Visual Background Choice and Light Environment Affect Male Guppy Visual Contrast

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

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

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

Revisiting old truths: The evolution of male coloration in guppies as a function of predation

The Trinidadian guppy, Poecilia reticulata, is a long-studied model for the evolution of trade-offs in male nuptial coloration as a function of female mating preferences versus predation risk. Previous work suggests that female mating preferences favor the evolution of increased conspicuous male coloration in low predation populations. In contrast, high predation risk shifts the balance towards reduced conspicuous coloration. In a From the Cover article in this issue of Molecular Ecology, Yong, Croft, Troscianko, Ramnarine, and Wilson (2021) use visual detection models to estimate the 'conspicuousness' of male color patterns as seen by guppies and their predators. The study fails to find robust patterns of increased conspicuousness in low predation populations. Only one of eight measures of conspicuousness showed parallel changes between high and low predation regimes, forcing us to re-consider the validity and repeatability of this classic example of parallel evolution.

The visual ecology of selective predation: Are unhealthy hosts less stealthy hosts?

Predators can strongly influence disease transmission and evolution, particularly when they prey selectively on infected hosts. Although selective predation has been observed in numerous systems, why predators select infected prey remains poorly understood. Here, we use a mathematical model of predator vision to test a long-standing hypothesis about the mechanistic basis of selective predation in a Daphnia-microparasite system, which serves as a model for the ecology and evolution of infectious diseases. Bluegill sunfish feed selectively on Daphnia infected by a variety of parasites, particularly in water uncolored by dissolved organic carbon. The leading hypothesis for selective predation in this system is that infection-induced changes in the transparency of Daphnia render them more visible to bluegill. Rigorously evaluating this hypothesis requires that we quantify the effect of infection on the visibility of prey from the predator's perspective, rather than our own. Using a model of the bluegill visual system, we show that three common parasites, Metschnikowia bicuspidata, Pasteuria ramosa, and Spirobacillus cienkowskii, decrease the transparency of Daphnia, rendering infected Daphnia darker against a background of bright downwelling light. As a result of this increased brightness contrast, bluegill can see infected Daphnia at greater distances than uninfected Daphnia-between 19% and 33% further, depending on the parasite. Pasteuria and Spirobacillus also increase the chromatic contrast of Daphnia. These findings lend support to the hypothesis that selective predation by fish on infected Daphnia could result from the effects of infection on Daphnia's visibility. However, contrary to expectations, the visibility of Daphnia was not strongly impacted by water color in our model. Our work demonstrates that models of animal visual systems can be useful in understanding ecological interactions that impact disease transmission.

Sequence Analysis and Ontogenetic Expression Patterns of Cone Opsin Genes in the Bluefin Killifish (Lucania goodei)

Sensory systems allow for the transfer of environmental stimuli into internal cues that can alter physiology and behavior. Many studies of visual systems focus on opsins to compare spectral sensitivity among individuals, populations, and species living in different lighting environments. This requires an understanding of the cone opsins, which can be numerous. The bluefin killifish is a good model for studying the interaction between environments and visual systems as they are found in both clear springs and tannin-stained swamps. We conducted a genome-wide screening and demonstrated that the bluefin killifish has 9 cone opsins: 1 SWS1 (354 nm), 2 SWS2 (SWS2B: 359 nm, SWS2A: 448 nm), 2 RH2 (RH2-2: 476 nm, RH2-1: 537 nm), and 4 LWS (LWS-1: 569 nm, LWS-2: 524 nm, LWS-3: 569 nm, LWS-R: 560 or 569 nm). These 9 cone opsins were located on 4 scaffolds. One scaffold contained the 2 SWS2 and 3 of the 4 LWS opsins in the same syntenic order as found in other cyprinodontoid fishes. We also compared opsin expression in larval and adult killifish under clear water conditions, which mimic springs. Two of the newly discovered opsins (LWS-2 and LWS-3) were expressed at low levels (<0.2%). Whether these opsins make meaningful contributions to visual perception in other contexts (i.e., swamp conditions) is unclear. In contrast, there was an ontogenetic change from using LWS-R to LWS-1 opsin. Bluefin killifish adults may be slightly more sensitive to longer wavelengths, which might be related to sexual selection and/or foraging preferences.

