Evidence for involvement of a transformer paralogue in sex determination of the wasp Leptopilina clavipes

Transformer (tra) is the central gear in many insect sex determination pathways and transduces a wide range of primary signals. Mediated by transformer-2 (tra2) it directs sexual development into the female or male mode. Duplications of tra have been detected in numerous Hymenoptera, but a function in sex determination has been confirmed only in Apis mellifera. We identified a tra2 orthologue (Lc-tra2), a tra orthologue (Lc-tra) and a tra paralogue (Lc-traB) in the genome of Leptopilina clavipes (Hymenoptera: Cynipidae). We compared the sequence and structural conservation of these genes between sexual (arrhenotokous) and asexual all-female producing (thelytokous) individuals. Lc-tra is sex-specifically spliced in adults consistent with its orthologous function. The male-specific regions of Lc-tra are conserved in both reproductive modes. The paralogue Lc-traB lacks the genomic region coding for male-specific exons and can only be translated into a full-length TRA-like peptide sequence. Furthermore, unlike LC-TRA, the LC-TRAB interstrain sequence variation is not differentiated into a sexual and an asexual haplotype. The LC-TRAB protein interacts with LC-TRA as well as LC-TRA2. This suggests that Lc-traB functions as a conserved element in sex determination of sexual and asexual individuals.

Maternal provision of non-sex-specific transformer messenger RNA in sex determination of the wasp Asobara tabida

In many insect species maternal provision of sex-specifically spliced messenger RNA (mRNA) of sex determination genes is an essential component of the sex determination mechanism. In haplodiploid Hymenoptera, maternal provision in combination with genomic imprinting has been shown for the parasitoid Nasonia vitripennis, known as maternal effect genomic imprinting sex determination (MEGISD). Here, we characterize the sex determination cascade of Asobara tabida, another hymenopteran parasitoid. We show the presence of the conserved sex determination genes doublesex (dsx), transformer (tra) and transformer-2 (tra2) orthologues in As. tabida. Of these, At-dsx and At-tra are sex-specifically spliced, indicating a conserved function in sex determination. At-tra and At-tra2 mRNA is maternally provided to embryos but, in contrast to most studied insects, As. tabida females transmit a non-sex-specific splice form of At-tra mRNA to the eggs. In this respect, As. tabida sex determination differs from the MEGISD mechanism. How the paternal genome can induce female development in the absence of maternal provision of sex-specifically spliced mRNA remains an open question. Our study reports a hitherto unknown variant of maternal effect sex determination and accentuates the diversity of insect sex determination mechanisms.

Phylogenetic distribution and evolutionary dynamics of the sex determination genes doublesex and transformer in insects

Sex determination in insects is characterized by a gene cascade that is conserved at the bottom but contains diverse primary signals at the top. The bottom master switch gene doublesex is found in all insects. Its upstream regulator transformer is present in the orders Hymenoptera, Coleoptera and Diptera, but has thus far not been found in Lepidoptera and in the basal lineages of Diptera. transformer is presumed to be ancestral to the holometabolous insects based on its shared domains and conserved features of autoregulation and sex-specific splicing. We interpret that its absence in basal lineages of Diptera and its order-specific conserved domains indicate multiple independent losses or recruitments into the sex determination cascade. Duplications of transformer are found in derived families within the Hymenoptera, characterized by their complementary sex determination mechanism. As duplications are not found in any other insect order, they appear linked to the haplodiploid reproduction of the Hymenoptera. Further phylogenetic analyses combined with functional studies are needed to understand the evolutionary history of the transformer gene among insects.

