The evolution of sex determination systems in dipteran insects other than Drosophila

The multitude of sex determination mechanisms displayed in dipteran insects has usually been described in terms of variations on a single principle in which the primary signal of the primitive pathway consists of a single allelic difference at one locus. Evolution of sex determination mechanisms is thought to have occurred by the addition of genes below the top gene of the pathway. The elucidation of the complex sex determination pathway of Drosophila melanogaster, as well as recent evidence that the basal genes of the pathway seem to be conserved across metazoan genera both in structure and, to a lesser degree, in function, points towards the possibility that sex determination pathways may have evolved from the bottom-up. Further to this is the question of whether the dominant male-determining factor, M, which is found in a number of insect species, represents part of the ancient sex determination pathway or is a later addition to the pathway. This, together with the possibility that the Mfactors found in numerous dipteran insect species may have a common origin, is discussed. The similarities of the sex determination pathways under the control of Mand the implications in relation to the construction of genetic sexing strains for biological control are also discussed.     

Sex determination in hymenoptera: A need for genetic and molecular studies

Sex-determining mechanisms appear to be very diverse in invertebrates. Haplodiploidy is a widespread mode of reproduction in insects: males are haploid and females are diploid. Several models have been proposed for the genetic mechanisms of sex determination in haplodiploid Hymenoptera. Although a one-locus multi-allele model is valid for several species, sex determination in other species cannot be explained by any of the existing models. Evidence for and predictions of two recently proposed models are discussed. Some genetic and molecular approaches are proposed to study sex determination in Hymenoptera.     

Genomic conflict in scale insects: the causes and consequences of bizarre genetic systems

It is now clear that mechanisms of sex determination are extraordinarily labile, with considerable variation across all taxonomic levels. This variation is often expressed through differences in the genetic system (XX‐XY, XX‐XO, haplodiploidy, and so on). Why there is so much variation in such a seemingly fundamental process has attracted much attention, with recent ideas concentrating on the possible role of genomic conflicts of interest. Here we consider the role of inter‐ and intra‐genomic conflicts in one large insect taxon: the scale insects. Scale insects exhibit a dizzying array of genetic systems, and their biology promotes conflicts of interest over transmission and sex ratio between male‐ and female‐expressed genes, parental‐ and offspring‐expressed genes (both examples of intra‐genomic conflict) and between scale insects and their endosymbionts (inter‐genomic conflict). We first review the wide range of genetic systems found in scale insects and the possible evolutionary transitions between them. We then outline the theoretical opportunities for genomic conflicts in this group and how these might influence sex determination and sex ratio. We then consider the evidence for these conflicts in the evolution of sex determination in scale insects. Importantly, the evolution of novel genetic systems in scale insects has itself helped create new conflicts of interest, for instance over sex ratio. As a result, a major obstacle to our understanding of the role of conflict in the evolution of sex‐determination and genetic systems will be the difficulty in identifying the direction of causal relationships. We conclude by outlining possible experimental and comparative approaches to test more effectively how important genomic conflicts have been.     

Sex determination in insects: a binary decision based on alternative splicing

The gene regulatory networks that control sex determination vary between species. Despite these differences, comparative studies in insects have found that alternative splicing is reiteratively used in evolution to control expression of the key sex-determining genes. Sex determination is best understood in Drosophila where activation of the RNA binding protein-encoding gene Sex-lethal is the central female-determining event. Sex-lethal serves as a genetic switch because once activated it controls its own expression by a positive feedback splicing mechanism. Sex fate choice in is also maintained by self-sustaining positive feedback splicing mechanisms in other dipteran and hymenopteran insects, although different RNA binding protein-encoding genes function as the binary switch. Studies exploring the mechanisms of sex-specific splicing have revealed the extent to which sex determination is integrated with other developmental regulatory networks.     

昆虫性别决定机制研究进展

阐述昆虫的性别决定机制是理解昆虫性别分化调控的理论基础,也为人类有效控制害虫开辟了新方向。昆虫性别决定机制存在复杂性和多样性,但主要是内因即性别决定基因级联互作调控的结果。本文对近年来基于性别决定基因级联互作的昆虫性别决定机制研究进行了综述,主要包括性别决定基因概况和重要性别决定相关基因的分子级联互作关系。目前发现昆虫重要性别决定相关基因主要集中在常染色体上,且部分基因之间存在紧密的级联互作,如Sxl,tra,dsx,csd和fem等。在这些基因中,tra/fem→dsx的调控模式在已报道的昆虫中存在共性,即tra和dsx相对较保守且tra通过性特异剪切来调控下游dsx的转录形式。目前大多数昆虫的性别决定机制还不清楚,但近年来模式昆虫性别决定机制取得了一定进展,对非模式昆虫的研究还处于起步阶段但却越来越受到重视。     

