Seasonal changes in cuticular hydrocarbons in response to polyphenism in the host-alternating aphid Prociphilus oriens

In most terrestrial arthropods, cuticular hydrocarbons (CHCs) function to assist in desiccation tolerance and chemical communications. However, few studies have clarified whether CHC profiles change among developmental stages or among different morphs in non-social insects. In the present study, we evaluated how CHC profiles change in accordance with polyphenism in the host-alternating aphid Prociphilus oriens, which exhibits a complex life cycle and five distinct morphs. These morphs are sexual or asexual and adapt to different host plants. We found that all generations of P. oriens shared high proportions of n-alkanes, but its composition varied among morphs. Three morphs that are attended by ants were characterized by relatively high proportions of n-C25 to n-C27, whereas two morphs that are not attended by ants had higher proportions of longer-chain n-alkanes, such as n-C27 and n-C29. The CHC profiles of sexual females were largely different from those of males. Considering that sexual females of Prociphilus spp. lack organs that secrete sex pheromones (scent plaques), the CHCs of sexual females are likely to function as a sex attractant. High proportions of methyl-branched alkanes were detected in the long and flocculent waxy substances of autumnal migrants. These methyl-branched alkanes are considered a cue to recognize conspecifics. We concluded that the functions and components of CHCs differ among morphs, and that those of sexual females differ from those of males and asexual generations because of their function in sexual communication.     

Battle of the Xs

Females and males often exhibit conspicuous morphological, physiological and behavioral differences. Similarly, gene expression profiles indicate that a large portion of the genome is sex‐differentially deployed, particularly in the germ line. Because males and females are so fundamentally different, each sex is likely to have a different optimal gene expression profile that is never fully achieved in either sex because of antagonistic selection in females versus males. Males are hemizygous for the X chromosome, which means that recessive male‐favorable de novo mutations on the X chromosome are subject to immediate selection. In females, a recessive female‐favorable mutation on one of two X chromosomes is not available for selection until it becomes frequent enough in the local population to result in homozygous individuals. Given that most mutations are recessive, one would expect that genes or alleles favoring males should accumulate on the X chromosome. Recent microarray work in Drosophila and C. elegans clearly shows the opposite. Why is the X chromosome a highly disfavored location for genes with male‐biased expression in these animals? BioEssays 26:543–548, 2004. Published 2004 Wiley Periodicals, Inc.     

Masculinization of the X Chromosome in the Pea Aphid

Evolutionary theory predicts that sexually antagonistic mutations accumulate differentially on the X chromosome and autosomes in species with an XY sex-determination system, with effects (masculinization or feminization of the X) depending on the dominance of mutations. Organisms with alternative modes of inheritance of sex chromosomes offer interesting opportunities for studying sexual conflicts and their resolution, because expectations for the preferred genomic location of sexually antagonistic alleles may differ from standard systems. Aphids display an XX/X0 system and combine an unusual inheritance of the X chromosome with the alternation of sexual and asexual reproduction. In this study, we first investigated theoretically the accumulation of sexually antagonistic mutations on the aphid X chromosome. Our results show that i) the X is always more favourable to the spread of male-beneficial alleles than autosomes, and should thus be enriched in sexually antagonistic alleles beneficial for males, ii) sexually antagonistic mutations beneficial for asexual females accumulate preferentially on autosomes, iii) in contrast to predictions for standard systems, these qualitative results are not affected by the dominance of mutations. Under the assumption that sex-biased gene expression evolves to solve conflicts raised by the spread of sexually antagonistic alleles, one expects that male-biased genes should be enriched on the X while asexual female-biased genes should be enriched on autosomes. Using gene expression data (RNA-Seq) in males, sexual females and asexual females of the pea aphid, we confirm these theoretical predictions. Although other mechanisms than the resolution of sexual antagonism may lead to sex-biased gene expression, we argue that they could hardly explain the observed difference between X and autosomes. On top of reporting a strong masculinization of the aphid X chromosome, our study highlights the relevance of organisms displaying an alternative mode of sex chromosome inheritance to understanding the forces shaping chromosome evolution.     

