Incomplete behavioural isolation and asymmetric female preference in darter sister species (Percidae: Etheostoma)

Identifying female phenotypes that promote behavioral isolation in a sexually dimorphic species of fish Etheostoma zonale

In sexually dimorphic species characterized by exaggerated male ornamentation, behavioral isolation is often attributed to female preferences for conspecific male signals. Yet, in a number of sexually dimorphic species, male mate choice also results in behavioral isolation. In many of these cases, the female traits that mediate species boundaries are unclear. Females in sexually dimorphic species typically lack many of the elaborate traits that are present in males and that are often used for taxonomic classification of species. In a diverse and largely sexually dimorphic group of fishes called darters (Percidae: Etheostoma), male mate choice contributes to behavioral isolation between a number of species; however, studies addressing which female traits males prefer are lacking. In this study, we identified the dominant female pattern for two sympatric species, Etheostoma zonale and Etheostoma barrenense, using pattern energy analysis, and we used discriminate function analysis to identify which aspects of female patterning can reliably classify species. We then tested the role of female features in male mate choice for E. zonale, by measuring male preference for computer animations displaying the identified (species-specific) conspecific features. We found that the region above the lateral line is important in mediating male mate preferences, with males spending a significantly greater proportion of time with animations exhibiting conspecific female patterning in this region than with animations exhibiting heterospecific female patterning. Our results suggest that the aspects of female phenotypes that are the target of male mate choice are different from the conspicuous male phenotypes that traditionally characterize 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.

Reinforcement in the banded darter Etheostoma zonale: The effect of sex and sympatry on preferences

Reinforcement occurs when selection against hybrid offspring strengthens behavioral isolation between parental species and may be an important factor in speciation. Theoretical models and experimental evidence indicate that both female and male preferences can be strengthened upon secondary contact via reinforcement. However, the question remains whether this process is more likely to affect the preferences of one sex or the other. Males of polygynous species are often predicted to exhibit weaker preferences than females, potentially limiting the ability for reinforcement to shape male preferences. Yet, in darters (Percidae: Etheostoma), male preference for conspecific mates appears to arise before female preferences during the early stages of allopatric speciation, and research suggests that male, but not female, preferences become reinforced upon secondary contact. In the current study, we aimed to determine whether the geographically widespread darter species Etheostoma zonale exhibits a signature of reinforcement, by comparing the strength of preference for conspecific mates between populations that are sympatric and allopatric with respect to a close congener, E. barrenense. We examined the strength of preference for conspecifics for males and females separately to determine whether the preferences of one or both sexes have been strengthened by reinforcement. Our results show that both sexes of E. zonale from sympatric populations exhibit stronger conspecific preferences than E. zonale from allopatric populations, but that female preferences appear to be more strongly reinforced than male preferences. Results therefore suggest that reinforcement of female preferences may promote behavioral isolation upon secondary contact, even in a genus that is characterized by pervasive male mate choice.

Preference for conspecifics evolves earlier in males than females in a sexually dimorphic radiation of fishes

Speciation by sexual selection is generally modeled as the coevolution of female preferences and elaborate male ornaments leading to behavioral (sexual) reproductive isolation. One prediction of these models is that female preference for conspecific males should evolve earlier than male preference for conspecific females in sexually dimorphic species with male ornaments. We tested that prediction in darters, a diverse group of freshwater fishes with sexually dimorphic ornamentation. Focusing on the earliest stages of divergence, we tested preference for conspecific mates in males and females of seven closely related species pairs. Contrary to expectation, male preference for conspecific females was significantly greater than female preference for conspecific males. Males in four of the 14 species significantly preferred conspecific females; whereas, females in no species significantly preferred conspecific males. Relationships between the strength of preference for conspecifics and genetic distance revealed no difference in slope between males and females, but a significant difference in intercept, also suggesting that male preference evolves earlier than females'. Our results are consistent with other recent studies in darters and suggest that the coevolution of female preferences and male ornaments may not best explain the earliest stages of behavioral isolation in this lineage.

