The influence of mitonuclear genetic variation on personality in seed beetles

Mitochondrial genetic variation as a potential mediator of intraspecific behavioural diversity

Mitochondrial genes play an essential role in energy metabolism. Variation in the mitochondrial DNA (mtDNA) sequence often exists within species, and this variation can have consequences for energy production and organismal life history. Yet, despite potential links between energy metabolism and the expression of animal behaviour, mtDNA variation has been largely neglected to date in studies investigating intraspecific behavioural diversity. We outline how mtDNA variation and interactions between mitochondrial and nuclear genotypes may contribute to the expression of individual-to-individual behavioural differences within populations, and why such effects may lead to sex differences in behaviour. We contend that integration of the mitochondrial genome into behavioural ecology research may be key to fully understanding the evolutionary genetics of animal behaviour.

Behavioural Repeatability and Behavioural Syndrome in the Dung Beetle Copris umbilicatus (Coleoptera, Scarabaeidae)

Although personality studies have primarily focused on vertebrates, the evidence showing invertebrates to be capable of displaying personalities has been steadily growing in recent years. In this study, we investigated the behavioural repeatability (repetition of a behaviour over time) and behavioural syndromes (a set of correlated behaviours) in Copris umbilicatus, which is a dung beetle species showing complex sub-social behaviour. We analysed three behaviours (activity, thanatosis and distress call emission) by measuring seven distinct behavioural traits (i.e., three activity-, one thanatosis- and three distress call-related traits). We found moderate to high levels of individual repeatability in all behavioural traits considered. The duration of thanatosis was inversely correlated with two activity traits, hinting a behavioural syndrome for thanatosis and activity, with bolder individuals exhibiting shorter thanatosis and higher locomotor activity in contrast with fearful individuals, which display longer thanatosis and poor locomotor activity. No relationships were found between the behavioural traits and body size or sex. Results of the principal component analysis (PCA) suggested personality differences among individuals. Dung beetles provide an impressive variety of ecosystem services. Since the provision of these services may depend on the personalities represented in local populations and communities, studies on the ecology of personality in dung beetles should be encouraged in future research.

Concerted evolution of metabolic rate, economics of mating, ecology, and pace of life across seed beetles

Coevolution between females and males has led to remarkable differences between the sexes but has taken very different routes, even in closely related animal species, for reasons that are not well understood. We studied the physiological processes that convert resources into offspring (metabolism) in males and females of several related beetle species. We found that ecological factors dictate metabolic rate, which, in turn, have predictable direct and indirect effects on male–female coevolution. Our findings suggest that a complete understanding of differences between the sexes requires an understanding of how ecology affects metabolic processes and how these differ in the sexes.

Male–female coevolution has taken different paths among closely related species, but our understanding of the factors that govern its direction is limited. While it is clear that ecological factors, life history, and the economics of reproduction are connected, the divergent links are often obscure. We propose that a complete understanding requires the conceptual integration of metabolic phenotypes. Metabolic rate, a nexus of life history evolution, is constrained by ecological factors and may exert important direct and indirect effects on the evolution of sexual dimorphism. We performed standardized experiments in 12 seed beetle species to gain a rich set of sex-specific measures of metabolic phenotypes, life history traits, and the economics of mating and analyzed our multivariate data using phylogenetic comparative methods. Resting metabolic rate (RMR) showed extensive evolution and evolved more rapidly in males than in females. The evolution of RMR was tightly coupled with a suite of life history traits, describing a pace-of-life syndrome (POLS), with indirect effects on the economics of mating. As predicted, high resource competition was associated with a low RMR and a slow POLS. The cost of mating showed sexually antagonistic coevolution, a hallmark of sexual conflict. The sex-specific costs and benefits of mating were predictably related to ecology, primarily through the evolution of male ejaculate size. Overall, our results support the tenet that resource competition affects metabolic processes that, in turn, have predictable effects on both life history evolution and reproduction, such that ecology shows both direct and indirect effects on male–female coevolution.