Manipulation of arthropod sex determination by endosymbionts: diversity and molecular mechanisms

Arthropods exhibit a large variety of sex determination systems both at the chromosomal and molecular level. Male heterogamety, female heterogamety, and haplodiploidy occur frequently, but partially different genes are involved. Endosymbionts, such as Wolbachia, Cardinium,Rickettsia, and Spiroplasma, can manipulate host reproduction and sex determination. Four major reproductive manipulation types are distinguished: cytoplasmic incompatibility, thelytokous parthenogenesis, male killing, and feminization. In this review, the effects of these manipulation types and how they interfere with arthropod sex determination in terms of host developmental timing, alteration of sex determination, and modification of sexual differentiation pathways are summarized. Transitions between different manipulation types occur frequently which suggests that they are based on similar molecular processes. It is also discussed how mechanisms of reproductive manipulation and host sex determination can be informative on each other, with a special focus on haplodiploidy. Future directions on how the study of endosymbiotic manipulation of host reproduction can be key to further studies of arthropod sex determination are shown.

Cuticular hydrocarbon divergence in the jewel wasp Nasonia: evolutionary shifts in chemical communication channels?

The evolution and maintenance of intraspecific communication channels constitute a key feature of chemical signalling and sexual communication. However, how divergent chemical communication channels evolve while maintaining their integrity for both sender and receiver is poorly understood. In this study, we compare male and female cuticular hydrocarbon (CHC) profiles in the jewel wasp genus Nasonia, analyse their chemical divergence and investigate their role as species-specific sexual signalling cues. Males and females of all four Nasonia species showed unique, nonoverlapping CHC profiles unambiguously separating them. Surprisingly, male and female phylogenies based on the chemical distances between their CHC profiles differed dramatically, where only male CHC divergence parallels the molecular phylogeny of Nasonia. In particular, N. giraulti female CHC profiles were the most divergent from all other species and very different from its most closely related sibling species N. oneida. Furthermore, although our behavioural assays indicate that female CHC profiles can generally be perceived as sexual cues attracting males in Nasonia, this function has apparently been lost in the highly divergent female N. giraulti CHC profiles. Curiously, N. giraulti males are still attracted to heterospecific, but not to conspecific female CHC profiles. We suggest that this striking discrepancy has been caused by an extensive evolutionary shift in female N. giraulti CHC profiles, which are no longer used as conspecific recognition cues. Our study constitutes the first report of an apparent abandonment of a sexual recognition cue that the receiver did not adapt to.

Adaptive latitudinal cline of photoperiodic diapause induction in the parasitoid Nasonia vitripennis in Europe

Living in seasonally changing environments requires adaptation to seasonal cycles. Many insects use the change in day length as a reliable cue for upcoming winter and respond to shortened photoperiod through diapause. In this study, we report the clinal variation in photoperiodic diapause induction in populations of the parasitoid wasp Nasonia vitripennis collected along a latitudinal gradient in Europe. In this species, diapause occurs in the larval stage and is maternally induced. Adult Nasonia females were exposed to different photoperiodic cycles and lifetime production of diapausing offspring was scored. Females switched to the production of diapausing offspring after exposure to a threshold number of photoperiodic cycles. A latitudinal cline was found in the proportion of diapausing offspring, the switch point for diapause induction measured as the maternal age at which the female starts to produce diapausing larvae, and the critical photoperiod for diapause induction. Populations at northern latitudes show an earlier switch point, higher proportions of diapausing individuals and longer critical photoperiods. Since the photoperiodic response was measured under the same laboratory conditions, the observed differences between populations most likely reflect genetic differences in sensitivity to photoperiodic cues, resulting from local adaptation to environmental cycles. The observed variability in diapause response combined with the availability of genomic tools for N. vitripennis represent a good opportunity to further investigate the genetic basis of this adaptive trait.

