Natural variation of the Y chromosome suppresses sex ratio distortion and modulates testis-specific gene expression in Drosophila simulans

X-linked sex-ratio distorters that disrupt spermatogenesis can cause a deficiency in functional Y-bearing sperm and a female-biased sex ratio. Y-linked modifiers that restore a normal sex ratio might be abundant and favored when a X-linked distorter is present. Here we investigated natural variation of Y-linked suppressors of sex-ratio in the Winters systems and the ability of these chromosomes to modulate gene expression in Drosophila simulans. Seventy-eight Y chromosomes of worldwide origin were assayed for their resistance to the X-linked sex-ratio distorter gene Dox. Y chromosome diversity caused males to sire ∼63% to ∼98% female progeny. Genome-wide gene expression analysis revealed hundreds of genes differentially expressed between isogenic males with sensitive (high sex ratio) and resistant (low sex ratio) Y chromosomes from the same population. Although the expression of about 75% of all testis-specific genes remained unchanged across Y chromosomes, a subset of post-meiotic genes was upregulated by resistant Y chromosomes. Conversely, a set of accessory gland-specific genes and mitochondrial genes were downregulated in males with resistant Y chromosomes. The D. simulans Y chromosome also modulated gene expression in XXY females in which the Y-linked protein-coding genes are not transcribed. The data suggest that the Y chromosome might exert its regulatory functions through epigenetic mechanisms that do not require the expression of protein-coding genes. The gene network that modulates sex ratio distortion by the Y chromosome is poorly understood, other than that it might include interactions with mitochondria and enriched for genes expressed in post-meiotic stages of spermatogenesis.     

Transmission ratio disruption, sterility, and control of t-complex function in sperm

Mouse t-complex located on chromosome 17 contains genes affecting solely male fertility. Some genes of this complex are recessive lethals; nonetheless, the high frequency of the t-complex carriers in a population is maintained due to a mechanism referred to as transmission ratio distortion (TRD), i.e., after crosses with wild-type females, males heterozygous for the t-complex transmit the t-bearing chromosome to nearly all their offspring, which suggests that the t-complex genes control sperm function. Analysis of this phenomenon shows that the resultant TRD is determined by the ratio between the distorter genes (Tcd) and a responder gene (Tcr) located within the t-complex region. Many authors believe that two to six distorter genes currently known have an additive effect. A genetic model of the non-Mendelian inheritance in the progeny of heterozygous male mice specifically explains sterility of animals carrying the t-complex with complementary lethal genes. The model suggests that some distorter gene products interacting with the responder gene have a selective effect on motility of both mutant and wild-type sperm. Insufficient sperm motility and/or their unsuccessful capacitation result in poor if any fertilization. Information on the t-complex genes is necessary for understanding the biological mechanisms of male sterility and may be used in medical practice.     

Male sterility of the mouse t-complex is due to homozygosity of the distorter genes

Evidence is presented that the male sterility produced by the mouse t-complex is due to interaction of at least three sterility factors. These factors are carried in the same partial haplotypes as the three distorter genes, Tcd-1, Tcd-2, and Tcd-3 and are suggested to be identical with them. When heterozygous, the distorter/sterility genes act on the wild-type form of the responder gene, rendering sperm carrying it nonfunctional, thus leading to high transmission of the t form of the responder. When homozygous, the harmful effects of the distorter genes are stronger and affect both forms of the responder, leading to sterility. If homozygous sterility is an inescapable part of ratio distortion, then the t-lethals confer a selective advantage in removing sterile males from the population. Thus, the relationship between the various properties of the t-complex can now be understood.     

Genetic Dissection of Segregation Distortion II. Mechanism of Suppression of Distortion by Certain Inversions

In(2L+2R)Cy and In(2LR)Pm² are inversion-bearing chromosomes, the former carrying a paracentric inversion in each arm and the latter carrying a long pericentric. Both chromosomes produce normal segregation ratios when present in heterozygous males with certain segregation distorter chromosomes. The apparent suppression of distortion by these chromosomes was long attributed to a failure of synapsis, but this hypothesis has fallen out of favor recently because a large number of chromosome aberrations, particularly translocations and inversions, suppress distortion even though their breakpoints fall into no recognizable pattern. Although failure of synapsis does not appear to be the mechanism of suppression of distortion, what is responsible for the suppression remains unknown. In this paper it is shown that In(2L+2R)Cy and In(2LR)Pm² suppress segregation distortion because they carry Rsp, a component of the segregation distorter system that renders a chromosome insensitive to distortion. Both chromosomes induce "suicide" of chromosomes carrying Sd Rsp⁺.     