Poison frog color morphs express assortative mate preferences in allopatry but not sympatry

The concurrent divergence of mating traits and preferences is necessary for the evolution of reproductive isolation via sexual selection, and such coevolution has been demonstrated in diverse lineages. However, the extent to which assortative mate preferences are sufficient to drive reproductive isolation in nature is less clear. Natural contact zones between lineages divergent in traits and preferences provide exceptional opportunities for testing the predicted evolutionary consequences of such divergence. The strawberry poison frog (Oophaga pumilio) displays extreme color polymorphism in and around the young Bocas del Toro archipelago. In a transition zone between red and blue allopatric lineages, we asked whether female preferences diverged along with coloration, and whether any divergent preferences persist in a zone of sympatry. When choosing among red, blue and phenotypically intermediate males, females from monomorphic red and monomorphic blue populations both expressed assortative preferences. However, red, blue, and intermediate females from the contact zone all preferred red males, suggesting that divergent preferences may be insufficient to effect behavioral isolation. Our results highlight the complexity of behavioral isolation, and the need for studies that can reveal the circumstances under which divergent preferences do and do not contribute to speciation.

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.

The ecological drivers of nuptial color evolution in darters (Percidae: Etheostomatinae)

Closely related animal lineages often vary in male coloration, and ecological selection is hypothesized to shape this variation. The role of ecological selection in inhibiting male color has been documented extensively at the population level, but relatively few studies have investigated the evolution of male coloration across a clade of closely related species. Darters are a diverse group of fishes that vary in the presence of elaborate male nuptial coloration, with some species exhibiting vivid color patterns and others mostly or entirely achromatic. We used phylogenetic logistic regression to test for correlations between the presence/absence of color traits across darter species and the ecological conditions in which these species occur. Environmental variables were correlated with the presence of nuptial color in darters with colorful species tending to inhabit environments that would support fewer predators and potentially transmit a broader spectrum of natural light compared to species lacking male coloration. We also tested the color preferences of a common darter predator, largemouth bass, and found that it exhibits a strong preference for red, providing further evidence of predation as a source of selection on color evolution in darters. Ecological selection therefore appears to be an important factor in dictating the presence or absence of male coloration in this group of fishes.

Epigenetic divergence as a potential first step in darter speciation

Recent studies show that epigenetic variation in the form of DNA methylation may serve as a substrate for selection. Theory suggests that heritable epigenetic marks that increase fitness should increase in frequency in a population, and these changes may result in novel morphology, behaviour, or physiology, and ultimately reproductive isolation. Therefore, epigenetic variation might provide the first substrate for selection during the course of evolutionary divergence. This hypothesis predicts that populations in the earliest stages of divergence will differentiate in their methylome prior to any genetic differentiation. While several studies have investigated natural epigenetic variation, empirical studies that test predictions about its role in speciation are surprisingly scarce. Here, we investigate DNA methylation variation using an isoschizomeric digest method, Methyl-Sensitive Amplified Polymorphism, across multiple stages of evolutionary divergence in natural populations of North American stream fishes. We show that epigenetic differentiation between methylomes is greater than genetic divergence among closely related populations across two river drainages. Additionally, we demonstrate that epigenetic divergence is a stronger predictor of the strength of behavioural reproductive isolation and suggest that changes in the methylome could influence the evolution of reproductive isolation between species. Our findings suggest a role for epigenetics not only in the initiation of divergence, but also in the maintenance of species boundaries over greater evolutionary timescales.

Changes in sexual signals are greater than changes in ecological traits in a dichromatic group of fishes

Understanding the mechanisms by which phenotypic divergence occurs is central to speciation research. These mechanisms can be revealed by measuring differences in traits that are subject to different selection pressures; greater influence of different types of selection can be inferred from greater divergence in associated traits. Here, we address the potential roles of natural and sexual selection in promoting phenotypic divergence between species of snubnose darters by comparing differences in body shape, an ecologically relevant trait, and male color, a sexual signal. Body shape was measured using geometric morphometrics, and male color was measured using digital photography and visual system-dependent color values. Differences in male color are larger than differences in body shape across eight allopatric, phylogenetically independent species pairs. While this does not exclude the action of divergent natural selection, our results suggest a relatively more important role for sexual selection in promoting recent divergence in darters. Variation in the relative differences between male color and body shape across species pairs reflects the continuous nature of speciation mechanisms, ranging from ecological speciation to speciation by sexual selection alone.