Takatoshi Ueno

Host-size-dependent sex ratio in a parasitoid wasp

Charnovs host-size model explains parasitoid host-size-dependent sex ratio as an adaptive consequence when there is a differential effect of host size on the offspring fitness of parasitoid males versus females. This article tests the predictions and the assumptions of the host-size model. The parasitoid wasp Pimpla nipponica Uchida (Hymenoptera: Ichneumonidae) laid more female eggs in larger or fresher host pupae when choice among hosts of different sizes or ages was allowed. Then, whether an asymmetrical effect of host size and age on the fitness of females versus males existed in P. nipponica was examined. Larger or fresher host pupae yielded larger wasps. Larger females lived longer, whereas male size did not influence male longevity. Large males mated successfully with relatively large females but failed with small females, whereas small males could mate successfully either with small or with large females. Thus, small-male advantages were found, and this held true even under male–male competition. Ovariole and egg numbers at any one time did not differ among females of different sizes. Larger females attained higher oviposition success and spent less time and energy for oviposition in hosts. Larger females produced more eggs from a single host meal. Taken together, females gained more, and males lost more, by being large. Host size and age thus asymmetrically affected the fitness of offspring males versus females through the relationships between host size or hast age and wasp size, which means the basic assumption of the host-size model was satisfied. Therefore, sex ratio control by P. nipponica in response to host size and age is adaptive.

Cloning of cDNA for Vitellogenin of the Parasitoid Wasp, Pimpla nipponica (Hymenoptera: Apocrita: Ichneumonidae): Vitellogenin Primary Structure and Evolutionary Considerations

The cDNA for vitellogenin (Vg) of the parasitoid wasp Pimpla nipponica (Hymenoptera: Apocrita) was cloned and sequenced. The deduced amino acid sequence with 1807 residues was obtained. The N-terminal 20 amino acids chemically determined for vitellin (Vn) agreed completely with the deduced 20 amino acids that follow the 16 amino acid residues for putative signal peptide. The cDNA clone for the Vg of the turnip sawfly Athalia rosae (Hymenoptera: Symphyta), previously obtained and partially sequenced, was also completely sequenced and the amino acid sequence deduced. Amino acid sequences were compared between these two species and also with known Vg sequences from other insects. Common to all these insects is the presence of two long regions with relatively well-conserved amino acid sequences, one near the N-terminal extending 267-282 residues (including two cysteines at conserved locations), and the other starting at position 450 to 655 and extending 279-283 residues, and of a region at the C-terminal extending some 200 residues (about 250 in Aedes aegypti due to the presence of a serine-rich stretch) with 10 cysteines at conserved locations. A molecular phylogenetic tree was constructed.

Self-recognition by the parasitic wasp Itoplectis naranyae (Hymenoptera: Ichneumonidae)

«Self-recognition», an ability of female parasitoids to discriminate between hosts parasitized by themselves and those parasitized by other conspecific females, was tested in the ichnuemonid wasp Itoplectis naranyae. The question of whether selfrecognition can be interpreted as kin recognition was then focused on. I. naranyae females were able to recognize self-parasitized hosts externally when they encountered them within a 30 min delay after the first parasitization; females strongly avoided superparasitism of self-parasitized hosts but not of conspecifically parasitized hosts. The intensity of self-superparasitism avoidance was strongly time dependent after the first parasitization, and also depended on the host density which females had experienced. Females avoided superparasitism either of conspecifically or of self-parasitized hosts internally gradually with time elapsed

Adaptiveness of sex ratio control by the pupal parasitoid Itoplectis naranyae (Hymenoptera: Ichneumonidae) in response to host size

Adaptiveness of sex ratio control by the solitary parasitoid wasp Itoplectis naranyae (Hymenoptera: Ichneumonidae) in response to host size was studied, by examining whether differential effects of host size on the fitness of resulting wasps are to be found between males and females. The offspring sex ratio (male ratio) decreased with increasing host size. Larger hosts yielded larger wasps. Male larvae were less efficient in consuming larger hosts than female larvae. No significant interaction in development time was found between parasitoid sex and host size. Larger female wasps lived longer than smaller females, while longevity of male wasps did not increase with increasing wasp size. Smaller males were able to mate either with small or with large females, while larger males failed to mate with small females. Larger female wasps had a greater number of ovarioles and mature eggs at any one time than smaller females, although the number of eggs produced per host-feeding was not influenced by female wasps. Thus, the differential effect of host size on the fitness of males and females exists in I. naranyae. The basic assumption of the host-size model was therefore satisfied, demonstrating that sex ratio control by I. naranyae in response to host size is adaptive.

