Precocene causes male determination in the aphid Myzus persicae

When adult apterous viviparous females of Myzus persicae, reared in short night conditions at 21–23°C, are treated with the precocene analogue 6-methoxy-7-ethoxy-2,2-dimethylchromene, they deposit males towards the end of their reproductive lives. The first-born normal daughters of the treated females also deposit some males at various times in their reproductive lives. Karyotypic analysis was used to investigate the sequence of male and female embryos in the ovarioles of precociously metamorphosed aphids. The experiments support the hypothesis (Mittler et al., 1979) that juvenile hormone level controls the sex determination process in aphids. Since male aphids have an XO sex chromosome constitution, this implies that juvenile hormone level influences the behaviour of the X-chromosomes at or before the single maturation division of the egg. At this division one X-chromosome is eliminated from eggs which will develop as males. Aphids provide the first example of a specific endocrine influence on chromosome behaviour in sex determination.     

Effect of Aedes aegypti (Diptera:Culicidae) Age on Sex Ratios in Romanomermis culicivorax (Nematoda:Mermithidae)

First, second, third, early and late fourth-instar larvae, and pupae of Aedes aegypti were infected with Romanomerrais culicivorax and reared at 20, 25, and 30 C. An increase in the ratio of male to female nematodes was observed with increase in host age at the time of infection at each temperature. The number of pupal and late fourth-instar infections was low, but R. culicivorax continued to develop in adult A. aegypti. Since male nematodes were recovered from both male and female hosts infected as late fourth instars or pupae, the sex of the host did not influence the sex of the nematode.     

Behavioral response of 12 ungulate species in captivity to the presence of humans

Twelve species of captive ungulates were studied to determine behavioral responses to the presence of a zookeeper within the exhibit and in front of the exhibit, with and without zoo visitors present. Significant differences in behavior occurred between species for nearly all behaviors observed. A significantly greater occurrence of vigilance and approach behavior was directed toward the zookeeper while within the exhibit relative to when the zookeeper stood in front of the exhibit. A significantly lower occurrence of eating or drinking occurred when the zookeeper was inside the exhibit. Significant differences occurred across size categories in the occurrence of approaching the zookeeper. The most frequently scored behavior was visual orientation. Statistically significant differences in the occurrence of visual orientation directed by females toward the zookeeper existed across size category, with more vigilance by species with larger body size. A statistically higher occurrence of vigilance toward the zookeeper was directed by female ungulates but not by males when the zoo was closed to the public. Although no statistical significance was found regarding the intraspecific vigilance of males, data on females revealed significant differences across species and across size categories. When data regarding vigilance toward the public were analyzed, statistically significant differences existed between species for females only. Likewise, when data regarding interspecific vigilance were examined, statistically significant differences were found across size categories for females, but not for males. The potential roles of vigilance in the wild are discussed in reference to its role in captivity.     

Is male back space limiting? An investigation into the reproductive demography of the giant water bug,Abedus indentatus (Heteroptera: Belostomatidae)

Because male giant water bugs in the subfamily Belostomatinae provide parental care by brooding eggs on their back, an accurate assessment can be made of both the actual and potential reproductive capacity of males. Two operational sex-ratio (OSR) indices were developed and empirically measured for a population of giant water bug, Abedus indentatus,in California. One index was based on reproductive rates measured in the laboratory; the other index was based on reproductive resources observed in the field. Both OSR indices suggest that the operational sex ratio fluctuates between maleskewed ratios in the summer and femaleskewed ratios in the winter. This pattern appears to be the consequence of two factors. First, the adult sex ratio is significantly female biased. Second, although males can outreproduce females at high ambient temperatures, the reverse is true at low temperatures. Possible reasons for the female-skewed adult sex ratio are examined, including differential recruitment, differential mortality, and sampling bias.     

The social development of free-ranging infantLemur catta at Berenty reserve,Madagascar

The social development of 11 free-ranging infantLemur catta was examined over the first 16 weeks of the infants' lives. By 16 weeks, infants still occasionally suckled and were carried dorsally, but on the whole, they were independent of their mothers. Sex or mother's rank was not found to affect frequency or type of play behavior. Mother's rank had no effect on frequency of maternal rejections, from the nipple or from riding, but female infants were rejected slightly more frequently than males were. Mothers tended to reject infants more severely and more frequently from dorsal riding than from the nipple. Sex and rank differences were not found with respect to behaviors determined as measures of independence; however, lower-ranking infants engaged in significantly more dependent behaviors than higher-ranking young did. It may be necessary for the infant of a low-ranking mother to maintain closer proximity to its mother for a longer period of time during infancy because such infants may be subject to abuse by higher-ranking group members and, furthermore, may not be as readily rescued in a stressful or dangerous situation as a higher-ranking infant. Sex was not found to be a factor in terms of measures of dependence. The lack of sex differences in developmental behaviors in this species may be related to female dominance, as well as to the fact that, as adults, both sexes engage in aggressive territorial behavirs.     

