Production Of Male Offspring Research Articles

Interaction between the entomopathogenic fungus, Beauveria bassiana (Ascomycota: Hypocreales) and the parasitoid wasp, Aphidius colemani Viereck (Hymenoptera: Braconidae)

The effect of the entomopathogenic fungus, <em>Beauveria bassiana</em>, on the biological characteristics of <em>Aphidius colemani</em>, a parasitoid of the green peach aphid, <em>Myzus persicae</em>, was studied under laboratory conditions. Third-instar nymphs of green peach aphid were infected with 5/3×105 conidia/mL of <em>B. bassiana</em>, which was determined to be the lethal concentration 50 dose. They were then offered to mated female parasitoids for 24 h at different intervals. Results showed that by prolonging the release intervals of parasitoids, the number of mummies and percent emergence of parasitoids were reduced. Moreover, production of male offspring increased in the F1 generation of parasitoids. The interference of <em>B. bassiana</em> with parasitoid development was also studied by first exposing the aphid hosts to the parasitoids for 24 h and subsequently spraying them with <em>B. bassiana</em> 24, 48, 72, and 96 h after exposure. Results showed that by prolonging fungal spraying intervals, the number of mummies and percent emergence of parasitoids were increased. It appeared that the best time for applying <em>B. bassiana</em> would be three to four days after parasitisation.

Sexual reproduction of Daphnia in a deep temperate reservoir: the phenology and genetics of male formation

Long-term persistence of a Daphnia population is allowed by the production of dormant stages, produced mostly through sexual reproduction. We investigated the spatio-temporal dynamics of male production in a spring population of Daphnia galeata in the reservoir, and compared the genetic structure of three groups within this population: male-, female-producing females, and adult males. With a fine resolution sampling design and the use of highly variable microsatellite markers we revealed that: (1) the spring period of male offspring production was delimited in time with minimum interannual variation to about 3 weeks, and in space to the upper 5-m water layer; (2) there were no remarkable changes in the clonal composition of male-producing females within the period of male production; (3) overall certain clones exhibited a higher tendency to produce male offspring and therefore the clonal structure of male-producing lineages was significantly different from that of female-producing lineages; and (4) the clonal structure of male-producing females was not significantly different from that of adult males occurring later in the reservoir. This suggests that males were not subjected to any significant selective forces till maturity and the male-producing females confer a long-term fitness advantage over female-producing females.

A Genetic Basis for a Postmeiotic X Versus Y Chromosome Intragenomic Conflict in the Mouse

Intragenomic conflicts arise when a genetic element favours its own transmission to the detriment of others. Conflicts over sex chromosome transmission are expected to have influenced genome structure, gene regulation, and speciation. In the mouse, the existence of an intragenomic conflict between X- and Y-linked multicopy genes has long been suggested but never demonstrated. The Y-encoded multicopy gene Sly has been shown to have a predominant role in the epigenetic repression of post meiotic sex chromatin (PMSC) and, as such, represses X and Y genes, among which are its X-linked homologs Slx and Slxl1. Here, we produced mice that are deficient for both Sly and Slx/Slxl1 and observed that Slx/Slxl1 has an opposite role to that of Sly, in that it stimulates XY gene expression in spermatids. Slx/Slxl1 deficiency rescues the sperm differentiation defects and near sterility caused by Sly deficiency and vice versa. Slx/Slxl1 deficiency also causes a sex ratio distortion towards the production of male offspring that is corrected by Sly deficiency. All in all, our data show that Slx/Slxl1 and Sly have antagonistic effects during sperm differentiation and are involved in a postmeiotic intragenomic conflict that causes segregation distortion and male sterility. This is undoubtedly what drove the massive gene amplification on the mouse X and Y chromosomes. It may also be at the basis of cases of F1 male hybrid sterility where the balance between Slx/Slxl1 and Sly copy number, and therefore expression, is disrupted. To the best of our knowledge, our work is the first demonstration of a competition occurring between X and Y related genes in mammals. It also provides a biological basis for the concept that intragenomic conflict is an important evolutionary force which impacts on gene expression, genome structure, and speciation.

