Genetic polymorphism for alternative mating behaviour in lekking male ruff Philomachus pugnax

Squid male alternative reproductive tactics are determined by birth date

Alternative reproductive tactics (ARTs) are discontinuous phenotypes associated with reproduction, observed in males of many species. Typically, large males adopt a tactic of competing with rivals for mating, while small males adopt a tactic of stealing fertilization opportunities from the large males. The 'birth date hypothesis', proposing that the date of birth influences the determination of each male's reproductive tactic, has been tested only in teleost fish to date. Here, the birth date hypothesis was tested in ARTs of Japanese spear squid Heterololigo bleekeri (consort/sneaker) by analysing statolith growth increments. The birth date significantly differed between consorts (early-hatched) and sneakers (late-hatched). However, no differences were detected in growth history up to 100 days from hatching. Most immature males caught during the reproductive season were larger than sneakers, and their hatch date was similar to that of consorts, suggesting that these immature males had already been following a life-history pathway as a consort. These results indicate that ARTs of H. bleekeri are determined based on their hatch date in early life. This study firstly suggests that the birth date hypothesis applies to aquatic invertebrates, suggesting that the mechanism by which birth date determines the individual phenotype is a phenomenon more common than previously believed.

Low Mutation Load in a Supergene Underpinning Alternative Male Mating Strategies in Ruff (Calidris pugnax)

A paradox in evolutionary biology is how supergenes can maintain high fitness despite reduced effective population size, the suppression of recombination, and the expected accumulation of mutational load. The ruff supergene involves 2 rare inversion haplotypes (satellite and faeder). These are recessive lethals but with dominant effects on male mating strategies, plumage, and body size. Sequence divergence to the wild-type (independent) haplotype indicates that the inversion could be as old as 4 million years. Here, we have constructed a highly contiguous genome assembly of the inversion region for both the independent and satellite haplotypes. Based on the new data, we estimate that the recombination event(s) creating the satellite haplotype occurred only about 70,000 yr ago. Contrary to expectations for supergenes, we find no substantial expansion of repeats and only a modest mutation load on the satellite and faeder haplotypes despite high sequence divergence to the non-inverted haplotype (1.46%). The essential centromere protein N (CENPN) gene is disrupted by the inversion and is as well conserved on the inversion haplotypes as on the noninversion haplotype. These results suggest that the inversion may be much younger than previously thought. The low mutation load, despite recessive lethality, may be explained by the introgression of the inversion from a now extinct lineage.

Multivariate Models of Animal Sex: Breaking Binaries Leads to a Better Understanding of Ecology and Evolution

"Sex" is often used to describe a suite of phenotypic and genotypic traits of an organism related to reproduction. However, these traits-gamete type, chromosomal inheritance, physiology, morphology, behavior, etc.-are not necessarily coupled, and the rhetorical collapse of variation into a single term elides much of the complexity inherent in sexual phenotypes. We argue that consideration of "sex" as a constructed category operating at multiple biological levels opens up new avenues for inquiry in our study of biological variation. We apply this framework to three case studies that illustrate the diversity of sex variation, from decoupling sexual phenotypes to the evolutionary and ecological consequences of intrasexual polymorphisms. We argue that instead of assuming binary sex in these systems, some may be better categorized as multivariate and nonbinary. Finally, we conduct a meta-analysis of terms used to describe diversity in sexual phenotypes in the scientific literature to highlight how a multivariate model of sex can clarify, rather than cloud, studies of sexual diversity within and across species. We argue that such an expanded framework of "sex" better equips us to understand evolutionary processes, and that as biologists, it is incumbent upon us to push back against misunderstandings of the biology of sexual phenotypes that enact harm on marginalized communities.

Modelling the impacts of male alternative reproductive tactics on population dynamics

Observations of male alternative reproductive tactics (ARTs) in a variety of species have stimulated the development of mathematical models that can account for the evolution and stable coexistence of multiple male phenotypes. However, little attention has been given to the population dynamic consequences of ARTs. We present a population model that takes account of the existence of two male ARTs (guarders and sneakers), assuming that tactic frequencies are environmentally determined and tactic reproductive success depends on the densities of both types. The presence of sneakers typically increases overall population density. However, if sneakers comprise a sufficiently large proportion of the population-or, equivalently, if guarders are sufficiently rare-then it is possible for the total population to crash to extinction (in this extreme regime, there is also an Allee effect, i.e. a threshold density below which the population will go extinct). We apply the model to the example of the invasive round goby (Neogobius melanostomus). We argue that ARTs can dramatically influence population dynamics and suggest that considering such phenotypic plasticity in population models is potentially important, especially for species of conservation or commercial importance.

Comparative phylogeography reveals widespread cryptic diversity driven by ecology in Panamanian birds

Widespread species often harbor unrecognized genetic diversity, and investigating the factors associated with such cryptic variation can help us better understand the forces driving diversification. Here, we identify potential cryptic species based on a comprehensive dataset of COI mitochondrial DNA barcodes from 2,333 individual Panamanian birds across 429 species, representing 391 (59%) of the 659 resident landbird species of the country, as well as opportunistically sampled waterbirds. We complement this dataset with additional publicly available mitochondrial loci, such as ND2 and cytochrome b, obtained from whole mitochondrial genomes from 20 taxa. Using barcode identification numbers (BINs), we find putative cryptic species in 19% of landbird species, highlighting hidden diversity in the relatively well-described avifauna of Panama. Whereas some of these mitochondrial divergence events corresponded with recognized geographic features that likely isolated populations, such as the Cordillera Central highlands, the majority (74%) of lowland splits were between eastern and western populations. The timing of these splits are not temporally coincident across taxa, suggesting that historical events, such as the formation of the Isthmus of Panama and Pleistocene climatic cycles, were not the primary drivers of cryptic diversification. Rather, we observed that forest species, understory species, insectivores, and strongly territorial species-all traits associated with lower dispersal ability-were all more likely to have multiple BINs in Panama, suggesting strong ecological associations with cryptic divergence. Additionally, hand-wing index, a proxy for dispersal capability, was significantly lower in species with multiple BINs, indicating that dispersal ability plays an important role in generating diversity in Neotropical birds. Together, these results underscore the need for evolutionary studies of tropical bird communities to consider ecological factors along with geographic explanations, and that even in areas with well-known avifauna, avian diversity may be substantially underestimated.