A potential role for overdominance in the maintenance of colour variation in the Neotropical tortoise beetle, Chelymorpha alternans

The presence of persistent polymorphisms within natural populations elicits the question of how such polymorphisms are maintained. All else equal, genetic drift and natural selection should remove genetic variants from populations. Disassortative mating and overdominance are potential mechanisms for maintaining variation within populations. Here, we consider the potential role of these mechanisms in maintaining variation in colour pattern in the tortoise beetle, Chelymorpha alternans. Five colour morphs distinguished by elytral and pronotal coloration are largely determined by a single locus of large effect with four segregating alleles. As many as four morphs co-occur in natural populations. We first assessed whether disassortative mating might maintain this polymorphism. To test for assortative and disassortative mating, we paired females with two males, one with the same colour pattern as the female and one with a different colour pattern and examined the colour patterns of the offspring. We found strong evidence for random mating as a function of colour pattern. We next assessed whether differences in offspring survival among assortative and disassortative male-female pairs maintain colour variation. Crosses involving disassortative pairings had significantly higher offspring survival during development and resulted in more adult progeny. This result is consistent with the effects of overdominance, whereby outcrossed individuals have higher fitness than their homozygous counterparts. Overall, differences in offspring survival appear to play a greater role in maintaining polymorphisms than nonrandom mating in species.

Asymmetric reinforcement in Lucania killifish: assessing reproductive isolation when both sexes choose

Reinforcement can occur when maladaptive hybridization in sympatry favors the evolution of conspecific preferences and target traits that promote behavioral isolation (BI). In many systems, enhanced BI is due to increased female preference for conspecifics. In others, BI is driven by male preference, and in other systems both sexes exert preferences. Some of these patterns can be attributed to classic sex-specific costs and benefits of preference. Alternatively, sex differences in conspecific preference can emerge due to asymmetric postzygotic isolation (e.g., hybrid offspring from female A × male B have lower fitness than hybrid offspring from female B × male A), which can lead to asymmetric BI (e.g., female A and male B are less likely to mate than female B and male A). Understanding reinforcement requires understanding how conspecific preferences evolve in sympatry. Yet, estimating conspecific preferences can be difficult when both sexes are choosy. In this study, we use Lucania killifish to test the hypothesis that patterns of reinforcement are driven by asymmetric postzygotic isolation between species. If true, we predicted that sympatric female Lucania goodei and sympatric male L. parva should have lower levels of BI compared with their sympatric counterparts, as they produce hybrid offspring with the highest fitness. To address the problem of measuring BI when both sexes are choosy, we inferred the contribution to BI of each partner using assays where one sex in the mating pair comes from an allopatric population with potentially low preference, whereas the other comes from a sympatric population with high preference. For one hybrid cross direction, we found that both female L. parva and male L. goodei have high contributions to BI in sympatry. In the other hybrid cross direction, we found that only female L. goodei contribute to BI. Sympatric male L. parva readily engaged in hybrid spawnings with allopatric L. goodei females. These results indicate that both asymmetric postzygotic isolation and the traditional sex-specific costs to preference likely affect the nature of selection on conspecific preferences and target traits.

Genomic landscape of reproductive isolation in Lucania killifish: The role of sex loci and salinity

Adaptation to different environments can directly and indirectly generate reproductive isolation between species. Bluefin killifish (Lucania goodei) and rainwater killifish (L. parva) are sister species that have diverged across a salinity gradient and are reproductively isolated by habitat, behavioural, extrinsic and intrinsic post-zygotic isolation. We asked if salinity adaptation contributes indirectly to other forms of reproductive isolation via linked selection and hypothesized that low recombination regions, such as sex chromosomes or chromosomal rearrangements, might facilitate this process. We conducted QTL mapping in backcrosses between L. parva and L. goodei to explore the genetic architecture of salinity tolerance, behavioural isolation and intrinsic isolation. We mapped traits relative to a chromosome that has undergone a centric fusion in L. parva (relative to L. goodei). We found that the sex locus appears to be male determining (XX-XY), was located on the fused chromosome and was implicated in intrinsic isolation. QTL associated with salinity tolerance were spread across the genome and did not overly co-localize with regions associated with behavioural or intrinsic isolation. This preliminary analysis of the genetic architecture of reproductive isolation between Lucania species does not support the hypothesis that divergent natural selection for salinity tolerance led to behavioural and intrinsic isolation as a by-product. Combined with previous studies in this system, our work suggests that adaptation as a function of salinity contributes to habitat isolation and that reinforcement may have contributed to the evolution of behavioural isolation instead, possibly facilitated by linkage between behavioural isolation and intrinsic isolation loci on the fused chromosome.