Colony pollen reserves affect body size, sperm production and sexual development in males of the stingless bee Melipona beecheii

The production of male sexual offspring by social insect colonies is often strongly seasonal or resource-dependent. In stingless bees, males are produced in smaller numbers under conditions of low colony food reserves; whether such males are negatively affected in traits related to reproductive success is not known. We compared body size, sperm production and sexual maturity in Melipona beecheii males reared with experimentally supplemented or reduced pollen reserves, but with otherwise equal numbers of workers and equal quantities of honey reserves. We also studied the same traits in males collected from non-manipulated colonies with pollen reserves intermediate between the supplemented or reduced groups but with more workers and honey reserves. Males reared under experimentally reduced pollen reserves had significantly smaller bodies and lower sperm counts compared to those reared in colonies with experimentally supplemented pollen reserves. There was also a significantly positive relationship between the number of sperm and body size in males across all colony treatments. The maximum number of sperm in seminal vesicles was recorded 2 days later in males from colonies with reduced pollen compared to males from colonies with supplementary pollen. Males from non-manipulated colonies were intermediate in size, sperm count and speed of maturation. Our study documents for the first time the existence of large size variation in males of stingless bees that is related with the amount of pollen reserves in their natal colony. We conclude that a colony’s pollen reserves have a major impact on male body size, sperm production and speed of sexual maturity in this stingless bee, which may be the case in other social insects. Stingless bees are a good model system to study the balance between colony-level selection and individual-level selection on male sexually selected traits such as body size.

Females first? Past, present and future variability in offspring sex ratio at a temperate sea turtle breeding area

AbstractThe long‐term survival of species with temperature‐dependant sex determination requires a sufficient range of incubation temperatures to ensure that both males and females are produced. The primary sex ratio of sea turtles is determined by the temperature experienced by eggs during the middle third of incubation. Here, we investigated the variability in the production of male and female offspring by loggerhead sea turtles Caretta caretta at six nesting beaches in the temperate breeding area of Zakynthos, Greece. Hatchling sex ratios were estimated using incubation durations and sand temperatures for 2007–2009, while the empirical relationship between air and sand temperature was used to infer historical (1875–2010) and future (2011–2100) hatchling sex ratios. First, all six beaches produced males; 55% of production was across five beaches, while 45% was focused on one beach (primarily in July). Second, male production varied across the season in different years; there was an initial peak in June, with production (rising, declining or plateauing) later in the season being regulated by August air temperatures. Interestingly, the annual male production rate estimated from the 3‐year dataset (23%) was half that estimated from the 135‐year reconstruction (50%), with the latter showing broad interannual variation. Finally, modelled predictions of future sex ratios ranged from a conservative 7.6% decline in male production by 2100 versus no production by 2038. This study provides a baseline from which predicted trends in hatchling sex ratio, in parallel to regular field assessments, could be used to provide guidelines for the selection of appropriate nest protection management protocols, to maintain required sex ratios that safeguard the future of this population.

Male offspring production by asexual Potamopyrgus antipodarum, a New Zealand snail.

As only females contribute directly to population growth, sexual females investing equally in sons and daughters experience a two-fold cost relative to asexuals producing only daughters. Typically, researchers have focused on benefits of sex that can counter this 'cost of males' and thus explain its predominance. Here, we instead ask whether asexuals might also pay a cost of males by quantifying the rate of son production in 45 experimental populations ('lineages') founded by obligately asexual female Potamopyrgus antipodarum. This New Zealand snail is a powerful model for studying sex because phenotypically similar sexual and asexual forms often coexist, allowing direct comparisons between sexuals and asexuals. After 2 years of culture, 23 of the 45 lineages had produced males, demonstrating that asexual P. antipodarum can make sons. We used maximum-likelihood analysis of a model of male production in which only some lineages can produce males to estimate that ~50% of lineages have the ability to produce males and that ~5% of the offspring of male-producing lineages are male. Lineages producing males in the first year of the experiment were more likely to make males in the second, suggesting that some asexual lineages might pay a cost of males relative to other asexual lineages. Finally, we used a simple deterministic model of population dynamics to evaluate how male production affects the rate of invasion of an asexual lineage into a sexual population, and found that the estimated rate of male production by asexual P. antipodarum is too low to influence invasion dynamics.

