Variable host quality, life-history invariants, and the reproductive strategy of a parasitoid wasp that produces single sex clutches

The parasitic wasp Achrysocharoides zwoelferi (Hymenoptera, Eulophidae) produces clutches consisting of only one sex. Moreover,male clutch size is invariably one while female clutches arein the range one to four. We designed field experiments todetermine the effect of host quality on clutch composition.We found that solitary male and solitary female clutches werereared from the same size mines, and that larger mines tendedto produce gregarious female clutches. A higher proportionof male clutches were placed in older hosts, despite theirlarge size. Variation in body size, both between and withinclutches, was measured in order to test the predictions of models that take into account the constraint that clutch size is aninteger trait, something of potential importance when absoluteclutch size is low. Our data supported several predictionsof these models, including the trade-off-invariant rule foroptimal offspring size developed by Charnov and Downhower.However, while most invertebrate clutch size models assume equal resource share among members of the same clutch, we found anincrease in inequality in larger clutches.     

Constrained sex allocation in a parasitoid due to variation in male quality

The theory of constrained sex allocation posits that when a fraction of females in a haplodiploid population go unmated and thus produce only male offspring, mated females will evolve to lay a female-biased sex ratio. I examined evidence for constrained sex ratio evolution in the parasitic hymenopteran Uscana semifumipennis. Mated females in the laboratory produced more female-biased sex ratios than the sex ratio of adults hatching from field-collected eggs, consistent with constrained sex allocation theory. However, the male with whom a female mated affected her offspring sex ratio, even when sperm was successfully transferred, suggesting that constrained sex ratios can occur even in populations where all females succeed in mating. A positive relationship between sex ratio and fecundity indicates that females may become sperm-limited. Variation among males occurred even at low fecundity, however, suggesting that other factors may also be involved. Further, a quantitative genetic experiment found significant additive genetic variance in the population for the sex ratio of offspring produced by females. This has only rarely been demonstrated in a natural population of parasitoids, but is a necessary condition for sex ratio evolution. Finally, matings with larger males produced more female-biased offspring sex-ratios, suggesting positive selection on male size. Because the great majority of parasitic hymenoptera are monandrous, the finding of natural variation among males in their capacity to fertilize offspring, even after mating successfully, suggests that females may often be constrained in the sex allocation by inadequate number or quality of sperm transferred.     

单爪螯蜂属四新种（膜翅目：螯蜂科）

报道中国单爪螯蜂属Ａｎｔｅｏｎ Ｊｕｒｉｎｅ４新种,即平头单爪螯蜂Ａ．ｐｌａｎｕｍ,ｓｐ．ｎｏｖ．;横单爪螯蜂Ａ．ｔｒａｎｓｖｅｒｕｍ,ｓｐ．ｎｏｖ,齿单爪螯蜂Ａ．ｓｅｒｒａｔｕｍ,ｓｐ,ｎｏｖ．和凸单爪螯蜂Ａ．ｃｏｎｖｅｘｕｍ,ｓｐ,ｎｏｖ．模式标本保存于浙江大学。     

中国双距螯蜂属一新种(膜翅目,螯蜂科)

记述采自山东平阴玫瑰上叶蝉寄生蜂1新种:长节双距螯蜂Gonatopus prolixus sp.nov..模式标本保存于浙江大学寄生蜂标本室.     

Sex ratio manipulation in response to host size in the parasitoid wasp Spalangia cameroni: is it adaptive?

Many species of parasitoid wasps produce a greater proportionof sons in small than in large hosts. As described by the host-sizemodel, natural selection is becoming a standard explanationfor the evolution of this phenomenon. We examined a criticalassumption of the host-size model, that host size has a morepositive effect on female than on male reproductive success.In laboratory experiments with the parasitoid wasp Spalangiacameroni, females that developed on larger hosts contained moreeggs at emergence. However, more eggs did not translate intomore offspring, at high or low density and regardless of whethera female had to burrow to reach hosts. The size of host on whicha female developed was also unrelated to her longevity, regardlessof the presence or absence of hosts. The size of host on whicha male developed had no effect on his sperm production or abilityto inseminate females, regardless of whether insemination abilitywas measured by the amount of sperm transferred to a female,by the proportion of a male's mates that produced any daughters,or by the proportion of daughters that a male's mates produced.Thus, despite data on multiple measures of fitness under a rangeof conditions, sex ratio manipulation in response to host sizein S. cameroni does not appear to be adaptive, and another explanationis needed.     

单爪螯蜂属二新种：膜翅目：螯蜂科;单爪螯蜂亚科

本文记述单爪螯蜂属二新种;梳垫单瓜螯蜂Ａｎｔｅｏｎ ｐｅｃｔｉｎａｒｏｌｉｕｍ,ｓｐ．ｎｏｖ。和滑盾单爪螯蜂Ａｎｔｅｏｎ ｂｌａｎｄｕｓｃｕｔｕｍ,ｓｐ．ｎｏｖ。新种的模式标本保存在浙江农业大学植物保护系。     

Humans at tropical latitudes produce more females

Skews in the human sex ratio at birth have captivated scientists for over a century. The accepted average human natal sex ratio is slightly male biased, at 106 males per 100 females or 51.5 per cent males. Studies conducted on a localized scale show that sex ratios deviate from this average in response to a staggering number of social, economical and physiological variables. However, these patterns often prove inconsistent when expanded to other human populations, perhaps because the nature of the influences themselves exhibit substantial cultural variation. Here, data collected from 202 countries over a decade show that latitude is a primary factor influencing the ratio of males and females produced at birth; countries at tropical latitudes produced significantly fewer boys (51.1% males) annually than those at temperate and subarctic latitudes (51.3%). This pattern remained strong despite enormous continental variation in lifestyle and socio-economic status, suggesting that latitudinal variables may act as overarching cues on which sex ratio variation in humans is based.     