A meta-analysis of sex differences in animal personality: no evidence for the greater male variability hypothesis

The notion that men are more variable than women has become embedded into scientific thinking. For mental traits like personality, greater male variability has been partly attributed to biology, underpinned by claims that there is generally greater variation among males than females in non-human animals due to stronger sexual selection on males. However, evidence for greater male variability is limited to morphological traits, and there is little information regarding sex differences in personality-like behaviours for non-human animals. Here, we meta-analysed sex differences in means and variances for over 2100 effects (204 studies) from 220 species (covering five broad taxonomic groups) across five personality traits: boldness, aggression, activity, sociality and exploration. We also tested if sexual size dimorphism, a proxy for sex-specific sexual selection, explains variation in the magnitude of sex differences in personality. We found no significant differences in personality between the sexes. In addition, sexual size dimorphism did not explain variation in the magnitude of the observed sex differences in the mean or variance in personality for any taxonomic group. In sum, we find no evidence for widespread sex differences in variability in non-human animal personality.

Mitochondrial polymorphism shapes intrapopulation behavioural variation in wild Drosophila

Intrapopulation variation in behaviour, including activity, boldness and aggressiveness, is becoming more widely recognized and is hypothesized to substantially affect ecological and evolutionary dynamics. Although previous studies used candidate-gene approaches and genome-wide association analyses to identify genes correlated with variations in activity and aggressiveness, behavioural variation may not be fully captured in the nuclear genome, as it does not account for mitochondrial genomes. Mitochondrial genes encode products that are key regulators of the cellular energy-producing pathways in metabolic processes and are thought to play a significant role in life-history and reproductive traits. In this study, we considered many isofemale lines of Drosophila immigrans established from two wild populations to investigate whether intrapopulation variation in the mitochondrial genome affected activity level within this species. We identified two major haplogroups in these populations, and activity levels in both larvae and adults differed significantly between the two haplogroups. This result indicated that intrapopulation variation in activity level may be partially controlled by mitochondrial genes, along with the interaction between nuclear and mitochondrial genes and the age of individual organisms.

Lithium influences whole-organism metabolic rate in Drosophila subobscura

Lithium is widely used to treat bipolar disorder. However, the efficacy and vulnerability as to its side effects are known to differ. Although the specific biochemical mechanism of action is still elusive, lithium may influence mitochondrial function, and consequently, metabolism. Lithium exposure in this study was conducted on a unique set of mito-nuclear introgression lines of Drosophila subobscura to disentangle the independent effects of mitochondrial DNA (mtDNA) against a common nuclear DNA background. The study addressed three issues: (a) whether lithium has a dose-dependent effect on whole-organism metabolic rate, (b) whether mtDNA haplotypes show divergent metabolic efficiency measured by metabolic rate to lithium exposure and (c) whether lithium influences the whole-organism metabolic rate across sexes. The results confirm that lithium influenced the whole-organism metabolic rate, showing a subtle balance between efficacy and adverse effects within a narrow dose range. In addition, lithium exposure was found to influence metabolism differently based on mtDNA haplotypes and sex. This preliminary research may have a range of biological implications for the role of mitochondrial variability in psychiatric disease and treatment by contributing to the understanding and predicting of the lithium treatment response and risk for toxic side effects.

Drosophila as a Model System to Investigate the Effects of Mitochondrial Variation on Innate Immunity

Understanding why the response to infection varies between individuals remains one of the major challenges in immunology and infection biology. A substantial proportion of this heterogeneity can be explained by individual genetic differences which result in variable immune responses, and there are many examples of polymorphisms in nuclear-encoded genes that alter immunocompetence. However, how immunity is affected by genetic polymorphism in an additional genome, inherited maternally inside mitochondria (mtDNA), has been relatively understudied. Mitochondria are increasingly recognized as important mediators of innate immune responses, not only because they are the main source of energy required for costly immune responses, but also because by-products of mitochondrial metabolism, such as reactive oxygen species (ROS), may have direct microbicidal action. Yet, it is currently unclear how naturally occurring variation in mtDNA contributes to heterogeneity in infection outcomes. In this review article, we describe potential sources of variation in mitochondrial function that may arise due to mutations in vital nuclear and mitochondrial components of energy production or due to a disruption in mito-nuclear crosstalk. We then highlight how these changes in mitochondrial function can impact immune responses, focusing on their effects on ATP- and ROS-generating pathways, as well as immune signaling. Finally, we outline how being a powerful and genetically tractable model of infection, immunity and mitochondrial genetics makes the fruit fly Drosophila melanogaster ideally suited to dissect mitochondrial effects on innate immune responses to infection.

