Heat shock sensitivity of adult male fertility in the parasitoid wasp Anisopteromalus calandrae (Hymenoptera, Pteromalidae).

In insects, decreased reproduction is a sublethal consequence of high temperatures, with males being more sensitive to this in many species. In hymenoptera, arrhenotokous parthenogenesis means that female offspring are produced using sperm and are thus diploid, while males are haploid. Consequently, sperm stocks in males and females (after copulation) are a key regulator of the sex ratio. Anisopteromalus calandrae is a parasitoid wasp in which males can suffer from subfertility due to a drastic decrease in sperm count after exposure to high temperatures during a critical early pupal stage. However, in this species spermatogenesis continues during adulthood, therefore the heat sensitivity of adult males remains to be studied. Laboratory studies were conducted on virgin and previously mated young adult males under control (30 °C) and heat shock (10 min at 48 °C) conditions to exhaust their initial sperm stock. After heat shock, in both virgin and already mated males, the individual sperm potential was half that of controls. Both groups continuously produced sperm, but sperm stock of heat shocked males' never reached that of the controls. Heat shock reduced survival at 10 days only in previously experienced males but had no impact on the mating ability in competition for a female compared to controls. Despite a reduced sperm count, heat shocked males had fully fertile spermatozoa. Such a physiological response to heat shock in a species with continuous sperm production could be of major interest for both wild populations in a context of temperature variations and parasitoid wasps introduced for agronomical purposes.

The Centrosomes of Haploid and Diploid Cells Have an Equal Number of Centrioles in the Parasitoid Wasp Anisopteromalus Calandrae.

BACKGROUND: The centrosome is the main center of the organization of microtubules (MT) in the cell, the origin for the formation of flagella and cilia, as well as the site of many regulatory intracellular processes. In diploid cells, the centrosome includes two centrioles connected to some additional structures and surrounded by pericentriolar material. METHODS: The ultrastructure of the cells was studied using transmission electron microscopy on serial ultrathin sections. RESULTS: Here, using transmission electron microscopy on a complete series of ultrathin sections of the centrosome region, we studied the relation between the number of centrioles and ploidy in diploid cells of female wasps and haploid cells of male in the parasitoid wasp Anisopteromalus calandrae (Hymenoptera). It showed that the haploid cells of the male insect have the same number of centrioles as the diploid cells of the female. CONCLUSIONS: It can be concluded that there is no strict correlation between the number of chromosome sets (ploidy) and the number of centrioles in haplodiploid insects.

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.

[Parasitic wasps, a model for studying male subfertility]. / Les hyménoptères parasitoïdes: des modèles pour l'étude de l'hypofertilité mâle.

Many studies have reported an increase in male reproductive disorders due to the environment. Despite their ecological importance invertebrates have been poorly studied; they are yet affected in the same way as vertebrates by changes in the environment. Different stresses made on parasitic wasps are able to induce a male subfertility. These insects are easy to rear in the laboratory, used to control insect pests, and have other benefits presented in this article. Parasitic wasps are good models for understanding the mechanisms inducing subfertility and may provide new areas for research in vertebrates.

Sperm Production Is Reduced after a Heatwave at the Pupal Stage in the Males of the Parasitoid Wasp Microplitisrufiventris Kok (Hymenoptera; Braconidae).

Understanding reproduction is essential for controlling pests and supporting beneficial insects. Among the many factors allowing optimal reproduction, sperm availability is key to sex ratio control in hymenopteran parasitoids since males are haploid and only females come from fertilization. Microplitisrufiventris (Hymenoptera; Braconidae) is a solitary endoparasitoid of some noctuids. This insect could be used for the control of the cotton leafworm Spodopteralittoralis. Under controlled conditions, sperm quantity was measured in virgin males at 1, 5, 10, and 15 days; it increases in adult males until the fifth day. Sperm stock of control males increased from 2500 at one day to 6700 at 15 days. With the control climatic condition being 25 °C, we tested the effects of a time-limited increase of temperature that can be found in Egypt (36 and 40 °C) during one day at the early pupal stage. Emerging males had 1500 and 420 sperm at 36 and 40 °C, respectively; both lived shorter than the control. The sperm potential of males is dependent on both age and temperature during the early pupal stage. It could have dramatic consequences on the sex ratio of M. rufiventris in natural and controlled populations.

