Molecular Ecology

Demographic history is expected to play a central role in shaping population vulnerability to climate change through its lasting effects on effective population sizes and genetic connectivity. However, existing studies report contrasting outcomes, and the consequences of alternative demographic histories have seldom been assessed concurrently across multiple taxa. Here, we analysed population genomic data from six of the major European forest tree species to address this gap. We found that, across all species, more genetically isolated populations showed reduced adaptive potential, as measured by standing levels of genetic diversity. Furthermore, populations with reduced historical gene flow and higher genetic differentiation exhibited higher genetic load and suboptimal climate adaptation, particularly in small, fragmented populations, potentially increasing their sensitivity to climate change. Finally, the beneficial effects of gene flow were evidenced by the absence of greater suboptimal climate adaptation in highly connected populations across the six species. Altogether, our results provide valuable insights into how genetic differentiation, reflecting the combined effects of genetic drift and limited historical gene flow, influences current vulnerability of forest tree populations to climate change.

Trait-selective harvesting by fisheries can impose strong selective pressures on fish populations, driving changes in life history traits affecting fisheries productivity and ecosystem functioning. While the genetic consequences of harvesting have been extensively studied, the extent to which phenotypic variation reflects genomic evolution versus environmentally-induced plasticity remains unclear. Epigenetic mechanisms, such as DNA methylation, may mediate between these processes, serving as a rapid and reversible response to the selective pressures imposed by harvesting. In this study, we implemented an improved laboratory and bioinformatics protocol, epiGBS3, to examine genomic variation and DNA methylation patterns in the marine fish Xyrichtys novacula. The study spanned three replicated geographical areas each comprising two adjacent locations: an intensively exploited fishery and a no-take Marine Protected Area (ntMPA). A nested analysis design across the three areas revealed strong gene flow and no evidence of genetic structure. Nevertheless, nucleotide diversity was significantly reduced in fisheries relative to ntMPAs. We also found that DNA methylation levels differed between protected and exploited sites after controlling for age, suggesting that fishing may influence epigenetic changes independently of fisheries-induced age-truncation effects. This represents one of the first lines of evidence that fisheries can potentially shape epigenetic variation, supporting DNA methylation as contributor to local adaptation under high gene flow and strong anthropogenic selection.

Laboratory and field populations of insects can experience a decline in fitness and loss of genetic diversity due to inbreeding depression and genetic drift, respectively. Matings among related individuals and small population size may also influence insect host microbiomes with consequences for fitness. In the dengue vector mosquito, Aedes aegypti, the bacterial microbiome is largely environmentally determined but recent studies have also revealed host genetic components. We generated a panel of 55 inbred lines from either of two founding outbred populations of Ae. aegypti to test for associations between life history traits, inbreeding, allelic diversity and microbiome composition using ddRADseq and bacterial 16S rRNA gene sequencing on pools of mosquitoes. Effects of inbreeding were diverse with severe composite fitness costs in many lines but minimal costs in others despite similar low levels of genetic diversity. We found no strong relationship between major life history traits across inbred lines, suggesting that any costs due to inbreeding were trait specific. Bacterial microbiome analysis of pooled samples from a subset of populations revealed common microbes across populations, with Elizabethkingia, Aeromonas and Ralstonia being the most abundant. Despite bacterial composition varying widely, there was no clear relationship between microbiome composition and fitness or population origin. However, there were several significant positive correlations between the relative abundance of different microbial taxa across lines. Our results demonstrate diverse impacts of inbreeding on fitness of mosquito populations but with limited impacts on the microbiome.

