Two Forms of Sexual Dimorphism in Gene Expression in Drosophila melanogaster: Their Coincidence and Evolutionary Genetics.

Phenotypic sexual dimorphism can be mediated by sex differences in gene expression. We examine two forms of sexual dimorphism in gene expression in Drosophila melanogaster: 1) sex-biased gene expression (SBGE) in which the sexes differ in the amount a gene is expressed and 2) sexual dimorphism in isoform usage, that is, sex-specific splicing (SSS). In whole body (but not the head) expression, we find a negative association between SBGE and SSS, possibly suggesting that these are alternate routes to resolving sexual antagonistic selection. Next, we evaluate whether expression dimorphism contributes to the heterogeneity among genes in rmf, the intersexual genetic correlation in body expression that constrains the extent to which a gene's expression can evolve independently between the sexes. We find lower rmf values for genes with than without SSS. We find higher rmf values for male- than female-biased genes (except genes with extreme male bias), even though male-biased genes are known to have greater evolutionary divergence in expression. Finally, we examine population genetic patterns in relation to SBGE and SSS because genes with expression dimorphism have likely experienced a history of sex differences in selection. SSS is associated with reduced values of Tajima's D and elevated direction of selection (DoS) values, suggestive of higher rates of adaptive evolution. Though DoS is highly elevated for genes with extreme male bias, DoS otherwise tends to decline from female-biased to unbiased to male-biased genes. Collectively, the results indicate that SBGE and SSS are differentially distributed across the genome and are associated with different forms of selection.

Predicting the Evolution of Sexual Dimorphism in Gene Expression.

Sexual dimorphism in gene expression is likely to be the underlying source of dimorphism in a variety of traits. Many analyses implicitly make the assumption that dimorphism only evolves when selection favors different phenotypes in the two sexes, although theory makes clear that it can also evolve as an indirect response to other kinds of selection. Furthermore, previous analyses consider the evolution of a single transcript or trait at a time, ignoring the genetic covariance with other transcripts and traits. We first show which aspects of the genetic-variance-covariance matrix, G, affect dimorphism when these assumptions about selection are relaxed. We then reanalyze gene expression data from Drosophila melanogaster with these predictions in mind. Dimorphism of gene expression for individual transcripts shows the signature of both direct selection for dimorphism and indirect responses to selection. To account for the effect of measurement error on evolutionary predictions, we estimated a G matrix for eight linear combinations of expression traits. Sex-specific genetic variances in female- and male-biased transcription, as well as one relatively unbiased combination, were quite unequal, ensuring that most forms of selection on these traits will have large effects on dimorphism. Predictions of response to selection based on the whole G matrix showed that sexually concordant and antagonistic selection are equally capable of changing sexual dimorphism. In addition, the indirect responses of dimorphism due to cross-trait covariances were quite substantial. The assumption that sexual dimorphism in transcription is an adaptation could be incorrect in many specific cases.

Sex differences in immune gene expression in the brain of a small shorebird.

Males and females often exhibit differences in behaviour, life histories, and ecology, many of which are typically reflected in their brains. Neuronal protection and maintenance include complex processes led by the microglia, which also interacts with metabolites such as hormones or immune components. Despite increasing interest in sex-specific brain function in laboratory animals, the significance of sex-specific immune activation in the brain of wild animals along with the variables that could affect it is widely lacking. Here, we use the Kentish plover (Charadrius alexandrinus) to study sex differences in expression of immune genes in the brain of adult males and females, in two wild populations breeding in contrasting habitats: a coastal sea-level population and a high-altitude inland population in China. Our analysis yielded 379 genes associated with immune function. We show a significant male-biased immune gene upregulation. Immune gene expression in the brain did not differ in upregulation between the coastal and inland populations. We discuss the role of dosage compensation in our findings and their evolutionary significance mediated by sex-specific survival and neuronal deterioration. Similar expression profiles in the coastal and inland populations suggest comparable genetic control by the microglia and possible similarities in pathogen pressures between habitats. We call for further studies on gene expression of males and females in wild population to understand the implications of immune function for life-histories and demography in natural systems.

Gender Dimorphism in Hepatic Carcinogenesis-Related Gene Expression Associated with Obesity as a Low-Grade Chronic Inflammatory Disease.