LAY SUMMARY

- What factors are common among bird species with cryptic diversity in Panama? What role do geography, ecology, phylogeographic history, and other factors play in generating bird diversity?- 19% of widely-sampled bird species form two or more distinct DNA barcode clades, suggesting widespread unrecognized diversity.- Traits associated with reduced dispersal ability, such as use of forest understory, high territoriality, low hand-wing index, and insectivory, were more common in taxa with cryptic diversity. Filogeografía comparada revela amplia diversidad críptica causada por la ecología en las aves de Panamá.

RESUMEN

Especies extendidas frecuentemente tiene diversidad genética no reconocida, y investigando los factores asociados con esta variación críptica puede ayudarnos a entender las fuerzas que impulsan la diversificación. Aquí, identificamos especies crípticas potenciales basadas en un conjunto de datos de códigos de barras de ADN mitocondrial de 2,333 individuos de aves de Panama en 429 especies, representando 391 (59%) de las 659 especies de aves terrestres residentes del país, además de algunas aves acuáticas muestreada de manera oportunista. Adicionalmente, complementamos estos datos con secuencias mitocondriales disponibles públicamente de otros loci, tal como ND2 o citocroma b, obtenidos de los genomas mitocondriales completos de 20 taxones. Utilizando los números de identificación de código de barras (en ingles: BINs), un sistema taxonómico numérico que proporcina una estimación imparcial de la diversidad potencial a nivel de especie, encontramos especies crípticas putativas en 19% de las especies de aves terrestres, lo que destaca la diversidad oculta en la avifauna bien descrita de Panamá. Aunque algunos de estos eventos de divergencia conciden con características geográficas que probablemente aislaron las poblaciones, la mayoría (74%) de la divergencia en las tierras bajas se encuentra entre las poblaciones orientales y occidentales. El tiempo de esta divergencia no coincidió entre los taxones, sugiriendo que eventos históricos tales como la formación del Istmo de Panamá y los ciclos climáticos del pleistoceno, no fueron los principales impulsores de la especiación. En cambio, observamos asociaciones fuertes entre las características ecológicas y la divergencia mitocondriale: las especies del bosque, sotobosque, con una dieta insectívora, y con territorialidad fuerte mostraton múltiple BINs probables. Adicionalmente, el índice mano-ala, que está asociado a la capacidad de dispersión, fue significativamente menor en las especies con BINs multiples, sugiriendo que la capacidad de dispersión tiene un rol importamente en la generación de la diversidad de las aves neotropicales. Estos resultos demonstran la necesidad de que estudios evolutivos de las comunidades de aves tropicales consideren los factores ecológicos en conjunto con las explicaciones geográficos. Palabras clave: biodiversidad tropical, biogeografía, códigos de barras, dispersión, especies crípticas.

Sex-specific morphs: the genetics and evolution of intra-sexual variation

Sex-specific morphs exhibit discrete phenotypes, often including many disparate traits, that are observed in only one sex. These morphs have evolved independently in many different animals and are often associated with alternative mating strategies. The remarkable diversity of sex-specific morphs offers unique opportunities to understand the genetic basis of complex phenotypes, as the distinct nature of many morphs makes it easier to both categorize and compare genomes than for continuous traits. Sex-specific morphs also expand the study of sexual dimorphism beyond traditional bimodal comparisons of male and female averages, as they allow for a more expansive range of sexualization. Although ecological and endocrinological studies of sex-specific morphs have been advancing for some time, genomic and transcriptomic studies of morphs are far more recent. These studies reveal not only many different paths to the evolution of sex-specific morphs but also many commonalities, such as the role of sex-determining genes and hormone signalling in morph development, and the mixing of male and female traits within some morphs.

Evolutionary Ecology of Fixed Alternative Male Mating Strategies in the Ruff (Calidris pugnax)

A few empirical examples document fixed alternative male mating strategies in animals. Here we focus on the polymorphism of male mating strategies in the ruff (Calidris pugnax, Aves Charadriiformes). In ruffs, three fixed alternative male mating strategies coexist and are signaled by extreme plumage polymorphism. We first present relevant data on the biology of the species. Then we review the available knowledge of the behavioral ecology of ruffs during the breeding season, and we detail the characteristics of each of the three known fixed male mating strategies. We next turn to the results of exceptional quality accumulated on both the structural and functional genomics of the ruff over the past few years. We show how much these genomic data can shed new, mechanistic light on the evolution and maintenance of the three fixed alternative male mating strategies. We then look if there is sufficient indication to support frequency-dependent selection as a key mechanism in maintaining these three strategies. Specifically, we search for evidence of equal fitness among individuals using each of the three strategies. Finally, we propose three lines of research avenues that will help to understand the eco-evolutionary dynamics of phenotypic differences within natural populations of this iconic model species.

