Evolutionary conservation of Ceratitis capitata transformer gene function

Transformer functions as a binary switch gene in the sex determination and sexual differentiation of Drosophila melanogaster and Ceratitis capitata, two insect species that separated nearly 100 million years ago. The TRA protein is required for female differentiation of XX individuals, while XY individuals express smaller, presumably nonfunctional TRA peptides and consequently develop into adult males. In both species, tra confers female sexual identity through a well-conserved double-sex gene. However, unlike Drosophila tra, which is regulated by the upstream Sex-lethal gene, Ceratitis tra itself is likely to control a feedback loop that ensures the maintenance of the female sexual state. The putative CcTRA protein shares a very low degree of sequence identity with the TRA proteins from Drosophila species. However, in this study we show that a female-specific Ceratitis Cctra cDNA encoding the putative full-length CcTRA protein is able to support the female somatic and germline sexual differentiation of D. melanogaster XX; tra mutant adults. Although highly divergent, CcTRA can functionally substitute for DmTRA and induce the female-specific expression of both Dmdsx and Dmfru genes. These data demonstrate the unusual plasticity of the TRA protein that retains a conserved function despite the high evolutionary rate. We suggest that transformer plays an important role in providing a molecular basis for the variety of sex-determining systems seen among insects.     

The autoregulatory loop: A common mechanism of regulation of key sex determining genes in insects

Sex determination in most insects is structured as a gene cascade, wherein a primary signal is passed through a series of sex-determining genes, culminating in a downstream double-switch known as doublesex that decides the sexual fate of the embryo. From the literature available on sex determination cascades, it becomes apparent that sex determination mechanisms have evolved rapidly. The primary signal that provides the cue to determine the sex of the embryo varies remarkably, not only among taxa, but also within taxa. Furthermore, the upstream key gene in the cascade also varies between species and even among closely related species. The order Insecta alone provides examples of astoundingly complex diversity of upstream key genes in sex determination mechanisms. Besides, unlike key upstream genes, the downstream double-switch gene is alternatively spliced to form functional sex-specific isoforms. This sex-specific splicing is conserved across insect taxa. The genes involved in the sex determination cascade such as Sex-lethal (Sxl) in Drosophila melanogaster, transformer (tra) in many other dipterans, coleopterans and hymenopterans, Feminizer (fem) in Apis mellifera, and IGF-II mRNA-binding protein (Bmimp) in Bombyx mori are reported to be regulated by an autoregulatory positive feedback loop. In this review, by taking examples from various insects, we propose the hypothesis that autoregulatory loop mechanisms of sex determination might be a general strategy. We also discuss the possible reasons for the evolution of autoregulatory loops in sex determination cascades and their impact on binary developmental choices.     

The sex-determining gene doublesex in the fly Megaselia scalaris: conserved structure and sex-specific splicing.

The well-known sex-determining cascade of Drosophila melanogaster serves as a paradigm for the pathway to sexual development in insects. But the primary sex-determining signal and the subsequent step, Sex-lethal (Sxl), have been shown not to be functionally conserved in non-Drosophila flies. We isolated doublesex (dsx), which is a downstream step in the cascade, from the phorid fly Megaselia scalaris, which is a distant relative of D. melanogaster. Conserved properties, e.g., sex-specific splicing, structure of the female-specific 3' splice site, a splicing enhancer region with binding motifs for the TRA2/RBP1/TRA complex that activates female-specific splicing in Drosophila, and conserved domains for DNA-binding and oligomerization in the putative DSX protein, indicate functional conservation of dsx in M. scalaris. Hence, the dsx step of the sex-determining pathway appears to be conserved among flies and probably in an even wider group of insects, as the analysis of a published cDNA from the silkmoth indicates.     

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.     

The evolution of the Drosophila sex-determination pathway

The molecular complexity of the Drosophila somatic sex-determination pathway poses formidable intellectual challenges for attempts to explain its evolutionary origins. Here we present a reconstruction of how this regulatory cascade might have evolved in a step-by-step fashion. We illustrate how mutations in genes, which were already part of the pathway or were recruited as new regulators of the pathway, were favored by sexual selection acting on the discriminatory sex-determining signal. This allows us to explain the major features of the pathway, including multiple promoter sites, alternative splicing patterns, autoregulation, and stop codons. Our hypothesis is built on the available data from Drosophila and other insect species, and we point out where it is amenable to further experimental and comparative tests.     

Medaka dmY/dmrt1Y is not the universal primary sex-determining gene in fish

An outstanding candidate for a primary male-determining gene equivalent to Sry of mammals has been recently described from a non-mammalian vertebrate, the medaka fish (Oryzias latipes). However, the universality of dmY/dmrt1Y as the master sex-determining gene in fish is questionable. Phylogenetic analysis shows that dmY/dmrt1Y is an evolutionarily young Y chromosome-specific duplicate of a gene involved in testis development in vertebrates, and that this duplicate cannot be the primary sex-determining gene in most other fish species. Study of alternative fish models will probably uncover new genetic strategies controlling sexual dimorphism in vertebrates.     

