlid Perissodus microlepis occurs during the early developmental stage

Behavioral laterality-typically represented by human handedness-is widely observed among animals. However, how laterality is acquired during development remains largely unknown. Here, we examined the effect of behavioral experience on the acquisition of lateralized predation at different developmental stages of the scale-eating cichlid fish Perissodus microlepis. Naïve juvenile fish without previous scale-eating experience showed motivated attacks on prey goldfish and an innate attack side preference. Following short-term predation experience, naïve juveniles learned a pronounced lateralized attack using their slightly skewed mouth morphology, and improved the velocity and amplitude of body flexion to succeed in foraging scales during dominant-side attack. Naïve young fish, however, did not improve the dynamics of flexion movement, but progressively developed attack side preference and speed to approach the prey through predation experience. Thus, the cichlid learns different aspects of predation behavior at different developmental stages. In contrast, naïve adults lost the inherent laterality, and they neither developed the lateralized motions nor increased their success rate of predation, indicating that they missed appropriate learning opportunities for scale-eating skills. Therefore, we conclude that behavioral laterality of the cichlid fish requires the integration of genetic basis and behavioral experiences during early developmental stages, immediately after they start scale-eating.

Biodiversity, resilience and the stability of evolutionary systems

Various macro-evolutionary phenomena, such as long-term stability punctuated by bursts of evolution, are difficult to explain via the micro-evolutionary process of weak selection acting steadily on individual mutations. In contrast, bursts of change are expected if evolutionary systems are complex and balanced, with occasional disruption of balance. Such disruption represents the collapse of resilience, akin to the snapping of an elastic band. It can be driven by external factors, or by self-propagating feedback loops internal to a system. Thus, evolutionary resilience could help explain how evolution generates broader patterns of biodiversity. We outline evidence and tests for this hypothesis, which emphasizes the processes balancing evolution, as urged fifty years ago in ecological genetics and via modern results in a range of systems.

Edge current and pairing order transition in chiral bacterial vortices

Bacterial suspensions show turbulence-like spatiotemporal dynamics and vortices moving irregularly inside the suspensions. Understanding these ordered vortices is an ongoing challenge in active matter physics, and their application to the control of autonomous material transport will provide significant development in microfluidics. Despite the extensive studies, one of the key aspects of bacterial propulsion has remained elusive: The motion of bacteria is chiral, i.e., it breaks mirror symmetry. Therefore, the mechanism of control of macroscopic active turbulence by microscopic chirality is still poorly understood. Here, we report the selective stabilization of chiral rotational direction of bacterial vortices in achiral circular microwells sealed by an oil/water interface. The intrinsic chirality of bacterial swimming near the top and bottom interfaces generates chiral collective motions of bacteria at the lateral boundary of the microwell that are opposite in directions. These edge currents grow stronger as bacterial density increases, and, within different top and bottom interfaces, their competition leads to a global rotation of the bacterial suspension in a favored direction, breaking the mirror symmetry of the system. We further demonstrate that chiral edge current favors corotational configurations of interacting vortices, enhancing their ordering. The intrinsic chirality of bacteria is a key feature of the pairing order transition from active turbulence, and the geometric rule of pairing order transition may shed light on the strategy for designing chiral active matter.

Dynamics of Laterality in Relation to the Predator-Prey Interaction between the Piscivorous Chub "Hasu" and Its Prey "Ayu" in Lake Biwa