Genomic Resources for Darters (Percidae: Etheostominae) Provide Insight into Postzygotic Barriers Implicated in Speciation

Comparative genomic approaches are increasingly being used to study the evolution of reproductive barriers in nonmodel species. Although numerous studies have examined prezygotic isolation in darters (Percidae), investigations into postzygotic barriers have remained rare due to long generation times and a lack of genomic resources. Orangethroat and rainbow darters naturally hybridize and provide a remarkable example of male-driven speciation via character displacement. Backcross hybrids suffer from high mortality, which appears to promote behavioral isolation in sympatry. To investigate the genomic architecture of postzygotic isolation, we used Illumina and PacBio sequencing to generate a chromosome-level, annotated assembly of the orangethroat darter genome and high-density linkage maps for orangethroat and rainbow darters. We also analyzed genome-wide RADseq data from wild-caught adults of both species and laboratory-generated backcrosses to identify genomic regions associated with hybrid incompatibles. Several putative chromosomal translocations and inversions were observed between orangethroat and rainbow darters, suggesting structural rearrangements may underlie postzygotic isolation. We also found evidence of selection against recombinant haplotypes and transmission ratio distortion in backcross hybrid genomes, providing further insight into the genomic architecture of genetic incompatibilities. Notably, regions with high levels of genetic divergence between species were enriched for genes associated with developmental and meiotic processes, providing strong candidates for postzygotic isolating barriers. These findings mark significant contributions to our understanding of the genetic basis of reproductive isolation between species undergoing character displacement. Furthermore, the genomic resources presented here will be instrumental for studying speciation in darters, the most diverse vertebrate group in North America.

Testing the potential mechanisms for the maintenance of a genetic color polymorphism in bluefin killifish populations

The maintenance of genetic variation in the face of natural selection is a long-standing question in evolutionary biology. In the bluefin killifish Lucania goodei, male coloration is polymorphic. Males can produce either red or yellow coloration in their anal fins, and both color morphs are present in all springs. These 2 morphs are heritable and how they are maintained in nature is unknown. Here, we tested 2 mechanisms for the maintenance of the red/yellow color morphs. Negative frequency-dependent mating success predicts that rare males have a mating advantage over common males. Spatial variation in fitness predicts that different color morphs have an advantage in different microhabitat types. Using a breeding experiment, we tested these hypotheses by creating populations with different ratios of red to yellow males (5 red:1 yellow; 1 red:5 yellow) and determining male mating success on shallow and deep spawning substrates. We found no evidence of negative frequency-dependent mating success. Common morphs tended to have higher mating success, and this was particularly so on shallow spawning substrates. However, on deep substrates, red males enjoyed higher mating success than yellow males, particularly so when red males were rare. However, yellow males did not have an advantage at either depth nor when rare. We suggest that preference for red males is expressed in deeper water, possibly due to alterations in the lighting environment. Finally, male pigment levels were correlated with one another and predicted male mating success. Hence, pigmentation plays an important role in male mating success.

Hybridization and postzygotic isolation promote reinforcement of male mating preferences in a diverse group of fishes with traditional sex roles

Behavioral isolation is thought to arise early in speciation due to differential sexual and/or natural selection favoring different preferences and traits in different lineages. Instead, behavioral isolation can arise due to reinforcement favoring traits and preferences that prevent maladaptive hybridization. In darters, female preference for male coloration has been hypothesized to drive speciation, because behavioral isolation evolves before F1 inviability. However, as with many long-lived organisms, the fitness of second-generation hybrids has not been assessed because raising animals to adulthood in the laboratory is challenging. Of late, reinforcement of male preferences has been implicated in darters because male preference for conspecific females is high in sympatry but absent in allopatry in multiple species pairs. The hypothesis that reinforcement accounts for behavioral isolation in sympatry assumes that hybridization and postzygotic isolation are present. Here, we used genomic and morphological data to demonstrate that hybridization is ongoing between orangethroat and rainbow darters and used hybrids collected from nature to measure postzygotic barriers across two hybrid generations. We observed sex ratio distortion in adult F1s and a dramatic reduction in backcross survival. Our findings indicate that selection to avoid hybridization promotes the evolution of male-driven behavioral isolation via reinforcement in this system.

