Recent advances in radioimmunoassay technology for the juvenile hormones

UHPLC-MS analysis of juvenile hormone II in Mediterranean corn borer (Sesamia nonagrioides) hemolymph using various ionization techniques

The juvenile hormones (JHs) have been considered the most versatile hormones in the animal kingdom. JH-II is the most abundant JH in Sesamia nonagrioides, important maize pests in the Mediterranean basin. This study compared the sensitivities and matrix effects of four ionization modes on analyzing JH-II in S. nonagrioides hemolymph using ultrahigh-performance liquid chromatography-mass spectrometry (UHPLC-MS) in single ion monitoring (SIM) mode. The ionization techniques tested were electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), and APPI with the lamp turned off, which corresponds to atmospheric pressure thermospray ionization (APTSI). ESI was discarded because of the high matrix effect. APPI was discarded because the correlation responses between solvent and matrix on the instrumental quality parameters were worse than those for APTSI and APCI. In our analytical conditions, APCI has shown the best validation parameter values. APCI ionization is widely available in instrumental laboratories.

Application of RNA interference in functional genomics studies of a social insect

Social insects represent a group of organisms that have dual importance from perspectives relating to both basic and applied science. From a basic perspective, social insects serve as excellent model systems for studying social organization, behavioral ecology, neurobiology, and phenotypic plasticity. From applied perspectives, social insects play important roles in the pollination of agricultural crops, in the damage of human structures and commodities, and in cellulose processing in natural ecosystems. With the advent of insect sociogenomics research (and the ability to identify dozens or hundreds of relevant candidate genes from a single experiment) has come a great demand for functional genomics tools for application in gene characterization. To date, RNAi is one of the most powerful tools to have become available for such functional characterizations, and it has broad relevance across a range of insect sociobiology research topics.

Regulation of polyphenic caste differentiation in the termite Reticulitermes flavipes by interaction of intrinsic and extrinsic factors

Polyphenism is a key strategy used by solitary insects to adapt to changing environmental conditions and by eusocial insects for existing collaboratively in a social environment. In social insects, the morphogenetic juvenile hormone(JH) is often involved in directing the differentiation of polyphenic behavioral castes. The present study examines the effects of JH, environment and feeding on caste polyphenism in a eusocial insect, the termite Reticulitermes flavipes (Kollar). Our approach included a combination of model JH bioassays, SDS-PAGE and western blotting. Our findings revealed significant temperature-dependent effects on (1) JH-induced soldier caste differentiation, (2) abundance of soldier-inhibitory hexamerin proteins and(3) JH-sequestration by hexamerin proteins. Additionally, although it appears to be dependent on a complex interaction of factors, feeding apparently plays a significant upstream role in enhancing hexamerin accumulation under normal colony conditions. These findings offer important new information on termite eusocial polyphenism by providing the first mechanistic evidence linking an intrinsic caste regulatory factor (hexamerin proteins) to an upstream extrinsic factor (environment) and a downstream response (caste differentiation). These observations are consistent with the hypothesis that the hexamerins serve as an environmentally and nutritionally responsive switching mechanism that regulates termite caste polyphenism.

Variation in endocrine signaling underlies variation in social life history

Variation in endocrine pathways can be a major mechanism underlying life-history evolution. Yet it is unclear whether this insight, derived primarily from solitary species, explains the origins of complex life-history traits in highly social taxa. Thus, we here document and study variation in social life-history syndromes of female fecundity, behavior, and life span in selectively bred honeybee (Apis mellifera) strains. Associated variation in endocrine signaling was uncovered by RNA interference (RNAi) silencing of the juvenile hormone (JH) suppressor gene vitellogenin. High versus low endocrine reactivity in response to vitellogenin knockdown consistently correlated with rapid social behavioral ontogeny and short life span versus slow social behavioral ontogeny and long life span. Variation in JH reactivity, furthermore, was a function of variation in fecundity (ovary size and follicle development). A JH-mediated pleiotropy of female life-history traits, including fecundity, behavior, and life span, characterizes the distantly related solitary insect Drosophila. For the first time, we document a similar regulatory principle in a highly social species where most females are alloparental helpers (workers) that seldom reproduce. We conclude that variation in endocrine pathways of solitary origin can underlie variation and evolvability of complex social life-history traits.