Hybrid incompatibilities in the parasitic wasp genus Nasonia: negative effects of hemizygosity and the identification of transmission ratio distortion loci

The occurrence of hybrid incompatibilities forms an important stage during the evolution of reproductive isolation. In early stages of speciation, males and females often respond differently to hybridization. Haldane's rule states that the heterogametic sex suffers more from hybridization than the homogametic sex. Although haplodiploid reproduction (haploid males, diploid females) does not involve sex chromosomes, sex-specific incompatibilities are predicted to be prevalent in haplodiploid species. Here, we evaluate the effect of sex/ploidy level on hybrid incompatibilities and locate genomic regions that cause increased mortality rates in hybrid males of the haplodiploid wasps Nasonia vitripennis and Nasonia longicornis. Our data show that diploid F(1) hybrid females suffer less from hybridization than haploid F(2) hybrid males. The latter not only suffer from an increased mortality rate, but also from behavioural and spermatogenic sterility. Genetic mapping in recombinant F(2) male hybrids revealed that the observed hybrid mortality is most likely due to a disruption of cytonuclear interactions. As these sex-specific hybrid incompatibilities follow predictions based on Haldane's rule, our data accentuate the need to broaden the view of Haldane's rule to include species with haplodiploid sex determination, consistent with Haldane's original definition.

DNA methylation plays a crucial role during early Nasonia development

Although the role of DNA methylation in insect development is still poorly understood, the number and role of DNA methyltransferases in insects vary strongly between species. DNA methylation appears to be widely present among the social hymenoptera and functional studies in Apis have suggested a crucial role for de novo methylation in a wide variety of developmental processes. The sequencing of three parasitoid Nasonia genomes revealed the presence of three Dnmt1 (Dnmt1a, Dnmt1b and Dnmt1c) genes and one Dnmt2 and Dnmt3 gene, suggesting a role of DNA methylation in Nasonia development. In the present study we show that in Nasonia vitripennis all Dnmt1 messenger RNAs (mRNAs) and Dnmt3 mRNA are maternally provided to the embryo and, of these, Dnmt1a is essential during early embryogenesis. Lowering of maternal Dnmt1a mRNA results in embryonic lethality during the onset of gastrulation. This dependence on maternal Dnmt1a during embryogenesis in an organismal group outside the vertebrates, suggests evolutionary conservation of the function of Dnmt1 during embryogenesis.

Haldane's rule in the 21st century

Haldane's Rule (HR), which states that 'when in the offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous (heterogametic) sex', is one of the most general patterns in speciation biology. We review the literature of the past 15 years and find that among the ∼85 new studies, many consider taxa that traditionally have not been the focus for HR investigations. The new studies increased to nine, the number of 'phylogenetically independent' groups that comply with HR. They continue to support the dominance and faster-male theories as explanations for HR, although due to increased reliance on indirect data (from, for example, differential introgression of cytoplasmic versus chromosomal loci in natural hybrid zones) unambiguous novel results are rare. We further highlight how research on organisms with sex determination systems different from those traditionally considered may lead to more insight in the underlying causes of HR. In particular, haplodiploid organisms provide opportunities for testing specific predictions of the dominance and faster X chromosome theory, and we present new data that show that the faster-male component of HR is supported in hermaphrodites, suggesting that genes involved in male function may evolve faster than those expressed in the female function.

A microsatellite marker linkage map of the housefly, Musca domestica: evidence for male recombination

We present the first molecular marker linkage map for Musca domestica containing 35 microsatellite plus six visible markers. We report the development of 33 new microsatellite markers of which 19 are included in the linkage map. Two hundred and thirty-six F2 individuals were genotyped from three crosses yielding a linkage map consisting of five linkage groups that represent the five autosomes of the housefly. The map covers a total of 229.6 cM with an average marker spacing of 4.4 cM spanning approximately 80.2% of the genome. We found up to 29% recombination in male houseflies in contrast to most previous studies. The linkage map will add to genetic studies of the housefly.