No evidence that Y‐chromosome differentiation affects male fitness in a Swiss population of common frogs

The canonical model of sex‐chromosome evolution assigns a key role to sexually antagonistic (SA) genes on the arrest of recombination and ensuing degeneration of Y chromosomes. This assumption cannot be tested in organisms with highly differentiated sex chromosomes, such as mammals or birds, owing to the lack of polymorphism. Fixation of SA alleles, furthermore, might be the consequence rather than the cause of recombination arrest. Here we focus on a population of common frogs (Rana temporaria) where XY males with genetically differentiated Y chromosomes (nonrecombinant Y haplotypes) coexist with both XY° males with proto‐Y chromosomes (only differentiated from X chromosomes in the immediate vicinity of the candidate sex‐determining locus Dmrt1) and XX males with undifferentiated sex chromosomes (genetically identical to XX females). Our study finds no effect of sex‐chromosome differentiation on male phenotype, mating success or fathering success. Our conclusions rejoin genomic studies that found no differences in gene expression between XY, XY° and XX males. Sexual dimorphism in common frogs might result more from the differential expression of autosomal genes than from sex‐linked SA genes. Among‐male variance in sex‐chromosome differentiation seems better explained by a polymorphism in the penetrance of alleles at the sex locus, resulting in variable levels of sex reversal (and thus of X‐Y recombination in XY females), independent of sex‐linked SA genes.     

Polymorphic fundatrices in thimbleberry aphid — ecology and maintenance

The thimbleberry aphid,Masonaphis maxima (Mason) lives on patches of plants that support 3,4 or 5 generations depending on site and weather. The life cycle requires sexual females and males to produce overwintering eggs. The eggs hatch in the spring to produce the first ’fundatrix’ generation; subsequent generations are produced parthenogenically. Males and other morphs are produced by wingless virginoparae, but sexual females are produced by ’gynoparae’, a winged morph that is specialized to produce only sexual females. The fundatrices have no indication of the number of generations that the plants will support in the current year. There are two fundatrix types that coexist in different ratios depending on the number of generations supported by the patch the previous year. One type produces sexual females in generations 3 and 5, and males in generations 4 and 5; the other type produces sexual females in generations 4 and 5, and males in generations 3, 4 and 5. The dimorphism adapts the aphid to its heterogeneous and somewhat unpredictable environment. The role of sex in the maintenance of the dimorphism is discussed. This is the first report of fundatrix polymorphism and consequent differential sex expression in aphids.     

Effects of age,sex, and genotype on high‐sensitivity metabolomic profiles in the fruit fly,Drosophila melanogaster

Researchers have used whole‐genome sequencing and gene expression profiling to identify genes associated with age, in the hope of understanding the underlying mechanisms of senescence. But there is a substantial gap from variation in gene sequences and expression levels to variation in age or life expectancy. In an attempt to bridge this gap, here we describe the effects of age, sex, genotype, and their interactions on high‐sensitivity metabolomic profiles in the fruit fly, Drosophila melanogaster. Among the 6800 features analyzed, we found that over one‐quarter of all metabolites were significantly associated with age, sex, genotype, or their interactions, and multivariate analysis shows that individual metabolomic profiles are highly predictive of these traits. Using a metabolomic equivalent of gene set enrichment analysis, we identified numerous metabolic pathways that were enriched among metabolites associated with age, sex, and genotype, including pathways involving sugar and glycerophospholipid metabolism, neurotransmitters, amino acids, and the carnitine shuttle. Our results suggest that high‐sensitivity metabolomic studies have excellent potential not only to reveal mechanisms that lead to senescence, but also to help us understand differences in patterns of aging among genotypes and between males and females.     