A Japanese piscivorous chub, "hasu" (Opsariichthys uncirostris), and its main prey, "ayu" (Plecoglossus altivelis), both have laterally asymmetric bodies, similar to other fishes; each population consists of righty morphs and lefty morphs. This antisymmetric dimorphism has a genetic basis. Temporal changes in the ratios of laterality (i.e., frequency of righty morphs in a population) of these predator and prey fish species were investigated for a 20-year period at a pelagic site in the southwestern area of Lake Biwa, Japan. The dimorphism of each species was maintained dynamically throughout the period, and the ratio of laterality was found to change periodically in a semi-synchronized manner. Direct inspection of the relationship between the ratios of laterality of the two species indicated that the ratio of ayu followed that of hasu, suggesting that the predator-prey interaction was responsible for the semi-synchronized change. Stomach contents analysis of each hasu revealed that cross-predation, in which righty predators catch lefty prey and lefty predators catch righty prey, occurred more frequently than the reverse combination (parallel-predation). This differential predation is presumed to cause frequency-dependent selection on the two morphs of the predator and prey, and to drive semi-synchronized changes in the laterality of the two species. Some discussion pertaining to the atypical form of the semi-synchronized change in laterality found in this study is presented from the viewpoint of predator-prey interaction in fishes.

Maternal Care-Providing Cichlid Neolamprologus furcifer Selectively Focuses on High-Threat Carnivorous Intruders, Limiting Attention to Other Threats

Animals adjust their behaviors based on information from multiple sources; however, the brain can effectively process limited amounts of information. Therefore, attention is restricted to a small portion of environmental stimuli. When animals process multiple information inputs, focusing on information that is deemed important improves detection probability. However, selective focus limits attention to other stimuli and associated behavioral responses. In this study, we examined how Tanganyikan cichlid, Neolamprologus furcifer, mothers selectively attack intruder fishes depending on the threat level and presence or absence of offspring. Species composition is complicated in Lake Tanganyika, and fish density is exceedingly high. Thus, parents must focus on high-threat-level intruders according to their parental care stage. Compared to females without offspring, mothers preferentially attacked carnivorous fishes farther from the nest over closer scale-eating fishes. Moreover, the percentage of females with injuries from scale-eating fish was significantly higher in those caring for offspring than those without offspring, demonstrating the cost of limited attention. Our results show that females focus on the early detection of carnivorous fishes because these predators dart in from long distances to forage eggs, fry, and juveniles, but this selective focus limits the attention placed on low-level threats. This study is the first to document the cost of limited attention in parents guarding offspring.

Surprising spatiotemporal stability of a multi-peak fitness landscape revealed by independent field experiments measuring hybrid fitness

The effect of the environment on fitness in natural populations is a fundamental question in evolutionary biology. However, experimental manipulations of both environment and phenotype at the same time are rare. Thus, the relative importance of the competitive environment versus intrinsic organismal performance in shaping the location, height, and fluidity of fitness peaks and valleys remains largely unknown. Here, we experimentally tested the effect of competitor frequency on the complex fitness landscape driving adaptive radiation of a generalist and two trophic specialist pupfishes, a scale-eater and molluscivore, endemic to hypersaline lakes on San Salvador Island (SSI), Bahamas. We manipulated phenotypes, by generating 3407 F4/F5 lab-reared hybrids, and competitive environment, by altering the frequency of rare transgressive hybrids between field enclosures in two independent lake populations. We then tracked hybrid survival and growth rates across these four field enclosures for 3-11 months. In contrast to competitive speciation theory, we found no evidence that the frequency of hybrid phenotypes affected their survival. Instead, we observed a strikingly similar fitness landscape to a previous independent field experiment, each supporting multiple fitness peaks for generalist and molluscivore phenotypes and a large fitness valley isolating the divergent scale-eater phenotype. These features of the fitness landscape were stable across manipulated competitive environments, multivariate trait axes, and spatiotemporal heterogeneity. We suggest that absolute performance constraints and divergent gene regulatory networks shape macroevolutionary (interspecific) fitness landscapes in addition to microevolutionary (intraspecific) competitive dynamics. This interplay between organism and environment underlies static and dynamic features of the adaptive landscape.

Does Asymmetrical Gonopodium Morphology Predict Lateralized Behavior in the Fish Xenophallus umbratilis?