Segregation distortion and the evolution of sex-determining mechanisms

Segregation distorters are alleles that distort normal segregation in their own favour. Sex chromosomal distorters lead to biased sex ratios, and the presence of such distorters, therefore, may induce selection for a change in the mechanism of sex determination. The evolutionary dynamics of distorter-induced changes in sex determination has only been studied in some specific systems. Here, we present a generic model for this process. We consider three scenarios: a driving X chromosome, a driving Y chromosome and a driving autosome with a male-determining factor. We investigate how the invasion prospects of a new sex-determining factor are affected by the strength of distortion and the fitness effect of the distorting allele. Our models show that in many cases, segregation distortion does create selection pressure, allowing novel sex-determining alleles to spread. When distortion leads to female-biased sex ratios, a new masculinizing gene can invade, leading to a new male heterogametic system. When distortion leads to male-biased sex ratios, a feminizing factor can invade and cause a switch to female heterogamety. In many cases, the distorter-induced change in the sex-determining system eventually leads to loss of the distorter from the population. Hence, the presence of sex chromosomal distorters will often only be transient, and the distorters may remain unnoticed. The role of segregation distortion in the evolution of sex determination may, therefore, be underestimated.

A comparison of recombination frequencies in intraspecific versus interspecific mapping populations of Nasonia

We present the first intraspecific linkage map for Nasonia vitripennis based on molecular markers. The map consists of 36 new microsatellite markers, extracted from the Nasonia genome sequence, and spans 515 cM. The five inferred linkage groups correspond to the five chromosomes of Nasonia. Comparison of recombination frequencies of the marker intervals spread over the whole genome (N=33 marker intervals) between the intraspecific N. vitripennis map and an interspecific N. vitripennis x N. giraulti map revealed a slightly higher (1.8%) recombination frequency in the intraspecific cross. We further considered an N. vitripennis x N. longicornis map with 29 microsatellite markers spanning 430 cM. Recombination frequencies in the two interspecific crosses differed neither between reciprocal crosses nor between mapping populations of embryos and adults. No major chromosomal rearrangements were found for the analyzed genomic segments. The observed differential F(2) hybrid male mortality has no significant effect on the genome-wide recombination frequency in Nasonia. We conclude that interspecific crosses between the different Nasonia species, a hallmark of Nasonia genetics, are generally suitable for mapping quantitative and qualitative trait loci for species differences.

Transcriptome and proteome analysis of ovaries of arrhenotokous and thelytokous Venturia canescens

Under arrhenotoky, unfertilized haploid eggs develop as males but under thelytoky they develop into diploid females after they have undergone diploidy restoration. In the parasitoid wasp Venturia canescens both reproductive modes occur. Thelytoky is genetically determined but the underlying genetics of diploidy restoration remain unknown. In this study we aim to identify the genes and/or proteins that control thelytoky. cDNA-amplified fragment length polymorphism (cDNA-AFLP) analysis of total ovarian RNA and two-dimensional protein electrophoresis in combination with mass spectrometry revealed putative transcripts and proteins involved in arrhenotokous and thelytokous development. The detected tubulin and actin protein differences are most likely functionally related to the two types of reproduction.

Host acceptance and sex allocation of Nasonia wasps in response to conspecifics and heterospecifics

Species recognition is an important aspect of an organism's biology. Here, we consider how parasitoid wasps vary their reproductive decisions when their offspring face intra- and interspecific competition for resources and mates. We use host acceptance and sex ratio behaviour to test whether female Nasonia vitripennis and Nasonia longicornis discriminate between conspecifics and heterospecifics when ovipositing. We tested pairs of conspecific or heterospecific females ovipositing either simultaneously or sequentially on a single host, using strains varying in their recent history of sympatry. Both N. vitripennis and N. longicornis rejected parasitized hosts more often than unparasitized hosts, although females were more likely to superparasitize their own species in the sequential treatment. However, sex ratio behaviour did not vary, suggesting similar responses towards conspecifics and heterospecifics. This contrasts with theory predicting that heterospecifics should not influence sex ratios as their offspring do not influence local mate competition, where conspecifics would. These non-adaptive sex ratios reinforce the lack of adaptive kin discrimination in N. vitripennis and suggest a behavioural constraint. Discrimination between closely related species is therefore context dependent in Nasonia. We suggest that isolating mechanisms associated with the speciation process have influenced behaviour to a greater extent than selection on sex ratios.