AO_SCPLOWBSTRACTC_SCPLOWMigration events can act as strong selective filters by spatially sorting individuals according to their migration ability, behaviour, and associated functional traits. The European eel, a panmictic and threatened fish, presents various estuarine migration patterns at juvenile stage (glass eel), ranging from sedentarization in brackish/saltwater of the estuary (non-migrant phenotype) to upstream colonisation of freshwater ecosystems (migrant phenotype). We hypothesize that migration propensity is partly genetically determined in glass eel, and that migration-related genotypes are spatially sorted during estuarine migration. To test these hypotheses, we first collected six pools of individuals over three years at two extreme sites along a gradient from ocean to Adour River tidal limit (Ocean vs. Upstream). Secondly, we collected additional glass eels and phenotypically sorted migrant vs. non-migrant individuals using an experimental device mimicking alternating tidal currents, producing two other pools. Whole genome pool sequencing and analysis of these eight pools generated 18.99 106 SNP variants. Controlling for linked selection through a local score approach, we found five best outlier SNPs with a significant genetic differentiation between Ocean vs. Upstream sites (average FST = 0.21) compared to the pangenomic estimate (FST = 0.0086). These five SNPs were all found in the same gene (gpb2), involved in interferon-mediated antiviral immune responses. We also found 28 best outlier SNPs with a significant genetic differentiation between migrant vs. non-migrant phenotypes (average FST = 0.51). They were located in genes mainly involved in neuronal development, cell migration and tissue remodelling, transcriptional regulation, and metabolic or stress-related processes. Our results support that variation in eel migration propensity is partly genetically determined and that, while panmixia maintains high level of genetic diversity, spatial sorting could promote intra-generational genetic divergence between habitats of European eels. However, the absence of shared genes among the best outliers between in-situ and experimental contrasts suggests a complex and context-dependent genetic control of migration.

Introgressive hybridization between wild and domestic animals is a widespread phenomenon with important implications for genetic diversity, local adaptation, and conservation management. The causes and consequences of this process are poorly understood. In Australia, hybridization between dingoes and domestic dogs presents a dual conservation challenge, threatening the genetic integrity of dingoes while allowing potential adaptive introgression. To investigate the environmental drivers of this process, we analyzed high-density SNP array data in 390 dingoes and 396 domestic dogs. Dingo populations showed regional genetic structure and were clearly differentiated from domestic dogs. Using local ancestry inference and genome-environment association analyses, we found low levels of dog introgression in dingoes from remote areas in Central and Western Australia, and moderate levels in Eastern and Southern populations. Climatic variables (maximum temperature of the warmest month, mean temperature of the driest quarter) and the Human Footprint Index (reflecting density of human populations and environmental modifications) were significant predictors of introgression. We identified four genomic regions with overrepresented dog ancestry, including a large introgressed block on chromosome 27, which contained an olfactory receptor gene showing signatures of positive selection, suggesting adaptive introgression. In addition, a chromosomal inversion previously described in dogs and absent in dingoes was initially identified as an introgressed block. We also detected eight genomic regions nearly free of dog ancestry, suggesting purifying selection against maladaptive variants. Together, these results highlight the complex interplay between introgression, human influence, and local adaptation in dingoes, offering valuable insights for conserving the evolutionary potential of this apex predator in increasingly modified landscapes.

Climate change is a growing threat to biodiversity, and the persistence of populations largely depends on their capacity to adapt to changing environmental conditions. Although there is an urgent need to forecast population adaptive potential, it is unclear how such predictions are affected by the genomic architectures underlying local adaptation across a species range. In this study, we examine the genomic basis of local adaptation of the short-distance dispersing coral Stylophora pistillata, sampled at forty-six sites across eight reefs of the Great Barrier Reef, Australia. Our results show that thermal adaptation for this species involves hundreds of genomic loci with combinations that differ across regions. Although adaptive loci were largely region-specific, genotype-environment relationships estimated across the range provided sensible predictions of regional-level local adaptation. Additionally, predicted shifts in genotype-environment associations under increased projected warming were highly spatially variable, both between and within geographic regions. While some Great Barrier Reef S. pistillata populations might be well-adapted for near-future (2050) and moderate (SSP1-2.6 and SSP2-4.5) climate warming, up to 30% may face severe maladaptation risk by 2100 under a high-emission (SSP5-8.5) scenario. Collectively, these findings offer new insights into the spatial distribution of adaptive potential in coral populations and how it might shape their resilience in a warming ocean.