Natural dissolved humic substances increase the lifespan and promote transgenerational resistance to salt stress in the cladoceran Moina macrocopa

Evidence has accumulated that humic substances (HS) are not inert biogeochemicals. Rather, they cause stress symptoms and may modulate the life history of aquatic organisms. Nevertheless, it is still not clear how HS interact with additional stressors and if their effects are transgenerational. We tested the interactive effects of HS and salt to cladocerans, discussing their consequences for the persistence in fluctuating environments, such as coastal lagoons. We used life-table experiments to test the effects of natural HS from a polyhumic coastal lagoon (0, 5, 10, 20, 50, and 100mg dissolved organic carbon (DOC) L(-1)) on the life-history of the cladoceran Moina macrocopa. We further tested the effects of HS (10mgDOCL(-1)), within and across generations, on the resistance of M. macrocopa to salt stress (5.5g L(-1)). HS at 5-20mgDOCL(-1) extended the mean lifespan of M. macrocopa by ~30%. HS also increased body length at maturity by ~4% at 5-50mg DOC L(-1) and stimulated male offspring production at all tested concentrations. Exposure to HS (even maternal only) alleviated the salt-induced reduction of somatic growth. Co-exposure to HS increased body volume by 12-22% relative to salt-only treatments, while pre-exposure to HS increased body volume by 40-56% in treatments with salt presence, when compared to non-pre-exposed animals. HS at environmentally realistic concentrations, by acting as mild chemical stressors, modify crucial life-history traits of M. macrocopa, favoring its persistence in fluctuating environments. Some of the effects of HS are even transgenerational.

CONTAGIOUS PARTHENOGENESIS, AUTOMIXIS, AND A SEX DETERMINATION MELTDOWN

Because of the twofold cost of sex, genes conferring asexual reproduction are expected to spread rapidly in sexual populations. However, in reality this simple prediction is often confounded by several complications observed in natural systems. Motivated by recent findings in the Cape honey bee and in the parasitoid wasp Lysiphlebus fabarum, we explore through mathematical models the spread of a recessive, parthenogenesis inducing allele in a haplodiploid population. The focus of these models is on the intricate interactions between the mode of parthenogenesis induction through automixis and complementary sex determination (CSD) systems. These interactions may result in asexual production of diploid male offspring and the spread of the parthenogenesis-inducing allele through these males. We demonstrate that if parthenogenetic females produce a substantial proportion of male offspring, this may prevent the parthenogenesis-inducing allele from spreading. However, this effect is weakened if these diploid males are at least partially fertile. We also predict a degradation of multilocus CSD systems during the spread of parthenogenesis, following which only a single polymorphic CSD locus is maintained. Finally, based on empirical parameter estimates from L. fabarum we predict that male production in parthenogens is unlikely to prevent the eventual loss of sexual reproduction in this system.

Estrogen represses SOX9 during sex determination in the red-eared slider turtle Trachemys scripta

Production of male offspring in viviparous eutherian mammals requires a sex-determining mechanism resistant to maternal hormones. This constraint is relaxed in egg-laying species, which are sensitive to hormones during sex determination and often use an increase in aromatase, the estrogen-synthesizing enzyme, as a key feminizing signal. In the turtle Trachemys scripta, sex is normally determined by temperature, but estrogen treatment overrides this cue and leads exclusively to female development. We assessed whether the expression of SOX9, a central male sex-determining gene in mammals, or three other conserved transcription factors (WT1, GATA4, and LHX9) was regulated by estrogen signaling in the turtle. As in mice, all somatic cell types in the immature turtle gonad initially expressed WT1 and GATA4, whereas SOX9 was restricted to the Sertoli precursors and LHX9 to the coelomic epithelium and interstitium. After the bipotential period, SOX9 was abruptly down-regulated at the female temperature. Strikingly, embryos treated with β-estradiol at the male temperature lost SOX9 expression more than two stages earlier than controls, though WT1, GATA4, and LHX9 were unaffected. Conversely, inhibition of estrogen synthesis and signaling prevented or delayed SOX9 down-regulation at the female temperature. These results suggest that endogenous estrogen feminizes the medulla of the bipotential turtle gonad by inhibiting SOX9 expression. This mechanism may be involved in the male-to-female sex reversal in wild populations exposed to environmental estrogens, and is consistent with results showing that the estrogen receptor represses Sox9 to block transdifferentiation of granulosa cells into Sertoli-like cells in the adult mouse ovary.