Why bilaterally symmetrical organisms express handedness remains an important question in evolutionary biology. In some species, anatomical asymmetries have evolved that accompany behavioral handedness, yet we know remarkably little about causal links between asymmetric morphological traits and behavior. Here, we explore if a dextral or sinistral orientation of the male intromittent organ predicts side preferences in male behaviors. Our study addresses this question in the Costa Rican livebearing fish, Xenophallus umbratilis. This fish has a bilaterally symmetrical body plan, with one exception—the male anal fin (gonopodium), used to inseminate females, terminates with a distinct left- or right-handed corkscrew morphology. We used a detour assay to test males for side biases in approach behavior when exposed to four different stimuli (predator, potential mate, novel object, empty tank control). We found that left morph males preferred using their right eye to view potential mates, predators, and the control, and that right morph males preferred to use their left eye to view potential mates and predators, and their right eye to view the control. Males of both morphs displayed no eye bias when approaching the novel object. Our results suggest that there is a strong link between behavior and gonopodium orientation, with right and left morph males responding with opposite directional behaviors when presented with the same stimuli. This presents the intriguing possibility that mating preferences—in this case constrained by gonopodial morphology—could be driving lateralized decision making in a variety of non-mating behaviors.

An experimental investigation of how intraspecific competition and phenotypic plasticity can promote the evolution of novel, complex phenotypes

Intraspecific competition has long been considered a key driver of evolutionary diversification, but whether it can also promote evolutionary innovation is less clear. Here we examined the interplay between competition and phenotypic plasticity in fuelling the origins of a novel, complex phenotype – a distinctive carnivore morph found in spadefoot toad tadpoles (genus Spea) that specializes on fairy shrimp. We specifically sought to explore the possible origins of this phenotype by providing shrimp to Scaphiopus holbrookii tadpoles (the sister genus to Spea that does not produce carnivores) while subjecting them to competition for their standard diet of detritus. Previous research had shown that this species will eat shrimp when detritus is limited, and that these shrimp-fed individuals produce features that are redolent of a rudimentary Spea carnivore. In this study, we found that: (1) behavioural and morphological plasticity enabled some individuals to expand their diet to include shrimp; (2) there was heritable variation in this plasticity; and (3) individuals received a growth and development benefit by eating shrimp. Thus, novel resource use can arise via plasticity as an adaptive response to intraspecific competition. More generally, our results show how competition and plasticity may interact to pave the way for the evolution of complex, novel phenotypes, such as the distinctive carnivore morph in present-day Spea.

The direction of genital asymmetry is expressed stochastically in internally fertilizing anablepid fishes

Animal genitalia vary considerably across taxa, with divergence in many morphological traits, including striking departures from symmetry. Different mechanisms have been proposed to explain this diversity, mostly assuming that at least some of the phenotypic variation is heritable. However, heritability of the direction of genital asymmetry has been rarely determined. Anablepidae are internally fertilizing fish where the anal fin of males has been modified into an intromittent organ that transfers sperm into the gonopore of females. Males of anablepid fishes exhibit asymmetric genitalia, and both left- and right-sided individuals are commonly found at similar proportions within populations (i.e. antisymmetry). Although this polymorphism was described over a century ago, there have been no attempts to determine if genital asymmetry has a genetic basis and whether the different morphs are accumulating genetic differences, as might be expected since in some species females have also asymmetric gonopores and thereby can only be fertilized by compatible asymmetric males. We address this issue by combining breeding experiments with genome-wide data (ddRAD markers) in representative species of the two anablepid genera with asymmetric genitalia: Anableps and Jenynsia. Breeding experiments showed that all offspring were asymmetric, but their morphotype (i.e. right- or left-sided) was independent of parental morphotype, implying that the direction of asymmetry does not have a strong genetic component. Consistent with this conclusion, association analyses based on approximately 25 000 SNPs did not identify markers significantly associated with the direction of genital asymmetry and there was no evidence of population structure between left- and right-sided individuals. These results suggest that the direction of genital asymmetry in anablepid fishes might be stochastic, a commonly observed pattern in species with antisymmetry in morphological traits.