Male alternative reproductive tactics and sperm competition: a meta-analysis

In many animal species, males may exhibit one of several discrete, alternative ways of obtaining fertilisations, known as alternative reproductive tactics (ARTs). Males exhibiting ARTs typically differ in the extent to which they invest in traits that improve their mating success, or the extent to which they face sperm competition. This has led to the widespread prediction that males exhibiting ARTs associated with a high sperm competition risk, or lower investment into traits that improve their competitiveness before mating, should invest more heavily into traits that improve their competitiveness after mating, such as large ejaculates and high-quality sperm. However, despite many studies investigating this question since the 1990s, evidence for differences in sperm and ejaculate investment between male ARTs is mixed, and there has been no quantitative summary of this field. Following a systematic review of the literature, we performed a meta-analysis examining how testes size, sperm number and sperm traits differ between males exhibiting ARTs that face either a high or low sperm competition risk, or high or low investment in traits that increase mating success. We obtained data from 92 studies and 67 species from across the animal kingdom. Our analyses showed that male fish exhibiting ARTs facing a high sperm competition risk had significantly larger testes (after controlling for body size) than those exhibiting tactics facing a low sperm competition risk. However, this effect appears to be due to the inappropriate use of the gonadosomatic index as a body-size corrected measure of testes investment, which overestimates the difference in testes investment between male tactics in most cases. We found no significant difference in sperm number between males exhibiting different ARTs, regardless of whether sperm were measured from the male sperm stores or following ejaculation. We also found no significant difference in sperm traits between males exhibiting different ARTs, with the exception of sperm adenosine triphosphate (ATP) content in fish. Finally, the difference in post-mating investment between male ARTs was not influenced by the extent to which tactics were flexible, or by the frequency of sneakers in the population. Overall, our results suggest that, despite clear theoretical predictions, there is little evidence that male ARTs differ substantially in investment into sperm and ejaculates across species. The incongruence between theoretical and empirical results could be explained if (i) theoretical models fail to account for differences in overall resource levels between males exhibiting different ARTs or fundamental trade-offs between investment into different ejaculate and sperm traits, and (ii) studies often use sperm or ejaculate traits that do not reflect overall post-mating investment accurately or affect fertilisation success.

Sex and morph differences in age-dependent trait changes in a polymorphic songbird

There is growing evidence of important variation in how animals age, in particular in how the expression of traits changes with age among different species and populations. However, less is known about variation within populations, which may include variation in ageing patterns between different types of individuals (e.g. sexes or distinct polymorphisms) and between different types of traits (e.g. general traits versus those used in social signalling contexts). We used 6 years of longitudinal data to examine age-related changes in trait expression in a captive population of Gouldian finches (Erythrura gouldiae), a socially monogamous songbird with genetically determined colour morphs that differ in behaviour and physiology. We contrasted ageing patterns of different types of traits (social signalling vs. size-related) in both sexes and in two colour morphs, using a mixed model approach to account for both within- and between-individual effects. We found pronounced sex differences in how social signalling traits change with age, showing a quadratic pattern in males, but not changing with age in females. In contrast, we observed no sex-specific ageing patterns in size traits. We also found subtle morph differences in how size-related traits changed with age, with black morphs stable or increasing with age while red morphs showing a decline with age. Finally, we found an interesting sex by morph interaction in one important social signal (headband width). These results highlight the importance of using within-individual approaches to understand ageing patterns across types of individuals (sex, morph, etc.) and the need for further research on the ageing patterns of traits that may experience different selective pressures.

Sexual selection on population-level mating opportunities drives morph ratios in a fig wasp with extreme male dimorphism

Background

Alternative mating tactics are widespread in animals and associated with extreme morphological polymorphism in some insects. Some fig wasps have both highly modified wingless males and dispersing winged males. Wingless males mate inside figs before females disperse, while winged males mate elsewhere after dispersal. Hamilton proposed a model for this system with morphs determined by alternative alleles. This has an equilibrium where the proportion of winged males equals the proportion of females dispersing unmated; i.e. the proportion of matings that they obtain. Previously, we have shown qualitative support for this prediction across nine wing-dimorphic fig wasp species. Here I test the quantitative prediction in the fig wasp Pseudidarnes minerva. In addition, some fig wasp species that lack winged males, but have two wingless morphs, show a conditional strategy with morph determination influenced by the number of wasps developing in a patch. I also test for this alternative pattern in the wing-dimorphic P. minerva.

Results

I sampled 114 figs that contained a mean of 2.1 P. minerva wasps from 44 trees across four sites in Sydney, Australia. At the whole population level, the proportion of winged males (0.84 or 0.79 corrected for sampling bias) did not differ significantly from the proportion of unmated females (0.84), providing strong quantitative support for the prediction of Hamilton’s model. In addition, there was no evidence for other factors, such as local mate competition or fighting between wingless males, that could violate simplifying assumptions of the model. Meanwhile, the proportion of winged males was not correlated with the number of wasps per fig, providing no evidence for a conditional strategy.

Conclusion

The morph ratio in P. minerva is consistent with Hamilton’s simple Mendelian strategy model, where morph ratios are set by average mating opportunities at the population level. This contrasts with some fig wasps from another subfamily that show conditional morph determination, allowing finer scale adaptation to fig-level mating opportunities. However, these conditional cases do not involve wing polymorphism. Male polymorphism is common and variable in fig wasps and has evolved independently in multiple lineages with apparently different underlying mechanisms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01898-3.

Extreme Y chromosome polymorphism corresponds to five male reproductive morphs of a freshwater fish

Loss of recombination between sex chromosomes often depletes Y chromosomes of functional content and genetic variation, which might limit their potential to generate adaptive diversity. Males of the freshwater fish Poecilia parae occur as one of five discrete morphs, all of which shoal together in natural populations where morph frequency has been stable for over 50 years. Each morph uses a different complex reproductive strategy and morphs differ dramatically in colour, body size and mating behaviour. Morph phenotype is passed perfectly from father to son, indicating there are five Y haplotypes segregating in the species, which encode the complex male morph characteristics. Here, we examine Y diversity in natural populations of P. parae. Using linked-read sequencing on multiple P. parae females and males of all five morphs, we find that the genetic architecture of the male morphs evolved on the Y chromosome after recombination suppression had occurred with the X. Comparing Y chromosomes between each of the morphs, we show that, although the Ys of the three minor morphs that differ in colour are highly similar, there are substantial amounts of unique genetic material and divergence between the Ys of the three major morphs that differ in reproductive strategy, body size and mating behaviour. Altogether, our results suggest that the Y chromosome is able to overcome the constraints of recombination loss to generate extreme diversity, resulting in five discrete Y chromosomes that control complex reproductive strategies.