Identification and characterization of the doublesex gene of Nasonia

The doublesex (dsx) gene of the parasitic wasp Nasonia vitripennis is described and characterized. Differential splicing of dsx transcripts has been shown to induce somatic sexual differentiation in Diptera and Lepidoptera, but not yet in other insect orders. Two spliceforms of Nasonia dsx mRNA are differentially expressed in males and females. In addition, in a gynandromorphic line that produces haploids (normally males) with full female phenotypes, these individuals show the female spliceform, providing the first demonstration of a direct association of dsx with somatic sex differentiation in Hymenoptera. Finally, the DNA binding (DM) domain of Nasonia dsx clusters phylogenetically with dsx from other insects, and Nasonia dsx shows microsynteny with dsx of Apis, further supporting identification of the dsx orthologue in Nasonia.

Species-diagnostic single-nucleotide polymorphism and sequence-tagged site markers for the parasitic wasp genus Nasonia (Hymenoptera: Pteromalidae)

Wasps of the genus Nasonia are important biological control agents of house flies and related filth flies, which are major vectors of human pathogens. Species of Nasonia (Hymenoptera: Pteromalidae) are not easily differentiated from one another by morphological characters, and molecular markers for their reliable identification have been missing so far. Here, we report eight single-nucleotide polymorphism and three sequence-tagged site markers derived from expressed sequenced tag libraries for the two closely related and regionally sympatric species N. giraulti and N. vitripennis. We studied variation of these markers in natural populations of the two species, and we mapped them in the Nasonia genome. The markers are species-diagnostic and evenly spread over all five chromosomes. They are ideal for rapid species identification and hybrid recognition, and they can be used to map economically relevant quantitative trait loci in the Nasonia genome.

Sex ratio selection and multi-factorial sex determination in the housefly: a dynamic model

Sex determining (SD) mechanisms are highly variable between different taxonomic groups and appear to change relatively quickly during evolution. Sex ratio selection could be a dominant force causing such changes. We investigate theoretically the effect of sex ratio selection on the dynamics of a multi-factorial SD system. The system considered resembles the naturally occurring three-locus system of the housefly, which allows for male heterogamety, female heterogamety and a variety of other mechanisms. Sex ratio selection is modelled by assuming cost differences in the production of sons and daughters, a scenario leading to a strong sex ratio bias in the absence of constraints imposed by the mechanism of sex determination. We show that, despite of the presumed flexibility of the SD system considered, equilibrium sex ratios never deviate strongly from 1 : 1. Even if daughters are very costly, a male-biased sex ratio can never evolve. If sons are more costly, sex ratio can be slightly female biased but even in case of large cost differences the bias is very small (<10% from 1 : 1). Sex ratio selection can lead to a shift in the SD mechanism, but cannot be the sole cause of complete switches from one SD system to another. In fact, more than one locus remains polymorphic at equilibrium. We discuss our results in the context of evolution of the variable SD mechanism found in natural housefly populations.

Sexual functionality of Leptopilina clavipes (Hymenoptera: Figitidae) after reversing Wolbachia-induced parthenogenesis

Females infected with parthenogenesis-inducing Wolbachia bacteria can be cured from their infection by antibiotic treatment, resulting in male production. In most cases, however, these males are either sexually not fully functional, or infected females have lost the ability to reproduce sexually. We studied the decay of sexual function in males and females of the parasitoid Leptopilina clavipes. In western Europe, infected and uninfected populations occur allopatrically, allowing for an investigation of both male and female sexual function. This was made by comparing females and males induced from different parthenogenetic populations with those from naturally occurring uninfected populations. Our results indicate that although males show a decay of sexual function, they are still able to fertilize uninfected females. Infected females, however, do not fertilize their eggs after mating with males from uninfected populations. The absence of genomic incompatibilities suggests that these effects are due to the difference in mode of reproduction.