Non-alcoholic fatty liver disease (NAFLD) and hepatocellular carcinoma (HCC) show clear evidence of sexual dimorphism, with a significantly higher incidence in males. Among the determining factors that could explain this sex-based difference, the specific distribution of fat by sex has been suggested as a primary candidate, since obesity is a relevant risk factor. In this context, obesity, considered a low-grade chronic inflammatory pathology and responsible for the promotion of liver disease, could lead to sexual dimorphism in the expression profile of genes related to tumor development. When we compared the expression levels of genes associated with the early stages of carcinogenesis in the liver between male and female diet-induced obesity (DIO) rats, we observed that the expression pattern was similar in obese male and female animals. Interestingly, the SURVIVIN/BIRC5 oncogene showed a higher expression in male DIO rats than in female DIO and lean rats. This trend related to sexual dimorphism was observed in leukocytes from patients with obesity, although the difference was not statistically significant. In conclusion, this study evidenced a similar pattern in the expression of most carcinogenesis-related genes in the liver, except SUVIVIN/BIRC5, which could be a predictive biomarker of liver carcinogenesis predisposition in male patients with obesity.

Sex Dimorphism of Nonalcoholic Fatty Liver Disease (NAFLD) in Pparg-Null Mice.

Men with nonalcoholic fatty liver disease (NAFLD) are more exposed to nonalcoholic steatohepatitis (NASH) and liver fibrosis than women. However, the underlying molecular mechanisms of NALFD sex dimorphism are unclear. We combined gene expression, histological and lipidomic analyses to systematically compare male and female liver steatosis. We characterized hepatosteatosis in three independent mouse models of NAFLD, ob/ob and lipodystrophic fat-specific (PpargFΔ/Δ) and whole-body PPARγ-null (PpargΔ/Δ) mice. We identified a clear sex dimorphism occurring only in PpargΔ/Δ mice, with females showing macro- and microvesicular hepatosteatosis throughout their entire life, while males had fewer lipid droplets starting from 20 weeks. This sex dimorphism in hepatosteatosis was lost in gonadectomized PpargΔ/Δ mice. Lipidomics revealed hepatic accumulation of short and highly saturated TGs in females, while TGs were enriched in long and unsaturated hydrocarbon chains in males. Strikingly, sex-biased genes were particularly perturbed in both sexes, affecting lipid metabolism, drug metabolism, inflammatory and cellular stress response pathways. Most importantly, we found that the expression of key sex-biased genes was severely affected in all the NAFLD models we tested. Thus, hepatosteatosis strongly affects hepatic sex-biased gene expression. With NAFLD increasing in prevalence, this emphasizes the urgent need to specifically address the consequences of this deregulation in humans.

Characterization of sexual size dimorphism and sex-biased genes expression profile in the olive flounder.

Sexual size dimorphism (SSD) is a widespread phenomenon in fish species, including in the olive flounder. Although it is well established that female olive flounders acquire more bone mass than males, the underlying mechanism and timing of this SSD remains controversial. Here, the gene expression profiles of adult male and female olive flounder fish were explored to better understand the SSD mechanisms. Using RNA sequencing, a total of 4784 sex-biased differentially expressed genes (DEGs) in the fin with asymptotic growth after maturity were identified, among which growth-related factors were found. Gene ontology and pathway enrichment studies were performed to predict potential SSD-related genes and their functions. According to functional analysis, negative regulation of cell proliferation was significantly enriched in males, and anabolism related genes were highly expressed in females. In addition, pathway analysis using the Kyoto Encyclopedia of Genes and Genomes database revealed that five sexual dimorphism-related candidate genes (bambia, smurf1, dvl2, cul1a, and dvl3) were enriched in osteogenesis-contributing pathways. These results suggest that these five candidate genes may be relevant for skeletal development in olive flounders. Altogether, this study adds new knowledge for a better understanding of SSD-related growth traits in olive flounder, which can be used for enhancing aquaculture productivity with reduced production costs.

Release from intralocus sexual conflict? Evolved loss of a male sexual trait demasculinizes female gene expression.

The loss of sexual ornaments is observed across taxa, and pleiotropic effects of such losses provide an opportunity to gain insight into underlying dynamics of sex-biased gene expression and intralocus sexual conflict (IASC). We investigated this in a Hawaiian field cricket, Teleogryllus oceanicus, in which an X-linked genotype ( flatwing) feminizes males' wings and eliminates their ability to produce sexually selected songs. We profiled adult gene expression across somatic and reproductive tissues of both sexes. Despite the feminizing effect of flatwing on male wings, we found no evidence of feminized gene expression in males. Instead, female transcriptomes were more strongly affected by flatwing than males', and exhibited demasculinized gene expression. These findings are consistent with a relaxation of IASC constraining female gene expression through loss of a male sexual ornament. In a follow-up experiment, we found reduced testes mass in flatwing males, whereas female carriers showed no reduction in egg production. By contrast, female carriers exhibited greater measures of body condition. Our results suggest sex-limited phenotypic expression offers only partial resolution to IASC, owing to pleiotropic effects of the loci involved. Benefits conferred by release from intralocus conflict could help explain widespread loss of sexual ornaments across taxa.

