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.