Righting behaviour in the European pond turtle (Emys orbicularis): relations between behavioural and morphological lateralization

Lateralization represents a key property of many behavioural traits, with the right and left sides of the brain providing different and integrative functions. Common ecological contexts where lateralization can be observed are foraging and predatory ones, where both visual and auditory lateralization may provide advantages such as faster response and increasing neural processing capacity. This is crucial in selecting a safe refuge during a predatory attack and may strongly affect the outcome of predator-prey interactions. For animals like turtles, a critical condition may occur when they are overturned on their own shell, which causes difficulties in breathing and thermoregulation, making them more vulnerable to predators. Therefore, the ability to right is a critical adaptive component related to survival in aquatic turtles, which has been observed to be lateralized. However, an overlooked feature of behavioural lateralization is its possible relationship with asymmetry in external morphology. Here we investigated this topic in freshwater European pond turtles Emys orbicularis, looking at a possible relation between lateralization in righting behaviour response and asymmetry in the shape of turtles' plastron and carapace. Righting performance (total time needed to completely turn) appeared to depend on shell shape. We found that none of the morphometric variables was related to a lateralization index calculated as the first side from which turtles tried to right. However, a strong negative correlation between the asymmetry index of plastron and the turning direction emerged, with more symmetric animals tending to turn to the right side.

Structural manipulations of a shelter resource reveal underlying preference functions in a shell-dwelling cichlid fish

Many animals can modify the environments in which they live, thereby changing the selection pressures they experience. A common example of such niche construction is the use, creation or modification of environmental resources for use as nests or shelters. Because these resources often have correlated structural elements, it can be difficult to disentangle the relative contribution of these elements to resource choice, and the preference functions underlying niche-construction behaviour remain hidden. Here, we present an experimental paradigm that uses 3D scanning, modelling and printing to create replicas of structures that differ with respect to key structural attributes. We show that a niche-constructing, shell-dwelling cichlid fish, Neolamprologus multifasciatus, has strong open-ended preference functions for exaggerated shell replicas. Fish preferred shells that were fully intact and either enlarged, lengthened or had widened apertures. Shell intactness was the most important structural attribute, followed by shell length, then aperture width. We disentangle the relative roles of different shell attributes, which are tightly correlated in the wild, but nevertheless differentially influence shelter choice and therefore niche construction in this species. We highlight the broad utility of our approach when compared with more traditional methods (e.g. two-choice tasks) for studying animal decision-making in a range of contexts.

Brain and Behavioral Asymmetry: A Lesson From Fish

It is widely acknowledged that the left and right hemispheres of human brains display both anatomical and functional asymmetries. For more than a century, brain and behavioral lateralization have been considered a uniquely human feature linked to language and handedness. However, over the past decades this idea has been challenged by an increasing number of studies describing structural asymmetries and lateralized behaviors in non-human species extending from primates to fish. Evidence suggesting that a similar pattern of brain lateralization occurs in all vertebrates, humans included, has allowed the emergence of different model systems to investigate the development of brain asymmetries and their impact on behavior. Among animal models, fish have contributed much to the research on lateralization as several fish species exhibit lateralized behaviors. For instance, behavioral studies have shown that the advantages of having an asymmetric brain, such as the ability of simultaneously processing different information and perform parallel tasks compensate the potential costs associated with poor integration of information between the two hemispheres thus helping to better understand the possible evolutionary significance of lateralization. However, these studies inferred how the two sides of the brains are differentially specialized by measuring the differences in the behavioral responses but did not allow to directly investigate the relation between anatomical and functional asymmetries. With respect to this issue, in recent years zebrafish has become a powerful model to address lateralization at different level of complexity, from genes to neural circuitry and behavior. The possibility of combining genetic manipulation of brain asymmetries with cutting-edge in vivo imaging technique and behavioral tests makes the zebrafish a valuable model to investigate the phylogeny and ontogeny of brain lateralization and its relevance for normal brain function and behavior.