Chromosomal scale assembly of parasitic wasp genome reveals symbiotic virus colonization.

Endogenous viruses form an important proportion of eukaryote genomes and a source of novel functions. How large DNA viruses integrated into a genome evolve when they confer a benefit to their host, however, remains unknown. Bracoviruses are essential for the parasitism success of parasitoid wasps, into whose genomes they integrated ~103 million years ago. Here we show, from the assembly of a parasitoid wasp genome at a chromosomal scale, that bracovirus genes colonized all ten chromosomes of Cotesia congregata. Most form clusters of genes involved in particle production or parasitism success. Genomic comparison with another wasp, Microplitis demolitor, revealed that these clusters were already established ~53 mya and thus belong to remarkably stable genomic structures, the architectures of which are evolutionary constrained. Transcriptomic analyses highlight temporal synchronization of viral gene expression without resulting in immune gene induction, suggesting that no conflicts remain between ancient symbiotic partners when benefits to them converge.

Microgynous queens in the Paleartic Ant, Manica rubida: dispersal morphs or social parasites?

In many ant species, queen size is dimorphic, with small microgynes and large macrogynes, which differ, for example, in size, insemination rate, ovary development, and dispersal tactics. These polymorphic queens often correspond with alternative reproductive strategies. The Palearctic ant, Manica rubida (Latreille) (Hymenoptera: Formicidae), lives mostly in mountainous regions in either monogynous colonies, containing one macrogynous queen or polygynous colonies, containing a few large macrogynous queens. In 1998, a colony of M. rubida was discovered containing macrogynes and many small alate microgynes that did not engage in a nuptial flight but, instead, stayed in the home nest the following winter. These microgynes were studied more closely by investigating their size, behavior, and spermatheca in relation to M. rubida macrogynes and workers. Mitochondrial DNA of macrogynes, microgynes and workers from four nests was sequenced to detect possible genetic differences between them. The microgynes were significantly smaller than the macrogynes, and the head width of the gynes was completely bimodal. The microgynes behaved like workers of the macrogynes in every experiment tested. Furthermore, the microgynes had a normal spermatheca and could be fecundated, but rarely (only one in several years). Finally, all the individuals were genetically identical, except three workers that differed by only one codon position. Because these microgynes have features of both queens and workers, their functional significance in the colony is not yet clear.

Sperm limitation affects sex allocation in a parasitoid wasp Nasonia vitripennis.

Insect reproduction is influenced by various external factors including temperature, a well-studied constraint. We investigated to what extent different levels of sperm limitation of males exposed to different heat stresses (34 and 36 °C) affect females' offspring production and sex allocation in Nasonia vitripennis. In this haplodiploid parasitoid wasp attacking different species of pest flies, we investigated the effect of the quantity of sperm females received and stored in their spermatheca on their sperm use decisions, hence sex allocation, over successive ovipositions. In particular, we compared the sex allocation of females presenting three levels of sperm limitation (i.e., mated with control, 34 °C heat-stressed or 36 °C heat-stressed males) on each host they parasitized. To disentangle the potential reduction of sperm quality after a heat stress exposure from that of sperm quantity, we also explored the clutch size and sex ratio produced by females that were partially sperm limited after copulating with multiply mated males. Independently of their sperm numbers, all types of females produced a similar total number of offspring, but the more limited ones had fewer daughters. Sperm limitation further affected the distribution of daughters' production across time. In addition to constraints acting on female physiology, male fertility should therefore be considered in studies measuring reproductive outputs of insects submitted to heat stresses.

Sperm stock and mating of males in a parasitoid wasp.