Extensive historical and contemporary hybridization suggests premating isolation in Vermivora warblers is not strong: A reply to Confer et al

We present comments on an article published by Confer et al. (Ecology and Evolution, 10, 2020). Confer et al. (2020) aggregate data from multiple studies of social pairing between Vermivora chrysoptera and V. cyanoptera, two wood warblers in the family Parulidae that hybridize extensively where they co‐occur. From analysis of these data, they conclude there is near‐complete reproductive isolation between these two species. In our reply, we show that this finding is not supported by other lines of evidence, and significant drawbacks of their study design preclude such strong conclusions. In our critique, we show that (a) coarse‐scale plumage classifications cannot be used to accurately estimate hybrid ancestry in Vermivora; (b) extra‐pair paternity is very high in Vermivora and is likely facilitating hybridization, yet was not considered by Confer et al. (2020), and we suggest this will have a substantial influence on the interpretation of reproductive isolation in the system; and (c) the central finding of strong total reproductive isolation is not compatible with the results of other long‐term studies, which demonstrate low isolation and high gene flow. We conclude with a more comprehensive interpretation of hybridization and reproductive isolation in Vermivora warblers.

Sex-specific associations between life-history traits and a novel reproductive polymorphism in the Pacific field cricket

Associations between heritable polymorphisms and life-history traits, such as development time or reproductive investment, may play an underappreciated role in maintaining polymorphic systems. This is because selection acting on a particular morph could be bolstered or disrupted by correlated changes in life history or vice versa. In a Hawaiian population of the Pacific field cricket (Teleogryllus oceanicus), a novel mutation (flatwing) on the X-chromosome is responsible for a heritable polymorphism in male wing structure. We used laboratory cricket colonies fixed for male wing morph to investigate whether males and females bearing the flatwing or normal-wing (wild-type) allele differed in their life-history traits. We found that flatwing males developed faster and had heavier testes than normal-wings, whereas flatwing homozygous females developed slower and had lighter reproductive tissues than normal-wing homozygous females. Our results advance our understanding of the evolution of polymorphisms by demonstrating that the genetic change responsible for a reproductive polymorphism can also have consequences for fundamental life-history traits in both males and females.

Trait differences among discrete morphs of a color polymorphic lizard, Podarcis erhardii

Color polymorphism defies evolutionary expectations as striking phenotypic variation is maintained within a single species. Color and other traits mediate social interactions, and stable polymorphism within a population is hypothesized to be related to correlational selection of other phenotypic traits among color morphs. Here, we report on a previously unknown throat color polymorphism in the Aegean Wall Lizard (Podarcis erhardii) and examine morph-correlated differences in traits important to social behavior and communication: maximum bite force capacity and chemical signal profile. We find that both sexes of P. erhardii have three color morphs: orange, yellow, and white. Moreover, orange males are significantly larger and tend to bite harder than yellow and white males. Although the established color polymorphism only partially matches the observed intraspecific variation in chemical signal signatures, the chemical profile of the secretions of orange males is significantly divergent from that of white males. Our findings suggest that morph colors are related to differences in traits that are crucial for social interactions and competitive ability, illustrating the need to look beyond color when studying polymorphism evolution.

Neural and molecular mechanisms underlying female mate choice decisions in vertebrates

Female mate choice is a dynamic process that allows individuals to selectively mate with those of the opposite sex that display a preferred set of traits. Because in many species males compete with each other for fertilization opportunities, female mate choice can be a powerful agent of sexual selection, often resulting in highly conspicuous traits in males. Although the evolutionary causes and consequences of the ornamentation and behaviors displayed by males to attract mates have been well studied, embarrassingly little is known about the proximate neural mechanisms through which female choice occurs. In vertebrates, female mate choice is inherently a social behavior, and although much remains to be discovered about this process, recent evidence suggests the neural substrates and circuits underlying other fundamental social behaviors (such as pair bonding, aggression and parental care) are likely similarly recruited during mate choice. Notably, female mate choice is not static, as social and ecological environments can shape the brain and, consequently, behavior in specific ways. In this Review, we discuss how social and/or ecological influences mediate female choice and how this occurs within the brain. We then discuss our current understanding of the neural substrates underlying female mate choice, with a specific focus on those that also play a role in regulating other social behaviors. Finally, we propose several promising avenues for future research by highlighting novel model systems and new methodological approaches, which together will transform our understanding of the causes and consequences of female mate choice.

Equal fitness among alternative mating strategies in a harem polygynous insect

Alternative mating strategies are widespread among animal taxa, with strategies controlled by a genetic polymorphism (Mendelian strategy) being rarer in nature than condition-dependent developmental strategies. Mendelian strategies are predicted to have equal average fitnesses and the proportion of offspring produced by a strategy should equal the equilibrium proportion of individuals representing the strategy in a population. Developmental strategies are not expected to produce offspring in equilibrium proportions; however, whether the alternative phenotypes should have equal average fitness is debated. The Wellington tree wētā (Hemideina crassidens) (Orthoptera: Anostostomatidae) is a harem polygynous insect in which intense sexual competition has favoured the evolution of three alternative mating strategies that differ in weapon size and the ability to fight for control of harems. Here, we use molecular genotyping to test the hypothesis that the alternative strategies in this species are maintained by having equal relative fitness and that morphs produce offspring in equilibrium proportions. As expected, the average relative fitness of the three strategies did not significantly differ and the proportion of offspring produced by each morph is equal to the frequency of that morph in the population. Our results support the hypothesis that the alternative male morphs in H. crassidens represent Mendelian strategies.