Comparative field cage tests of the population suppressing efficiency of three genetic control systems for Aedes Aegypti.

Cycling populations of Aedes aegypti were set up in cages and managed in such a way that the populations had a maximum of threefold recovery potential in response to control measures. Into three such populations daily releases were made of males which had been chemosterilised, or were double translocation heterozygotes (T1T3) or T1T3 with sex ration distortion (DT1T3). Eradication of the populations was achieved with all cases, but the rate of suppression was markedly slower with T1T3 than the other two systems, with which the rates were similar. T1T3 and DT1T3 releases introduced considerable inherited genetic loads into the target populations. The results were in general agreement with computer predictions.     

Transmission Ratio Distortion,Sterility, and Control of the t-Complex Function in Sperm

Mouse t-complex located on chromosome 17 contains genes affecting only male fertility. Some genes of this complex are recessive lethals; nonetheless, the high frequency of the t-complex carriers in a population is maintained due to a mechanism referred to as transmission ratio distortion (TRD), i.e., after crosses with wild-type females, males heterozygous for the t-complex transmit the t-bearing chromosome to nearly all their offspring, which suggests that the t-complex genes control sperm function. Analysis of this phenomenon shows that the resultant TRD is determined by the ratio between the distorter genes (Tcd) and a responder gene (Tcr) located within the t-complex region. Many authors believe that two to six distorter genes currently known have an additive effect. A genetic model of the non-Mendelian inheritance in the progeny of heterozygous male mice specifically explains sterility of animals carrying the t-complex with complementary lethal genes. The model suggests that some distorter gene products interacting with the responder gene have a selective effect on motility of both mutant and wild-type sperm. Insufficient sperm motility and/or their unsuccessful capacitation result in poor if any fertilization. Information on the t-complex genes is necessary for understanding the biological mechanisms of male sterility and may be used in medical practice.     

Identification of quantitative trait loci affecting sex determination in the Eastern treehole mosquito (Ochlerotatus triseriatus)

Laboratory colonies of the eastern treehole mosquito (Ochlerotatus triseriatus (Say)) exhibit a consistent female-biased sex ratio. This is unusual among mosquito species, in which heritable sex ratio distortion is usually male biased and mediated by meiotic drive. Quantitative trait loci (QTL) affecting sex were mapped in an F(1) intercross to better understand the genetics underlying this female bias. In P(1) and F(1) parents and in 146 F(2) individuals with a female-biased sex ratio (106 females:40 males), regions of seven cDNA loci were analyzed with single-strand conformation polymorphism (SSCP) analysis to identify and orient linkage groups. Genotypes were also scored at 73 random amplified polymorphic DNA (RAPD)-SSCP loci. In addition to the sex locus, at least four QTL affecting sex determination were detected with interval mapping on linkage groups I and II. Alleles at the sex locus cumulatively accounted for approximately 61-77% of the genetic variance in sex. Alleles at QTL adjacent to the sex locus and at a QTL on the opposite end of linkage group I increased the proportion of females, but alleles at a QTL on linkage group I and a second QTL on linkage group II increased the proportion of males. The female-biased sex ratio observed in laboratory colonies of O. triseriatus is most easily explained by the existence of multiple female biased distorter loci, as have been observed in other Diptera.     

The evolution of costly mate choice against segregation distorters

The evolution of female preference for male genetic quality remains a controversial topic in sexual selection research. One well‐known problem, known as the lek paradox, lies in understanding how variation in genetic quality is maintained in spite of natural selection and sexual selection against low‐quality alleles. Here, we theoretically investigate a scenario where females pay a direct fitness cost to avoid males carrying an autosomal segregation distorter. We show that preference evolution is greatly facilitated under such circumstances. Because the distorter is transmitted in a non‐Mendelian fashion, it can be maintained in the population despite directional sexual selection. The preference helps females avoid fitness costs associated with the distorter. Interestingly, we find that preference evolution is limited if the choice allele induces a very strong preference or if distortion is very strong. Moreover, the preference can only persist in the presence of a signal that reliably indicates a male's distorter genotype. Hence, even in a system where the lek paradox does not play a major role, costly preferences can only spread under specific circumstances. We discuss the importance of distorter systems for the evolution of costly female choice and potential implications for the use of artificial distorters in pest control.     