Short Day Lengths Skew Prenatal Sex Ratios toward Males in Siberian Hamsters

For decades, researchers have documented significant skews in the production of male versus female offspring in many species. Because males and females are differentially susceptible to environmental challenges and also represent different fitness benefits, it may be beneficial to exert control over the offspring sex ratio when environmental conditions become challenging. Some of the most dramatic environmental challenges occur on a seasonal basis. Indeed, seasonal variation in offspring sex ratios has been documented in both mammalian and nonmammalian species. The seasonal environmental factor (or factors) that drives the skews in sex ratios is unknown; however, it is essential that such a cue be predictable and reliable and that it does not vary from year to year. We hypothesized that photoperiod, a stable cue of seasonal changes in temperature and resource availability, may underlie seasonal variation in offspring sex ratios of mammals. We predicted that short day lengths in particular, which signal impending winter conditions and related energetic demands, would stimulate an anticipatory skew in the offspring sex ratio. We used Siberian hamsters as models because they are phenotypically responsive to photoperiod but up to 60% of females continue to breed during the winter. The sexes of weanling hamsters conceived and raised in short, winter like day lengths were significantly skewed toward males. Furthermore, these skews occurred before birth; embryos collected from pregnant females maintained in short-day conditions were also significantly male biased. Thus, photoperiod functions as an effective seasonal cue, stimulating sex ratio skews toward males when day lengths are short.

Reproductive effects and bioaccumulation of chlordane in Daphnia magna

Acute and chronic toxicity of high-grade chlordane (98%) and bioaccumulation were investigated in Daphnia magna at water soluble concentrations obtained without cosolvent. The measured effective concentrations immobilizing 50% of the microcrustacea (95% confidence interval) were 22.6 (19.7-26.1) microg/L at 24 h and 13.4 (11.3-15.8) microg/L at 48 h. This indicated an increase of chlordane toxicity with time of exposure as confirmed in chronic studies. After 21 d of exposure, significant effects on survival were recorded at a chlordane concentration greater than 2.9 microg/L, whereas reproduction (number of offspring per adult, brood size) and length of adults decreased at 0.7 microg/L or more in a concentration- and time-dependent manner. The production of male offspring and developmental abnormalities, consisting of underdeveloped second antennae and shell spines in live neonates, were recorded. The chlordane concentration tested with no significant adverse effect (NOEC) on reproduction of daphnids after 21 d compared with controls was 0.18 microg/L. The bioaccumulation factor of chlordane by daphnids exposed at a level of concentration close to the 21-d NOEC reached 10,600, wet weight, and 244,000, dry weight, after 40 d. The trans-chlordane bioaccumulated to a greater extent than the cis isomer in daphnids, whereas the cis isomer was predominant in the test medium. The results suggest a crucial role of the invertebrates in transfer of chlordane in aquatic food webs and can be used to derive a freshwater guideline for environmental protection accounting for bioaccumulation.

Revisiting the variance-based selection model of diploid drone production for multiple mating in honey bees

The high number of mates of honeybee queens has lead to the proposal of several adaptive explanations. The competing hypotheses to explain multiple mating in honeybees and some other social insects have been mostly evaluated empirically with comprehensive theoretical analysis lacking behind. We report on the mathematical analysis of the diploid drone hypothesis for multiple mating, which suggests that multiple mating evolved as a safeguard against the production of infertile male offspring. In accordance with earlier models, our analysis shows that multiple mating does not reduce the average value of diploid drone production but reduces its variance. We combine this observation with a colony growth model to assess the impact of this reduction in variance to the colony fitness. Considering a plausible parameter space for the honeybee, we conclude that the reduction in variance of diploid drone production can be a significant selective force for multiple mating. We have also described rules of a game for which a problem of finding the best strategy is equivalent to the above biological problem of bee mating. We made a significant progress in the general solution of this game and conjectured that the best strategy is strongly related to the geometry of rational numbers.