Sex-specific changes in the aphid DNA methylation landscape.

Aphids present an ideal system to study epigenetics as they can produce diverse, but genetically identical, morphs in response to environmental stimuli. Here, using whole genome bisulphite sequencing and transcriptome sequencing of the green peach aphid (Myzus persicae), we present the first detailed analysis of cytosine methylation in an aphid and investigate differences in the methylation and transcriptional landscapes of male and asexual female morphs. We found that methylation primarily occurs in a CG dinucleotide (CpG) context and that exons are highly enriched for methylated CpGs, particularly at the 3' end of genes. Methylation is positively associated with gene expression, and methylated genes are more stably expressed than unmethylated genes. Male and asexual female morphs have distinct methylation profiles. Strikingly, these profiles are divergent between the sex chromosome and the autosomes; autosomal genes are hypomethylated in males compared to asexual females, whereas genes belonging to the sex chromosome, which is haploid in males, are hypermethylated. Overall, we found correlated changes in methylation and gene expression between males and asexual females, and this correlation was particularly strong for genes located on the sex chromosome. Our results suggest that differential methylation of sex-biased genes plays a role in aphid sexual differentiation.

Sexual dimorphism and rapid turnover in gene expression in pre-reproductive seedlings of a dioecious herb.

BACKGROUND AND AIMS: Sexual dimorphism in morphology, physiology or life history traits is common in dioecious plants at reproductive maturity, but it is typically inconspicuous or absent in juveniles. Although plants of different sexes probably begin to diverge in gene expression both before their reproduction commences and before dimorphism becomes readily apparent, to our knowledge transcriptome-wide differential gene expression has yet to be demonstrated for any angiosperm species. METHODS: The present study documents differences in gene expression in both above- and below-ground tissues of early pre-reproductive individuals of the wind-pollinated dioecious annual herb, Mercurialis annua, which otherwise shows clear sexual dimorphism only at the adult stage. KEY RESULTS: Whereas males and females differed in their gene expression at the first leaf stage, sex-biased gene expression peaked just prior to, and after, flowering, as might be expected if sexual dimorphism is partly a response to differential costs of reproduction. Sex-biased genes were over-represented among putative sex-linked genes in M. annua but showed no evidence for more rapid evolution than unbiased genes. CONCLUSIONS: Sex-biased gene expression in M. annua occurs as early as the first whorl of leaves is produced, is highly dynamic during plant development and varies substantially between vegetative tissues.

Recent Sex Chromosome Divergence despite Ancient Dioecy in the Willow Salix viminalis.

Sex chromosomes can evolve when recombination is halted between a pair of chromosomes, and this can lead to degeneration of the sex-limited chromosome. In the early stages of differentiation sex chromosomes are homomorphic, and even though homomorphic sex chromosomes are very common throughout animals and plants, we know little about the evolutionary forces shaping these types of sex chromosomes. We used DNA- and RNA-Seq data from females and males to explore the sex chromosomes in the female heterogametic willow, Salix viminalis, a species with ancient dioecy but with homomorphic sex chromosomes. We detected no major sex differences in read coverage in the sex determination (SD) region, indicating that the W region has not significantly degenerated. However, single nucleotide polymorphism densities in the SD region are higher in females compared with males, indicating very recent recombination suppression, followed by the accumulation of sex-specific single nucleotide polymorphisms. Interestingly, we identified two female-specific scaffolds that likely represent W-chromosome-specific sequence. We show that genes located in the SD region display a mild excess of male-biased expression in sex-specific tissue, and we use allele-specific gene expression analysis to show that this is the result of masculinization of expression on the Z chromosome rather than degeneration of female-expression on the W chromosome. Together, our results demonstrate that insertion of small DNA fragments and accumulation of sex-biased gene expression can occur before the detectable decay of the sex-limited chromosome.

Purifying and Positive Selection Influence Patterns of Gene Loss and Gene Expression in the Evolution of a Plant Sex Chromosome System.

