The spatio-temporal partitioning of sperm by males of the prospermatogenic parasitoid Nasonia vitripennis is in line with its gregarious lifestyle

Maternal body size affects sex allocation in a gregarious parasitoid

In parasitoid wasps, body size is a key predictor of reproductive success, as size is often correlated with increased gamete production. In Hymenoptera, due to haplodiploid sex determination, sperm are required to produce daughters but not sons, so sex allocation relies on a female's ability to obtain and manage a finite supply of sperm. Body size may, therefore, affect sex allocation if smaller males provide fewer sperm, or if larger, more fertile females deplete their sperm supplies by fertilizing more eggs. We examined the importance of male and female body size on offspring production and sex allocation in the parasitoid, Nasonia vitripennis (Walker). We predicted larger females would experience constrained sex allocation due to limited sperm, especially if mated with smaller males. We categorized 80 males and 80 females by size (large or small) into 4 mating groups: large females with large or small males, and small females with large or small males. We analyzed brood size and offspring sex ratios after each female oviposited on 12 pupae of Sarcophaga bullata (Parker). Overall, larger females produced more offspring and a greater proportion of males. Sex ratios became more male-biased with increased offspring production, indicating that sperm depletion had affected sex allocation in larger females. In contrast, male size did not affect sex allocation, but exhibited a small positive effect on females' offspring production. Our study highlights a complex relationship between body size, fecundity, and sex allocation, enhancing our understanding of reproductive strategies in haplodiploid organisms.

Hot and cold waves decrease sperm production and bias sex ratio in the parasitoid wasp Cotesia typhae (Hymenoptera, Braconidae)

Parasitoid wasps are haplodiploid, meaning that sperm stored by egg laying females are only used to produce daughters. Thus, the sex ratio of the offspring depends on the availability of sperm after mating. In these insects, males are sensitive to temperature at the pupal stage. This stress leads to subfertility due to a drastic reduction in the number of sperm produced and transferred to females. Experiments were conducted under controlled conditions on the parasitoid wasp Cotesia typhae (Hymenoptera, Braconidae), a natural enemy of the invading pest Sesamia nonagrioides (Lepidoptera, Noctuidae). At 25-27 °C, sperm production was measured for 7 days, and found to reach a plateau at the third day of adult life. It leads to a final amount around 25,000 sperm per male. A male can successfully inseminate at least 10 females, producing predominantly female offspring. Sperm production decreased significantly after 1 day of pupal exposure to heat at 34 or 36 °C and 7 days of cold at 0, 5 or 10 °C. This highlights that both cold and heat are stressful. After mating with one male treated at 10 or 34 °C, females store fewer sperm than the control, and produce fewer daughters. The sex ratio of the offspring is male biased when males experienced temperature stresses during development, like other parasitoid wasps. In the field, C. typhae populations would be affected by heat and cold, at least at the pupal stage. This lowers overwintering risk in case this biological agent was introduced in Europe. This risk is both economical, as companies seek to establish costly continuous production to sell beneficial insects, and ecological as the introduced population would not settle in the ecosystem. Lastly, the transport and storage of this insect of agronomic interest would need to consider temperature variations to ensure successful application.

Bacterial supergroup-specific "cost" of Wolbachia infections in Nasonia vitripennis

The maternally inherited endosymbiont, Wolbachia, is known to alter the reproductive biology of its arthropod hosts for its own benefit and can induce both positive and negative fitness effects in many hosts. Here, we describe the effects of the maintenance of two distinct Wolbachia infections, one each from supergroups A and B, on the parasitoid host Nasonia vitripennis. We compare the effect of Wolbachia infections on various traits between the uninfected, single A-infected, single B-infected, and double-infected lines with their cured versions. Contrary to some previous reports, our results suggest that there is a significant cost associated with the maintenance of Wolbachia infections where traits such as family size, fecundity, longevity, and rates of male copulation are compromised in Wolbachia-infected lines. The double Wolbachia infection has the most detrimental impact on the host as compared to single infections. Moreover, there is a supergroup-specific negative impact on these wasps as the supergroup B infection elicits the most pronounced negative effects. These negative effects can be attributed to a higher Wolbachia titer seen in the double and the single supergroup B infection lines when compared to supergroup A. Our findings raise important questions on the mechanism of survival and maintenance of these reproductive parasites in arthropod hosts.