Variation in individual temperature preferences, not behavioural fever, affects susceptibility to chytridiomycosis in amphibians

The ability of wildlife populations to mount rapid responses to novel pathogens will be critical for mitigating the impacts of disease outbreaks in a changing climate. Field studies have documented that amphibians preferring warmer temperatures are less likely to be infected with the fungal pathogen Batrachochytrium dendrobatidis (Bd). However, it is unclear whether this phenomenon is driven by behavioural fever or natural variation in thermal preference. Here, we placed frogs in thermal gradients, tested for temperature preferences and measured Bd growth, prevalence, and the survival of infected animals. Although there was significant individual- and species-level variation in temperature preferences, we found no consistent evidence of behavioural fever across five frog species. Interestingly, for species that preferred warmer temperatures, the preferred temperatures of individuals were negatively correlated with Bd growth on hosts, while the opposite correlation was true for species preferring cooler temperatures. Our results suggest that variation in thermal preference, but not behavioural fever, might shape the outcomes of Bd infections for individuals and populations, potentially resulting in selection for individual hosts and host species whose temperature preferences minimize Bd growth and enhance host survival during epidemics.

The vertical distributions and spawning site choices of red and yellow bluefin killifish Lucania goodei colour morphs

A genetic colour polymorphism is present in bluefin killifish Lucania goodei, where red and yellow anal-fin morphs coexist in clear springs, but the source of balancing selection is unknown. In a field study, vertical distributions did not differ between the morphs and there was little evidence that light environments differed qualitatively over the 200 cm at which fish were collected. A greenhouse study showed that both morphs preferred to spawn at shallow depths and hence vertical distribution and spawning site choice are unlikely to explain the maintenance of the colour polymorphism.

Agonistic character displacement of genetically based male colour patterns across darters

Agonistic character displacement (ACD) occurs when selection to avoid maladaptive interspecific aggression leads to the evolution of agonistic signals and/or associated behavioural biases in sympatry. Here, we test for a pattern consistent with ACD in male colour pattern in darters (Percidae: Etheostoma). Male colour pattern has been shown to function in male-male competition rather than female mating preferences in several darter species. Additionally, males bias their aggression towards conspecific over heterospecific males in sympatry but not in allopatry, consistent with divergent ACD in male behavioural biases. We use a common garden approach to show that differences in male colour pattern among four closely related darter species are genetically based. Additionally, we demonstrate that some aspects of male colour pattern exhibit enhanced differences in sympatric compared to allopatric populations of two darter species, consistent with ACD. However, other male colour traits are more similar between species in sympatry compared with allopatry, indicating that not all signal components are under strong divergent selection in sympatry. This study provides evidence that interspecific male-male aggressive interactions alone can promote elaborate male signal evolution both between and within species. We discuss the implications this has for male-driven ACD and cascade ACD.

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

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

Seeing red: color vision in the largemouth bass

How animals visually perceive the environment is key to understanding important ecological behaviors, such as predation, foraging, and mating. This study focuses on the visual system properties and visual perception of color in the largemouth bass Micropterus salmoides. This study (1) documents the number and spectral sensitivity of photoreceptors, (2) uses these parameters to model visual perception, and (3) tests the model of color perception using a behavioral assay. Bass possess single cone cells maximally sensitive at 535 nm, twin cone cells maximally sensitive at 614 nm, and rod cells maximally sensitive at 528 nm. A simple model of visual perception predicted that bass should not be able to discern between chartreuse yellow and white nor between green and blue. In contrast, bass should be able to discern red from all achromatic (i.e., gray scale) stimuli. These predictions were partially upheld in behavioral trials. In behavioral trials, bass were first trained to recognize a target color to receive a food reward, and then tested on their ability to differentiate between their target color and a color similar in brightness. Bass trained to red and green could easily discern their training color from all other colors for target colors that were similar in brightness (white and black, respectively). This study shows that bass possess dichromatic vision and do use chromatic (i.e., color) cues in making visual-based decisions.