Hymenoptera are haplodiploid insects, consequently sex ratio depends on female's sperm management which itself arises from the reproductive capacity of neighbouring males. To study the influence of ageing on male reproductive potential, laboratory experiments were conducted on Dinarmus basalis (Hymenoptera, Pteromalidae) males, a tropical wasp in which sperm counts are known to constrain sex ratio. Two groups of virgin males were compared: 1-day and 30-days old. Parameters recorded were sperm quantity and viability in seminal vesicles, shape of testis, mating ability in both individual and competitive situations and sperm stored by females after male multiple mating. Older males had twice as much sperm as young males, but their reproductive capacities did not differ. They were able to copulate with 20 successive virgin females in a short period. Sperm stored in spermathecae decreased with female mating order. In competition, old and young males had the same access to females. The difference between old and young males was visible at the level of reproductive tract: young males have functional testis and old males have empty non-functional testis. Spermatozoa are kept viable in male seminal vesicles for long periods. In this species, the reproductive potential of males is not altered by ageing. At the population level this may represent an adaptation for maintaining continuous reserves of sperm at the disposal of females.

Centrioles without microtubules: a new morphological type of centriole.

The centrosome is the organizing center of microtubules in the cell, the basis for the origin of cilia and flagella and a site for the concentration of a regulatory proteins multitude. The centrosome comprises two centrioles surrounded by pericentriolar material. Centrioles in the cells of different organisms can contain nine triplets, doublets or singlets of microtubules. Here, we show that in somatic cells of male wasp larvae Anisopteromalus calandrae, centrioles do not contain microtubules and are composed of nine electron-dense prongs, which together form a cogwheel structure. These microtubule-free centrioles can be the platform for procentriole formation and form microtubule-free cilia-like structures. In nymph and imago cells centrioles have a microtubule triplet structure. Our study describes how centriole structure differs in a development-stage-dependent and a cell-type-dependent manner. The discovery of a centriole without microtubules casts a new light on the centriole formation process and the evolution of this organelle.

Comparison of reproductive traits of foundresses in a native and an invasive hornet in Europe.

The population dynamics of annual social hymenoptera such as vespids depend largely on the fertility of the foundresses, which, in turn, is a key factor in the context of biological invasions. The native European hornet Vespa crabro (Vc) and the invasive Asian hornet Vespa velutina (Vv) have generally similar ecological traits, e.g. nesting and feeding habits, although they differ in colony size, which is higher in Vv. Furthermore, in contrast to Vc, Vv is more specialized in its predatory habits, intensively hunting honey bees at the hive. Comparing the morphological and reproductive traits of two closely related species occupying the same ecological niche, one of which is a native species and the other an alien, can help us to gain an understanding of the invasion process. To this end, we here compare reproductive (ovarian size and maturation, fat level, spermatheca size and sperm stock, fecundity) and morphological traits (head size, weight) of the foundresses of these two hornet species. We observed that ovarian maturation began approximately one month earlier in Vv than in Vc, and that the fat level in the former was lower. We found twice the number of sperm in the mated foundresses of Vv than in those of Vc (more than 100 × 103 and less than 50 × 103 sperm, respectively), in a 16% smaller spermatheca in Vc. Furthermore, the sperm of Vv was found to be 65% shorter than that of Vc. The precocity and higher potential fecundity of Vv queens may have favoured this species over Vc in terms of predatory behaviour, and thereby contributed to its invasiveness.

Delayed sexual maturity in males of Vespa velutina.

Vespa velutina var nigrithorax (Lepelletier, 1835) is an invasive predator of bees accidentally introduced in France in 2004, and it is having a serious impact on apiculture and ecosystems. Studying the reproduction of an invasive species is key to assess its population dynamic. This study explores the sexual maturation of V. velutina males and the evolution of their fertility. The main studied parameters were physiologic (spermiogenesis, spermatogenesis) and anatomic (testes size and structure, head width). Two populations of males were described based on their emergence period: early males in early summer or classic males in autumn. Each testis has an average of 108 testicular follicles. Spermatogenesis is synchronous, with only 1 sperm production wave, and completed, on average, at 10.3 d after emergence with the degeneration of the testes. The sperm counts in seminal vesicles of mature males are 3 × 106 in October/November and 0.8 × 106 in June. In comparison, females store 0.1 × 106 sperm in their spermathecae. The early males emerged from colonies made by fertilized queens. The reproductive potential of these early males seemed limited, and their function in the colony is discussed. The sperm stock evolution in autumn males suggests the occurrence of a reproductive pattern of male competition for the access to females and a single copulation per male. The synchronicity of male and foundress emergences and sexual maturation is of primary importance for the mating success and the future colony development.