Kelp forests are widely distributed along temperate and polar coastlines worldwide and are among the worlds most productive and diverse marine ecosystems. Yet, due in part to ocean warming, they are declining and even disappearing in many parts of the world. While genomic tools can identify local adaptation and predict species responses to global change, these predictions have rarely been validated in the field, hampering their widespread use in conservation practice. Here, we applied a seascape genomics approach to investigate environmental adaptation in the two main canopy-forming species of the Northeast Pacific, Macrocystis tenuifolia and Nereocystis luetkeana. We leveraged whole-genome sequences of 598 individuals across 94 sites along the British Columbia and Washington coasts, together with 37 environmental variables. Both species showed genomic signatures of local adaptation, with distinct environmental drivers shaping adaptation in each species despite their co-occurrence across much of the studied area. Using gradient forests, we modelled the genetic turnover across environmental gradients and predicted populations vulnerability (genomic offset) under projected environmental conditions. Genomic offsets differed greatly among regions and were positively correlated with kelp declines observed to date, especially in Macrocystis, validating the link between genomic models and outcomes in the field and allowing us to translate genomic predictions into an ecologically meaningful metric: the risk of extirpation under global change. Our models predict that assisted migration could significantly attenuate kelps vulnerability to global change. Across environmentally heterogenous coastlines, short-distance migration can often substantially reduce future genomic-environmental mismatches, but in many cases, long-distance migration would be most beneficial. Our results highlight the potential of seascape genomics to predict vulnerability of populations to global change. Importantly, the validated link between our genomic models and ecological outcomes allows quantification of climate-driven extirpation risk and can inform conservation strategies to improve the resilience and sustainable management of these vulnerable ecosystems.

Structural variants (SVs) are important components of genetic architecture, expanding beyond traditionally used single nucleotide polymorphisms (SNPs). Though growing, our understanding of the evolutionary forces maintaining SVs in natural populations is limited. Inversions in particular can facilitate local adaptation in populations with high gene flow, including many marine species. The purple sea urchin (Strongylocentrotus purpuratus) spans a broad latitudinal range with diverse environmental conditions and is known for its high genetic diversity, making it an ideal system for studying inversion polymorphism, as well as being a crucial foundation species for marine ecosystems. We used low-coverage whole genome sequence data from 140 individuals from seven populations to identify structural variants using a range of methods including local PCA. We integrated Bayesian selection scans to test for local adaptation in putative inversions. We identified nine loci of interest as putative inversions, three of which overlap with areas of the genome associated with local adaptation scans. Additionally, we find functional enrichment within these regions associated with biomineralisation and development. Our results are the first instance of identifying putative inversions in the purple sea urchin and add to the genomic repertoire of model species. This study offers a valuable resource for future research and reinforces the growing evidence that chromosomal inversions represent a fundamental component of standing genetic variation in natural populations, with important implications for the study of local adaptation. Significance statementChromosomal inversions are an important part of the repertoire of standing genetic variation in wild populations and can facilitate adaptation despite strong gene flow. The purple sea urchin, a widely studied marine species, exhibits high gene flow, large population sizes, and extensive genetic diversity across diverse environmental conditions, making it an ideal model for evolutionary genomics. We identified nine putative inversions, three with signatures of selection, adding echinoderms to the growing list of phyla with putatively adaptive inversions. These findings provide new insights into structural variation in a highly dispersive marine species and highlight potential evolutionary mechanisms maintaining these polymorphisms.

Understanding the genetic basis of polygenic adaptation is challenging. Populations that undergo parallel evolution serve as experimental replications enabling the study of molecular mechanism. Apis cerana is widely-distributed in Asia and has repeatedly colonized the Qinghai-Tibet Plateau. The highland populations are derived from lowland colonies while showed not admixture with each other, representing independent events of colonization. We investigated resequencing genomes of five populations to study the genetic basis of highland adaptation in A. cerana. Using two complementary methods, we isolated genes with adaptive signals in each lineage. Although large proportion of adaptive loci are unique to each population, most potentially adaptive polymorphism shared with genetic variation in lowland populations, consistent with widespread signals of soft selection sweeps. Whereas parallelism was low at the level of adaptive loci, it was greater for functional pathways and greatest for phenotypes. Further enrichment analysis found the shared adaptive loci were overrepresented in pathways related to the development of sensory system and body morphogenesis. These highly connected pathways and loci could buffer different genetic paths and maintain developmental stability. Adaptive signals in the development-related loci suggest stabilizing selection further drive phenotypic convergence under similar stress. However, lineage-specific loci and pathways facilitated adaptive divergence in each lineage within the broadly similar highland environment. Our results demonstrate the genetic redundancy of highland adaptation and lineage-specific evolution via independent colonization in A. cerana. This underscores that predicting a populations adaptive potential requires understanding its full adaptive architecture within the ecological context--including abiotic and biotic interactions.