Infanticide in a Dryinid Parasitoid, Haplogonatopus atratus

When a parasitized host is superparasitized, the second comer—an immature parasitoid originating from the second oviposition—is usually handicapped relative to the first comer in the competition for the host. In Haplogonatopus atratus, however, the survival rate of second comers is frequently higher than that of first comers. Infanticide is assumed to be responsible for this phenomenon. The present report reveals that, when the female wasp of H. atratus superparasitizes, she examines several membranes between the host's abdominal terga with her stinger before laying a supernumerary egg and that, when she locates the first comer, she pierces it with her stinger to kill it. Moreover, once she catches a host she performs infanticide irrespective of whether the host is self- or conspecifically parasitized.     

Queen control over reproductive decisions--no sexual deception in the ant Lasius niger

Queen-worker conflicts in social insect societies have received much attention in the past decade. In many species workers modify the colony sex ratio to their own advantage or produce their own male offspring. In some other species, however, queens seem to be able to prevent workers from making selfish reproductive decisions. So far, little effort has been made to find out how queens may keep control over sex ratio and male parentage. In this study we use a Lasius niger population under apparent queen control to show that sexual deception cannot explain queen dominance in this population. The sexual deception hypothesis postulates that queens should prevent workers from discriminating against males by disguising male brood as females. Contrary to the predictions of this hypothesis, we found that workers are able to distinguish male and female larvae early in their development: in early spring workers generally placed only either female or male larvae in the uppermost chambers of the nest, although both types of larvae must have been present. At this time males were only at 11% of their final dry weight, a developmental stage at which (according to two models) workers would still have benefited from replacing queen-produced males by females or worker-produced males. This study thus demonstrates that sexual deception cannot account for the apparent queen control over colony sex ratio and male parentage in L. niger.     

毛角螯蜂属中国新纪录及一新种记述(膜翅目:螯蜂科:螯蜂亚科)

本文描述毛角螯蜂属一新种：脊毛角螯蜂Ｔｈａｕｍａｔｏｄｒｙｉｎｕｓ ｃａｒｉｎａｔｕｓ ｓｐ．ｎｏｖ．。这是毛角螯蜂属在中国的首次纪录。     

Sex ratio, sex-specific chick mortality and sexual size dimorphism in birds

It has been suggested that sexual size dimorphism (SSD) may influence sex ratios at different life stages. Higher energy requirements during growth associated with larger body size could lead to a greater mortality of the larger sex and ultimately to an overproduction of the smaller sex. To explore the associations between SSD and hatching and fledging sex ratio we performed a species-level analysis and a phylogenetically controlled analysis, based on 83 bird species. Overall, there was a significant inverse relationship between the degree of SSD and the proportion of males at hatching and fledging. Sex-specific mortality related to SSD showed a weak but persistent negative tendency, suggesting a mortality bias towards the larger sex. These results suggest that changes in relation to SSD may take place mainly at the conception stage, but could be adjusted during growth. However, conclusions should be treated cautiously as these relationships weaken when additional variables are considered.     

中国新记录异螫蜂属及广东一新种记述（膜翅目：螯蜂科）

本文报道了采自我国南方广东省的异螫蜂属１新种──扁角异螫蜂ＡｄｒｙｉｎｕｓＰｌａｔｙｃｏｒｎｉｓｓｐ．ｎｏｖ,模式标本存浙江农业大学植物保护系生物防治标本室。     

Colony-level sex ratio selection in the eusocial Hymenoptera

We present an inclusive fitness model on worker-controlled sex investments in eusocial Hymenoptera which expands the existing theory for random mating populations as formulated by Trivers and Hare (1976) and Benford (1978). We assume that relatedness asymmetry is variable among colonies — owing to multiple mating, worker reproduction and polygyny — and that workers are able to assess the relatedness asymmetry in their own colony. A simple marginal value argument shows that “assessing” workers maximize their inclusive fitness by specializing on the production of the sex to which they are relatively more related than the average worker in the population is related to that sex. The model confirms our earlier verbal argument on this matter (Boomsma and Grafen, 1990) and gives further quantitative predictions of the optimal sex ratio of relatedness-asymmetry classes for both infinite and finite, random mating populations. It is shown that in large populations all but one of the relatedness-asymmetry classes should specialize on the production of one sex only. The remaining, balancing class is selected to compensate any bias induced by the other class(es) such that the population sex ratio reflects the relatedness asymmetry of that balancing class. In the absence of worker-reproduction, the sex ratio compensation by the balancing-class is generally close to 100%, unless the population is very small. In the Discussion we address explicitly the likelihood of our relatedness-assessment hypothesis and other assumptions made in the model. The relationship of our model with previous theory on sex allocation in eusocial Hymenoptera is worked out in the Appendix.