Asymmetric and Spiraled Genitalia Coevolve with Unique Lateralized Mating Behavior

Asymmetric genitalia and lateralized mating behaviors occur in several taxa, yet whether asymmetric morphology in one sex correlates or coevolves with lateralized mating behavior in the other sex remains largely unexplored. While lateralized mating behaviors are taxonomically widespread, among mammals they are only known in the harbor porpoise (Phocoena phocoena). Males attempt copulation by approaching a female exclusively on her left side. To understand if this unusual lateralized behavior may have coevolved with genital morphology, we quantified the shape of female and male harbor porpoise reproductive tracts using 2D geometric morphometrics and 3D models of the vaginal lumen and inflated distal penis. We found that the vaginas varied individually in shape and that the vaginas demonstrated both significant directional and fluctuating asymmetry. This asymmetry resulted from complex 3D spirals and vaginal folds with deep recesses, which may curtail the depth or direction of penile penetration and/or semen movement. The asymmetric shapes of the vaginal lumen and penis tip were both left-canted with similar angular bends that mirrored one another and correspond with the left lateral mating approach. We suggest that the reproductive anatomy of both sexes and their lateral mating behavior coevolved.

Parallel evolution of regressive and constructive craniofacial traits across distinct populations of Astyanax mexicanus cavefish

Life in complete darkness has driven the evolution of a suite of troglobitic features in the blind Mexican cavefish Astyanax mexicanus, such as eye and pigmentation loss. While regressive evolution is a hallmark of obligate cave-dwelling organisms, constructive (or augmented) traits commonly arise as well. The cavefish cranium has undergone extensive changes compared with closely-related surface fish. These alterations are rooted in both cranial bones and surrounding sensory tissues such as enhancements in the gustatory and lateral line systems. Cavefish also harbor numerous cranial bone asymmetries: fluctuating asymmetry of individual bones and directional asymmetry in a dorsal bend of the skull. This asymmetry is mirrored by the asymmetrical patterning of mechanosensory neuromasts. We explored the relationship between facial bones and neuromasts using in vivo fluorescent colabeling and microcomputed tomography. We found an increase in neuromast density within dermal bone boundaries across three distinct populations of cavefish compared to surface-dwelling fish. We also show that eye loss disrupts early neuromast patterning, which in turn impacts the development of dermal bones. While cavefish exhibit alterations in cranial bone and neuromast patterning, each population varied in the severity. This variation may reflect observed differences in behavior across populations. For instance, a bend in the dorsal region of the skull may expose neuromasts to water flow on the opposite side of the face, enhancing sensory input and spatial mapping in the dark.

A function for the bicameral mind

Why do the left and right sides of the brain have different functions? Having a lateralized brain, in which each hemisphere processes sensory inputs differently and carries out different functions, is common in vertebrates, and it has now been reported for invertebrates too. Experiments with several animal species have shown that having a lateralized brain can enhance the capacity to perform two tasks at the same time. Thus, the different specializations of the left and right sides of the brain seem to increase brain efficiency. Other advantages may involve control of action that, in Bilateria, may be confounded by separate and independent sensory processing and motor outputs on the left and right sides. Also, the opportunity for increased perceptual training associated with preferential use of only one sensory or motoric organ may result in a time advantage for the dominant side. Although brain efficiency of individuals can be achieved without the need for alignment of lateralization in the population, lateral biases (such as preferences in the use of a laterally-placed eye) usually occur at the population level, with most individuals showing a similar direction of bias. Why is this the case? Not only humans, but also most non-human animals, show a similar pattern of population bias (i.e., directional asymmetry). For instance, in several vertebrate species (from fish to mammals) most individuals react faster when a predator approaches from their left side, although some individuals (a minority usually ranging from 10 to 35%) escape faster from predators arriving from their right side. Invoking individual efficiency (lateralization may increase fitness), evolutionary chance or simply genetic inheritance cannot explain this widespread pattern. Using mathematical theory of games, it has been argued that the population structure of lateralization (with either antisymmetry or directional asymmetry) may result from the type of interactions asymmetric organisms face with each other.