Self-host discrimination by a parasitic wasp : The role of short-term memory

Ovipositing parasitoid wasps deposit marking pheromones on the host that deter conspecifics from superparasitism. In addition to recognizing previously parasitized hosts, female wasps of some species discriminate between hosts parasitized by themselves and by conspecifics (self–host discrimination). The mechanism of self–host discrimination in the parasitic waspPimpla nipponicaUchida was examined. A series of experiments, however, failed to show that females of this species deposit chemical markers on a host during oviposition. Females failed to recognize hosts covered either with pieces of cocoon derived from, or plastic tubes used to conceal, hosts they had previously parasitized. They also recognized hosts that had been superparasitized by conspecifics. Two further experiments supported an alternative hypothesis, that females remember hosts they have parasitized before. Rejection of the first parasitized host decreased markedly after females had attacked two or three hosts, and females cooled to induce amnesia failed to recognize self-parasitized hosts. It is suggested that hosts could smell different enough for a female wasp to discriminate between self- and conspecifically parasitized hosts. The role of short-term memory in the foraging strategies of parasitoid wasps is discussed.

Comparison between primary and secondary sex ratios in parasitoid wasps using a method for observing chromosomes

Hymenopteran insects, including parasitoid wasps, usually have a haplo-diploid sex-determination system. This enables an ovipositing female to choose the sex of her egg by controlling sperm access to the egg (Flanders, 1956). The potential for sex ratio control makes hymenopteran insects ideal experimental organisms for testing sex ratio theory (Charnov, 1982; Godfray, 1994). Although there are many sex ratio studies in parasitoid wasps, the data are based on sex ratio at wasp emergence. However, if differential mortality between immature sexes occurs, the sex ratio at wasp emergence will differ from that at oviposition, i.e., the sex ratio determined by the female wasps. In fact, in several parasitoid wasps, unsuitability of hosts and shortage of food are suggested to result in higher mortality of female larva than male larva (King, 1987). Because most sex ratio theories deal with sex ratio at oviposition, any studies using sex ratio at wasp emergence can be incorrect to test sex ratio theories, if different sex mortality occurs. Therefore, effects of differential sex mortality need to be clarified in sex ratio studies. To date, the effects of differential mortality on secondary sex ratio have been estimated by: 1) comparing mortality of offspring from mated females with that from unmated females (Sandlan, 1979); 2) egg transfer experiments between small and large hosts (Heinz & Parrella, 1990); 3) using a predictive model of primary sex ratio (Wellings et al., 1986); and 4) using differences in female behaviour as an index for predicting primary sex ratio (Suzuki et al., 1984; van Dijken & Waage, 1987; Ueno, 1995). These methods, however, are not suitable for determining exact sex ratio and are not always applicable. Since most Hymenoptera are haplo-diploid, cytological methods for observing chromosomes at the egg stage are available to determine exact primary sex ratio and are applicable to most parasitoid wasps. However, little research using cytological methods to determine primary sex ratio has been done (van Dijken et al., 1989, 1993). In this paper, we apply a cytological method to determine primary sex ratios in two idiobiont parasitoid wasps Pimpla nipponica Uchida and Itoplectis naranyae Ashmead (Hymenoptera: Ichneumonidae). These polyphagous parasitoids attack a variety of host species which differ in size (Ueno & Tanaka, 1994). In such a case, sex ratio control in response to host size could evolve (Charnov, 1982). Here we test whether these parasitoids change the sex ratio in response to host size and whether host size is responsible for differential mortality between immature sexes.