Social Games and Genic Selection Drive Mammalian Mating System Evolution and Speciation

Mating system theory based on economics of resource defense has been applied to describe social system diversity across taxa. Such models are generally successful but fail to account for stable mating systems across different environments or shifts in mating system without a change in ecological conditions. We propose an alternative approach to resource defense theory based on frequency-dependent competition among genetically determined alternative behavioral strategies characterizing many social systems (polygyny, monogamy, sneak). We modeled payoffs for competition, neighborhood choice, and paternal care to determine evolutionary transitions among mating systems. Our model predicts four stable outcomes driven by the balance between cooperative and agonistic behaviors: promiscuity (two or three strategies), polygyny, and monogamy. Phylogenetic analysis of 288 rodent species supports assumptions of our model and is consistent with patterns of evolutionarily stable states and mating system transitions. Support for model assumptions include that monogamy and polygyny evolve from promiscuity and that paternal care and monogamy are coadapted in rodents. As predicted by our model, monogamy and polygyny occur in sister taxa among rodents more often than by chance. Transitions to monogamy also favor higher speciation rates in subsequent lineages, relative to polygynous sister lineages. Taken together, our results suggest that genetically based neighborhood choice behavior and paternal care can drive transitions in mating system evolution. While our genic mating system theory could complement resource-based theory, it can explain mating system transitions regardless of resource distribution and provides alternative explanations, such as evolutionary inertia, when resource ecology and mating systems do not match.

Alternative reproductive strategies in white-throated sparrows are associated with differences in parasite load following experimental infection

Immune defences often trade off with other life-history components. Within species, optimal allocation to immunity may differ between the sexes or between alternative life-history strategies. White-throated sparrows (Zonotrichia albicollis) are unusual in having two discrete plumage morphs, white-striped and tan-striped. Within each sex, white-striped individuals are more aggressive and provide less parental care than tan-striped individuals. We extended immunocompetence handicap models, which predict sex differences in immunity and parasitism, to hypothesize that infection susceptibility should be greater in white-striped than tan-striped birds. We inoculated birds of both morphs with malarial parasites. Contrary to our prediction, among birds that became infected, parasite loads were higher in tan-striped than white-striped individuals and did not differ between the sexes. Circulating androgen levels did not differ between morphs but were higher in males than females. Our findings are not consistent with androgen-mediated immunosuppression. Instead, morph differences in immunity could reflect social interactions or life-history-related differences in risk of injury, and/or genetic factors. Although plumage and behavioural morphs of white-throated sparrow may differ in disease resistance, these differences do not parallel sex differences that have been reported in animals, and do not appear to be mediated by differences in androgen levels.

The role of genetic diversity in the evolution and maintenance of environmentally-cued, male alternative reproductive tactics

BACKGROUND

Alternative reproductive tactics (ARTs) are taxonomically pervasive strategies adopted by individuals to maximize reproductive success within populations. Even for conditionally-dependent traits, consensus postulates most ARTs involve both genetic and environmental interactions (GEIs), but to date, quantifying genetic variation underlying the threshold disposing an individual to switch phenotypes in response to an environmental cue has been a difficult undertaking. Our study aims to investigate the origins and maintenance of ARTs within environmentally disparate populations of the microscopic bulb mite, Rhizoglyphus robini, that express 'fighter' and 'scrambler' male morphs mediated by a complex combination of environmental and genetic factors.

RESULTS

Using never-before-published individual genetic profiling, we found all individuals across populations are highly inbred with the exception of scrambler males in stressed environments. In fact within the poor environment, scrambler males and females showed no significant difference in genetic differentiation (Fst) compared to all other comparisons, and although fighters were highly divergent from the rest of the population in both poor or rich environments (e.g., Fst, STRUCTURE), fighters demonstrated approximately three times less genetic divergence from the population in poor environments. AMOVA analyses further corroborated significant genetic differentiation across subpopulations, between morphs and sexes, and among subpopulations within each environment.

CONCLUSION

Our study provides new insights into the origin of ARTs in the bulb mite, highlighting the importance of GEIs: genetic correlations, epistatic interactions, and sex-specific inbreeding depression across environmental stressors. Asymmetric reproductive output, coupled with the purging of highly inbred individuals during environmental oscillations, also facilitates genetic variation within populations, despite evidence for strong directional selection. This cryptic genetic variation also conceivably facilitates stable population persistence even in the face of spatially or temporally unstable environmental challenges. Ultimately, understanding the genetic context that maintains thresholds, even for conditionally-dependent ARTs, will enhance our understanding of within population variation and our ability to predict responses to selection.

Behavioural tactic predicts preoptic-hypothalamic gene expression more strongly than developmental morph in fish with alternative reproductive tactics

Reproductive success relies on the coordination of social behaviours, such as territory defence, courtship and mating. Species with extreme variation in reproductive tactics are useful models for identifying the neural mechanisms underlying social behaviour plasticity. The plainfin midshipman (Porichthys notatus) is a teleost fish with two male reproductive morphs that follow widely divergent developmental trajectories and display alternative reproductive tactics (ARTs). Type I males defend territories, court females and provide paternal care, but will resort to cuckoldry if they cannot maintain a territory. Type II males reproduce only through cuckoldry. We sought to disentangle gene expression patterns underlying behavioural tactic, in this case ARTs, from those solely reflective of developmental morph. Using RNA-sequencing, we investigated differential transcript expression in the preoptic area-anterior hypothalamus (POA-AH) of courting type I males, cuckolding type I males and cuckolding type II males. Unexpectedly, POA-AH differential expression was more strongly coupled to behavioural tactic than morph. This included a suite of transcripts implicated in hormonal regulation of vertebrate social behaviour. Our results reveal that divergent expression patterns in a conserved neuroendocrine centre known to regulate social-reproductive behaviours across vertebrate lineages may be uncoupled from developmental history to enable plasticity in the performance of reproductive tactics.

Actuarial senescence in a dimorphic bird: different rates of ageing in morphs with discrete reproductive strategies

It is often hypothesized that intra-sexual competition accelerates actuarial senescence, or the increase in mortality rates with age. However, an alternative hypothesis is that parental investment is more important to determining senescence rates. We used a unique model system, the white-throated sparrow (Zonotrichia albicollis), to study variation in actuarial senescence. In this species, genetically determined morphs display discrete mating strategies and disassortative pairing, providing an excellent opportunity to test the predictions of the above hypotheses. Compared to tan-striped males, white-striped males are more polygynous and aggressive, and less parental. Tan-striped females receive less parental support, and invest more into parental care than white-striped females, which are also more aggressive. Thus, higher senescence rates in males and white-striped birds would support the intra-sexual competition hypothesis, whereas higher senescence rates in females and tan-striped birds would support the parental investment hypothesis. White-striped males showed the lowest rate of actuarial senescence. Tan-striped females had the highest senescence rate, and tan-striped males and white-striped females showed intermediate, relatively equal rates. Thus, results were inconsistent with sexual selection and competitive strategies increasing senescence rates, and instead indicate that senescence may be accelerated by female-biased parental care, and lessened by sharing of parental duties.