Description of a personality syndrome in a common and invasive ground beetle (Coleoptera: Carabidae)

Animal personality, defined as consistent differences among individuals in their behaviour, is being increasingly studied as it might lead to a new understanding of the evolution of behaviours. Despite a clear interest in studying personality in a wide range of taxa for comparative analyses, studies on invertebrates are still scarce. Here, we investigated the personality of a ground beetle, Nebria brevicollis, which is widespread in Europe and invasive in North America. We measured seven behavioural traits from an array of three different tests: (i) activity and exploration related traits; (ii) reaction to a threat, and (iii) phototaxis. The repeatability was tested by measuring all behaviours twice, on different days. All behavioural traits were consistent through time, highlighting the presence of personality in the beetle. In addition, we analysed the relationship between the different traits and highlighted two clusters of behaviours (behavioural syndrome), one grouping activity, exploration and boldness traits, and a second one consisting of responses to a threat. This study is the first to our knowledge to provide evidence for personality dimensions within the vast group of the Carabidae. It also constitutes a preliminary step in the experimental investigation of the importance of animal personality in invasive species.

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.

The Evolution of Dark Matter in the Mitogenome of Seed Beetles

Animal mitogenomes are generally thought of as being economic and optimized for rapid replication and transcription. We use long-read sequencing technology to assemble the remarkable mitogenomes of four species of seed beetles. These are the largest circular mitogenomes ever assembled in insects, ranging from 24,496 to 26,613 bp in total length, and are exceptional in that some 40% consists of non-coding DNA. The size expansion is due to two very long intergenic spacers (LIGSs), rich in tandem repeats. The two LIGSs are present in all species but vary greatly in length (114-10,408 bp), show very low sequence similarity, divergent tandem repeat motifs, a very high AT content and concerted length evolution. The LIGSs have been retained for at least some 45 my but must have undergone repeated reductions and expansions, despite strong purifying selection on protein coding mtDNA genes. The LIGSs are located in two intergenic sites where a few recent studies of insects have also reported shorter LIGSs (>200 bp). These sites may represent spaces that tolerate neutral repeat array expansions or, alternatively, the LIGSs may function to allow a more economic translational machinery. Mitochondrial respiration in adult seed beetles is based almost exclusively on fatty acids, which reduces the need for building complex I of the oxidative phosphorylation pathway (NADH dehydrogenase). One possibility is thus that the LIGSs may allow depressed transcription of NAD genes. RNA sequencing showed that LIGSs are partly transcribed and transcriptional profiling suggested that all seven mtDNA NAD genes indeed show low levels of transcription and co-regulation of transcription across sexes and tissues.

Personality in the cockroach Diploptera punctata: Evidence for stability across developmental stages despite age effects on boldness

Despite a recent surge in the popularity of animal personality studies and their wide-ranging associations with various aspects of behavioural ecology, our understanding of the development of personality over ontogeny remains poorly understood. Stability over time is a central tenet of personality; ecological pressures experienced by an individual at different life stages may, however, vary considerably, which may have a significant effect on behavioural traits. Invertebrates often go through numerous discrete developmental stages and therefore provide a useful model for such research. Here we test for both differential consistency and age effects upon behavioural traits in the gregarious cockroach Diploptera punctata by testing the same behavioural traits in both juveniles and adults. In our sample, we find consistency in boldness, exploration and sociality within adults whilst only boldness was consistent in juveniles. Both boldness and exploration measures, representative of risk-taking behaviour, show significant consistency across discrete juvenile and adult stages. Age effects are, however, apparent in our data; juveniles are significantly bolder than adults, most likely due to differences in the ecological requirements of these life stages. Size also affects risk-taking behaviour since smaller adults are both bolder and more highly explorative. Whilst a behavioural syndrome linking boldness and exploration is evident in nymphs, this disappears by the adult stage, where links between other behavioural traits become apparent. Our results therefore indicate that differential consistency in personality can be maintained across life stages despite age effects on its magnitude, with links between some personality traits changing over ontogeny, demonstrating plasticity in behavioural syndromes.