The European Pleistocene populations of the narrow-headed vole (Stenocranius gregalis), an index species of the Palearctic glacial communities, were recently found to differ from the extant Asian species by a deep genetic divergence and are to be considered a separate species, Stenocranius anglicus, which had to persist through the interglacial stages in local European refugia. Here, we analyze over 2000 first lower molars from 14 stratified localities in the Czech Republic and Slovakia, spanning the Middle Pleistocene to Holocene, employing geometric morphometrics, biometric measurements, and morphotype classifications to assess molar shape variation. Our results demonstrate persistent morphological variability, with particularly high morphotype diversity during MIS 5-3, followed by simplification and reduced variance in post-LGM populations. Morphological divergence was greater among geographic localities than stratigraphic stages, suggesting strong regional and ecological influences. Stratified sequences reveal diverse evolutionary trajectories from long-term morphological stability in refugia to gradual simplification preceding extinction in the early Holocene. These patterns align with broader Eurasian trends but also highlight regionally specific responses to climatic and ecological change accompanying the species extinction dynamics during the early to middle Holocene. The paper underscores the importance of integrating detailed morphometrics with stratigraphic and ecological evidence to shed light on these topics.

Physical barriers are well known to restrict gene flow and generate population structure, yet what drives genetic differentiation in the absence of such barriers remains less understood. In these cases, long-term climatic fluctuations may shape genomic variation by altering habitat connectivity through time. The Western Ghats mountains in Peninsular India, marked by high endemism and in-situ radiations, provide a compelling natural laboratory to understand how historical climate change can shape genetic diversity. While the role of topographic barriers in generating diversity in this landscape is well documented, the influence of paleoclimatic processes has rarely been examined, especially from a genomic perspective. Here, we combine genome-wide SNPs and ecological niche modelling with present and past climatic data to test the role of elevation, geographic distance, environment and paleohabitat dynamics in shaping the genetic diversity and structure in a wet-adapted tarantula species endemic mostly to the central Western Ghats. Despite overall genomic admixture and continuity, populations show little north-south structuring, and genetically distinct central populations. Paleoclimate projections from the present to the Last Glacial Maximum reveal that northern and southern regions maintained stable habitat suitability for Thrigmopoeus, whereas central regions experienced high temporal variability. Linear mixed models identify historical stability of suitable habitats as the strongest predictor of genetic structure. Central populations occupying historically unstable habitats also show reduced heterozygosity, elevated inbreeding, and smaller historical effective population sizes. These results demonstrate that, even in the absence of physical barriers, long-term climate dynamics can generate and maintain fine-scale, within-species genetic diversity.

Despite the coppery titi monkey (Plecturocebus cupreus) being a model system for the study of neurodevelopment and behavior, the evolutionary forces shaping observed levels and patterns of genetic variation in the species have remained poorly studied. In order to illuminate the pervasive eCects of purifying and background selection, we have fit a distribution of fitness eCects of newly arising exonic mutations, utilizing patterns of polymorphism and divergence based on a recently published high-quality genome assembly. To further characterize episodically acting selective processes, we additionally performed the first whole-genome scans for recent positive and balancing selection in this species, reducing false-positive rates by incorporating the demographic history of the population into an evolutionary null model. These scans identified a small number of biomedically-relevant genes with strong statistical support for having experienced recent selective sweeps or long-term balancing selection. In addition, we identified four genomic deletions bearing the signatures of balancing selection. Taken together, this study provides the first insights into patterns of persistent and episodic selective processes in this species.

Adaptive tracking is an evolutionary process in which allele frequencies and phenotypes shift in response to temporally fluctuating environments. Currently it is unclear whether adaptive tracking causes predictable evolution of complex traits such as thermal tolerance. We investigated seasonal adaptive tracking of critical thermal minimum (CTmin) and genome-wide allele frequencies over multiple years in the invasive fly Drosophila suzukii. CTmin increased throughout the growing season, showing a lag of several generations between increasing temperature and evolutionary change. Genetic analyses indicate CTmin is highly polygenic, with little overlap between alleles associated with CTmin and other seasonally fluctuating alleles. Thus, polygenic traits may track seasonal environments without leaving strong genomic signals. By contrast, there were strong seasonal genomic signatures for alleles associated with oligogenic traits as such pesticide resistance and olfactory behavior. These findings suggest that seasonal adaptive tracking shapes a broad suite of traits that contribute to D. suzukiis invasion success.