膜翅目昆虫的性别决定机制

昆虫性别决定机制存在多样性和复杂性,其中膜翅目昆虫的性别决定由单双倍体决定,单倍体为雄性,二倍体为雌性。本文就膜翅目昆虫的性别决定模式和分子机制进行综述。膜翅目昆虫性别决定有6种模式,即互补性性别决定(complementary sex determination, CSD)、多位点互补性性别决定(multiple-locus CSD, ml-CSD)、基因组印记、母体效应、内共生体诱导产雌单性生殖、父本遗传基因组消除(paternal genome elimination, PGE)。其中,CSD机制是目前在膜翅目昆虫中普遍接受的性别决定模式。而蜜蜂的CSD性别决定机制是膜翅目昆虫性别决定模式中的典型代表,受csd→fem→dsx这一调控级联的控制。     

膜翅目昆虫补偿性性别决定机制

膜翅目昆虫单双倍体性别决定机制(雄性是单倍体、雌性是二倍体)在昆虫纲的进化中有非常重要的作用。通常膜翅目昆虫的性别由单一位点的等位基因决定,杂合体发育成雌性,半合体发育成雄性。在近亲繁殖的情况下,一定数目的雄性会出现纯合二倍体,由于遗传阻隔这种二倍体的雄性通常是不育的。csd基因的发现为膜翅目昆虫性别决定机制提供了分子生物学证据。文章探讨CSD的分子生物学基础,对膜翅目昆虫sl-CSD的分布进行综述并且探讨膜翅目昆虫降低二倍体雄性消耗的策略以及可能存在的进化机制,最后提出几点建议以便从遗传学、生态学以及进化生物学角度全面的了解sl-CSD。     

Molecular players involved in temperature-dependent sex determination and sex differentiation in Teleost fish

The molecular mechanisms that underlie sex determination and differentiation are conserved and diversified. In fish species, temperature-dependent sex determination and differentiation seem to be ubiquitous and molecular players involved in these mechanisms may be conserved. Although how the ambient temperature transduces signals to the undifferentiated gonads remains to be elucidated, the genes downstream in the sex differentiation pathway are shared between sex-determining mechanisms. In this paper, we review recent advances on the molecular players that participate in the sex determination and differentiation in fish species, by putting emphasis on temperature-dependent sex determination and differentiation, which include temperature-dependent sex determination and genetic sex determination plus temperature effects. Application of temperature-dependent sex differentiation in farmed fish and the consequences of temperature-induced sex reversal are discussed.     

Function and evolution of sex determination mechanisms, genes and pathways in insects

Animals have evolved a bewildering diversity of mechanisms to determine the two sexes. Studies of sex determination genes--their history and function--in non-model insects and Drosophila have allowed us to begin to understand the generation of sex determination diversity. One common theme from these studies is that evolved mechanisms produce activities in either males or females to control a shared gene switch that regulates sexual development. Only a few small-scale changes in existing and duplicated genes are sufficient to generate large differences in sex determination systems. This review summarises recent findings in insects, surveys evidence of how and why sex determination mechanisms can change rapidly and suggests fruitful areas of future research.     

Male‐specific alleles in the Ryukyu drywood termite Neotermes sugioi

Sex‐determination systems often show remarkable diversity in upstream signals, although downstream genes are broadly conserved. Therefore, the downstream genes have been investigated in various taxa, but the most upstream signals determining sex in insects have been well‐described mainly in model organisms, including fruit flies and honey bees, and not in hemimetabolous insects such as termites. Identification of sex‐linked genetic markers in termites is important to the survey of primary sex‐determination signals. Here, we report male‐specific alleles at the microsatellite locus NK12‐1 in the Ryukyu drywood termite Neotermes sugioi (Kalotermitidae). This study provides the third example of a genetic marker linked with sexual phenotype in termites, which is a small but important step to elucidate the evolutionary process of the sex‐determination system in termites.     

What generates the diversity of Wolbachia—arthropod interactions?