Two hexamerin genes from the termite Reticulitermes flavipes: Sequence, expression, and proposed functions in caste regulation

Previous molecular studies on the termite Reticulitermes flavipes have revealed that two hexamerin proteins serve an important status quo role in the regulation of juvenile hormone (JH)-dependent caste differentiation. Here, we report sequence data and other experimental evidence suggesting how these two hexamerins function in achieving caste regulation. The two hexamerin genes, named Hex-1 and Hex-2, encode highly unique sequence features relative to the 100+ other known insect hexamerins. These features include a long hydrophobic tail and prenylation motif in Hex-1, and a long hydrophilic insertion plus several putative protease cleavage sites in Hex-2. Both hexamerin genes are primarily expressed in fat body tissue, but only Hex-2 expression is substantially induced by JH. SDS-PAGE showed that the hexamerin proteins constitute a major proportion of total soluble termite protein. Also, although each protein occurs in both the membrane and soluble protein fractions, Hex-2 has stronger membrane affinity. Anti-JH antiserum specifically recognizes hemolymph-soluble Hex-1 protein, supporting that the unique prenylation site in Hex-1 facilitates covalent JH binding to the primary amino acid chain. Finally, increased ratios of Hex-2 to Hex-1 transcription occur in caste phenotypes and developmental stages that differentiate in response to rising JH titers. Two main conclusions can be taken from these studies. First, elevated ratios of Hex-2 to Hex-1 expression are associated with caste phenotypes that differentiate in response to rising JH titers (i.e., workers, presoldiers and soldiers). Second, due to their unique structural features and other observed characteristics, our findings support the hypothesis that the two hexamerins participate in the regulation of caste-differentiation by modulating JH availability.

Social exploitation of hexamerin: RNAi reveals a major caste-regulatory factor in termites

Lower termites express a unique form of eusocial polyphenism in that totipotent workers can differentiate into either soldier or reproductive caste phenotypes. In this initial effort using RNA interference in termites, we found that two hexamerin genes, Hex-1 and Hex-2, participate in the regulation of caste polyphenism. Our methodology involved a dual gene-silencing approach that used a single short-interfering RNA fragment to silence the two homologous hexamerin genes. We performed validation studies that evaluated effects on nontarget housekeeping genes, silencing of a nonhousekeeping control gene, and effects at the protein level. We found that the two hexamerin proteins, which are inducible by the morphogenetic juvenile hormone and which constitute a significant proportion of total termite protein, suppress juvenile-hormone-dependent worker differentiation to the soldier caste phenotype. This mechanism allows termite colonies to retain high proportions of altruistic worker caste members, thus apparently enhancing colony-inclusive fitness. These findings demonstrate a unique status quo regulatory mechanism for termite worker caste retention and provide an example of previously undescribed preadult developmental/caste-regulatory genes from any social insect.

Social reversal of immunosenescence in honey bee workers

A striking example of immunosenescence is seen in the honey bee (Apis mellifera) worker caste. The bees' age-associated transition from hive duties to more risky foraging activities is linked to a dramatic decline in immunity. Explicitly, it has been shown that an increase in the juvenile hormone (JH) level, which accompanies onset of foraging behavior, induces extensive hemocyte death through nuclear pycnosis. Here, we demonstrate that foragers that are forced to revert to hive-tasks show reversal of immunosenescence, i.e. a recovery of immunity with age. This recovery, which is triggered by a social manipulation, is accompanied by a drop in the endogenous JH titer and an increase in the hemolymph vitellogenin level. Vitellogenin is a zinc binding glycolipoprotein that has been implicated in the regulation of honey bee immune integrity. We also establish that worker immunosenescence is mediated by apoptosis, corroborating that reversal of immunosenescence emerges through proliferation of new cells. The results presented here, consequently, reveal a unique flexibility in honey bee immunity--a regulatory plasticity that may be of general biological interest.