Negative frequency dependent selection on sympatric mtDNA haplotypes in Drosophila subobscura

BACKGROUND

Recent experimental evidence for selection on mitochondrial DNA (mtDNA) has prompted the question as to what processes act to maintain within-population variation in mtDNA. Balancing selection though negative frequency dependent selection (NFDS) among sympatric haplotypes is a possibility, but direct empirical evidence for this is very scarce.

FINDINGS

We extend the previous findings of a multi-generation replicated cage experiment in Drosophila subobscura, where mtDNA polymorphism was maintained in a laboratory setting. First, we use a set of Monte Carlo simulations to show that the haplotype frequency dynamics observed are inconsistent with genetic drift alone and most closely match those expected under NFDS. Second, we show that haplotype frequency changes over time were significantly different from those expected under either genetic drift or positive selection but were consistent with those expected under NFSD.

CONCLUSIONS

Collectively, our analyses provide novel support for NFDS on mtDNA haplotypes, suggesting that mtDNA polymorphism may at least in part be maintained by balancing selection also in natural populations. We very briefly discuss the possible mechanisms that might be involved.

Divergent evolution of life span associated with mitochondrial DNA evolution

Mitochondria play a key role in ageing. The pursuit of genes that regulate variation in life span and ageing have shown that several nuclear-encoded mitochondrial genes are important. However, the role of mitochondrial encoded genes (mtDNA) is more controversial and our appreciation of the role of mtDNA for the evolution of life span is limited. We use replicated lines of seed beetles that have been artificially selected for long or short life for >190 generations, now showing dramatic phenotypic differences, to test for a possible role of mtDNA in the divergent evolution of ageing and life span. We show that these divergent selection regimes led to the evolution of significantly different mtDNA haplotype frequencies. Selection for a long life and late reproduction generated positive selection for one specific haplotype, which was fixed in most such lines. In contrast, selection for reproduction early in life led to both positive selection as well as negative frequency-dependent selection on two different haplotypes, which were both present in all such lines. Our findings suggest that the evolution of life span was in part mediated by mtDNA, providing support for the emerging general tenet that adaptive evolution of life-history syndromes may involve mtDNA.

Sexually antagonistic selection on genetic variation underlying both male and female same-sex sexual behavior

BACKGROUND

Intralocus sexual conflict, arising from selection for different alleles at the same locus in males and females, imposes a constraint on sex-specific adaptation. Intralocus sexual conflict can be alleviated by the evolution of sex-limited genetic architectures and phenotypic expression, but pleiotropic constraints may hinder this process. Here, we explored putative intralocus sexual conflict and genetic (co)variance in a poorly understood behavior with near male-limited expression. Same-sex sexual behaviors (SSBs) generally do not conform to classic evolutionary models of adaptation but are common in male animals and have been hypothesized to result from perception errors and selection for high male mating rates. However, perspectives incorporating sex-specific selection on genes shared by males and females to explain the expression and evolution of SSBs have largely been neglected.

RESULTS

We performed two parallel sex-limited artificial selection experiments on SSB in male and female seed beetles, followed by sex-specific assays of locomotor activity and male sex recognition (two traits hypothesized to be functionally related to SSB) and adult reproductive success (allowing us to assess fitness consequences of genetic variance in SSB and its correlated components). Our experiments reveal both shared and sex-limited genetic variance for SSB. Strikingly, genetically correlated responses in locomotor activity and male sex-recognition were associated with sexually antagonistic fitness effects, but these effects differed qualitatively between male and female selection lines, implicating intralocus sexual conflict at both male- and female-specific genetic components underlying SSB.

CONCLUSIONS

Our study provides experimental support for the hypothesis that widespread pleiotropy generates pervasive intralocus sexual conflict governing the expression of SSBs, suggesting that SSB in one sex can occur due to the expression of genes that carry benefits in the other sex.