Codiversification often arises when hosts and their endosymbionts share a linked evolutionary history, exhibit vertical transmission, or share ecological and biogeographic processes. Most studies on the codiversification of carpenter ants (genus Camponotus) have focused on the co-phylogeny of hosts and endosymbionts across multiple species; however, no studies have examined the intraspecific population-level phylogeographic patterns of codiversification within Camponotus. California has been a geographic focus for phylogeographic studies due to its high endemism and complex geographic structure, and Camponotus laevigatus is a carpenter ant primarily found there. Here, we used whole-genome sequencing from C. laevigatus and its endosymbiont, Blochmaniella to investigate phylogeographic patterns of host-endosymbiont codiversification and estimated kinship of ants sampled near one another. We identified three phylogeographic clusters and isolation-by-distance analyses indicated a positive relationship between genetic and geographic distance in C. laevigatus and Blochmaniella. Using estimates of effective migration surfaces, we found that the Central Valley in California acts as a significant barrier to gene flow among populations. Our phylogenetic analyses revealed the congruent phylogenies of C. laevigatus and Blochmaniella, supporting codiversification. We also estimated kinship among individuals from the same and nearby sampling sites; kinship results indicated full-sister relationships among individuals from the same sampling site, except for three pairwise comparisons, and foragers from nearby sampling sites displayed some shared kinship. Lastly, our demographic analysis revealed a Pleistocene divergence, highlighting the role of Quaternary climatic cycles in shaping the population structure of C. laevigatus.

Immunity against pathogens is costly and can adversely affect fitness through resource allocation or physiological trade-offs. Infection by multiple pathogens may further worsen the effects if hosts require the activation of multiple immune responses, which in turn can elevate the overall immunological costs. Poor body condition and stressful environments can also exacerbate these trade-offs. In this study, we experimentally tested these possibilities using Tribolium castaneum populations that were evolving against either a single or a set of coinfecting bacterial pathogens. Contrary to our expectations, none of these evolved beetles showed any measurable trade-offs between pathogen resistance vs major fitness traits, such as reproduction or lifespan. Instead, they appeared to increase reproductive success and resistance to starvation, suggesting improved body condition that could mask underlying fitness costs. However, evolved beetles showed reduced quinone production, an externally secreted antimicrobial defence, indicating trade-offs between internal vs external immunity. Finally, we identified reproductive costs only under limited resource availability, but not under suboptimal resource quality, suggesting that trade-offs can be highly condition-dependent. Overall, this study provides a unique comparison across pathogens and infection types, highlighting the importance of analysing variation in life-history traits within relevant ecological contexts to understand fitness costs of evolving immunity.

Population genetic theory predicts that natural selection will be more efficient in large than small populations because genetic drift reduces the efficiency of selection in small populations. Small populations adapting to new environments can also be expected to evolve higher recombination rates to facilitate adaptation as well as to dissociate and purge harmful mutations. We tested these hypotheses (1) by investigating differences in the strength of association between nucleotide diversity ({pi}) and recombination rate across the genomes of nine-spined sticklebacks (Pungitius pungitius) from four small freshwater (mean Ne {approx} 2 578) and four large marine (mean Ne = 86 742) populations, as well as (2) by comparing recombination rates between small and large populations using population specific linkage maps. We found the predicted positive correlation of{pi} with recombination rate from all but the smallest freshwater populations, suggesting prevalent linked selection even after accounting for variation in GC/CpG content, and gene density. Mean recombination rates did not differ between freshwater and marine populations, except that the smallest Ne freshwater population exhibited significantly elevated recombination rate. GWAS analyses suggested a polygenic basis for recombination rates. These results suggest an important role for linked selection in reducing{pi} in low recombination regions especially in large populations. Moreover, as predicted by theory, at least one of the small freshwater populations appears to have evolved a higher recombination rate than its marine ancestors.

Restoration and management of natural populations often assume that local genotypes are best suited for transplantation to their local environment. Prioritizing a single local genotype, however, contrasts with the framework of maximizing intraspecific diversity to increase population resilience to environmental change. Local populations may also become maladapted to a rapidly changing environment, motivating alternative frameworks that instead minimize environmental distance between source and transplantation sites. Here, we tested the predictive power of the local is best, maximize intraspecific diversity, and minimize environmental distance frameworks on the survival and growth of Eastern oyster (Crassostrea virginica) genotypes in field common gardens that differed in salinity and disease pressure. Although a genome scan revealed patterns of adaptation to disease, heat stress, and salinity among source populations, we did not find strong support for the local is best framework: geographically distant southern genotypes performed comparably to local selection lines and a local wild population. Higher genetic diversity within monocultures was associated with higher survival, yet highly diverse polycultures survived at lower rates than the best-performing monocultures, providing mixed support for the maximize intraspecific diversity framework. Temperature and salinity of the environments-of-origin of parents predicted the survival of their offspring in common gardens, but the relationship between survival and environmental distance was context-dependent, leading to mixed support for the minimize environmental distance framework. Together, these results demonstrate that no single framework reliably predicted transplantation success, suggesting that effective management strategies may need to integrate genomic and environmental lines of evidence to guide genotype selection.

