Compensation of depleted neuronal subsets by new neurons in a local area of the adult olfactory bulb.

In the olfactory bulb (OB), loss of preexisting granule cells (GCs) and incorporation of adult-born new GCs continues throughout life. GCs consist of distinct subsets. Here, we examined whether the loss and incorporation of GC subsets are coordinated in the OB. We classified GCs into mGluR2-expressing and -negative subsets and selectively ablated mGluR2-expressing GCs in a local area of the OB with immunotoxin-mediated cell ablation method. The density of mGluR2-expressing GCs showed considerable recovery within several weeks after the ablation. During recovery, an mGluR2-expressing new GC subset was preferentially incorporated over an mGluR2-negative new GC subset in the area of ablation, whereas the preferential incorporation was not observed in the intact area. The area-specific preferential incorporation of mGluR2-expressing new GCs occurred for BrdU analog- and retrovirus-labeled adult-born cells as well as for neonate-derived transplanted cells. The mGluR2-expressing new GCs in the ablated area were synaptically incorporated into the local bulbar circuit. The spine size of mGluR2-expressing new GCs in the ablated area was larger than that of those in the intact area. In contrast, mGluR2-negative new GCs did not show ablated area-specific spine enlargement. These results indicate that local OB areas have a mechanism to coordinate the loss and incorporation of GC subsets by compensatory incorporation of new GC subsets, which involves subset-specific cellular incorporation and subset-specific regulation of spine size.

Gene up-regulation in response to predator kairomones in the water flea, Daphnia pulex.

BACKGROUND: Numerous cases of predator-induced polyphenisms, in which alternate phenotypes are produced in response to extrinsic stimuli, have been reported in aquatic taxa to date. The genus Daphnia (Branchiopoda, Cladocera) provides a model experimental system for the study of the developmental mechanisms and evolutionary processes associated with predator-induced polyphenisms. In D. pulex, juveniles form neckteeth in response to predatory kairomones released by Chaoborus larvae (Insecta, Diptera). RESULTS: Previous studies suggest that the timing of the sensitivity to kairomones in D. pulex can generally be divided into the embryonic and postembryonic developmental periods. We therefore examined which of the genes in the embryonic and first-instar juvenile stages exhibit different expression levels in the presence or absence of predator kairomones. Employing a candidate gene approach and identifying differentially-expressed genes revealed that the morphogenetic factors, Hox3, extradenticle and escargot, were up-regulated by kairomones in the postembryonic stage and may potentially be responsible for defense morph formation. In addition, the juvenile hormone pathway genes, JHAMT and Met, and the insulin signaling pathway genes, InR and IRS-1, were up-regulated in the first-instar stage. It is well known that these hormonal pathways are involved in physiological regulation following morphogenesis in many insect species. During the embryonic stage when morphotypes were determined, one of the novel genes identified by differential display was up-regulated, suggesting that this gene may be related to morphotype determination. Biological functions of the up-regulated genes are discussed in the context of defense morph formation. CONCLUSIONS: It is suggested that, following the reception of kairomone signals, the identified genes are involved in a series of defensive phenotypic alterations and the production of a defensive phenotype.

Elaborate regulations of the predator-induced polyphenism in the water flea Daphnia pulex: kairomone-sensitive periods and life-history tradeoffs.

Adaptive polyphenism produces alternative phenotypes depending on environmental stimuli. The water flea Daphnia pulex shows predator-induced polyphenism, facultatively forming neckteeth in response to kairomones released by Chaoborus larvae. This study was designed to reveal the regulatory systems producing the defensive morph during embryonic and postembryonic development. As noted previously, the crest epithelium at the site of neckteeth is shown to thicken earlier the neckteeth formation, and the neckteeth number increased until the third instar, and later disappeared. Exposure to kairomone at various time points and intervals during development showed that the signal was required even at early postembryonic stages to maintain neckteeth. Moreover, two different induction methods, i.e. embryonic and maternal exposures, enabled us to discriminate maternal and zygotic effects in response to kairomone. Direct embryonic exposure is shown to be sufficient to form neckteeth without maternal effect although their growth was diminished; namely, there is a trade-off for neckteeth production. However, maternal exposures resulted in larger progenies in smaller numbers, suggesting that the mother daphnids change their reproductive strategy depending on kairomone signals. Taken together, the developmental responses to the presence of predators are regulated elaborately at various levels.