Transmission ratio distortion in mouse t-haplotypes is due to multiple distorter genes acting on a responder locus

Transmission ratios of male mice heterozygous for various combinations of partial t-haplotypes provide evidence in support of a model for the genetic basis of ratio distortion, involving two or more distorter genes acting on a responder locus. The t form of the responder locus, Tcr, in the medial part of the haplotype, must be present and heterozygous for distortion to occur. When the responder alone is present, as in t^low haplotypes, the chromosome carrying it is transmitted in a low ratio (<50%). The t forms of the distorter loci act additively, in cis or trans, to raise the transmission of whichever chromosome carries Tcr. Identified distorter loci are Tcd-1, in the proximal part of the haplotype, Tcd-2, distal to Tcr, and probably Tcd-3, lying between Tcr and Tcd-2. In the absence of Tcr the distorters are transmitted normally. The system is compared with the SD system of Drosophila.     

Fitness consequences of the selfish supergene Segregation Distorter

Segregation distorters are selfish genetic elements that subvert Mendelian inheritance, often by destroying gametes that do not carry the distorter. Simple theoretical models predict that distorter alleles will either spread to fixation or stabilize at some high intermediate frequency. However, many distorters have substantially lower allele frequencies than predicted by simple models, suggesting that key sources of selection remain to be discovered. Here, we measured the fitness of Drosophila melanogaster adults and juveniles carrying zero, one or two copies of three different variants of the naturally occurring supergene Segregation Distorter (SD), in order to investigate why SD alleles remain relatively rare within populations despite being preferentially inherited. First, we show that the three SD variants differ in the severity and dominance of the fitness costs they impose on individuals carrying them. Second, SD‐carrying parents produced less fit offspring in some crosses, independent of offspring genotype, indicating that SD alleles can have nongenetic, transgenerational costs in addition to their direct costs. Third, we found that SD carriers sometimes produce a biased offspring sex ratio, perhaps due to off‐target effects of SD on the sex chromosomes. Finally, we used a theoretical model to investigate how sex ratio and transgenerational effects alter the population genetics of distorter alleles; accounting for these additional costs helps to explain why real‐world segregation distorter alleles are rarer than predicted.     

Incomplete feminisation by the microsporidian sex ratio distorter,Nosema granulosis,and reduced transmission and feminisation efficiency at low temperatures

We investigated the effects of temperature on transovarial transmission and feminisation by Nosema granulosis, a microsporidian sex ratio distorter of the brackish water amphipod Gammarus duebeni. There was no difference in parasite transmission efficiency to the F(1) eggs of infected females maintained under two temperature conditions, 5 and 10 degrees C (89 and 86%, respectively). When F(1) individuals were screened as adults, the proportion infected was also similar at both temperatures (74 and 75%, respectively). However, transmission to the eggs of the F(2) generation was significantly reduced at low temperatures (61% at 5 degrees C and 91% at 10 degrees C). In addition, feminisation efficiency was reduced substantially at low temperatures; at 10 degrees C, a calculated 85% of infected males were feminised, but at 5 degrees C only 49% were feminised. This is the first evidence for incomplete feminisation and temperature-dependent transmission and feminisation by this sex ratio distorter. We examine the consequences for parasite spread and maintenance in natural populations using a model to predict parasite prevalence in large populations. Reduced feminisation at low temperatures impedes the spread of the parasite so that it attains a substantially lower frequency, or may even be excluded, from host populations.     