Modulation of sex ratios in Daphnia magna following multigenerational exposure to sewage treatment plant effluents

The influence of sewage treatment plant effluents on sex ratios in Daphnia magna was investigated. Female daphnids were acclimated for several generations to effluents from a municipal sewage treatment plant and a residential oxidation lagoon and then placed under conditions to maximize male offspring production. Both effluents resulted in a statistically significant decrease in male production and a shift in male broods from earlier broods to later broods near the end of the adult life cycle. For example, sex ratios in control daphnids ranged from 0.43 to 0.67 in broods 3–4 compared with 0.0–0.13 in daphnids exposed to the residential oxidation lagoon. Secondary sexual characteristics of both sexes were statistically significantly increased by the sewage lagoon effluent but not the municipal effluent. These preliminary results suggest that alteration in timing of sexual determination due to exposure to sewage treatment plant effluents could severely impact the survival of daphnid populations.

Ecological genomics in Daphnia: stress responses and environmental sex determination

Ecological genomics is the study of adaptation of natural populations to their environment, and therefore seeks to link organism and population level processes through an understanding of genome organization and function. The planktonic microcrustacean Daphnia, which has long been an important system for ecology, is now being used as a genomic model as well. Here we review recent progress in selected areas of Daphnia genomics research. Production of parthenogenetic male offspring occurs through environmental cues, which clearly involves endocrine regulation and has also been studied as a toxicological response to juvenoid hormone analog insecticides. Recent progress has uncovered a putative juvenoid cis-response element, which together with microarray analysis will stimulate further research into nuclear hormone receptors and their associated transcriptional regulatory networks. Ecotoxicological studies indicate that mRNA profiling is a sensitive and specific research tool with promising applications in environmental monitoring and for uncovering conserved cellular processes. Rapid progress is expected to continue in these and other areas, as genomic tools for Daphnia become widely available to investigators.

An experimental test for indirect benefits in Drosophila melanogaster.

BackgroundDespite much empirical attention, tests for indirect benefits of mate choice have rarely considered the major components of sexual and nonsexual offspring fitness relevant to a population. Here we use a novel experimental design to test for the existence of any indirect benefits in a laboratory adapted population of D. melanogaster. Our experiment compared the fitness (mating success, longevity, and productivity) of individuals possessing genomes that derived two generations previously from males that were either entirely successful (studs) or wholly unsuccessful (duds) at achieving mates in three subsequent rounds of mating trials.ResultsMales from the stud treatment were 30% more successful on average at securing mates than males from the dud treatment. In contrast, we found no difference between treatments in measures of productivity or of longevity when measured in a mixed-sex environment. In the absence of females, however, males in the stud treatment outlived males in the dud treatment.ConclusionOur results suggest that mating with successful males in this population provides an indirect benefit to females and that, at least in this environment, the benefit arises primarily through the production of more attractive male offspring. However, it is unclear whether this represents solely a traditional sexy sons benefit or whether there is an additional good genes component (with male offspring simply allocating their surplus condition to traits that enhance their mating success). The lack of any detectable differences in female fitness between the two treatments suggests the former, although the longevity advantage of males in the stud treatment when females were absent is consistent with the latter. Determining the effect of this indirect benefit on the evolution of female mate preferences (or resistance) will require comparable data on the direct costs of mating with various males, and an understanding of how these costs and benefits integrate across generations and vary among environments.

Colony genetic structure in a facultatively eusocial hover wasp

The degree of genetic heterogeneity among the individuals in an animal society depends on the society's genetic structure. Genetic heterogeneity, in turn, means that group members will differ in their reproductive objectives and conflicts over reproduction may arise. The resolution of these conflicts may be reflected in the way that reproduction is partitioned between potential reproductives. We used 5 microsatellite loci to investigate genetic structure and reproductive skew in 17 nests of the Malaysian hover wasp, Parischnogaster alternata. Parischnogaster alternata colonies are small (1?10 females), and all adult colony members are capable of mating and producing offspring. We found that colonies tended to consist of closely related individuals and that at any one time the production of both female and male offspring was nearly always monopolized by a single dominant female, despite considerable variation between nests in parameters predicted to affect skew. Subordinate females that remained in their natal colonies obtained indirect fitness benefits by helping to raise offspring to which they were related. Subordinate females also appeared to be positioned within an age-based queue for inheritance of the dominant egg-laying position. We suggest that the high skew in P. alternata may result from strong ecological constraints on solitary nesting, high relatedness, and a relatively high probability that subordinates will eventually inherit the position of dominance.