Mapping of paternal-sex-ratio deletion chromosomes localizes multiple regions involved in expression and transmission

The paternal-sex-ratio (PSR) chromosome in the parasitic wasp Nasonia vitripennis is a submetacentric supernumerary (B chromosome). Males transmit PSR, but after fertilization it causes the loss of the paternal autosomes. Paternal genome loss caused by PSR results in the conversion of a female (diploid) zygote into a male (haploid) under haplodiploid sex determination. In this study, site-specific markers were developed to assay deletion derivatives of PSR. Both polymerase chain reaction and Southern hybridization were used to detect the presence/absence of 16 single-site markers on a set of 20 functional and nine nonfunctional deletion chromosomes. Based on the pattern of marker loss on the deletion chromosomes, the basic organization of PSR was revealed. Two sets of markers were deleted independently, apparently representing the two arms of the submetacentric chromosome. The presence or absence of specific regions was examined in relation to phenotypic characteristics of the deletion chromosomes; ability to cause paternal genome loss, and stability in mitotic cell divisions. Rather than identifying a single region on PSR as being responsible for PSR function, the results suggest that the retention of one of two chromosomal regions is sufficient for causing paternal genome loss. Furthermore, a region was identified that is tightly correlated with mitotic stability, as measured from chromosomal transmission rates. Functional chromosomes with short-arm deletions had high (approximately 100%) transmission rates, whereas functional chromosomes with long-arm deletions had low (approximately 85%) transmission rates.

Chromosomal anchoring of linkage groups and identification of wing size QTL using markers and FISH probes derived from microdissected chromosomes in Nasonia (Pteromalidae: Hymenoptera)

Nasonia vitripennis is a small parasitic hymenopteran with a 50-year history of genetic work including linkage mapping with mutant and molecular markers. For the first time we are now able to anchor linkage groups to specific chromosomes. Two linkage maps based on a hybrid cross (N. vitripennis x N. longicornis) were constructed using STS, RAPD and microsatellite markers, where 17 of the linked STS markers were developed from single microdissected banded chromosomes. Based on these microdissections we anchored all linkage groups to the five chromosomes of N. vitripennis. We also verified the chromosomal specificity of the microdissection through in situ hybridization and linkage analyses. This information and technique will allow us in the future to locate genes or QTL detected in different mapping populations efficiently and fast on homologous chromosomes or even chromosomal regions. To test this approach we asked whether QTL responsible for the wing size in two different hybrid crosses (N. vitripennis x N. longicornis and N. vitripennis x N.giraulti) map to the same location. One QTL with a major effect was found to map to the centromere region of chromosome 3 in both crosses. This could indicate that indeed the same gene/s is involved in the reduction of wing in N. vitripennis and N. longicornis.

Gene flow between arrhenotokous and thelytokous populations of Venturia canescens (Hymenoptera)

In the solitary parasitoid wasp Venturia canescens both arrhenotokously (sexual) and thelytokously (parthenogenetical) reproducing individuals occur sympatrically. We found in the laboratory that thelytokous wasps are able to mate, receive and use sperm of arrhenotokous males. Using nuclear (amplified fragment length polymorphism, virus-like protein) and mitochondrial (restriction fragment length polymorphism) markers, we show the occurrence of gene flow from the arrhenotokous to the thelytokous mode in the field. Our results reinforce the paradox of sex in this species.

Why girls want to be boys

The mechanisms by which sex is genetically determined are bewilderingly diverse and appear to change rapidly during evolution.(1) What makes the sex-determining process so prone to perturbations? Two recent articles(2,3) explore theoretically the role of genetic conflict in sex determination evolution. Both studies use the idea that selection on sex-determining genes may act differently in parents and in offspring and they suggest that the resulting conflict can drive changes in sex-determining mechanisms.

Courtship and mating behaviour of interspecific Nasonia hybrids (Hymenoptera, Pteromalidae): a grandfather effect

Nasonia courtship behaviour includes easily quantifiable, stereotyped components. We analysed displays of N. vitripennis x N. longicornis hybrid males. Most of them performed well-organised displays that were intermediate between the parental species. However, in both reciprocal crosses, a significant bias towards the behaviour of the grandpaternal species was observed. Possible explanations for this effect are a biased recovery of genotypes, either due to nucleo-cytoplasmic interaction or non-mendelian transmission, or differential activation of genes in hybrid females. This study is a first step towards unravelling the genetic architecture of courtship behaviour of Nasonia, which may provide information about factors responsible for species isolation.