Egg viability decreases rapidly with time since ovulation in the rainbow darter Etheostoma caeruleum: implications for the costs of choosiness

Egg viability in the rainbow darter Etheostoma caeruleum, a fish apparently lacking female mate choice, was found to decline rapidly after ovulation. It was observed that the majority of a female's clutch may fail to hatch if she is prevented from mating for as little as 6 h. These data suggest that exercising female mate preferences may be selectively disfavoured in E. caeruleum due to the high cost of delaying mating.

Male-driven reproductive and agonistic character displacement in darters and its implications for speciation in allopatry

Selection against hybridization can cause mating traits to diverge between species in sympatry via reproductive character displacement (RCD). Additionally, selection against interspecific fighting can cause aggressive traits to diverge between sympatric species via agonistic character displacement (ACD). By directly affecting conspecific recognition traits, RCD and ACD between species can also incidentally cause divergence in mating and fighting traits among populations within a species [termed cascade RCD (CRCD) and cascade ACD]. Here, we demonstrate patterns consistent with male-driven RCD and ACD in 2 groups of darters (orangethroat darter clade Ceasia and rainbow darter Etheostoma caeruleum). In both groups, males that occur in sympatry (between Ceasia and E. caeruleum) have higher levels of preference for mating and fighting with conspecifics over heterospecifics than do males from allopatry. This is consistent with RCD and ACD. We also found patterns consistent with CRCD and cascade ACD among species of Ceasia. Ceasia males that are sympatric to E. caeruleum (but allopatric to one another) also have heightened preferences for mating and fighting with conspecific versus heterospecific Ceasia. In contrast, Ceasia males that are allopatric to E. caeruleum readily mate and fight with heterospecific Ceasia. We suggest that RCD and ACD between Ceasia and E. caeruleum has incidentally led to divergence in mating and fighting traits among Ceasia species. This study is unique in that male preferences evolve via both RCD (male preference for conspecific females) and ACD (male preference to fight conspecific males) which leads to subsequent divergence among allopatric lineages.

Male and female contributions to behavioral isolation in darters as a function of genetic distance and color distance

Determining which reproductive isolating barriers arise first between geographically isolated lineages is critical to understanding allopatric speciation. We examined behavioral isolation among four recently diverged allopatric species in the orangethroat darter clade (Etheostoma: Ceasia). We also examined behavioral isolation between each Ceasia species and the sympatric rainbow darter Etheostoma caeruleum. We asked (1) is behavioral isolation present between allopatric Ceasia species, and how does this compare to behavioral isolation with E. caeruleum, (2) does male color distance and/or genetic distance predict behavioral isolation between species, and (3) what are the relative contributions of female choice, male choice, and male competition to behavioral isolation? We found that behavioral isolation, genetic differentiation, and male color pattern differentiation were present between allopatric Ceasia species. Males, but not females, discerned between conspecific and heterospecific mates. Males also directed more aggression toward conspecific rival males. The high levels of behavioral isolation among Ceasia species showed no obvious pattern with genetic distance or male color distance. However, when the E. caeruleum was included in the analysis, an association between male aggression and male color distance was apparent. We discuss the possibility that reinforcement between Ceasia and E. caeruleum is driving behavioral isolation among allopatric Ceasia species.

Intrasexual competition underlies sexual selection on male breeding coloration in the orangethroat darter, Etheostoma spectabile

Elaborate, sexually dimorphic traits are widely thought to evolve under sexual selection through female preference, male–male competition, or both. The orangethroat darter (Etheostoma spectabile) is a sexually dichromatic fish in which females exhibit no preferences for male size or coloration. We tested whether these traits affect individual reproductive success in E. spectabile when multiple males are allowed to freely compete for a female. The quality and quantity of male coloration were associated with greater success in maintaining access to the female and in spawning as the primary male (first male to participate). On the other hand, sneaking behavior showed little correlation with coloration. Male breeding coloration in E. spectabile may therefore demonstrate how intrasexual competition can be a predominant factor underlying the evolution of male ornaments.