Neuroendocrine factors affecting the steady-state levels of early trypsin mRNA in Aedes aegypti

Transcription of the early trypsin gene occurs in the midgut after adult emergence under control of juvenile hormone (JH). We tested the hypothesis that factors that affect the steady-state levels of early trypsin mRNA do so by influencing the levels of JH. We investigated the effect of ingesting different meals on early trypsin mRNA levels as well as on JH levels. We also studied how early trypsin mRNA levels changed when the midgut was isolated from different components of the neuroendocrine system by abdominal ligation and decapitation. Early trypsin transcripts levels are high in unfed females; feeding different meals had three distinct effects on the changes of steady-state levels of early trypsin mRNA: (1) blood and protein meals caused the level to decrease drastically and remained low for at least 24 h; (2) amino acid meals caused a transient decrease in the mRNA level, but it returned to high levels after 12-18 h; and (3) sugar, latex and saline meals had no effect on the early trypsin mRNA steady-state levels. The changes in JH levels after ingesting blood and amino acid meals show profiles resembling the changes in early trypsin mRNA levels for the corresponding meal. Decapitation at 1, 2 and 3 days after emergence does not affect the steady-state levels of early trypsin in unfed females. In contrast, 24 h after feeding, transcript levels were significantly higher in decapitated females when compared with non-decapitated fed females. We propose that the changes in the steady-state levels of early trypsin mRNA observed after the ingestion of different meals, ligations and decapitations are generated by changes in the levels of juvenile hormone.

Functional significance of parasitism-induced suppression of juvenile hormone esterase activity in developmentally delayed Choristoneura fumiferana larvae

The parasitic wasp Tranosema rostrale transmits a polydnavirus (PDV) to its host, Choristoneura fumiferana, during oviposition. Last-instar C. fumiferana larvae parasitized by T. rostrale early in the stadium fail to undergo metamorphosis, and injection of the wasp's calyx fluid (CxF; contains PDV) into healthy caterpillars induces a dose-dependent delay in initiation of metamorphosis (D. Doucet and M. Cusson, 1996, Entomol. Exp. Appl. 81, 21-30). In the present work, parasitization and injection of CxF (0.5 female equivalent) on the first day of the last stadium both prevented the rise in hemolymph 20-hydroxyecdysone (20HE) titer observed between day 4 and day 7 in control and saline-injected larvae. Similarly, juvenile hormone esterase (JHE) activity was depressed following parasitization or CxF injection, whereas control larvae displayed a peak on day 4. However, neither parasitism nor injection of CxF on day 1 prevented the JH-producing glands from turning off during the first half of the last stadium. Likewise, low but clearly detectable JH titers were observed in the first hours following the molt but very low titers, at or near the detection limit of our radioimmunoassay, were seen in both control and parasitized larvae on day 4. Prothoracic glands showed no apparent sign of degeneration 4 days after injection of CxF but had significantly smaller cells than saline-injected larvae 7 days postinjection. It is not clear whether this was a direct effect of T. rostrale PDV. Thus, disruption of spruce budworm metamorphosis by T. rostrale CxF involves depression of 20HE titers but is not associated with a measurable increase in the level of JH, as shown for some other host-parasitoid systems. In view of the latter observation, we put forward three hypotheses regarding the functional significance of the observed suppression of JHE activity in developmentally arrested C. fumiferana larvae.

Juvenile hormone titers in virgin and mated Choristoneura fumiferana and C. rosaceana females: assessment of the capacity of males to produce and transfer JH to the female during copulation

We used a radioimmunoassay (RIA) to assess the effect of mating on juvenile hormone (JH) titer in females of the tortricid moths Choristoneura fumiferana and C. rosaceana. Virgins had undetectable levels of JH in their hemolymph on the 5th day of the pupal stage but titers rose to 1-4 and 0.2-0.5 ng JH II eq./ml, respectively, after emergence. On days 1, 3 and 5 following copulation, females of both species had higher JH titers than virgins of the same ages, with the greatest difference between virgin and mated females observed on day 3 for C. fumiferana and on day 5 for C. rosaceana. This increase was apparently not the result of a male-to-female transfer of JH during copulation since: (i) the accessory sex glands (ASGs) of males of both species displayed a very limited ability to convert JH acid into JH, (ii) ASGs produced no JH when incubated in vitro in the presence of L-[methyl-(3)H]-methionine, (iii) ASGs of males injected with L-[methyl-(3)H]-methionine 24 h prior to dissection contained no JH-associated radioactivity, and (iv) freshly formed spermatophores dissected out of females mated to similarly injected males contained no trace of radioactive JH. In addition, the JH content of ASGs and spermatophores, as measured by RIA, was not higher than that of virgin-female hemolymph, on a per-mg basis. However, in contrast with earlier findings in other species of moths, the CA of male C. fumiferana and C. rosaceana maintained in vitro in the presence of tritiated methionine produced and released JH I, JH II and JH III in quantities and proportions similar to those reported for female glands.