Regulation and function of the melanization reaction in Drosophila

The melanization reaction, involving the synthesis of melanin to encapsulate pathogens, is a prominent immune response in Drosophila, the mosquito, and other insects and arthropods. Biochemical studies with large insects have defined a basic model for how melanization is activated and regulated upon microbial infection. In this model, recognition of a microorganism triggers a serine protease cascade that activates phenol oxidase (PO), a key enzyme in the melanin biosynthetic pathway, and serpin-type protease inhibitors are involved in inhibiting the cascade. In the past few years, genetic studies in Drosophila have identified serine proteases and serpins that regulate activation of PO and melanization in vivo. These studies, along with molecular genetic analysis of melanization in the mosquito, have provided new insight into the role that melanization plays in fighting microbial infection.     

X chromosome dosage compensation and its relationship to sex determination in C. elegans

A genetic regulatory hierarchy controls all aspects of Caenorhabditis elegans sex determination and X chromosome dosage compensation in response to the primary sex-determining signal, the X/A ratio. Initially, these two processes are coordinately regulated by a group of genes that transmit this primary signal to downstream genes that preferentially control either sex determination or dosage compensation. The relationship between these two processes is complex: not only are they coordinately controlled, a feedback mechanism operates to allow a disruption in dosage compensation to affect sexual fate. We describe our genetic and molecular understanding of the regulatory hierarchy, the feedback control and the dosage compensation process itself.     

Evidence That Sisterless-a and Sisterless-B Are Two of Several Discrete ``numerator Elements'''' of the X/a Sex Determination Signal in Drosophila That Switch Sxl between Two Alternative Stable Expression States

The primary signal for Drosophila sex determination is the number of X chromosomes relative to the number of sets of autosomes. The present report shows that the numerator of this X/A signal appears to be determined by the cumulative dose of a relatively limited number of discrete X-linked genetic elements, two of which are sisterless-a and sisterless-b. This discovery regarding the nature of the sex determination signal grew out of previous studies of both the likely X/A signal target (the feminizing switch gene, Sex-lethal) and two positive regulators of that target gene (sis-a and daughterless). Combinations of genetic perturbations in these three genes had been shown to have synergistic effects. A model proposed in part to account for these interactions generated a large variety of strong predictions for sex-specific synergistic interactions that would be diagnostic for X/A numerator elements and could distinguish them from other components of the sex determination system. All these predictions, as well as other predictions for X/A numerator elements, are shown here to be fulfilled. The most compelling observations involve sexually reciprocal viability effects of duplications of wild-type genes: combinations of sis-a+, sis-b+ and/or Sxl+ duplications are lethal to males but rescue females from the otherwise lethal effects of changes in other components of the sex determination machinery. The many interactions described here illustrate an important principle that may seem counter-intuitive: perturbations of the sex determination signal for Drosophila generally will not appear to affect adult sexual phenotype. This principle follows from the fact that Sxl is involved in dosage compensation as well as sex determination, and from important aspects of the nature and timing of Sxl's regulation both by the X/A signal and by Sxl's own products (positive autoregulation). These factors mask potential effects on adult sexual differentiation by causing the premature death of cells and/or individuals. The fact that the vast array of results presented here conform to this principle is strong evidence in favor of a "binary state" model for Sxl regulation by the X/A signal. This model is favored over an alternative "multiple state" hypothesis that was proposed by others in a different study of the X/A signal. In that same study it was concluded that region 3E8-4F11 of the X chromosome contained especially potent X/A numerator elements.(ABSTRACT TRUNCATED AT 400 WORDS)     

The dice of fate: the csd gene and how its allelic composition regulates sexual development in the honey bee, Apis mellifera

Perhaps 20% of known animal species are haplodiploid: unfertilized haploid eggs develop into males and fertilized diploid eggs into females. Sex determination in such haplodiploid species does not rely on a difference in heteromorphic sex chromosome composition but the genetic basis has been elucidated in some hymenopteran insects (wasps, sawflies, ants, bees). In these species, the development into one sex or the others depends on an initial signal whether there is only one allele or two different alleles of a single gene, the complementary sex determiner (csd), in the zygotic genome. The gene has been most-recently identified in the honey bee and has been found to encode an arginine serine-rich (SR) type protein. Heterozygosity generates an active protein that initiates female development while hemizygosity/homozygosity results in a non-active CSD protein and default male development. I will discuss plausible models of how the molecular decision of male and female is made and implemented. Comparison to hierarchies of dipteran insects suggests that SR-type protein has facilitated the differentiation of sex-determining systems and hierarchies.     

Rapid evolution of reproductive proteins in abalone and Drosophila

Observations from different taxa, including plants, protozoa, insects and mammals, indicate that proteins involved in reproduction evolve rapidly. Several models of adaptive evolution have been proposed to explain this phenomenon, such as sexual conflict, sexual selection, self versus non-self recognition and pathogen resistance. Here we discuss the potential role of sexual conflict in the rapid evolution of reproductive genes in two different animal systems, abalone (Haliotis) and Drosophila. In abalone, we reveal how specific interacting sperm-egg proteins were identified and discuss this identification in the light of models for rapid protein evolution and speciation. For Drosophila, we describe the genomic approaches taken to identify male accessory gland proteins and female reproductive tract proteins. Patterns of protein evolution from both abalone and Drosophila support the predicted patterns of rapid protein evolution driven by sexual conflict. We stress however that other selective pressures may contribute to the rapid evolution that is observed. We conclude that the key to distinguishing between sexual conflict and other mechanisms of protein evolution will be an integration of genetic, experimental and theoretical data.