Sex chromosomes are unique regions of the genome, with a host of properties that distinguish them from autosomes and from each other. Although there is extensive theory describing sex chromosome formation and subsequent degeneration of the Y chromosome, the relative importance of processes governing degeneration is poorly understood. In particular, it is not known whether degeneration occurs solely as a direct result of inefficient selection due to loss of recombination, or whether adaptive gene silencing on the Y chromosome results in most degeneration occurring neutrally. We used comparative transcriptome data from two related annual plants with highly heteromorphic sex chromosomes, Rumex rothschildianus and Rumex hastatulus, to investigate the patterns and processes underlying Y chromosome degeneration. The rate of degeneration varied greatly between the two species. In R. rothschildianus, we infer widespread gene loss, higher than previously reported for any plant. Gene loss was not random: genes with lower constraint and those not expressed during the haploid phase were more likely to be lost. There was indirect evidence of adaptive evolution on the Y chromosome from the over-expression of Y alleles in certain genes with sex-biased gene expression. There was no complete dosage compensation, but there was evidence for targeted dosage compensation occurring in more selectively constrained genes. Overall, our results are consistent with selective interference playing the dominant role in the degeneration of the Y chromosome, rather than adaptive gene silencing.

Global Dosage Compensation Is Ubiquitous in Lepidoptera, but Counteracted by the Masculinization of the Z Chromosome.

While chromosome-wide dosage compensation of the X chromosome has been found in many species, studies in ZW clades have indicated that compensation of the Z is more localized and/or incomplete. In the ZW Lepidoptera, some species show complete compensation of the Z chromosome, while others lack full equalization, but what drives these inconsistencies is unclear. Here, we compare patterns of male and female gene expression on the Z chromosome of two closely related butterfly species, Papilio xuthus and Papilio machaon, and in multiple tissues of two moths species, Plodia interpunctella and Bombyx mori, which were previously found to differ in the extent to which they equalize Z-linked gene expression between the sexes. We find that, while some species and tissues seem to have incomplete dosage compensation, this is in fact due to the accumulation of male-biased genes and the depletion of female-biased genes on the Z chromosome. Once this is accounted for, the Z chromosome is fully compensated in all four species, through the up-regulation of Z expression in females and in some cases additional down-regulation in males. We further find that both sex-biased genes and Z-linked genes have increased rates of expression divergence in this clade, and that this can lead to fast shifts in patterns of gene expression even between closely related species. Taken together, these results show that the uneven distribution of sex-biased genes on sex chromosomes can confound conclusions about dosage compensation and that Z chromosome-wide dosage compensation is not only possible but ubiquitous among Lepidoptera.

Sex-biased gene expression, sexual antagonism and levels of genetic diversity in the collared flycatcher (Ficedula albicollis) genome.

Theoretical work suggests that sexual conflict should promote the maintenance of genetic diversity by the opposing directions of selection on males and females. If such conflict is pervasive, it could potentially lead to genomic heterogeneity in levels of genetic diversity an idea that so far has not been empirically tested on a genomewide scale. We used large-scale population genomic and transcriptomic data from the collared flycatcher (Ficedula albicollis) to analyse how sexual conflict, for which we use sex-biased gene expression as a proxy, relates to genetic variability. Here, we demonstrate that the extent of sex-biased gene expression of both male-biased and female-biased genes is significantly correlated with levels of nucleotide diversity in gene sequences and that this correlation extends to diversity levels also in intergenic DNA and introns. We find signatures of balancing selection in sex-biased genes but also note that relaxed purifying selection could potentially explain part of the observed patterns. The finding of significant genetic differentiation between males and females for male-biased (and gonad-specific) genes indicates ongoing sexual conflict and sex-specific viability selection, potentially driven by sexual selection. Our results thus indicate that sexual antagonism could potentially be considered as one viable explanation to the long-standing question in evolutionary biology of how genomes can remain so genetically variable in face of strong natural and sexual selection.

Slow evolution of sex-biased genes in the reproductive tissue of the dioecious plant Salix viminalis.

