Cyclic parthenogenesis (heterogony) is a widespread reproductive mode found in diverse taxa such as digenean trematodes, gall wasps, gall midges, aphids, cladocerans and rotifers. It is of particular interest as it combines the advantages of asexual reproduction (rapid population growth) and sexual reproduction (recombination). Usually sexual reproduction is initiated when, or slightly before, environmental conditions deteriorate, and often results in the production of resting stages. The optimal timing of diapause induction must thus be under strong natural selection. Using the cladoceran Daphnia as a model system, we show here for the first time that the switch from parthenogenetic to sexual reproduction in a cyclical parthenogenetic organism can be influenced by the chemical composition of food. Under crowding conditions Daphnia reproduced parthenogenetically with subitaneous eggs when fed the algal species Cryptomonas sp., but started the production of resting eggs when fed with the green algal species Scenedesmus obliquus. Supplementation experiments with lipids and especially proteins showed that the induction of resting egg production in two clones of different Daphnia species was due to a dietary deficiency in the green alga. Hence, the low food quality induced a switch in the reproductive mode that may contribute to optimal timing of the sexual reproduction of Daphnia in nature. Furthermore, our results have two other major implications: first, they suggest that protein compounds should be added to the list of diet constituents potentially limiting or influencing Daphnia reproduction. Second, we show that the role of food quality goes far beyond the up to now documented effects of food quality on somatic growth and trophic transfer efficiency of herbivores: due to its effects on sexual reproduction and the production of resting eggs, food quality might influence genetic diversity and long-term persistence of Daphnia in lakes.
The enormous success of the genus Daphnia in freshwater ecosystems is at least partially due to their cyclical parthenogenetic life cycle, in which asexual and sexual reproduction alternate periodically. This temporal change between reproductive strategies allows for (1) rapid population growth via subitaneously developing eggs when environmental conditions are appropriate and (2) the maintenance of genetic diversity via sexual reproduction and the production of resting eggs when environmental conditions deteriorate. We show here that dietary amino acids are involved in triggering the switch between reproductive modes in Daphnia pulex. Supplementation experiments demonstrate that specific dietary amino acids, in particular arginine and histidine, avert crowding-induced resting egg production, enhance subitaneous reproduction by increasing algal food quality and, as a combined effect of both processes, increase population growth rates. These findings suggest that the availability of single dietary amino acids potentially affects the seasonal dynamics and long-term persistence of Daphnia populations in the field, which may have consequences for the efficiency of carbon transfer and thus the trophic structure of freshwater food webs.
Predator avoidance via alternative microhabitat choice by the freshwater amphipod Gammarus roeseli was examined in laboratory y-maze experiments, where predator presence was simulated by fish-conditioned water. The amphipod responses to three benthivorous fish species, which differ in their mode of feeding, were exam ined. G. roeseli avoided habitats with water conditioned by juvenile burbots (Lota lata) and crucian carp (Carassius earassius), but not by juvenile Eurasian perch (Perea fluviatilis). Responses to predator-conditioned water only took place at high fish densities. Thus the amphipods were capable of distinguishing between acute pred ation risk, represented by high infochemical concentrations, and the low background level of infochemicals. The results of the study imply that proper microhabitat choice is an alternative to decreased locomotory activity, for enhanced survival of amphipods in the presence of fish predators in both lentic and lotic environments. A preliminary characterisation of the infochemical suggested that it is different from that which cau ses responses in Daphnia, as it could not be extracted by lipophilic solid phase extrac tion.
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