Cytoplasmic incompatibility and multiple symbiont infection in the ash whitefly parasitoid, Encarsia inaron

Abstract Terrestrial arthropods are commonly infected with maternally inherited symbionts that cause reproductive incompatibilities between hosts with differing infection status. Such symbionts can have major effects on the efficacy of a biological control program if releases are comprised of mixtures of differentially infected individuals. In this study, the ash whitefly parasitoid, Encarsia inaron (Hymenoptera: Aphelinidae) from Arizona was surveyed for the presence of heritable bacterial symbionts; experiments were also performed to test for two phenotypes known to be caused by Encarsia symbionts—cytoplasmic incompatibility and changes in oviposition behavior and host use. E. inaron has successfully reduced ash whitefly to non-pest status in all three locations it has been released (California, Arizona, and North Carolina) and is also notable as one of the only Encarsia species that is not autoparasitic, with both male and female wasps developing as primary parasitoids of whiteflies. We show that E. inaron is infected with both Wolbachia and Cardinium. While there was no effect of the symbionts on oviposition behavior or host use, crosses between doubly infected male wasps and uninfected females resulted in a severe reduction in the number of female offspring; male offspring production was unaffected. This study thus serves as a further warning that ascribing a phenotype to a symbiont with confidence depends on eliminating the possibility of a mixed infection, and establishes E. inaron as a useful model for dissecting Wolbachia–Cardinium interactions.

Importance of Development of Ecotoxicogenomics in Understanding Molecular Mechanisms of Chemicals in Developing Animals

Developing organisms are sensitive to estrogenic chemicals. Exposure to estrogens or estrogenic chemicals during critical periods of development induces persistent changes in reproductive as well as nonreproductive organs, including persistent molecular alterations. Chemicals released into the environment potentially disrupt the endocrine system in wild animals and humans, some of which exhibit estrogenic or anti-androgenic activity by binding to estrogen or androgen receptors. Estrogen-responsive genes and critical developmental windows of various animal species, therefore, should be identified to understand the molecular basis of estrogenic activity during embryonic development. To understand molecular mechanisms of estrogenic chemicals in developing organisms, we identified estrogen-responsive genes using cDNA microarrays and a quantitative reverse transcriptase-polymerase chain reaction, and genes related to estrogen-independent vaginal changes in mice induced by estrogens during a critical window. We also analyzed genes in Xenopus laevis embryos related to abnormalities induced by estrogenic chemicals using cDNA microarrays. Estrogen receptors and other steroid hormone receptors were cloned in various species, including the American alligator, crocodile, Florida red-bellied turtle, Silurana tropicalis, roach, Fundulus and mosquito fish. An environmental androgen, trenbolone, was found to induce persistent changes in the gonopodium and ovotestis in mosquito fish. In Daphnia magna, juvenile hormone agonists induce the production of male offspring. Tributyltin (TBT) induces imposex in rockshell through retinoid X receptor (RXR) activation. Microarray technology is a powerful tool for understanding the molecular mechanism of receptor-mediated toxicology in various animal species. Ecotoxicogenomics is a multidisciplinary field concerned with elucidating how the entire genome is involved in the biological response of wild animals exposed to environmental toxicants and stressors. Ecotoxicogenomics combines information from studies of genomic-scale mRNA profiling by microarray analysis (transcriptomics), protein profiling (proteomics), metabolite profiling (metabolonomics) and computational models to understand the roles of gene-environment interaction in the chemical toxicity. To understand the molecular mechanism of chemicals in developing wildlife, the establishment of ecotoxicogenomics is essential in the near future.