Reproduction and host-feeding in the solitary parasitoid wasp pimpla nipponica (Hymenoptera: Ichneumonidae)

Summary Many parasitoid wasps feed on host body fluids in order to achieve maximal lifetime reproduction, a phenomenon termed “host-feeding”. This study examines egg morphology, embryonic development, ovarian dynamics and lifetime oviposition of the parasitoid wasp Pimpla nipponica, which uses the host as adult food, and highlights the contribution of host-feeding to female reproductive success. Females emerge without mature eggs and produce relatively large yolk-rich eggs at a constant rate throughout the remainder of life, indicating that P. nipponica is a synovigenic parasitoid. Females have relatively few number of ovarioles, and the lateral oviducts are narrow, suggesting low potential fecundity at any one time. In fact, dissection revealed that females have only 4–6 eggs at any one time throughout their lifetime. Eggs do not increase in volume in hosts during development and hatch successfully in Ringer solution, indicating that P. nipponica produces anhydropic eggs. Although females produce about 2...

The effects of host-feeding on synovigenic egg development in an endoparasitic wasp, Itoplectis naranyae.

Abstract Many adult parasitoids feed on host insects, a behavior known as host-feeding. Feeding on hosts is essential to maximizing female fecundity, but its contribution to reproduction varies from species to species. The relationship between fecundity and host-feeding was examined in the solitary endoparasitoid wasp Itoplectis naranyae Ashmead, (Hymenoptera: Ichneumonidae) to assess the significance of host-feeding in female reproduction. Adult female wasps did not respond to hosts when they were 0–1 days old, but subsequently increased their oviposition and host-feeding activities with increasing female age. While newly emerging females had no mature eggs in their ovary, the number of mature eggs increased rapidly thereafter, a process termed synovigeny. Female wasps were capable of maturing eggs without host-feeding, and this suggested that they produced a certain portion of eggs from nutritional reserves that had been stored during the larval stage. Behavioral observations revealed that I. naranyae was a destructive host-feeder as the host was damaged during feeding. Female fecundity was greater in females that had previously fed on hosts than those did not, indicating that host-feeding was involved in egg production. There was a time-delayed relation between host-feeding events and additional egg production; at least 3 days were required to mature eggs from nutrients gained via feeding on hosts. The significance of host-feeding in I. naranyae reproduction is discussed in the context of its life history traits.

Trophic eggs compensate for poor offspring feeding capacity in a subsocial burrower bug

Various animals produce inviable eggs or egg-like structures called trophic eggs, which are presumed to be an extended maternal investment for the offspring. However, there is little knowledge about the ecological or physiological constraints associated with their evolutionary origin. Trophic eggs of the seminivorous subsocial burrower bug (Canthophorus niveimarginatus) have some unique characteristics. Trophic eggs are obligate for nymphal survival, and first-instar nymphs die without them. To identify the cause of nymphal death, we hypothesized that first-instar nymphs starve to death because they cannot feed on anything but trophic eggs. Although first-instar nymphs fed on artificially exposed endosperm did survive, nymphs that were provided with intact seed were not able to penetrate the seed vessel and starved to death. Another hypothesis that trophic eggs play a role in transferring the midgut symbiont, essential for survival in heteropteran bugs, from mother to offspring was rejected because almost all nymphs had retained the symbiont without feeding on trophic eggs. These results suggest that poor feeding capacity of the offspring is the cause of nymphal death, and the important constraint that promotes the evolution of the curious trophic egg system in C. niveimarginatus.

Sex Allocation by a Parasitoid Wasp (Hymenoptera: Ichneumonidae) to Different Host Species: A Question for the Mechanism of Host Size Estimation

Sex allocation by the polyphagous solitary pupal parasitoid wasp Pimpla luctuosa Smith to a small host species, Galleria mellonella (L.), and a large host species, Mamestra brassicae L., was investigated to test whether female wasps responded to hosts of different sizes across different host species. In the experiments, both host species were presented to each test female wasp. Primary and secondary sex ratio experiments revealed that female wasps laid more female eggs in larger pupae of each host species, indicating that female wasps recognized size differences within host species. The wasp sex ratio (male ratio) from M. brassicae, however, was much higher than that expected on the basis of the sex ratio curve from different-sized G. mellonella. Larger hosts of each host species yielded larger wasps, indicating that the host size estimation by female wasps across different host species was incomplete or was not simple. These results suggested that P. luctuosa evaluated host size not only by physical measures such as dimension but also by other unknown measures. A possible explanation for the adaptiveness of different sex ratio responses by Pimpla luctuosa to different host species was discussed.