Behavioral Isolation due to Cascade Reinforcement in Lucania Killifish

Reinforcement occurs when behavioral isolation is strengthened between species due to selection against hybridization in sympatry. Mate preferences and their target traits may change in sympatry as a consequence of reinforcement. This can potentially generate further behavioral isolation within species if sympatric populations evolve extreme preferences or traits that cause them to reject individuals from foreign populations as mates or be rejected as mates. This process is known as cascade reinforcement. We measured behavioral isolation between sympatric and allopatric populations of Lucania killifish to determine whether isolation evolves due to reinforcement between species and whether reinforcement affects preferences within species, consistent with the cascade reinforcement hypothesis. We measured mate preferences in both sexes between species (Lucania parva vs. Lucania goodei) and within species (among populations of L. parva). Between species, both male and female preferences for conspecifics were highest in sympatric populations. Within species, L. parva females from sympatric populations preferred their own native males over foreign males. Allopatric L. parva females and all L. parva males showed no preferences within species. Within species, behavioral isolation showed no association with ecological variables, such as salinity. Thus, reinforcement is a primary factor generating behavioral isolation in Lucania killifish, creating strong preferences in both sexes among species and leading to cascade reinforcement of female mate preference within species.

Influence of sex and habitat on the size and shape of anal and dorsal fins of the blackstripe topminnow Fundulus notatus

The size and shape of the anal and dorsal fin in the blackstripe topminnow Fundulus notatus from lake and stream habitats across multiple ages and sexes were examined. Differences in the size and shape of anal and dorsal fins were sex-specific and not related to habitat differences. Males have longer and more pointed anal fins and longer, larger and more pointed dorsal fins than females. These sex differences occur predominantly in the older age class. The angle (i.e. pointedness) of the dorsal and anal fins is tightly correlated suggesting that fins follow a similar growth trajectory as individuals become sexually mature.

Fish and robot dancing together: bluefin killifish females respond differently to the courtship of a robot with varying color morphs

The experimental integration of bioinspired robots in groups of social animals has become a valuable tool to understand the basis of social behavior and uncover the fundamental determinants of animal communication. In this study, we measured the preference of fertile female bluefin killifish (Lucania goodei) for robotic replicas whose aspect ratio, body size, motion pattern, and color morph were inspired by adult male killifish. The motion of the fish replica was controlled via a robotic platform, which simulated the typical courtship behavior observed in killifish males. The positional preferences of females were measured for three different color morphs (red, yellow, and blue). While variation in preference was high among females, females tend to spend more time in the vicinity of the yellow painted robot replicas. This preference may have emerged because the yellow robot replicas were very bright, particularly in the longer wavelengths (550–700 nm) compared to the red and blue replicas. These findings are in agreement with previous observations in mosquitofish and zebrafish on fish preference for artificially enhanced yellow pigmentation.

Insight into genomic changes accompanying divergence: genetic linkage maps and synteny of Lucania goodei and L. parva reveal a Robertsonian fusion

Linkage maps are important tools in evolutionary genetics and in studies of speciation. We performed a karyotyping study and constructed high-density linkage maps for two closely related killifish species, Lucania parva and L. goodei, that differ in salinity tolerance and still hybridize in their contact zone in Florida. Using SNPs from orthologous EST contigs, we compared synteny between the two species to determine how genomic architecture has shifted with divergence. Karyotyping revealed that L. goodei possesses 24 acrocentric chromosomes (1N) whereas L. parva possesses 23 chromosomes (1N), one of which is a large metacentric chromosome. Likewise, high-density single-nucleotide polymorphism-based linkage maps indicated 24 linkage groups for L. goodei and 23 linkage groups for L. parva. Synteny mapping revealed two linkage groups in L. goodei that were highly syntenic with the largest linkage group in L. parva. Together, this evidence points to the largest linkage group in L. parva being the result of a chromosomal fusion. We further compared synteny between Lucania with the genome of a more distant teleost relative medaka (Oryzias latipes) and found good conservation of synteny at the chromosomal level. Each Lucania LG had a single best match with each medaka chromosome. These results provide the groundwork for future studies on the genetic architecture of reproductive isolation and salinity tolerance in Lucania and other Fundulidae.