STRESSFUL SEX

Sex and stress may be more closely linked than you might think. Gerald LeBlanc of North Carolina State University is intrigued by the relationship between environmental stress responses and reproduction. Reproduction in the tiny freshwater crustacean Daphnia is normally an asexual all-female affair. But a stressful environment can trigger a switch to sexual reproduction in these creatures, producing freeze- and desiccation-tolerant eggs with better chances of coping with a hostile environment. Exploring this relationship between sex and stress, LeBlanc and colleagues found that the same hormonal pathway is responsible for two common environmental stress responses in Daphnia (p. 15).LeBlanc already knew that a hormone called methyl farnesoate causes Daphnia eggs to develop into males. He had also noticed that Daphnia exposed to this hormone turned a copper colour. `Then I received a serendipitous phone call from Thomas Gorr of Harvard Medical School' LeBlanc recalls, `who suggested that the animals might be turning red because the hormone was triggering production of the oxygen-transporter haemoglobin.' Puzzled by this apparent link between haemoglobin accumulation and male offspring production, LeBlanc and Gorr began a joint venture to see if the methyl farnesoate hormone was regulating both of these stress responses in the little crustacean.`If the same hormonal signalling pathway regulates haemoglobin production and male offspring production' LeBlanc reasoned, `exposing Daphnia to the hormone should trigger both haemoglobin and male offspring production.'The team decided to examine the response of 11 Daphnia clones to the hormone; five of these could produce male offspring, but the other six could not produce males. Sure enough, after the hormone treatment, haemoglobin levels in the five male-producing clones shot up and all but one of the clones also began producing male offspring. Clearly, the hormone was triggering both haemoglobin and male offspring production in these clones. But would the non-male-producing clones also increase their haemoglobin and male offspring production in response to the hormone? The team found that five of the six non-male-producing clones did not produce male offspring or haemoglobin after hormone treatment. This suggests that these clones lack a common component of the signalling pathway, most likely a receptor for the hormone. The co-responsiveness of haemoglobin accumulation and male offspring production in these 11 clones indicates that the same hormonal pathway regulates these two processes. `Discovering that haemoglobin is hormonally regulated is exciting'says LeBlanc, `because we already know a lot about the haemoglobin gene that responds to methyl farnesoate. So we now have an array of genetic tools to study this new stress signalling pathway.'But why are haemoglobin and male offspring production linked in this way?`Stressful environmental signals that tell the animals to switch to sex might coincidentally be correlated with low environmental oxygen levels' LeBlanc suggests, `for example, overcrowding may result in a drop in oxygen levels. When the environment has suddenly turned hostile, increased haemoglobin production to help the animals transport oxygen and male offspring production resulting in eggs with better survival chances might both come in handy.'

Stress signaling: coregulation of hemoglobin and male sex determination through a terpenoid signaling pathway in a crustacean

Environmental signals can activate neuro-endocrine cascades that regulate various physiological processes. In the present study, we demonstrate that two responses to environmental stress signaling in the crustacean Daphnia magna--hemoglobin accumulation and male offspring production--are co-elevated by the crustacean terpenoid hormone methyl farnesoate and several synthetic analogs. Potency of the hormones with respect to the induction of both hemoglobin and male offspring was highly correlated, suggesting that both processes are regulated by the same terpenoid signaling pathway. Six clones of the D. pulex/pulicaria species complex that were previously characterized as unable to produce male offspring and five clones that were capable of producing males were evaluated for both hemoglobin induction and male offspring production in response to methyl farnesoate. Four of the five male-producing clones produced both hemoglobin and male offspring in response to the hormone. Five of the six non-male-producing clones produced neither hemoglobin nor males in response to the hormone. These results provide additional evidence that both physiological processes are regulated by the same signaling pathway. Furthermore, the results indicate that the non-male-producing clones are largely defective in some methyl farnesoate signaling component, downstream from methyl farnesoate synthesis but upstream from the genes regulated by the hormone. A likely candidate for the site of the defect is the methyl farnesoate receptor. As a consequence of this defect, non-male-producing clones have lost their responsiveness to environmental signals that are transduced by this endocrine pathway. This defect in signaling would be likely to enhance population growth in stable environments due to the elimination of males from the population, assuming that other processes critical to population growth are not also compromised by this defect.