Fitness Consequences of Female Alternative Reproductive Tactics in House Mice (Mus musculus domesticus)

Alternative reproductive tactics (ARTs) are defined as discrete differences in morphological, physiological, and/or behavioral traits associated with reproduction that occur within the same sex and population. House mice provide a rare example of ARTs in females, which can rear their young either solitarily or together with one or several other females in a communal nest. We assessed the fitness consequences of communal and solitary breeding in a wild population to understand how the two tactics can be evolutionarily stable. Females switched between the two tactics (with more than 50% of all females having two or more litters using both tactics), pointing toward communal and solitary breeding being two tactics within a single strategy and not two genetically determined strategies. Communal breeding resulted in reduced pup survival and negatively impacted female reproductive success. Older and likely heavier females more often reared their litters solitarily, indicating that females use a condition-dependent strategy. Solitary breeding seems the more successful tactic, and only younger and likely less competitive females might opt for communal nursing, even at the cost of increased pup mortality. This study emphasizes the importance of analyzing phenotypic plasticity and its role in cooperation in the context of female ARTs.

Physiological differences between female limited, alternative life history strategies: The Alba phenotype in the butterfly Colias croceus

Across a wide range of taxa, individuals within populations exhibit alternative life history strategies (ALHS) where their phenotypes dramatically differ due to divergent investments in growth, reproduction and survivorship, with the resulting trade-offs directly impacting Darwinian fitness. Though the maintenance of ALHS within populations is fairly well understood, little is known regarding the physiological mechanisms that underlie ALHS and how environmental conditions can affect the evolution and expression of these phenotypes. One such ALHS, known as Alba, exists within females of many species in the butterfly genus Colias. Previous works in New World species not only found that female morphs differ in their wing color due to a reallocation of resources away from the synthesis of wing pigments to other areas of development, but also that temperature played an important role in these trade-offs. Here we build on previous work conducted in New World species by measuring life history traits and conducting lipidomics on individuals reared at hot and cold temperatures in the Old World species Colias croceus. Results suggest that the fitness of Alba and orange morphs likely varies with rearing temperature, where Alba females have higher fitness in cold conditions and orange in warm. Additionally shared traits between Old and New World species suggest the Alba mechanism is likely conserved across the genus. Finally, in the cold treatment we observe an intermediate yellow morph that may have decreased fitness due to slower larval development. This cost may manifest as disruptive selection in the field, thereby favoring the maintenance of the two discrete morphs. Taken together these results add insights into the evolution of, and the selection on, the Alba ALHS.

Does the handicap principle explain the evolution of dimorphic ornaments?

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We reinvestigate a new model based on the handicap hypothesis.

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We show the handicap hypothesis does not explain male dimorphisms.

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The results are due to the ‘playing-the-field’ assumption of the model.

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The generality of the ‘playing-the-field’ assumption is suspect.

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The evolutionary stability of the proposed new equilibrium is questionable.

Signal function drives phenotypic and genetic diversity: the effects of signalling individual identity, quality or behavioural strategy

Animal coloration is influenced by selection pressures associated with communication. During communication, signallers display traits that inform receivers and modify receiver behaviour in ways that benefit signallers. Here, we discuss how selection on signallers to convey different kinds of information influences animal phenotypes and genotypes. Specifically, we address the phenotypic and genetic consequences of communicating three different kinds of information: individual identity, behavioural strategy and quality. Previous work has shown signals that convey different kinds of information differ in terms of the (i) type of selection acting on signallers (e.g. directional, stabilizing, or negative frequency dependent), and (ii) developmental basis of signals (i.e. heritability, genetic architecture). These differences result in signals that convey different information having consistently different phenotypic properties, including the amount, modality and continuity of intraspecific variation. Understanding how communication influences animal phenotypes may allow researchers to quickly identify putative functions of colour variation prior to experimentation. Signals that convey different information will also have divergent evolutionary consequences. For example, signalling individual identity can increase genetic diversity, signalling quality may decrease diversity, and signalling strategy can constrain adaptation and contribute to speciation. Considering recent advances in genomic resources, our framework highlights new opportunities to resolve the evolutionary consequences of selection on communication across diverse taxa and signal types.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Evolution of sex-specific pace-of-life syndromes: genetic architecture and physiological mechanisms

Sex differences in life history, physiology, and behavior are nearly ubiquitous across taxa, owing to sex-specific selection that arises from different reproductive strategies of the sexes. The pace-of-life syndrome (POLS) hypothesis predicts that most variation in such traits among individuals, populations, and species falls along a slow-fast pace-of-life continuum. As a result of their different reproductive roles and environment, the sexes also commonly differ in pace-of-life, with important consequences for the evolution of POLS. Here, we outline mechanisms for how males and females can evolve differences in POLS traits and in how such traits can covary differently despite constraints resulting from a shared genome. We review the current knowledge of the genetic basis of POLS traits and suggest candidate genes and pathways for future studies. Pleiotropic effects may govern many of the genetic correlations, but little is still known about the mechanisms involved in trade-offs between current and future reproduction and their integration with behavioral variation. We highlight the importance of metabolic and hormonal pathways in mediating sex differences in POLS traits; however, there is still a shortage of studies that test for sex specificity in molecular effects and their evolutionary causes. Considering whether and how sexual dimorphism evolves in POLS traits provides a more holistic framework to understand how behavioral variation is integrated with life histories and physiology, and we call for studies that focus on examining the sex-specific genetic architecture of this integration.