The relative rate of evolution for sex-biased genes has often been used as a measure of the strength of sex-specific selection. In contrast to studies in a wide variety of animals, far less is known about the molecular evolution of sex-biased genes in plants, particularly in dioecious angiosperms. Here, we investigate the gene expression patterns and evolution of sex-biased genes in the dioecious plant Salix viminalis. We observe lower rates of sequence evolution for male-biased genes expressed in the reproductive tissue compared to unbiased and female-biased genes. These results could be partially explained by the lower codon usage bias for male-biased genes leading to elevated rates of synonymous substitutions compared to unbiased genes. However, the stronger haploid selection in the reproductive tissue of plants, together with pollen competition, would also lead to higher levels of purifying selection acting to remove deleterious variation. Future work should focus on the differential evolution of haploid- and diploid-specific genes to understand the selective dynamics acting on these loci.

Temporal variation in brain transcriptome is associated with the expression of female mimicry as a sequential male alternative reproductive tactic in fish.

Distinct patterns of gene expression often underlie intra- and intersexual differences, and the study of this set of coregulated genes is essential to understand the emergence of complex behavioural phenotypes. Here, we describe the development of a de novo transcriptome and brain gene expression profiles of wild-caught peacock blenny, Salaria pavo, an intertidal fish with sex-role reversal in courtship behaviour (i.e., females are the courting sex) and sequential alternative reproductive tactics in males (i.e., larger and older nest-holder males and smaller and younger sneaker males occur). Sneakers mimic both female's courtship behaviour and nuptial coloration to get access to nests and sneak fertilizations, and later in life transition into nest-holder males. Thus, this species offers the unique opportunity to study how the regulation of gene expression can contribute to intersex phenotypes and to the sequential expression of male and female behavioural phenotypes by the same individual. We found that at the whole brain level, expression of the sneaker tactic was paralleled by broader and divergent gene expression when compared to either females or nest-holder males, which were more similar between themselves. When looking at sex-biased transcripts, sneaker males are intersex rather than being either nest-holder or female-like, and their transcriptome is simultaneously demasculinized for nest-holder-biased transcripts and feminized for female-biased transcripts. These results indicate that evolutionary changes in reproductive plasticity can be achieved through regulation of gene expression, and in particular by varying the magnitude of expression of sex-biased genes, throughout the lifetime of the same individual.

Sexual selection drives evolution and rapid turnover of male gene expression.

The profound and pervasive differences in gene expression observed between males and females, and the unique evolutionary properties of these genes in many species, have led to the widespread assumption that they are the product of sexual selection and sexual conflict. However, we still lack a clear understanding of the connection between sexual selection and transcriptional dimorphism, often termed sex-biased gene expression. Moreover, the relative contribution of sexual selection vs. drift in shaping broad patterns of expression, divergence, and polymorphism remains unknown. To assess the role of sexual selection in shaping these patterns, we assembled transcriptomes from an avian clade representing the full range of sexual dimorphism and sexual selection. We use these species to test the links between sexual selection and sex-biased gene expression evolution in a comparative framework. Through ancestral reconstruction of sex bias, we demonstrate a rapid turnover of sex bias across this clade driven by sexual selection and show it to be primarily the result of expression changes in males. We use phylogenetically controlled comparative methods to demonstrate that phenotypic measures of sexual selection predict the proportion of male-biased but not female-biased gene expression. Although male-biased genes show elevated rates of coding sequence evolution, consistent with previous reports in a range of taxa, there is no association between sexual selection and rates of coding sequence evolution, suggesting that expression changes may be more important than coding sequence in sexual selection. Taken together, our results highlight the power of sexual selection to act on gene expression differences and shape genome evolution.

Comparative transcriptome analysis reveals differentially expressed genes associated with sex expression in garden asparagus (Asparagus officinalis).

BACKGROUND: Garden asparagus (Asparagus officinalis) is a highly valuable vegetable crop of commercial and nutritional interest. It is also commonly used to investigate the mechanisms of sex determination and differentiation in plants. However, the sex expression mechanisms in asparagus remain poorly understood. RESULTS: De novo transcriptome sequencing via Illumina paired-end sequencing revealed more than 26 billion bases of high-quality sequence data from male and female asparagus flower buds. A total of 72,626 unigenes with an average length of 979 bp were assembled. In comparative transcriptome analysis, 4876 differentially expressed genes (DEGs) were identified in the possible sex-determining stage of female and male/supermale flower buds. Of these DEGs, 433, including 285 male/supermale-biased and 149 female-biased genes, were annotated as flower related. Of the male/supermale-biased flower-related genes, 102 were probably involved in anther development. In addition, 43 DEGs implicated in hormone response and biosynthesis putatively associated with sex expression and reproduction were discovered. Moreover, 128 transcription factor (TF)-related genes belonging to various families were found to be differentially expressed, and this finding implied the essential roles of TF in sex determination or differentiation in asparagus. Correlation analysis indicated that miRNA-DEG pairs were also implicated in asparagus sexual development. CONCLUSIONS: Our study identified a large number of DEGs involved in the sex expression and reproduction of asparagus, including known genes participating in plant reproduction, plant hormone signaling, TF encoding, and genes with unclear functions. We also found that miRNAs might be involved in the sex differentiation process. Our study could provide a valuable basis for further investigations on the regulatory networks of sex determination and differentiation in asparagus and facilitate further genetic and genomic studies on this dioecious species.