Disruptive selection of shell colour in land snails: a mark–recapture study of Euhadra peliomphala simodae

Many theoretical studies have suggested that disruptive selection plays an important role in phenotypic divergence, but few studies have determined the action of disruptive selection on phenotypic divergence via field studies. This study investigated the effect of disruptive selection on shell colour polymorphism in the Japanese land snail Euhadra peliomphala simodae to determine whether extreme phenotypes of snail shell colour are favoured over intermediate phenotypes. We conducted field surveys on an oceanic island with black, yellow and intermediate-coloured E. p. simodae snails. We captured and marked ~1800 individual snails and monitored their survival over 18 months. We quantified shell colours against images and examined the frequency distribution of shell colour variation. The variation exhibited a bimodal distribution with a far lower frequency of intermediate-coloured snails than of black or yellow snails. The population sizes of the three snail groups fluctuated synchronously with the changing seasons. Bayesian estimates showed lower survival rates for juvenile intermediate-coloured snails than for juvenile black and yellow snails, implying there was disruptive selection associated with shell colour. We suggest this disruptive selection may have resulted in the evolutionary divergence of the snail’s shell colour within the lineage having high shell colour variation.

Bi-parental mucus provisioning in the scale-eating cichlid Perissodus microlepis (Cichlidae)

Although parental care is known to occur in a wide range of teleost fishes, postnatal provisioning of nutrition has been documented rarely. Here, we describe a novel example of bi-parental care in a teleost, i.e. mucus-provisioning behaviour in the scale-eating cichlid Perissodus microlepis endemic to Lake Tanganyika. Field observations revealed that young guarded by their parents frequently glanced towards the body surface of both parents. Furthermore, analyses of stomach contents of the young found the presence of ingested mucus, confirming that the young feed on the mucus secretions of their parents. The frequency of glancing behaviour increased with size of the young up to ~13 mm in standard length, but then declined with further growth. Additionally, the frequency of glancing of young towards their parents was higher when the frequency of foraging on plankton was lower. Underwater cage experiments revealed a higher rate of growth in the young kept in direct contact with their parents than in those not allowed direct contact. We conclude that glancing behaviour in young P. microlepis is a form of direct parental nourishment that confers growth benefits to the young when food abundance is low.

Intergenerational fluctuations in colour morph frequencies may maintain elytral polymorphisms in the ladybird beetle Cheilomenes sexmaculata (Coleoptera: Coccinellidae)

Phenotypic polymorphisms are found in a wide array of taxa, and unravelling the mechanisms that maintain them is of great interest to evolutionary and ecological biologists. Temporal environmental heterogeneity may play a role in the maintenance of polymorphisms but is poorly understood. In the present study, we analysed trends in intergenerational elytral colour morph frequencies in relation to changes in fitness and life history traits (i.e. body size, mortality, fecundity, hatching rate and mate preference) in the ladybird beetle Cheilomenes sexmaculata (Coleoptera: Coccinellidae). A long-term field survey spanning nine years showed that the frequency of dark morphs increases over winter and then decreases in spring. Dark morphs may have an advantage in winter due to their higher tolerance of low temperatures compared with light morphs. Light-morph females were heavier in winter than dark-morph females. They also mated more frequently and had higher hatching rates, potentially causing an increase in light morphs in spring. These results suggest that fluctuations in morph frequencies resulting from the conflicting directions of selection pressures between overwintering and spring generations may help to maintain genetic polymorphism.

A way forward with eco evo devo: an extended theory of resource polymorphism with postglacial fishes as model systems

A major goal of evolutionary science is to understand how biological diversity is generated and altered. Despite considerable advances, we still have limited insight into how phenotypic variation arises and is sorted by natural selection. Here we argue that an integrated view, which merges ecology, evolution and developmental biology (eco evo devo) on an equal footing, is needed to understand the multifaceted role of the environment in simultaneously determining the development of the phenotype and the nature of the selective environment, and how organisms in turn affect the environment through eco evo and eco devo feedbacks. To illustrate the usefulness of an integrated eco evo devo perspective, we connect it with the theory of resource polymorphism (i.e. the phenotypic and genetic diversification that occurs in response to variation in available resources). In so doing, we highlight fishes from recently glaciated freshwater systems as exceptionally well-suited model systems for testing predictions of an eco evo devo framework in studies of diversification. Studies on these fishes show that intraspecific diversity can evolve rapidly, and that this process is jointly facilitated by (i) the availability of diverse environments promoting divergent natural selection; (ii) dynamic developmental processes sensitive to environmental and genetic signals; and (iii) eco evo and eco devo feedbacks influencing the selective and developmental environments of the phenotype. We highlight empirical examples and present a conceptual model for the generation of resource polymorphism - emphasizing eco evo devo, and identify current gaps in knowledge.