Sperm production and mating potential of males after a cold shock on pupae of the parasitoid wasp Dinarmus basalis (Hymenoptera: Pteromalidae).

For ectothermic species, temperature is a key environmental factor influencing several aspects of their physiology and ecology, acting particularly on reproduction. To measure the consequences of a severe thermal stress during development on male reproduction, a cold shock (1h at -18 degrees C) was tested on Dinarmus basalis pupae. D. basalis (Hymenoptera: Pteromalidae) is a parasitoid wasp in which sperm management in both male and female is of prime importance. After a cold shock, developmental success was reduced, with a quarter of cold-shocked males not emerging correctly. The stress effects were estimated at the level of sperm stock in seminal vesicles of males at different ages and on the ability of 2-day-old males to access females in single and multiple mating and in male-male competition. Cold-shocked males had a reduced sperm stock compared to control males and this difference persisted with age. The rate of sperm production was similar in both groups. The consequences of a cold shock on male reproductive ability were perceptible in multiple mating and male-male competition but not in single mating. Cold-shocked males were at a disadvantage, inseminating fewer females and copulating less frequently. Finally, male pupae of D. basalis were able to withstand severe temperature stresses and their reproductive functions were partially preserved.

The length of a short sperm: Elongation and shortening during spermiogenesis in Cotesia congregata (Hymenoptera, Braconidae).

The spermatozoon of the parasitoid wasp Cotesia congregata is an extremely short gamete measuring less than 7 µm; it is as yet the shortest flagellated sperm to be identified. The mature sperm consists of an acrosome, surrounded by an extra cellular coat, a condensed nucleus, two uncoiled mitochondrial derivatives and a short axoneme. Testes of young adults contain a continuum of differentiation stages. Initially, the flagellum is approximately 5 µm long. It conserves its length in round, elongated and mature spermatids, but is reduced to less than 3 µm in mature spermatozoa. The nucleus is 2 µm in diameter when round, 10 µm long when it becomes a long boat-hull shaped filament, and then reduces to 3.6 µm. Thus, during development the gamete reaches a total length of 15 µm before finally reducing to less than half that length. Some traits of mature sperm anatomy are similar to related species of the Braconidae family, but others seem to be specific and could be due to the shortness of the cell. This uncommon elongation and subsequent shortening of such a tiny flagellated cell constitutes a model for both nucleus and cilium development.

Effect of host size on male fitness in the parasitoid wasp Dinarmus basalis.

The effect of host size on male fitness was tested in the parasitoid wasp Dinarmus basalis (Hymenoptera, Pteromalidae) using hosts of different fresh weight. Fitness was measured as the sperm stock in seminal vesicles, and the ability to access females in single or competition situations. Both body size and sperm in seminal vesicles increased with host fresh weight. Males from small hosts had a reduced size and sperm stock compared to those from larger hosts. In single situations, males from both small and large hosts had similar reproductive capacities, whereas in multiple mating or competition situations, males from small hosts were at a disadvantage, inseminating fewer females and copulating less frequently. However, females did not appear to choose between males, and no effect on sperm stored in the spermatheca was observed. Being small does not prevent a D. basalis male mating and producing progeny in single situations, although more offspring could be expected from larger males because of their better competitive abilities.

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

Male fitness depends on the number of lifetime progeny of their mates and could be constrained by the chance of finding a mate, lifespan and temporal patterns of sperm production and allocation. Here, we used the parasitic wasp Nasonia vitripennis with a two-week lifespan and a gregarious lifestyle, to analyze how the reproductive system is organized to allocate spermatozoa over consecutive matings. Results show that spermatogenesis is synchronized and completed one day before emergence so that males emerge with a full sperm complement. We also found a regulation of spermatozoa transfer between testis and seminal vesicles that allows males to partition small ejaculates over multiple matings. Overall, this study shows that for N. vitripennis, male fertilization potential is determined (1) at the pupal stage, when spermatogenesis takes place to generate a complete life-long stock, (2) on emergence, when transport of spermatozoa from testes to seminal vesicles is initiated and (3) in adulthood, during which spermatozoa are partitioned over successive copulations. Such life history-traits are consistent with the gregarious lifestyle of N. vitripennis.