Heat Shock Protein 90 (HSP90) functions as an evolutionary capacitor, allowing populations to store cryptic genetic variation that can be released under stress. While former studies have described the release of morphological variation, its behavioural consequences remain unexplored. In the red flour beetle, Tribolium castaneum, HSP90 inhibition released a phenotype with much smaller, less defined eyes that confers fitness benefits in continuous light and was subsequently assimilated. We hypothesized that altered eye morphology affects light perception and thereby changes light-dependent behaviours. To test whether phenotypes released via evolutionary capacitance can beneficially alter behaviour, we examined locomotor activity rhythm entrainment to light-dark cycles as well as individual and group light choice behaviour. Males of the reduced-eye phenotype exhibited a diminished startle response to sudden light exposure in locomotor activity assays. We also found reduced negative phototaxis in groups of beetles with reduced eyes. This modified behaviour, indicating reduced light sensitivity, may stem from impaired light perception caused by altered eye morphology. Lower light sensitivity could be beneficial under stressful environmental conditions by promoting the exploration of alternative niches. Therefore, this study provides the first evidence for potentially beneficial behavioural changes in a HSP90-released phenotype, reinforcing HSP90s role as an evolutionary capacitor.

O_LIDisentangling the relative influence of abiotic and biotic factors on plant population differentiation is a major challenge. Orchids often occur in patchily distributed populations, and all orchids depend on orchid mycorrhizal fungi (OrM) for seed germination. Hence, local abiotic conditions together with OrM may influence population differentiation and adaptation. C_LIO_LIBased on 316,952 polymorphic SNPs sampled from 21 populations throughout the range of the Mediterranean orchid, Orchis italica (Poir.), we performed a suite of analyses to test how population differentiation and potential adaptation is influenced by the interplay between abiotic factors and OrM. C_LIO_LIWe found strong differentiation at the regional level, while loci under selection were associated with temperature, precipitation regime, soil texture, and overall OrM abundance. Outlier SNP functions were associated with stress responses and metabolic processes in the presence of OrM. C_LIO_LIAbiotic conditions and OrM combined determines differentiation in O. italica. Identifying selective pressures underlying differentiation and adaptive variation is critical for understanding plant responses to ongoing environmental change. C_LI

In sex-role reversed species, females are socially polyandrous and compete for multiple mates, whereas males conduct the majority of parental care. To understand the extent to which physiological differences between females and males are shaped by sex roles, we examined sex differences in gene expression in sex-role reversed northern jacanas (Jacana spinosa). Given that females compete for mating opportunities, and males cycle between courtship and parental care, we predicted that transcriptomic profiles would be more similar between females and courting males, in contrast to female and parenting males. Leveraging a high quality de novo genome assembly, we conducted RNA-seq on two brain regions associated with the regulation of social behavior: the preoptic area of the hypothalamus and the nucleus taeniae. The majority of genes differentially expressed between the sexes were male-biased. Of these male-biased genes, the majority were located on the Z-chromosome. Contrary to our prediction, the greatest difference in autosomal gene expression was between females and courting males, in the preoptic area of the hypothalamus. Several differentially expressed genes related to elements of hormone signaling that are likely to be behaviorally salient, including higher expression of androgen receptor in females relative to parenting males, and higher expression of prolactin receptor in males, regardless of breeding stage. Some sex-associated gene networks were also associated with competitive traits, whereas others were associated with aggressive behaviors, regardless of sex. Few genes were differentially expressed between courting and parenting males, yet some nonetheless had connections to behavioral endocrinology, including prolactin, thyroid and insulin-like growth factor pathways. Our investigation of sex differences in gene expression can help to reveal the molecular mechanisms underlying female competition and male parental care in socially polyandrous species. We conclude that social polyandry is not a simple reversal in the direction of sex-biased gene expression in the brain, but rather a result of complex genetic and hormonal interactions that warrants further study.