Playing out Liem's Paradox: Opportunistic Piscivory across Lake Tanganyikan Cichlids

Trophic specialization is a key feature of the diversity of cichlid fish adaptive radiations. However, K. F. Liem observed that even species with highly specialized trophic morphologies have dietary flexibility, enabling them to exploit episodic food resources opportunistically. Evidence for dietary flexibility comes largely from laboratory studies, and it is unclear whether cichlid fishes undergo diet shifts in the wild. We report observations of diet switching by multiple cichlid species in Lake Tanganyika as a consequence of unusual concentrations of schooling juvenile clupeid fishes. Fish species with varying degrees of trophic specialization converged on a single prey: juvenile sardines that are also endemic to Lake Tanganyika (Stolothrissa tanganicae and Limnothrissa miodon). We provide evidence for cichlid species acting as jacks-of-all-trades and discuss this evidence in the framework of Liem's classic paradox: that trophic specialization does not preclude dietary flexibility.

Male-specific asymmetric curvature of anal fin in a viviparous teleost, Xenotoca eiseni

Xenotoca eiseni is a viviparous teleost from the family Goodeidae. Internal fertilization occurs in this species; therefore, the male fish must transfer their sperm to the females. To this end, the males use their anal fins as external genitalia during mating. However, in goodeid species, there are a few reports of anal fin modifications functioning as genitalia. In the present study, I describe a male-specific lateral curvature with fin robe thickening on the two most posterior rays of the anal fin of X. eiseni. Morphological and behavioral analyses suggested that the lateral curvature provides a directional preference for coupling of male X. eiseni. The modification appears to be a male secondary sex characteristic. Other goodeid species Xenotoca melanosoma and Chapalichthys pardalis also possess the lateral curvature, but without robe thickening. Furthermore, Zoogoneticus quitzeoensis exhibited only curvature structure either smaller than those of other species or absent. Therefore, the anal fin curvature may not necessarily be used for internal fertilization in all species investigated in this study. However, it is still possible that it provides novel advantages during mating in certain goodeid species.

Asymmetry in genitalia is in sync with lateralized mating behavior but not with the lateralization of other behaviors

Asymmetries in bilateral organisms attract a lot of curiosity given that they are conspicuous departures from the norm. They allow the investigation of the integration at different levels of biological organization. Here we study whether and how behavioral and asymmetrical anatomical traits co-evolved and work together. We ask if asymmetry is determined locally for each trait or at a whole individual level in a species bearing conspicuous asymmetrical genitalia. Asymmetric genitalia evolved in many species; however, in most cases the direction of asymmetry is fixed. Therefore, it has been rarely determined if there is an association between the direction of asymmetry in genitalia and other traits. In onesided livebearer fish of the genus Jenynsia (Cyprinodontiformes, Anablepidae), the anal fin of males is modified into a gonopodium, an intromittent organ that serves to inseminate females. The gonopodium shows a conspicuous asymmetry, with its tip bending either to the left or the right. By surveying 13 natural populations of Jenynsia lineata, we found that both genital morphs are equally common in wild populations. In a series of experiments in a laboratory population, we discovered asymmetry and lateralization for multiple other traits; yet, the degree of integration varied highly among them. Lateralization in exploratory behavior in response to different stimuli was not associated with genital morphology. Interestingly, the direction of genital asymmetry was positively correlated with sidedness of mating preference and the number of neuromasts in the lateral line. This suggests integration of functionally linked asymmetric traits; however, there is no evidence that asymmetry is determined at the whole individual level in our study species.