Density‐dependent sex‐biased development of macroptery in a water strider

In wing‐polymorphic insects, wing morphs differ not only in dispersal capability but also in life history traits because of trade‐offs between flight capability and reproduction. When the fitness benefits and costs of producing wings differ between males and females, sex‐specific trade‐offs can result in sex differences in the frequency of long‐winged individuals. Furthermore, the social environment during development affects sex differences in wing development, but few empirical tests of this phenomenon have been performed to date. Here, I used the wing‐dimorphic water strider Tenagogerris euphrosyne to test how rearing density and sex ratio affect the sex‐specific development of long‐winged dispersing morphs (i.e., sex‐specific macroptery). I also used a full‐sib, split‐family breeding design to assess genetic effects on density‐dependent, sex‐specific macroptery. I reared water strider nymphs at either high or low densities and measured their wing development. I found that long‐winged morphs developed more frequently in males than in females when individuals were reared in a high‐density environment. However, the frequency of long‐winged morphs was not biased according to sex when individuals were reared in a low‐density environment. In addition, full‐sib males and females showed similar macroptery incidence rates at low nymphal density, whereas the macroptery incidence rates differed between full‐sib males and females at high nymphal density. Thus complex gene‐by‐environment‐by‐sex interactions may explain the density‐specific levels of sex bias in macroptery, although this interpretation should be treated with some caution. Overall, my study provides empirical evidence for density‐specific, sex‐biased wing development. My findings suggest that social factors as well as abiotic factors can be important in determining sex‐biased wing development in insects.     

Epigenetic inheritance and genome regulation: is DNA methylation linked to ploidy in haplodiploid insects?

Organisms show great variation in ploidy level. For example, chromosome copy number varies among cells, individuals and species. One particularly widespread example of ploidy variation is found in haplodiploid taxa, wherein males are typically haploid and females are typically diploid. Despite the prevalence of haplodiploidy, the regulatory consequences of having separate haploid and diploid genomes are poorly understood. In particular, it remains unknown whether epigenetic mechanisms contribute to regulatory compensation for genome dosage. To gain greater insights into the importance of epigenetic information to ploidy compensation, we examined DNA methylation differences among diploid queen, diploid worker, haploid male and diploid male Solenopsis invicta fire ants. Surprisingly, we found that morphologically dissimilar diploid males, queens and workers were more similar to one another in terms of DNA methylation than were morphologically similar haploid and diploid males. Moreover, methylation level was positively associated with gene expression for genes that were differentially methylated in haploid and diploid castes. These data demonstrate that intragenic DNA methylation levels differ among individuals of distinct ploidy and are positively associated with levels of gene expression. Thus, these results suggest that epigenetic information may be linked to ploidy compensation in haplodiploid insects. Overall, this study suggests that epigenetic mechanisms may be important to maintaining appropriate patterns of gene regulation in biological systems that differ in genome copy number.     

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.     

Effects of ploidy and sex-locus genotype on gene expression patterns in the fire ant Solenopsis invicta

Males in many animal species differ greatly from females in morphology, physiology and behaviour. Ants, bees and wasps have a haplodiploid mechanism of sex determination whereby unfertilized eggs become males while fertilized eggs become females. However, many species also have a low frequency of diploid males, which are thought to develop from diploid eggs when individuals are homozygous at one or more sex determination loci. Diploid males are morphologically similar to haploids, though often larger and typically sterile. To determine how ploidy level and sex-locus genotype affect gene expression during development, we compared expression patterns between diploid males, haploid males and females (queens) at three developmental timepoints in Solenopsis invicta. In pupae, gene expression profiles of diploid males were very different from those of haploid males but nearly identical to those of queens. An unexpected shift in expression patterns emerged soon after adult eclosion, with diploid male patterns diverging from those of queens to resemble those of haploid males, a pattern retained in older adults. The finding that ploidy level effects on early gene expression override sex effects (including genes implicated in sperm production and pheromone production/perception) may explain diploid male sterility and lack of worker discrimination against them during development.