Wolbachia are strictly endocellular, vertically transmitted bacteria associated with insects and crustaceans. This group of parasites modify their hosts' reproduction so as to increase their own fitness. This paper reviews the variability of these parasitic alterations and their consequences for host biology and populations. Wolbachia induce cytoplasmic incompatibility (a characteristic apparently specific to Wolbachia) in several insects and one isopod crustacean; parthenogenesis (thelytoky) in haplo-diploid insects; feminization in various isopods. The consequences of these phenomena on speciation, population dynamics and genetic polymorphism are discussed. The variability of the mechanisms of host sex determination is one important factor responsible for the diversity of Wolbachia-host interactions. However, parasite characteristics, such as the capacity to disturb host mitosis, and the ability to be horizontally transferred between hosts, also appear to play a role in this diversity.     

Sex determination in the silkworm, Bombyx mori: a female determinant on the W chromosome and the sex-determining gene cascade

In insects, the sex is determined completely by genetic mechanisms, which at least in somatic tissues, are cell autonomous. The sex of the silkworm, Bombyx mori, is strongly controlled by the presence of the W chromosome. Genetic studies using translocations and deletions of W suggested that a presumptive feminizing gene (Fem) is located in a limited region of the W chromosome. Recent genomic studies revealed a small number of potential candidates for the Fem gene in this region. In addition, a Bombyx homologue of the Drosophila sex determining gene doublesex has been identified on an autosome and analyzed. Whereas the Drosophila doublesex gene is regulated by activation of splicing in females, the Bombyx doublesex gene (Bmdsx) encodes female- and male-specific mRNAs regulated via male-specific repression of splicing. The vitellogenin gene (Vg) is a target of the BmDSX protein, which directly binds to the Vg promoter. Furthermore, as ectopic expression of the male-type Bmdsx induces male-like transformation of the sexual organs, BmDSX may control sex-specific morphological characteristics in Bombyx. This suggests that although upstream events in Drosophila and Bombyx sex determination differ, similarities between the two species do exist in downstream genetic control of sex determination.     

棕色田鼠性别决定研究进展

哺乳动物性别决定方式属于雄性异配型性别决定, 依赖于Y染色体, SRY基因是性别决定中最重要的基因。文章报道了棕色田鼠指名亚种有Y染色体, 但是Y染色体上没有SRY基因, 性别决定不依赖于SRY基因, 排除了R-spondin 1基因是性别决定基因, 同时讨论了棕色田鼠指名亚种SRY基因缺失后可能的性别决定 机制。     

Population structure and genetic diversity analysis in Gynaikothrips uzeli (Zimerman, 1909) (Thysanoptera: Phlaeothripidae) by RAPD markers

Thrips are small insects (0.5-3.0 mm) with distinct habits and life histories characterized by haplodiploid sex determination. In general, low levels of genetic diversity have been reported in haplodiploid insects, although most reports focus on the order Hymenoptera. Therefore, we used RAPD markers to evaluate the structure and both inter- and intra-population genetic variability of Gynaikothrips uzeli (Thysanoptera: Phlaeothripidae). Six populations, three from Paraná state, southern Brazil, and three from Bahia, northeastern Brazil, were studied. Similarly to other haplodiploid insects, the genetic diversity of G. uzeli was reduced. This result is putatively related to the haplodiploid sex determination system, which yields little genetic variation, and to ecological traits of the studied species, such as the low dispersal abilities and life mode in leaf galls. All individuals were homogeneously clustered in their respective collection sites, forming two main groups in which populations from similar environments were more closely related. The analyzed populations were highly structured, and the genetic variation was higher among than within populations.     

Sex determination: the amphibian models

We review and discuss current knowledge about sex determination in amphibians. The astonishing wide variety of mechanisms of genotypic sex determination is presented and discussed in an evolutionary context. We recall the natural occurrence of transitory juvenile hermaphroditism in some species. Our present knowledge of the mechanisms of sex determination in amphibians is compared to that in mammals. The influence of epigenetic factors, and especially temperature is highlighted. In amphibians, the influence of temperature on sexual differentiation, that can prevail over genotypic sex determination, remains poorly considered in publications. We suggest that studies on genetic and epigenetic factors of gonadal sex differentiation in amphibians could provide substantial information on the evolutionary process of sex determination mechanisms in current living vertebrates.     