Sex-biased transcriptome divergence along a latitudinal gradient.

Sex-dependent gene expression is likely an important genomic mechanism that allows sex-specific adaptation to environmental changes. Among Drosophila species, sex-biased genes display remarkably consistent evolutionary patterns; male-biased genes evolve faster than unbiased genes in both coding sequence and expression level, suggesting sex differences in selection through time. However, comparatively little is known of the evolutionary process shaping sex-biased expression within species. Latitudinal clines offer an opportunity to examine how changes in key ecological parameters also influence sex-specific selection and the evolution of sex-biased gene expression. We assayed male and female gene expression in Drosophila serrata along a latitudinal gradient in eastern Australia spanning most of its endemic distribution. Analysis of 11 631 genes across eight populations revealed strong sex differences in the frequency, mode and strength of divergence. Divergence was far stronger in males than females and while latitudinal clines were evident in both sexes, male divergence was often population specific, suggesting responses to localized selection pressures that do not covary predictably with latitude. While divergence was enriched for male-biased genes, there was no overrepresentation of X-linked genes in males. By contrast, X-linked divergence was elevated in females, especially for female-biased genes. Many genes that diverged in D. serrata have homologs also showing latitudinal divergence in Drosophila simulans and Drosophila melanogaster on other continents, likely indicating parallel adaptation in these distantly related species. Our results suggest that sex differences in selection play an important role in shaping the evolution of gene expression over macro- and micro-ecological spatial scales.

Environmental Plasticity in the Intersexual Correlation and Sex Bias of Gene Expression.

Intersexual genetic correlations are expected to constrain the evolution of sexual dimorphic traits, including the degree of sex-biased gene expression. Consistent with that expectation, studies in fruit flies and birds have reported that genes whose expression has a strong intersexual genetic correlation (rMF) show a lower level of sex-biased expression (SBE). However, it is known that both rMF and SBE can be affected by the environment. It is therefore unclear whether there is a consistent relationship between these 2 quantities across multiple environments. In this paper, we study this relationship in the African malaria mosquito Anopheles gambiae. We show that both rMF and SBE change between environments. The change in SBE across environments is significantly correlated with dN/dS: greater changes in SBE are associated with higher values of dN/dS. Furthermore, the relationship between rMF and SBE is sensitive to the environment. We conclude that this relationship is sufficiently plastic that environmental effects should be considered in future studies.

Sexual dimorphism and the evolution of sex-biased gene expression in the brown alga ectocarpus.

Males and females often have marked phenotypic differences, and the expression of these dissimilarities invariably involves sex differences in gene expression. Sex-biased gene expression has been well characterized in animal species, where a high proportion of the genome may be differentially regulated in males and females during development. Male-biased genes tend to evolve more rapidly than female-biased genes, implying differences in the strength of the selective forces acting on the two sexes. Analyses of sex-biased gene expression have focused on organisms that exhibit separate sexes during the diploid phase of the life cycle (diploid sexual systems), but the genetic nature of the sexual system is expected to influence the evolutionary trajectories of sex-biased genes. We analyze here the patterns of sex-biased gene expression in Ectocarpus, a brown alga with haploid sex determination (dioicy) and a low level of phenotypic sexual dimorphism. In Ectocarpus, female-biased genes were found to be evolving as rapidly as male-biased genes. Moreover, genes expressed at fertility showed faster rates of evolution than genes expressed in immature gametophytes. Both male- and female-biased genes had a greater proportion of sites experiencing positive selection, suggesting that their accelerated evolution is at least partly driven by adaptive evolution. Gene duplication appears to have played a significant role in the generation of sex-biased genes in Ectocarpus, expanding previous models that propose this mechanism for the resolution of sexual antagonism in diploid systems. The patterns of sex-biased gene expression in Ectocarpus are consistent both with predicted characteristics of UV (haploid) sexual systems and with the distinctive aspects of this organism's reproductive biology.