Contrasting patterns of X‐chromosome divergence underlie multiple sex‐ratio polymorphisms in stalk‐eyed flies

Sex‐linked segregation distorters cause offspring sex ratios to differ from equality. Theory predicts that such selfish alleles may either go to fixation and cause extinction, reach a stable polymorphism or initiate an evolutionary arms race with genetic modifiers. The extent to which a sex ratio distorter follows any of these trajectories in nature is poorly known. Here, we used X‐linked sequence and simple tandem repeat data for three sympatric species of stalk‐eyed flies (Teleopsis whitei and two cryptic species of T. dalmanni) to infer the evolution of distorting X chromosomes. By screening large numbers of field and recently laboratory‐bred flies, we found no evidence of males with strongly female‐biased sex ratio phenotypes (SR) in one species but high frequencies of SR males in the other two species. In the two species with SR males, we find contrasting patterns of X‐chromosome evolution. T. dalmanni‐1 shows chromosome‐wide differences between sex‐ratio (X^SR) and standard (X^ST) X chromosomes consistent with a relatively old sex‐ratio haplotype based on evidence including genetic divergence, an inversion polymorphism and reduced recombination among X^SR chromosomes relative to X^ST chromosomes. In contrast, we found no evidence of genetic divergence on the X between males with female‐biased and nonbiased sex ratios in T. whitei. Taken with previous studies that found evidence of genetic suppression of sex ratio distortion in this clade, our results illustrate that sex ratio modification in these flies is undergoing recurrent evolution with diverse genomic consequences.     

Evolutionary consequences of cytoplasmic sex ratio distorters

Summary Computer simulations of diploid genetic models were used to examine the consequences of the spread of a cytoplasmic sex ratio distorter on the frequencies of nuclear sex-determination alleles and the spread of nuclear resistance alleles in female biased populations. The cytoplsmic elements considered here override the expression of the nuclear sex-determination genes, turning genetic males into females. When homozygous male genotypes are viable, a cytoplasmic sex ratio historter spreads in a population if the proportion of daughters produced by infected females exceeds the proportion of daughters produced by uninfected females. The equilibrium frequency of male phenotypes is the proportion of uninfected progeny produced by infected females. When homozygous male genotypes are lethal, the conditions for the spread of the cytoplasmic element are more stringent. The spread of a cytoplasmic sex ratio distorter causes an increase in the frequency of nuclear male sex-determination alleles as a result of the unusual combinations of genotypes which mate in infected populations. Eventually, a cytoplasmic element may replace the nuclear gene as the sex-determination mechanism. This occurs without selection. Nuclear genes conferring resistance to cytoplasmic sex ratio distorters generally increase in female biased populations and often restore a 11 sex ratio despite continual selection on the cytoplasmic element to increase its transmission efficiency.     

Pest control by genetic manipulation of sex ratio

We model the release of insects carrying an allele at multiple loci that shifts sex ratios in favor of males. We model two approaches to sex ratio alteration. In the first (denoted SD), meiotic segregation (or sperm fertility) is distorted in favor of gametes carrying the male-determining genetic element (e.g., Y-chromosome). It is assumed that any male carrying at least one copy of the SD allele produces only genotypically male offspring. In the second approach (denoted PM), the inserted allele alters sex ratio by causing genetically female individuals to become phenotypically male. It is assumed that any insect carrying at least one copy of the PM allele is phenotypically male. Both approaches reduce future population growth by reducing the number of phenotypic females. The models allow variation in the number of loci used in the release, the size of the release, and the negative fitness effect caused by insertion of each sex ratio altering allele. We show that such releases may be at least 2 orders of magnitude more effective than sterile male releases (SIT) in terms of numbers of surviving insects. For example, a single SD release with two released insects for every wild insect and a 5% fitness cost per inserted allele could reduce the target population to 1/1000th of the no-release population size, whereas a similar-sized SIT release would only reduce the population to one-fifth of its original size. We also compare these two sex ratio alteration approaches to a female-killing (FK) system and the sterile male technique when there are repeated releases over a number of generations. In these comparisons, the SD approach is the most efficient with equivalent pest suppression achieved by release of approximately 1 SD, 1.5-20 PM, 2-70 FK, and 16-3,000 SIT insects, depending on conditions. We also calculate the optimal number of SD and PM allele insertions to be used under various conditions, assuming that there is an additional genetic load incurred for each allelic insertion.     