[Reproductive capacity of females Eupelmus vuilleti (Eupelmidae) inseminated by hyperparasitoid males developed upon the primary parasitoid Dinarmus basalis (Pteromalidae)]. / Capacité de reproduction des femelles Eupelmus vuilleti (Eupelmidae), inséminées par des mâles développés en hyperparasitoïdes de Dinarmus basalis (Pteromalidae).

Eupelmus vuilleti is a primary and solitary ectoparasitoid of the larval stages of Bruchids (Callosobruchus maculatus, Bruchidius atrolineatus). In a context of intense competition for healthy hosts, E. vuilleti displays ovicide and larvicide behaviours towards the Pteromalid D. basalis during its development (kleptoparasitism), and in an extreme expression of kleptoparasitism the E. vuilleti females hyperparasitize the final larval stage (L5 stage) of D. basalis. In this study, we compared the variability of reproductive success in males that had developed in the context of hyperparasitism to that in males that had developed on primary hosts. The adaptation capacity of the males when 24 h old was analysed in terms of their weight, of the quantity of spermatozoids stored in the seminal vesicles, of the quality of insemination determined from the quantity of spermatozoids stored in the spermatheca of the females after the first mating, and of the number of daughters produced. Adults of E. vuilleti, the larvae of which had developed as hyperparasitoids, are smaller than those that have developed on primary hosts, but they keep all the abilities required to parasite a population of primary hosts once the competitive pressure is reduced.

A combined approach to heat stress effect on male fertility in Nasonia vitripennis: from the physiological consequences on spermatogenesis to the reproductive adjustment of females mated with stressed males.

In recent years, several studies have shown a decline in reproductive success in males in both humans and wildlife. Research on male fertility has largely focused on vertebrates, although invertebrates constitute the vast majority of terrestrial biodiversity. The reduction of their reproductive capacities due to environmental stresses can have strong negative ecological impacts, and also dramatic consequences on world food production if it affects the reproductive success of biological control agents, such as parasitic wasps used to control crop pests. Here Nasonia vitripennis, a parasitic wasp of various fly species, was studied to test the effects of 24h-heat stress applied during the first pupal stage on male fertility. Results showed that only primary spermatocytes were present at the first pupal stage in all cysts of the testes. Heat stress caused a delay in spermatogenesis during development and a significant decrease in sperm stock at emergence. Females mated with these heat-stressed males showed a reduce sperm count stored in their spermatheca. Females did not appear to distinguish heat-stressed from control males and did not remate more frequently to compensate for the lack of sperm transferred. As a result, females mated with heat-stressed males produced a suboptimal lifetime offspring sex ratio compared to those mated with control males. This could further impact the population dynamics of this species. N. vitripennis appears to be an interesting biological model to study the mechanisms of subfertility and its consequence on female reproductive strategies and provides new research perspectives in both invertebrates and vertebrates.

Heat stress affects male reproduction in a parasitoid wasp.

In insects, reproductive success and survival are affected by temperature. Reproduction is more sensitive than other physiological traits. While the effects of heat stress on females are well known, the effects on males are less clear. Hymenopteran parasitoids are valuable for studying the consequences of heat stress on male reproduction. In these species, through arrhenotoquous parthenogenesis, the sex ratio of the offspring is directly dependent on the sperm stock acquired by females during copulation. In the lab, heat temperature treatments (32-44°C) were applied for 3 days in the pupal stage of Anisopteromalus calandrae males, and development was completed at 30°C. Three different effects were observed depending on the temperature: mortality above 42°C, sterility of emerging males at 40°C, and sub-fertility at 38°C. This sub-fertility is characterized by a dramatic decrease in male sperm supplies, of up to 7% compared to control males. In the course of ageing, the sperm stock of sub-fertile males increases but never reaches the level of control males. Survival was significantly higher in control (30°C) males than those treated at 38°C. Male mating ability was similar whatever the treatment (control and 38°C), but females mated with 38°C-treated males stored 100 times less sperm on average than those mated with control males. The offspring sex ratio of females mated with 38°C-treated males was strongly male biased. The physiological mechanisms are as yet unknown. The relationship between temperature, sperm stock and sex ratio should be taken into account in the management of parasitoids for integrated pest management.