Influence of the egg parasitoidPlatytelenomus busseolae [Hym.: Scelionidae] on the population ofSesamia nonagrioides [Lep.: Noctuidae] in central greece

The egg parasitoid,Platytelenomus busseolae (Gahan) (Hymenoptera: Scelionidae) was recorded from egg masses of the corn stalk borer,Sesamia nonagrioides (Lefebvre) (Lepidoptera: Noctuidae) on maize in the area of Istiaea, Central Greece. Natural parasitism was studied during the years 1986, 1987. A total of 3,382 egg masses, containing 205,227 eggs, was collected from corn fields. Parasitized eggs were found from end July to mid-October. In 1986. 76.2% of the egg masses and 42.8% of all eggs were parasitized, while in 1987, respective values were 27.6% and 12.9%. The inferior performance of the parasitoid in 1987 may be due to the unfavorable winter and summer conditions. Parasitism decreased when the host population was at its peak, but it increased again within 1 to 3 weeks. A small percentage of egg masses was completely parasitized (9.5% and 4.7% for the 2 years respectively). In 1987, the sex ratio, ♀♀/♂♂ was 1.5: 1 and 26.2% of the egg masses produced only ♂♂. Mated ♀♀ produced 3 times more ♀♀ than ♂♂. It seems that the parasitoid, under favorable weather conditions, can play a significant role in the control of the corn stalk borer.      

Presence of primary and secondary males in a population of the protogynous Synbranchus marmoratus

Synbranchus marmoratus , the 'swamp eel', is a protogynous diandric fish. The primary and secondary males can be distinguished from each other easily by differences in gonadal morphology. Primary males have lobular, unrestricted testes with central efferent ducts. The secondary male has a 'lamellar' testis, efferent ducts are present in the ventral region (new formation), lateral supports and it is covered by the former ovarian capsule. The length of primary male varies from 13 to 88 cm while secondary males range from 56 to 91 cm. Transitional individuals vary between 45 and 60 cm in length. The swamp eel population studied is composed of 80% primary males and 20% secondary males. Although gonadosomatic indices are always higher in primary males, they increase in secondary ones as the newly acquired stage progresses. The lamellar organization of the gonad of secondary males, the absence of this kind of testis in individuals smaller than 56 cm, and the presence of hermaphroditic fish, is evidence for protogyny in this species.     

Testicular development and serum sex steroid profiles during the annual sexual cycle of the male sturgeon hybrid the bester

Testicular development in the adult male F₁ sturgeon hybrid, the bester ( Huso huso L. female x Acipenser ruthenus L. male), was examined monthly in relation to serum sex steroid levels. Spermatogenesis lasted for 1 year, with meiosis generally starting in September and spermiogenesis in November, although there was considerable variation in testicular developmental stages between fish sampled monthly. Testicular development continued, slowly, during the winter months until April. Fish did not exhibit spontaneous spermiation, and phagocytotic activity of Sertoli cells became prominent from May onwards. Androgen levels increased during Spermatogenesis and remained high throughout the pre-spermiation period. In the degeneration stage, 11-ketotestosterone concentrations declined to low levels, while testoster- one levels remained high. The serum concentration of 17,20β-dihydroxy-4-pregnen-3-one was low throughout the reproductive cycle. Based on these results, it is suggested that the time appropriate for induction of final maturation would be from November–December to April when the testes are in the late stage of development.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

Body size,litter size,timing of reproduction,and juvenile survival in the Unita ground squirrel,Spermophilus armatus

The timing of reproduction affected litter size, offspring mass, and offspring survival in the Uinta ground squirrel, Spermophilus armatus, in Grand Teton National Park, Wyoming. Survival of juvenile females to yearling age varied negatively with date of weaning and positively with individual offspring mass. At the same time, juveniles weaned early in the season were lighter, and juveniles weaned later in the season were heavier. The coefficient of variation for juvenile body mass, originally measured at weaning, significantly decreased by the time juveniles entered hibernation, indicating that individuals weaned early and light caught up in body mass to individuals weaned later and heavier. From the perspective of the mother's investment in the litter, litter size (corrected for mother's mass) decreased with later wcaning dates, while the relationship of weaning date to litter mass (corrected for mother's mass) was significant in only one year. Maternal allocation of resources in litters changed over the season so that mothers produced many, small offspring early in the season, and fewer, large offspring late in the season.