A comparative genomic approach using mouse and fruit fly data to discover genes involved in testis function in hymenopterans with a focus on Nasonia vitripennis

Background

Spermatogenesis appears to be a relatively well-conserved process even among distantly related animal taxa such as invertebrates and vertebrates. Although Hymenopterans share many characteristics with other organisms, their complex haplodiploid reproduction system is still relatively unknown. However, they serve as a complementary insect model to Drosophila for studying functional male fertility. In this study, we used a comparative method combining taxonomic, phenotypic data and gene expression to identify candidate genes that could play a significant role in spermatogenesis in hymenopterans.

Results

Of the 546 mouse genes predominantly or exclusively expressed in the mouse testes, 36% had at least one ortholog in the fruit fly. Of these genes, 68% had at least one ortholog in one of the six hymenopteran species we examined. Based on their gene expression profiles in fruit fly testes, 71 of these genes were hypothesized to play a marked role in testis function. Forty-three of these 71 genes had an ortholog in at least one of the six hymenopteran species examined, and their enriched GO terms were related to the G2/M transition or to cilium organization, assembly, or movement. Second, of the 379 genes putatively involved in male fertility in Drosophila, 224 had at least one ortholog in each of the six Hymenoptera species. Finally, we showed that 199 of these genes were expressed in early pupal testis in Nasonia vitripennis; 86 exhibited a high level of expression, and 54 displayed modulated expression during meiosis.

Conclusions

In this study combining phylogenetic and experimental approaches, we highlighted genes that may have a major role in gametogenesis in hymenopterans; an essential prerequisite for further research on functional importance of these genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01825-6.

Reproductive context of extremely short sperm in the parasitic wasp Cotesia congregata (Hymenoptera: Braconidae)

Among adaptive traits under sexual selection, the length of spermatozoa shows high interspecific variation. In insects, extremes exist for both short and long sperm. The spermatozoa of the endoparasitic wasp Cotesia congregata (Say) are the shortest flagellated sperm described in animals, 6.6 µm in length. By comparison, the sperm of Drosophila bifurca are almost 6000 times longer. Thus, C. congregata has the potential to shed light on the selection pressures that drive variation in sperm length in relation to their production and use. The reproductive organs, sperm counts, controlled oviposition and sex ratios were investigated. The testes showed stratified differentiation stages of spermatogenesis, and sperm counts revealed continuous spermatogenesis in the late pupal stage. The small female spermatheca stored ~1000 sperm, resulting in an extremely high sperm concentration. The number of progeny per brood decreased over time until depletion of eggs. Females produced up to 370 daughters, corresponding to the effective use of 34% of the average sperm stock. Haploid males made up a greater proportion of broods in later parasitisms. Sperm miniaturization may be an adaptation to transfer increased quantities for the entire reproductive life of females in the absence of sperm competition but in the reduced space offered by the spermatheca.

Exposure to opposing temperature extremes causes comparable effects on Cardinium density but contrasting effects on Cardinium-induced cytoplasmic incompatibility

Terrestrial arthropods, including insects, commonly harbor maternally inherited intracellular symbionts that confer benefits to the host or manipulate host reproduction to favor infected female progeny. These symbionts may be especially vulnerable to thermal stress, potentially leading to destabilization of the symbiosis and imposing costs to the host. For example, increased temperatures can reduce the density of a common reproductive manipulator, Wolbachia, and the strength of its crossing incompatibility (cytoplasmic incompatibility, or CI) phenotype. Another manipulative symbiont, Cardinium hertigii, infects ~ 6–10% of Arthropods, and also can induce CI, but there is little homology between the molecular mechanisms of CI induced by Cardinium and Wolbachia. Here we investigated whether temperature disrupts the CI phenotype of Cardinium in a parasitic wasp host, Encarsia suzannae. We examined the effects of both warm (32°C day/ 29°C night) and cool (20°C day/ 17°C night) temperatures on Cardinium CI and found that both types of temperature stress modified aspects of this symbiosis. Warm temperatures reduced symbiont density, pupal developmental time, vertical transmission rate, and the strength of both CI modification and rescue. Cool temperatures also reduced symbiont density, however this resulted in stronger CI, likely due to cool temperatures prolonging the host pupal stage. The opposing effects of cool and warm-mediated reductions in symbiont density on the resulting CI phenotype indicates that CI strength may be independent of density in this system. Temperature stress also modified the CI phenotype only if it occurred during the pupal stage, highlighting the likely importance of this stage for CI induction in this symbiosis.