Ligand regulation of juvenile hormone binding protein mRNA in mutant Manduca sexta

Insect hemolymph juvenile hormone binding protein (hJHBP) regulates peripheral titers of its ligands, the juvenile hormones. In larvae of the black (bl) strain of the tobacco hornworm, Manduca sexta, treatment with small doses of juvenile hormone I (JH I) can also regulate titers of hJHBP. To further investigate this regulation, responsiveness of hJHBP mRNA expression to JH I was characterized in vivo. RNA analyzes revealed that transcript levels in fat body, the site of hJHBP synthesis, increased fivefold within several hours of treatment with physiological doses of hormone and remained elevated for approximately 16 h. Sensitivity to JH treatment was found to vary temporally. To ensure transcript identity, a wild-type cDNA clone and a bl RT-PCR fragment were sequenced and found to be 99% homologous. Together, these results suggest that JH participates in regulating expression of its transport protein in bl larvae by modifying the in vivo abundance of hJHBP's mRNA transcript.

Development changes in juvenile hormone and juvenile hormone acid titers in the hemolymph and in-vitro juvenile hormone synthesis by corpora allata of the silkworm, Bombyx mori

A simple method was developed to quantify hemolymph juvenile hormone (JH) and JH acid in hemolymph extracts from Bombyx mori with an established radioimmunoassay (RIA) for JH I. When various organic solvent extracts of hemolymph were assayed by RIA, levels of non-specific binding of the labeled ligand in the assay were determined to be greater than 50% of the maximum amount of the label bound by the antiserum. When hemolymph was diluted with methanol:water:8.4N ammonium hydroxide (10:9:1) and extracted with isooctane, non-specific binding was only 50% higher than control levels obtained with the assay buffer alone. The organic phase contained only JH and aqueous phase, JH acid. Consequently, this extraction method was used to prepare samples for RIA and enabled the separate measurement of JH and JH acid in hemolymph. With this method, changes in the hemolymph titers of JH and JH acid were determined from the third instar through early pupal stage of Bombyx mori. Changes in the in vitro secretory activity of corpora allata and brain-corpora cardiaca-corpora allata complexes from fifth instar larvae were also determined by using JH I RIA of the incubation medium.

Characterization of antibody 444 using chromatographically purified enantiomers of juvenile hormones I, II, and III: implications for radioimmunoassays

Optically pure (> 99.5%) enantiomers of insect juvenile hormones (JH) I, II, and III were obtained by injection of racemic mixtures onto a chiral HPLC column using hexane:2-propanol (99.5:0.5) as the mobile phase. The enantiomers of JH III were the best resolved (R = 4.26), followed by those of JH II (R = 2.29) and JH I (R = 1.47). These purified natural and unnatural enantiomers were used to further characterize an antiserum (444) developed for JH radioimmunoassays (RIAs). Based on ED50 values generated using optically pure [methyl-3H]-10R,11S-JH II as a tracer, the natural isomers of JH I, JH II, and JH III were 30, 87, and 36 times more immunoreactive, respectively, than the unnatural isomers. When compared with the racemates, the natural isomers were approximately twice as immunoreactive. In competitive displacement studies where the natural enantiomers of the three JHs were compared, immunoreactivities were in the order JH II > JH I > JH III (ED50 = 109, 198, and 300, respectively). Availability of pure natural enantiomers of JH, both as tracers and competitors, should improve the sensitivity and accuracy of JH titer determinations made by RIA and facilitate various enzyme, binding protein, and receptor studies.