Automictic parthenogenesis in the parasitoid Venturia canescens (Hymenoptera: Ichneumonidae) revisited

Both arrhenotokous and thelytokous reproduction are known to occur in the parasitoid wasp Venturia canescens. The cytological mechanism of thelytoky was previously reported to involve the formation of a restitution metaphase after the reduction division, but the exact nature of the subsequent divisions, whether reductional or equational, remained unclear. We reinvestigated the cytological mechanisms in a thelytokous strain collected in France. Our observations confirm previous results, but an equational and not a reduction division was observed after restitution. This type of reproduction can be classified as central fusion automictic parthenogenesis. In two arrhenotokous strains the normal pattern of oogenesis and syngamy of Hymenoptera was observed. In addition, we used PCR amplification to show that thelytoky in V. canescens is not caused by Wolbachia bacteria. The results are discussed in relation to maintenance of heterozygosity and female sex.

B-chromosome evolution

B chromosomes are extra chromosomes to the standard complement that occur in many organisms. They can originate in a number of ways including derivation from autosomes and sex chromosomes in intra- and interspecies crosses. Their subsequent molecular evolution resembles that of univalent sex chromosomes, which involves gene silencing, heterochromatinization and the accumulation of repetitive DNA and transposons. B-chromosome frequencies in populations result from a balance between their transmission rates and their effects on host fitness. Their long-term evolution is considered to be the outcome of selection on the host genome to eliminate B chromosomes or suppress their effects and on the B chromosome's ability to escape through the generation of new variants. Because B chromosomes interact with the standard chromosomes, they can play an important role in genome evolution and may be useful for studying molecular evolutionary processes.

Absence of single-locus complementary sex determination in the braconid wasps Asobara tabida and Alysia manducator

In species with single-locus complementary sex determination (sl-CSD), sex is determined by multiple alleles at a single locus. In the haplodiploid Hymenoptera, sl-CSD results in females, if individuals are heterozygous at the sex locus, and in males, if individuals are hemizygous (haploid males) or homozygous (diploid males). Several hymenopteran species have been shown to have sl-CSD, but in several others sl-CSD is absent and the phylogenetic distribution remains unclear. In the family Braconidae, all four species tested so far were shown to possess sl-CSD. In this study, inbreeding experiments were used to test for the presence of sl-CSD in two species belonging to a subfamily of the Braconidae, Asobara tabida and Alysia manducator (Alysiinae). In both species inbreeding experiments showed no difference in brood size or sex ratio compared to the (outbred) control group. Furthermore, the sex ratios found in the inbreeding treatment differed significantly from the sex ratios expected under sl-CSD. Therefore, we conclude that sl-CSD is absent in these species. This study is the first to show the lack of sl-CSD in species of the Braconidae family and that hymenopteran sex-determining mechanisms can vary, even within a family.

Is the Y chromosome of Drosophila an evolved supernumerary chromosome?

The Y chromosomes of most Drosophila species are necessary for male fertility but they are not involved in sex determination. They have many puzzling properties that resemble the effects caused by B chromosomes. Classical genetic and molecular studies reveal substantial affinities between Y and B chromosomes and suggest that the Y chromosomes of Drosophila are not degenerated homologues of the X chromosomes, but rather that their Y chromosomes evolved as specialized supernumeraries similar to classical B chromosomes.

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.