Direct and indirect genetic effects of sex-specific mitonuclear epistasis on reproductive ageing

Mitochondria are involved in ageing and their function requires coordinated action of both mitochondrial and nuclear genes. Epistasis between the two genomes can influence lifespan but whether this also holds for reproductive senescence is unclear. Maternal inheritance of mitochondria predicts sex differences in the efficacy of selection on mitonuclear genotypes that should result in differences between females and males in mitochondrial genetic effects. Mitonuclear genotype of a focal individual may also indirectly affect trait expression in the mating partner. We tested these predictions in the seed beetle Callosobruchus maculatus, using introgression lines harbouring distinct mitonuclear genotypes. Our results reveal both direct and indirect sex-specific effects of mitonuclear epistasis on reproductive ageing. Females harbouring coadapted mitonuclear genotypes showed higher lifetime fecundity due to slower senescence relative to novel mitonuclear combinations. We found no evidence for mitonuclear coadaptation in males. Mitonuclear epistasis not only affected age-specific ejaculate weight, but also influenced male age-dependent indirect effects on traits expressed by their female partners (fecundity, egg size, longevity). These results demonstrate important consequences of sex-specific mitonuclear epistasis for both mating partners, consistent with a role for mitonuclear genetic constraints upon sex-specific adaptive evolution.

Bolder Takes All? The Behavioral Dimension of Biogeography

Animal personality can be seen as behavioral polymorphism that could play a direct and active role in driving evolutionary pathways. We argue here that consistent individual differences in key personality traits affecting dispersal and other density-dependent processes have provided substantial contributions to molding biogeographic patterns. Building upon opportunities recently opened by genomics and other novel approaches, we explore the hypothesis that Pleistocene range expansions, island colonizations, and other historical biogeographic processes could have been promoted by non-random samples of behavioral types of the founder populations. We provide context and testable hypotheses, based on case studies, that could bring new implications to our understanding of the processes shaping spatial and temporal patterns of variation in animal biodiversity.

Sex-specific effects of sympatric mitonuclear variation on fitness in Drosophila subobscura

Background

A number of recent studies have shown that the pattern of mitochondrial DNA variation and evolution is at odds with a neutral equilibrium model. Theory has suggested that selection on mitonuclear genotypes can act to maintain stable mitonuclear polymorphism within populations. However, this effect largely relies upon selection being either sex-specific or frequency dependent. Here, we use mitonuclear introgression lines to assess differences in a series of key life-history traits (egg-to-adult developmental time, viability, offspring sex-ratio, adult longevity and resistance to desiccation) in Drosophila subobscura fruit flies carrying one of three different sympatric mtDNA haplotypes.

Results

We found functional differences between these sympatric mtDNA haplotypes, but these effects were contingent upon the nuclear genome with which they were co-expressed. Further, we demonstrate a significant mitonuclear genetic effect on adult sex ratio, as well as a sex × mtDNA × nuDNA interaction for adult longevity.

Conclusions

The observed effects suggest that sex specific mitonuclear selection contributes to the maintenance of mtDNA polymorphism and to mitonuclear linkage disequilibrium in this model system.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0421-2) contains supplementary material, which is available to authorized users.

Within-population genetic effects of mtDNA on metabolic rate in Drosophila subobscura

A growing body of research supports the view that within-species sequence variation in the mitochondrial genome (mtDNA) is functional, in the sense that it has important phenotypic effects. However, most of this empirical foundation is based on comparisons across populations, and few studies have addressed the functional significance of mtDNA polymorphism within populations. Here, using mitonuclear introgression lines, we assess differences in whole-organism metabolic rate of adult Drosophila subobscura fruit flies carrying either of three different sympatric mtDNA haplotypes. We document sizeable, up to 20%, differences in metabolic rate across these mtDNA haplotypes. Further, these mtDNA effects are to some extent sex specific. We found no significant nuclear or mitonuclear genetic effects on metabolic rate, consistent with a low degree of linkage disequilibrium between mitochondrial and nuclear genes within populations. The fact that mtDNA haplotype variation within a natural population affects metabolic rate, which is a key physiological trait with important effects on life-history traits, adds weight to the emergent view that mtDNA haplotype variation is under natural selection and it revitalizes the question as to what processes act to maintain functional mtDNA polymorphism within populations.