Physical mapping of male fertility and meiotic drive quantitative trait loci in the mouse t complex using chromosome deficiencies

The t complex spans 20 cM of the proximal region of mouse chromosome 17. A variant form, the t haplotype (t), exists at significant frequencies in wild mouse populations and is characterized by the presence of inversions that suppress recombination with wild-type (+) chromosomes. Transmission ratio distortion and sterility are associated with t and affect males only. It is hypothesized that these phenomena are caused by trans-acting distorter/sterility factors that interact with a responder locus (Tcr(t)) and that the distorter and sterility factors are the same because homozygosity of the distorters causes male sterility. One factor, Tcd1, was previously shown to be amorphic using a chromosome deletion. To overcome limitations imposed by recombination suppression, we used a series of deletions within the t complex in trans to t chromosomes to characterize the Tcd1 region. We find that the distorter activity of Tcd1 is distinct from a linked sterility factor, originally called tcs1. YACs mapped with respect to deletion breakpoints localize tcs1 to a 1.1-Mb interval flanked by D17Aus9 and Tctex1. We present evidence for the existence of multiple proximal t complex regions that exhibit distorter activity. These studies demonstrate the utility of chromosome deletions for complex trait analysis.     

Linkage and segregation distortion in Drosophila melanogaster

Segregation distortion is caused by a group of genetic elements in and near the centric heterochromatin of chromosome 2 of Drosophila melanogaster. These elements promote their preferential recovery in heterozygous males by rendering sperm bearing the homologous chromosome dysfunctional. Previous work has shown that numerous Y-autosome translocations are associated with the suppression of the segregation distorter phenotype. The present study examined the effects of translocations between the major autosomes upon the expression of segregation distortion. Autosomal translocations involving either the segregation distorter chromosome or its sensitive homologue had no significant effect upon the expression of segregation distortion. These results argue that linkage arrangement per se may not have a major effect on segregation distortion. The suppression of SD by specific Y-autosomal translocations may be due to the disruption of elements on the Y chromosome that are important for the expression of SD.     

Extremely female-biased sex ratio and lethal male-male combat in a parasitoid wasp, Melittobia australica (Eulophidae)

Melittobia australica (Hymenoptera: Eulophidae) is a gregariousectoparasitoid of the prepupae and pupae of solitary wasps andbees. The males never disperse from their natal patch, and matingtakes place only on the host from which they emerged. We measuredthe offspring sex ratio of M. australica with differing foundressnumbers and examined combat between emerged males. The offspringsex ratios were extremely female biased and almost independentof foundress number in all cases. The population of M. australicaused in the experiment was infected with the cytoplasmicallyinherited symbiotic bacterium Wolbachia. However, although Wolbachiais a potential sex-ratio distorter, noninfected individualsshowed the same sex ratio patterns as the Wolbachia-infectedindividuals. An arena experiment showed that younger males werealmost always killed by older males that had eclosed earlier.These results suggested that lethal male–male combat isan additional factor distorting the sex ratio toward a morefemale-biased sex ratio. This provides a new perspective oncurrent local mate competition models.     

Evidence for differential maturation of reciprocal sperm segregants in the murine Rb(6.16) translocation heterozygote.

The fertilizing ability of unaged sperm and those aged experimentally in the cauda by surgically ligating the corpus epididymis in males carrying the Rb(6.16) translocation was studied. Chromosomally normal females were inseminated with unaged sperm delivered by males mating at 3-day intervals, and aged sperm were studied after matings on 6-14 postoperative days. The sperm chromosome complement was analyzed in first-cleavage metaphase zygotes after sequential G- and C-banding of the chromosomes. Of 283 metaphasic zygotes in the control group, 183 (or 64.7%) were analyzed and showed a ratio of 2.7:1 for chromosomally normal and balanced segregants of the translocation, deviating significantly (P less than 0.001) from the expected 1:1. The ratio of X- to Y-bearing sperm also deviated from expected (P less than 0.01) mostly due to a significant deficiency (P less than 0.05) of balanced sperm that were X-bearing. Fertilized oocytes were recovered from matings of 10 males on days 6-8 postoperatively, and, of 139 metaphasic one-cell zygotes, 101 (or 72.3%) were analyzed. These showed a Mendelian ratio of 1:1 for normal and balanced segregants. The sex ratio in the aged group (57Y:41X) also showed no deviation from 1:1. The results, which reveal significant physiological distortions for both the segregation and the sex ratios in males heterozygous for the Rb(6.16) translocation, suggest that differential maturation of the translocation-bearing sperm and the chromosomally normal reciprocal exists. The findings further support the concept that sperm chromosomal complement affects their maturation and function, and that factors on chromosome 6 and the X or Y chromosome additively affect sperm function.