Junctions between repetitive DNAs on the PSR chromosome of Nasonia vitripennis: association of palindromes with recombination

The Paternal-Sex-Ratio (PSR) chromosome of Nasonia vitripennis contains several families of repetitive DNAs that show significant sequence divergence but share two palindromic regions. This study reports on the analysis of junctions between two of these repetitive DNA families (psr2 and psr18). Three lambda clones that hybridized to both repeat families were isolated from PSR-genomic DNA libraries through multiple screenings and analyzed by Southern blots. Analysis of clones showed a region in which the two repeat types are interspersed, flanked by uniform blocks of each repeat type. PCR amplification of genomic DNA confirmed the contiguous arrangement of psr2 and psr18 on PSR and identified an additional junction region between these repeats that was not present in the lambda inserts. We isolated and sequenced 41 clones from the lambda inserts and genomic PCR products containing junction sequences. Sequence analysis showed that all transitions between psr2 and psr18 repeats occurred near one of the two palindromes. Based on the inheritance pattern of PSR, recombination between repeats on this chromosome must be mitotic (rather than meiotic) in origin. The occurrence of exchanges near the palindromes suggests that these sequences enhance recombination between repeat units. Rapid amplification of repetitive DNA may have been an important factor in the evolution of the PSR chromosome.

Deletion analysis of the selfish B chromosome, Paternal Sex Ratio (PSR), in the parasitic wasp Nasonia vitripennis

Paternal Sex Ratio (PSR) is a "selfish" B chromosome in the parasitoid wasp Nasonia vitripennis. It is transmitted via sperm, but causes supercondensation and destruction of the paternal chromosomes in early fertilized eggs. Because this wasp has haplodiploid sex determination, the effect of PSR is to convert diploid (female) eggs into haploid (male) eggs that carry PSR. Characterizing its genetic structure is a first step toward understanding mechanisms of PSR action. The chromosome is largely heterochromatic and contains several tandemly repeated DNA sequences that are not present on the autosomes. A deletion analysis of PSR was performed to investigate organization of repeats and location of functional domains causing paternal chromosome destruction. Deletion profiles using probes to PSR-specific repetitive DNA indicate that most repeats are organized in blocks on the chromosome. This study shows that the functional domains of PSR can be deleted, resulting in nonfunctional PSR chromosomes that are transmitted to daughters. A functional domain may be linked with the psr22 repeat, but function may also depend on abundance of PSR-specific repeats on the chromosome. It is hypothesized that the repeats act as a "sink" for a product required for proper paternal chromosome processing. Almost all deletion chromosomes remained either functional of nonfunctional in subsequent generations following their creation. One chromosome was exceptional in that it reverted from nonfunctionality to functionality in one lineage. Transmission rates of nonfunctional deletion chromosomes were high through haploid males, but low through diploid females.

Effects of deletions on mitotic stability of the paternal-sex-ratio (PSR) chromosome from Nasonia

Paternal-Sex-Ratio (PSR) is a B chromosome that causes all-male offspring in the parasitoid wasp Nasonia vitripennis. It is only transmitted via sperm of carrier males and destroys the other paternal chromosomes during the first mitotic division of the fertilized egg. Because of haplodiploidy, the effect of PSR is to convert diploid (female) eggs into haploid eggs that develop into PSR-bearing males. The PSR chromosome was previously found to contain several families of repetitive DNA, which appear to be present in local blocks. PSR chromosomes with irradiation-induced deletions have decreased rates of transmission and increased variation in transmission. This study investigates whether these differences in transmission of deletion chromosomes are due to mitotic instability. Two deletion chromosomes (E306 and F316) and the wild-type PSR chromosome were examined. A cytogenetic assay of testes revealed that wild-type PSR males contained the chromosome in 98%-100% of their spermatocytes. Similar counts from carriers of two deletion chromosomes were lower and varied between individuals from 50%-100%. One F316 male did not contain the chromosome in any of its spermatocytes although the chromosome was present in somatic tissues based on hybridization to PSR-specific repetitive DNA. A molecular analysis of males found the wild-type PSR chromosome to be present in all somatic tissues. Tissue specific differences in the presence of PSR were found in several males from the two deletion lines. The results show that deletions can result in mosaicism due to increased mitotic instability of PSR. Such individuals sometimes partially or completely fail to transmit the chromosome.(ABSTRACT TRUNCATED AT 250 WORDS)