Reproductive isolation between two darter species is enhanced and asymmetric in sympatry

Robust reproductive isolation was found between the rainbow darter Etheostoma caeruleum and the orangethroat darter Etheostoma spectabile, as more offspring were produced when conspecific males and females were crossed as compared with heterospecific crosses. Furthermore, fewer eggs resulted from heterospecific crosses involving sympatric E. spectabile females than those using allopatric E. spectabile females, while a similar pattern was not observed in heterospecific crosses using E. caeruleum females. These results suggest that reinforcement, i.e. selection for pre-zygotic reproductive barriers driven by reduced hybrid fitness, may have contributed to the evolution and maintenance of reproductive barriers between these potentially hybridizing species in sympatry.

Functional and population genomic divergence within and between two species of killifish adapted to different osmotic niches

Adaptation to salinity affects species distributions, promotes speciation, and guides many evolutionary patterns in fishes. To uncover the basis of a complex trait like osmoregulation, genome-level analyses are sensible. We combine population genomic scans with genome expression profiling to discover candidate genes and pathways associated with divergence between osmotic environments. We compared transcriptome sequence divergence between multiple freshwater and saltwater populations of the rainwater killifish, Lucania parva. We also compared sequence divergence between L. parva and its sister species, Lucania goodei, a freshwater specialist. We found highly differentiated single nucleotide polymorphisms (SNPs) between freshwater and saltwater L. parva populations in cell junction and ion transport genes, including V-type H(+) ATPase. Between species, we found divergence in reproduction and osmotic stress genes. Genes that were differentially expressed between species during osmotic acclimation included genes involved in ion transport and cell volume regulation. Gene sets that were divergent in coding sequence and divergent in expression did not overlap, although they did converge in function. Like many studies using genomic scans, our approach may miss some loci that contribute to adaptation but have complicated patterns of allelic variation. Our study suggests that gene expression and coding sequence may evolve independently as populations adapt to a complex physiological challenge.

Differences in offspring size predict the direction of isolation asymmetry between populations of a placental fish

Crosses between populations or species often display an asymmetry in the fitness of reciprocal F1 hybrids. This pattern, referred to as isolation asymmetry or Darwin's Corollary to Haldane's Rule, has been observed in taxa from plants to vertebrates, yet we still know little about which factors determine its magnitude and direction. Here, we show that differences in offspring size predict the direction of isolation asymmetry observed in crosses between populations of a placental fish, Heterandria formosa. In crosses between populations with differences in offspring size, high rates of hybrid inviability occur only when the mother is from a population characterized by small offspring. Crosses between populations that display similarly sized offspring, whether large or small, do not result in high levels of hybrid inviability in either direction. We suggest this asymmetric pattern of reproductive isolation is due to a disruption of parent-offspring coadaptation that emerges from selection for differently sized offspring in different populations.

Diurnal lighting patterns and habitat alter opsin expression and colour preferences in a killifish

Spatial variation in lighting environments frequently leads to population variation in colour patterns, colour preferences and visual systems. Yet lighting conditions also vary diurnally, and many aspects of visual systems and behaviour vary over this time scale. Here, we use the bluefin killifish (Lucania goodei) to compare how diurnal variation and habitat variation (clear versus tannin-stained water) affect opsin expression and the preference to peck at different-coloured objects. Opsin expression was generally lowest at midnight and dawn, and highest at midday and dusk, and this diurnal variation was many times greater than variation between habitats. Pecking preference was affected by both diurnal and habitat variation but did not correlate with opsin expression. Rather, pecking preference matched lighting conditions, with higher preferences for blue at noon and for red at dawn/dusk, when these wavelengths are comparatively scarce. Similarly, blue pecking preference was higher in tannin-stained water where blue wavelengths are reduced. In conclusion, L. goodei exhibits strong diurnal cycles of opsin expression, but these are not tightly correlated with light intensity or colour. Temporally variable pecking preferences probably result from lighting environment rather than from opsin production. These results may have implications for the colour pattern diversity observed in these fish.