Maternal-zygotic gene conflict over sex determination: effects of inbreeding

There is growing evidence that sex determination in a wide range of organisms is determined by interactions between maternal-effect genes and zygotically expressing genes. Maternal-effect genes typically produce products (e.g., mRNA or proteins) that are placed into the egg during oogenesis and therefore depend upon maternal genotype. Here it is shown that maternal-effect and zygotic genes are subject to conflicting selective pressures over sex determination in species with partial inbreeding or subdivided populations. The optimal sex ratios for maternal-effect genes and zygotically expressing genes are derived for two models: partial inbreeding (sibmating) and subdivided populations with local mating in temporary demes (local mate competition). In both cases, maternal-effect genes are selected to bias sex determination more toward females than are zygotically expressed genes. By investigating the invasion criteria for zygotic genes in a population producing the maternal optimum (and vice versa), it is shown that genetic conflict occurs between these genes. Even relatively low levels of inbreeding or subdivision can result in maternal-zygotic gene conflict over sex determination. The generality of maternal-zygotic gene conflict to sex determination evolution is discussed; such conflict should be considered in genetic studies of sex-determining mechanisms.     

双翅目害虫性别分离技术的研究进展及展望

在过去的几十年中,昆虫不育技术(sterile insect technique, SIT)已被用于防治农业害虫和人类健康相关的病媒害虫。相较于传统的农药控制策略,昆虫不育技术具有物种特异性和环境友好型等特点。通过释放不育雄虫的昆虫不育技术的主要障碍是在大规模饲养阶段将雄性与雌性分离,从而提高这些防治方法的成本效率,并防止释放携带和传播疾病的雌性群体。目前大多数针对双翅目害虫的遗传防治策略没有进行性别分离,少数害虫性别分离方法是基于蛹的大小或者雌雄蛹羽化时间差异进行人工识别和机械识别分离。双翅目昆虫性别决定及分化分子机制多种多样,其性别决定主要信号差异巨大,其多种性别决定基因已用于性别分离系统的开发。性比失衡性别分离策略通过破坏性别决定途径关键基因的表达获得雄性偏向后代,雌性条件性致死分离策略利用性别决定关键基因的雌雄选择性剪接差异实现性别分离,这两种性别分离策略目前正在害虫不育防治中接受大规模饲养应用评估,而基于双翅目昆虫雌雄性二态和基因标记发展的可视化性别分离策略也已成功实现多种害虫的性别分离。我们对性比失衡分离策略、雌性条件性致死分离策略和可视化性别分离策略在双翅目害虫中的研究进展进行了综述,重点评估了这些方法在雄虫大规模饲养和释放的应用潜力,以期在更完善的性别分离技术支持下为害虫防治研究取得更多突破性进展。     

Ecdysteroids： the overlooked sex steroids of insects？ Males： the black box

The paradigm, still around in textbooks, that ＇in insects sex is strictly genetic, thus that they do not have sex hormones＇, is mainly based on a wrong interpretation of the ＇gynandromorph argument＇. It is no longer tenable. Given the fact that vertebrates and invertebrates probably had a common, sexually reproducing ancestor, there is no reason to assume that only vertebrates need sex hormones. The major function of sex hormones is to inform the somatoplasm about developmental changes that take place in the gonads. In contrast to juvenile hormone and neuropeptides, ecdysteroids meet all criteria to act as sex hormones, which was probably their ancient role. Their much better documented role in moulting and metamorphosis was a secondary acquisition that enabled arthropods to cope with growth problems, imposed by a rigid cuticle. Female insects use 20-hydroxyecdysone （20E）, secreted by the follicle cells of the ovary, in a similar way as females of egg-laying vertebrates use estrogens. For a variety of reasons, the possibility that ecdysteroids, in particular ecdysone （E）, might also act as sex hormones in male insects, thus as the counterpart of testosterone of vertebrates, has been very much overlooked. Thanks to the recent discovery of the molecular basis of the haploid-diploid system of sex determination in the honeybee, the characterization of Halloween genes, proteomics, RNAi and so on, it now becomes possible to verify whether in insects, as with vertebrates, males are the endocrinologically default gender form.     

Genetic and epigenetic effects in sex determination

Sex determination is a complex and dynamic process with multiple genetic and environmental causes, in which germ and somatic cells receive various sex‐specific features. During the fifth week of fetal life, the bipotential embryonic gonad starts to develop in humans. In the bipotential gonadal tissue, certain cell groups start to differentiate to form the ovaries or testes. Despite considerable efforts and advances in identifying the mechanisms playing a role in sex determination and differentiation, the underlying mechanisms of the exact functions of many genes, gene–gene interactions, and epigenetic modifications that are involved in different stages of this cascade are not completely understood. This review aims at discussing current data on the genetic effects via genes and epigenetic mechanisms that affect the regulation of sex determination. Birth Defects Research (Part C) 108:321–336, 2016. © 2016 Wiley Periodicals, Inc.