Alternative reproductive strategies and the maintenance of female color polymorphism in damselflies

Genetic polymorphisms are powerful model systems to study the maintenance of diversity in nature. In some systems, polymorphisms are limited to female coloration; these are thought to have arisen as a consequence of reducing male mating harassment, commonly resulting in negative frequency-dependent selection on female color morphs. One example is the damselfly Ischnura elegans, which shows three female color morphs and strong sexual conflict over mating rates. Here, we present research integrating male tactics, and female evolutionary strategies (female mating behavior and morph-specific female fecundity) in populations with different morph-specific mating frequencies, to obtain an understanding of mating rates in nature that goes beyond the mere measure of color frequencies. We found that female morph behavior differed significantly among but not within morphs (i.e., female morph behavior was fixed). In contrast, male tactics were strongly affected by the female morph frequency in the population. Laboratory work comparing morph-specific female fecundity revealed that androchrome females have lower fecundity than both of the gynochrome female morphs in the short term (3-days), but over a 10-day period one of the gynochrome female morphs became more fecund than either of the other morphs. In summary, our study found sex-specific dynamics in response to different morph frequencies and also highlights the importance of studying morph-specific fecundities across different time frames to gain a better understanding of the role of alternative reproductive strategies in the maintenance of female-limited color polymorphism.

GENETIC, ENVIRONMENTAL, AND CONDITION-DEPENDENT EFFECTS ON FEMALE AND MALE ORNAMENTATION IN THE BARN OWL TYTO ALBA

Secondary sexual characters are thought to indicate individual quality. Expression of sex-limited traits in an extravagant state may require both the underlying genes and the available nutrient resources. The assessment of the relative contribution of genes, environment, and body condition is relevant for understanding to that extent the extravagant trait may signal genotypic or phenotypic quality of the individual. In birds, usually only the males are ornamented. In the barn owl, Tyto alba, both females and males display sex-limited plumage traits. Males are commonly lighter colored and females spottier. In an experiment with combined cross-fostering and brood size manipulation we determined the relative contribution of genes, environment, and body condition to the variation in plumage coloration and plumage spottiness. The partial cross-fostering experiment tested the relative importance of shared genes and a shared environment for the resemblance of related birds. Siblings raised in different nests converged toward similar trait values, offspring resembled the true but not the foster parents, and plumage traits of unrelated nestlings sharing the same nest were not correlated. Results were not inflated by maternal effects detectable in the mother's phenotype, because middaughter to mother resemblance was not higher than midson to father resemblance. This suggests that plumage coloration and spottiness are largely genetically inherited traits, and that the rearing environment does not have a strong impact on the expression of these traits. To further investigate whether the two sex-limited traits are condition dependent, brood sizes were manipulated. Enlargement or reduction of broods by two nestlings resulted in lower and higher body mass of nestlings, respectively. However, nestlings raised in enlarged or reduced broods did not show either a significantly darker or lighter or a more or less spotted plumage. We did not detect any genotype-by-environment interaction. In conclusion, simultaneous cross-fostering and brood size manipulation demonstrate that additive genetic variance for plumage coloration and spottiness is maintained and that both the rearing environment and body condition do not account for a large proportion of the phenotypic variance in female and male ornamentations.

Are there consistent behavioral differences between sexes and male color morphs in Pelvicachromis pulcher?

Adult sex ratios in the kribensis cichlid (Pelvicachromis pulcher) are influenced by environmental conditions during early development. These environmental sex-determining factors may also organize life-long variation in social behavior within each sex. If this is true, then individual differences in behavior may be, at least in part, expressions of the relative strength of sexual differentiation of that individual. As adults, kribensis males take on one of four alternative color morphs. Males of the yellow morph tend toward breeding monogamously and are produced at higher frequency under female-biasing environmental conditions, while males of the red morph tend more towards breeding polygynously and are produced more frequently by male-biasing early environments. Here we test whether these two alternative kribensis male color morphs show consistent behavioral differences as predicted by an underlying behavioral syndrome of relative feminization to masculinization. We compare these males to females in five different behavioral tests: an aggression assay, an open field exploration task, a novel environment emergence task, and three cerebral lateralization tests. We hypothesize that red males will show more exaggerated sex differences across all behaviors. We find that red males are hypermasculinized as predicted with respect to aggressive behavior and activity levels, but not all behaviors follow this pattern. We find no evidence for a common behavioral syndrome underlying personality traits across females, yellow males and red males.

A model for conditional male trimorphisms

Conditional dimorphisms are widespread in color, morphology, behavior, and life history. Such traits have been successfully modeled in game theory as conditional strategies, and in quantitative genetics as threshold traits. Conditional trimorphisms have recently been unveiled, and here we combine the rock-paper-scissors (RPS) model of game theory and the environmental threshold (ET) model of quantitative genetics to model trimorphisms that are environmentally induced and result from the expression of two thresholds. We investigated the tactic fitness structure for maintenance of alternative reproductive tactics in scarab dung beetles that constitute the first known examples of conditional male trimorphism. We parameterized a novel ternary fitness landscape that explains how conditional male trimorphism in these beetles can be maintained. We tracked changes in tactic frequencies in a wild population of Phanaeus triangularis and detected fitness intransitivity consistent with RPS dynamics. Quantitative predictions of our model compare favorably with corresponding observed parameters. The ternary landscape further reveals how geographic populations of these beetles can evolve between conditional trimorphism and dimorphism. The ternary model also suggests that polyphenic systems could potentially evolve between conditional and purely genetic mediation.