Insects often harbor heritable symbiotic bacteria that infect their cells and/or bodily fluids. These heritable bacteria are passed from mother to offspring and can have substantial effects on host insect biology, and include bacteria like Cardinium that cause mating incompatibilities between symbiont-infected and uninfected individuals. Often, the extent of these symbiont-conferred modifications correlates with the bacterial density in the host. The appearance of these phenotypes is also affected by temperature stress, which often reduces bacterial density. However, here we find that temperature-altered strength of Cardinium-induced mating incompatibility in a whitefly parasitoid wasp can be independent of Cardinium density. While heat treatment reduced the symbiont density and the phenotype, as expected, cold treatment also reduced symbiont density but increased the degree of mating incompatibility. Here, the prolonged duration of the host pupal development in the cold treatments appeared to be more important for phenotype strength. These results suggest that the connection between bacterial density and phenotype strength may not be as general as previously thought. Furthermore, the modification of this manipulative phenotype has implications for the effectiveness of the host, Encarsia suzannae, as a biological control agent.

Physiological and biochemical responses to thermal stress vary among genotypes in the parasitic wasp Nasonia vitripennis

In ectotherm species such as insects, thermal fluctuations represent a major environmental factor driving development, survival and reproduction of individuals. Reproductive traits are particularly sensitive to heat stress that can induce a permanent sterility, or at least hypofertility, of adult males. This study aims to compare physiological and biochemical responses associated to male performances to an exposure of 24 h to moderately high temperature (36 °C) among three inbred lines of N. vitripennis (AsymC, Cor, Oul). Cor males showed very specific metabolic adjustments compared to the two other lines. By contrast, Oul males showed stronger phenotypic adjustment of its life cycle, and produced metabolic water to compensate water loss by heat stress. Finally, AsymC males had probably more difficulties to acclimate at 36 °C, even for a short period, as their adult longevity was significantly reduced. Thus, the ability of developmental plasticity in N. vitripennis males exposed to heat stress appears to be dependent of their genotypes.

Life-history traits of the Whiting polyploid line of the parasitoid Nasonia vitripennis

In hymenopterans, males are normally haploid (1n) and females diploid (2n), but individuals with divergent ploidy levels are frequently found. In species with 'complementary sex determination' (CSD), increasing numbers of diploid males that are often infertile or unviable arise from inbreeding, presenting a major impediment to biocontrol breeding. Non-CSD species, which are common in some parasitoid wasp taxa, do not produce polyploids through inbreeding. Nevertheless, polyploidy also occurs in non-CSD Hymenoptera. As a first survey on the impacts of inbreeding and polyploidy of non-CSD species, we investigate life-history traits of a long-term laboratory line of the parasitoid Nasonia vitripennis (Walker) (Hymenoptera: Pteromalidae) ('Whiting polyploid line') in which polyploids of both sexes (diploid males, triploid females) are viable and fertile. Diploid males produce diploid sperm and virgin triploid females produce haploid and diploid eggs. We found that diploid males did not differ from haploid males with respect to body size, progeny size, mate competition, or lifespan. When diploid males were mated to many females (without accounting for mating order), the females produced a relatively high proportion of male offspring, possibly indicating that these males produce less sperm and/or have reduced sperm functionality. In triploid females, parasitization rate and fecundity were reduced and body size was slightly increased, but there was no effect on lifespan. After one generation of outbreeding, lifespan as well as parasitization rate were increased, and a body size difference was no longer apparent. This suggests that outbreeding has an effect on traits observed in an inbred polyploidy background. Overall, these results indicate some phenotypic detriments of non-CSD polyploids that must be taken into account in breeding.