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.

Differential lipid biosynthesis underlies a tradeoff between reproduction and flight capability in a wing-polymorphic cricket

The biochemical basis of life-history tradeoffs is a poorly studied aspect of life-history evolution. We used radiotracer and endocrine approaches to investigate the extent to which morphs of a wing-polymorphic insect differ in the biosynthesis of lipid classes important for dispersal capability vs. reproduction (ovarian growth). The flight-capable genotype of Gryllus firmus biosynthesized a greater amount of total lipid and triglyceride (main flight fuel), which was preferentially allocated to somatic tissue during early adulthood. By contrast, the flightless genotype biosynthesized a significantly greater amount of phospholipid (important in egg development), which was preferentially allocated to ovaries. Topical application of a juvenile-hormone mimic to the flight-capable morph caused it to express all aspects of lipid metabolism seen in the flightless morph. Differences in biosynthesis between morphs (i) occur coincident with 100-400% greater ovarian growth in the flightless morph, (ii) result from alterations of both de novo biosynthesis of fatty acid and downstream partitioning of fatty acids into triglyceride vs. phospholipid, and (iii) possibly result from genetically polymorphic hormonal regulators with negative pleiotropic effects on lipid biosynthesis and ovarian growth. The present study provides direct documentation of genetically based alterations of in vivo flux through pathways of intermediary metabolism leading to the differential production of end products central to the specialization of phenotypes for alternate life histories.

The role of low levels of juvenile hormone esterase in the metamorphosis of Manduca sexta

The activity of juvenile hormone esterase (JHE) in feeding fifth instar larvae of Manduca sexta increases gradually with larval weight and rises to a peak after larvae pass the critical weight when juvenile hormone secretion ceases. Starvation of larvae of Manduca sexta (L.) that had exceeded the critical weight inhibited peak levels of JHE, but did not delay entry into the wandering stage when larvae leave the plant in search of a pupation site. This suggests that peak levels of JHE may not be essential for the normal timing of metamorphosis. Starved larvae pupated normally, indicating the peak of JHE was not necessary for a morphologically normal pupation. Treatments of larvae with the selective JHE inhibitor O-ethyl-S-phenyl phosphoramidothiolate (EPPAT) that began immediately after larvae achieved the critical weight (6.0 to 6.5 grams for our strain of Manduca) delayed entry into the wandering stage. By contrast, EPPAT treatment of larvae at weights above 8.0 g had no effect on the subsequent timing of the onset of wandering. Therefore, although the normal timing of the onset of wandering does not require peak levels of JHE, it requires low to moderate levels of JHE to be present until larvae reach a weight of about 8.0 g.

EVOLUTIONARY ENDOCRINOLOGY OF JUVENILE HORMONE ESTERASE: FUNCTIONAL RELATIONSHIP WITH WING POLYMORPHISM IN THE CRICKET, GRYLLUS FIRMUS

The existence, nature, and physiological consequences of genetic variation for juvenile hormone esterase (JHE) activity was studied in the wing-polymorphic cricket, Gryllus firmus. Hemolymph (blood) JHE activity was sixfold lower in nascent short-winged (SW) females, relative to nascent long-winged (LW) females during the last juvenile stadium (stage). Morph-associated genetic variation for JHE activity had two causes, variation in loci: (1) regulating whole-organism enzyme activity; and (2) controlling the degree to which JHE is secreted into the blood Reduced JHE activity in nascent SW-selected individuals was associated with reduced in vivo juvenile hormone catabolism. This suggests that variation in JHE activity during juvenile development may have important physiological consequences with respect to the regulation of blood levels of juvenile hormone and consequent specification of wing morph. This is the first definitive demonstration of genetic variation for hormonal metabolism in any insect and a genetic association between hormone metabolism and the subsequent expression of morphological variation (wing morph). However, we have not yet firmly established whether these associations represent causal relationships In contrast to the clear association between JHE activity and wing morph development, we observed no evidence indicating that variation in JHE activity plays any direct or indirect role in causing the dramatic differences in ovarian growth between adult wing morphs. Variation in JHE activity also does not appear to be important in coordinating the development of wing morph with the subsequent expression of reproductive differences between adult morphs. Finally genetic variation for the developmental profiles of JHE activity during juvenile and adult stages are remarkably similar in three Gryllus species. This suggests that genetic correlations between JHE activities during different periods of development, which underlie these activity profiles, have been conserved since the divergence of the three Gryllus species.

Activity-dependent changes to the brain and behavior of the honey bee, Apis mellifera (L.)

To explore the origins and possible behavioral consequences of structural plasticity in an insect brain, we have taken advantage of the following: (1) the highly compartmentalized nature of the primary antenno-sensory centers (antennal lobes) of the brain, (2) the ease with which individual compartments, or glomeruli, within the antennal-lobe neuropil can be identified, and (3) the predictability of changes to readily identifiable glomeruli in the antennal lobes of the adult worker honey bee. Treatment with the juvenile hormone analog methoprene and hive manipulation techniques are used to induce precocious foraging behavior in young worker honey bees. The impact of these treatments on the ontogeny of olfactory learning performance and on the volumes of readily identifiable glomeruli in the antennal lobes of the bee brain are examined in parallel. The study reveals that (1) significant changes in glomerular volume are activity dependent and (2) associative learning of floral odors improves with experience. Improvements in associative learning performance coincide temporally with increases in glomerular volume. This raises an important question: are changes in glomerular volume that result from shifts in behavior simply a consequence of changes in the use of peripheral sensory pathways, or are they associated with events that underlie learning and the formation of long-term memories?