Testosterone production in response to exogenous gonadotropin releasing hormone (GnRH challenge) depends on social environment and color polymorphism

Testosterone is an important mediator of behavior, morphology and physiology. A cascade of signals regulates the amount of testosterone (T) circulating in the plasma; in response to stimulus the hypothalamus releases gonadotropin-releasing hormone (GnRH), which triggers secretion of gonadotropins from the pituitary, stimulating the synthesis and release of T from the gonads. Previous work has shown that changes to the social environment can alter circulating T-levels, which may have important fitness consequences, but it is currently unclear whether these changes are due to alterations in the signal from the brain, or changes in the ability of the pituitary and gonads to respond to this signal. Further, the strength and direction of response to a changing environment may differ according to life-history strategy. Species with genetically determined alternative strategies offer a pathway for examining these differences. Here we use a finch with a genetically determined polymorphism, the Gouldian finch (Erythrura gouldiae), to determine whether T-levels change in response to social environment. We also use injections of GnRH to determine whether these changes are due to alterations in the ability of the pituitary and gonads to respond to this signal. We found that social environment (presence of females) had a rapid effect on male circulating T-levels, and that this difference was reflected in responsiveness to GnRH. We observed no overall morph differences in T-levels, but we did observe morph differences in the pattern of T secretion across environments, and morph differences in the repeatability of T-levels across time and environment.

Brain Transcriptional Profiles of Male Alternative Reproductive Tactics and Females in Bluegill Sunfish

Bluegill sunfish (Lepomis macrochirus) are one of the classic systems for studying male alternative reproductive tactics (ARTs) in teleost fishes. In this species, there are two distinct life histories: parental and cuckolder, encompassing three reproductive tactics, parental, satellite, and sneaker. The parental life history is fixed, whereas individuals who enter the cuckolder life history transition from sneaker to satellite tactic as they grow. For this study, we used RNAseq to characterize the brain transcriptome of the three male tactics and females during spawning to identify gene ontology (GO) categories and potential candidate genes associated with each tactic. We found that sneaker males had higher levels of gene expression differentiation compared to the other two male tactics. Sneaker males also had higher expression in ionotropic glutamate receptor genes, specifically AMPA receptors, compared to other males, which may be important for increased spatial working memory while attempting to cuckold parental males at their nests. Larger differences in gene expression also occurred among male tactics than between males and females. We found significant expression differences in several candidate genes that were previously identified in other species with ARTs and suggest a previously undescribed role for cAMP-responsive element modulator (crem) in influencing parental male behaviors during spawning.

Advances in finding Alba: the locus affecting life history and color polymorphism in a Colias butterfly

Although alternative life-history strategies exist within many populations, very little is known about their genetic basis and mechanistic insight into these traits could greatly advance the understanding of eco-evolutionary dynamics. Many species of butterfly within the genus Colias exhibit a sex-limited wing colour polymorphism, called Alba, which is correlated with an alternative life-history strategy. Here, we have taken the first steps in localizing the region carrying Alba in Colias croceus, a species with no genomic resources, by generating whole genome sequence of a single Alba mother and two sequencing pools, one for her Alba and another for her orange, offspring. These data were used in a bulk-segregant analysis wherein SNPs fulfilling the Mendelian inheritance expectations of Alba were identified. Then, using the conserved synteny in Lepidoptera, the Alba locus was assigned to chromosome 15 in Bombyx mori. We then identified candidate regions within the chromosome by investigating the distribution of Alba SNPs along the chromosome and the difference in nucleotide diversity in exons between the two pools. A region spanning ~ 5.7 Mbp at the 5' end of the chromosome was identified as likely to contain the Alba locus. These insights set the stage for more detailed genomic scans and mapping of the Alba phenotype, and demonstrate an efficient use of genomic resources in a novel species.

The genetic basis of discrete and quantitative colour variation in the polymorphic lizard, Ctenophorus decresii

Background

Colour polymorphic species provide invaluable insight into processes that generate and maintain intra-specific variation. Despite an increasing understanding of the genetic basis of discrete morphs, sources of colour variation within morphs remain poorly understood. Here we use the polymorphic tawny dragon lizard Ctenophorus decresii to test simple Mendelian models for the inheritance of discrete morphs, and to investigate the genetic basis of continuous variation among individuals across morphs. Males of this species express either orange, yellow, orange surrounded by yellow, or grey throats. Although four discrete morphs are recognised, the extent of orange and yellow varies greatly. We artificially elevated testosterone in F0 females and F1 juveniles to induce them to express the male throat colour polymorphism, and quantified colour variation across the pedigree.

Results

Inheritance of discrete morphs in C. decresii best fit a model whereby two autosomal loci with complete dominance respectively determine the presence of orange and yellow. However, a single locus model with three co-dominant alleles for orange, yellow and grey could not be definitively rejected. Additionally, quantitative expression of the proportion of orange and yellow on the throat was strongly heritable (orange: h² = 0.84 ± 0.14; yellow: h² = 0.67 ± 0.19), with some evidence for covariance between the two.

Conclusions

Our study supports the theoretical prediction that polymorphism should be governed by few genes of major effect, but implies broader genetic influence on variation in constituent morph traits.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0757-2) contains supplementary material, which is available to authorized users.

A flamboyant behavioral polymorphism is controlled by a lethal supergene

Two new studies show how highly divergent modes of male reproduction in a wading bird are controlled by alternate alleles at a single locus encompassing a 4.5-Mb inversion in the genome. The locus is an example of a 'supergene' controlling multiple complex phenotypes.

Flexible timing of reproductive effort as an alternative mating tactic in black grouse (Lyrurus tetrix) males

Alternative reproductive tactics often take the form of dichotomous behavioural phenotypes. Focusing attention on such obvious dichotomy means that flexible patterns of behaviour within tactics is largely ignored. Using a long-term dataset of black grouse (Lyrurus tetrix) lek behaviours, we tested whether there were fine-scale differences in reproductive effort (lek attendance, fighting rates) and whether these were related to age and phenotype. Yearling males increased their lek attendance and fighting rate to a peak when adult male effort was declining. Adults and yearlings allocated reproductive effort according to their body mass but this was unrelated to differences in timing of effort. In adult males, different patterns of lek attendance were associated with different costs of reproduction, measured by mass loss or gain. Overall, our work demonstrates that individuals can use flexible patterns of reproductive effort both in terms of their own condition, their age and the likely costs of behaviours.