A test for environmental effects on behavioral isolation in two species of killifish

Behavioral isolation is a common and potent mechanism of reproductive isolation. Determining the extent to which behavioral isolation varies with environmental conditions is critical to understanding speciation and the maintenance of species boundaries. Here, we tested the effect of salinity on behavioral isolation (female species recognition, male-male competition, and male species recognition) between two closely related killifish (Lucania goodei and L. parva) that differ in salinity tolerance. We performed no-choice assays and behavioral trials where males could compete and court females in fresh water (0 ppt) and brackish water (15 ppt). We found high levels of behavioral isolation that did not vary as a function of salinity. In behavioral trials, male species recognition of females was strong and asymmetric between the two species. Lucania goodei males preferred conspecifics and rarely courted or mated with L. parva females. Lucania parva males preferred conspecifics but readily courted and mated with L. goodei females. This asymmetry matches previously documented asymmetries in hybrid offspring fitness. Crosses between L. parva males and L. goodei females produce fully viable/fertile hybrids, but crosses between L. goodei males and L. parva females produce males with reduced fertility. Hence, behavioral isolation may have evolved in part due to reinforcement.

Rapid light-induced shifts in opsin expression: finding new opsins, discerning mechanisms of change, and implications for visual sensitivity

Light-induced shifts in cone frequency and opsin expression occur in many aquatic species. Yet little is known about how quickly animals can alter opsin expression and, thereby, track their visual environments. Similarly, little is known about whether adult animals can alter opsin expression or whether shifts in opsin expression are limited to critical developmental windows. We took adult wild-caught bluefin killifish (Lucania goodei) from three different lighting environments (spring, swamp and variable), placed them under two different lighting treatments (clear vs. tea-stained water) and monitored opsin expression over 4 weeks. We measured opsin expression for five previously described opsins (SWS1, SWS2B, SWS2A, RH2-1 and LWS) as well as RH2-2 which we discovered via 454 sequencing. We used two different metrics of opsin expression. We measured expression of each opsin relative to a housekeeping gene and the proportional expression of each opsin relative to the total pool of opsins. Population and lighting environment had large effects on opsin expression which were present at the earliest time points indicating rapid shifts in expression. The two measures of expression produced radically different patterns. Proportional measures indicated large effects of light on SWS1 expression, whereas relative measures indicated no such effect. Instead, light had large effects on the relative expression of SWS2B, RH2-2, RH2-1 and LWS. We suggest that proportional measures of opsin expression are best for making inferences about colour vision, but that measures relative to a housekeeping gene are better for making conclusions about which opsins are differentially regulated.

The effects of water depth and light on oviposition and egg cannibalism in the bluefin killifish Lucania goodei

This study showed that sex and depth had strong effects on egg cannibalism, whereas water clarity (clear v. tea-stained) had no effect on cannibalism or oviposition in the bluefin killifish Lucania goodei. These results are consistent with the extreme levels of iteroparity in L. goodei where females appear to spread their eggs across multiple locations and depths presumably to avoid egg predation.

Teasing apart the many effects of lighting environment on opsin expression and foraging preference in bluefin killifish

Coloration and color vision covary with lighting in many taxa. Determining the mechanisms underlying these patterns is difficult because lighting environments can have multiple effects on signaling that occur at multiple timescales. Lighting environments can (1) immediately affect signal propagation and transmission, which determine the radiance spectrum reaching the receiver; (2) induce variation in visual systems via developmental plasticity; and (3) lead to genetic differences in visual systems due to a history of selection in different habitats. We tease apart these effects on pecking preference and examine the relationship between pecking preference and opsin expression. Using killifish from two visually distinct populations (clear vs. tea-stained water), we performed crosses (genetics), raised animals under different lighting conditions (developmental plasticity), and assayed the preference to peck at different-colored dots under different lighting conditions (immediate effects). Pecks are interpreted as foraging preference. Developmental plasticity affected both pecking preference and opsin expression. Lighting environments also had immediate effects on pecking preference, but these depended on the lighting conditions animals experienced during development. Genetic effects were detected in opsin expression, but there were no corresponding effects on pecking preference. Overall, only 3.36% of the variation in pecking preference was accounted for by opsin expression.