Parasite driven time-lagged negative frequency-dependent selection of hosts has been studied in natural populations by following changes in host genotype frequencies over time. However, such dynamics have not been considered at higher taxonomic levels, for example, between parental species and their hybrids. In a field study on a Daphnia hybrid system, we observed that one Daphnia taxon first was relatively underinfected, but became over-infected after a strong increase in frequency. This finding is consistent with the idea of parasite evolution towards the most frequent host taxon. In two experiments, we investigated whether the assumptions made by a model of negative frequency-dependent selection apply to our host taxa system. First, we showed that the parasite can change the outcome of taxa competition and secondly, we confirmed that the over-infection of one host taxon observed in the field has a genetic basis. Our results indicate that the incorporation of host-parasite interactions at the species level may allow us to gain a more complete picture of forces driving dynamic taxa coexistence in Daphnia hybrid systems. More generally, we suggest that if hybrids coexist in sympatry with parental taxa, the infection patterns as observed under natural conditions may be rather temporal and unstable.

Abstract-It has been repeatedly suggested that parasites play an important role in the ecology and evolution of Daphnia populations; however, little is known about Daphnia-parasite interactions in lakes with vertebrate predation on Daphnia. Between September 1997 and April 1998, an epidemic of the protist parasite Caullerya mesnili in Daphnia galeata and Daphnia hyalina occurred in Lake Constance (Germany), infecting up to 50% of all individuals. Using laboratory experiments, we investigated the epidemiological interactions between this parasite and its host D. galeata at the individual and the population level. C. mesnili was found to be transmitted directly and horizontally through waterborne infection stages. Transmission of the parasite was dependent on the host density, and all life stages of female D. galeata were susceptible to infection. In a life table experiment at low and high food levels, the life expectancy and fecundity of infected D. galeata were dramatically reduced at both food levels as compared to the uninfected controls. Additionally, we found a significant interaction between infection and food level, indicating a stronger parasitic effect in well-fed hosts. To test the effects of the parasite at the population level, we compared the size of D. galeata populations infected with C. mesnili with the size of parasite-free microcosm populations. The population size of infected D. galeata was significantly lower than that of uninfected populations after 4 weeks. In all four infected replicate populations, the parasite drove the host population to extinction.

In Greifensee (Switzerland), Daphnia galeata ϫ hyalina hybrids cooccur with both parental taxa. Hybrids are the most abundant taxon, suggesting that hybrids have greater fitness. In addition to many known factors that promote hybrids, specific environmental conditions favoring parentals must also exist to explain their cooccurrence. We investigated the influence of the protozoan gut parasite Caullerya mesnili on the D. galeata ϫ hyalina species complex. Up to 22% of the Daphnia population was infected with C. mesnili in October 2002. C. mesnili dramatically reduced the fecundity of its hosts. Only 2% of infected individuals carried eggs compared with 70% in the uninfected group, which suggests that C. mesnili exerts a strong selection pressure. Our results indicate that hybrids were frequently infected, whereas parental D. galeata were almost never infected. We also found genetic variation for infection within hybrids, evidenced by significant differences in clonal composition between the infected and uninfected parts of the taxon. Resistance of D. galeata might counterbalance the greater fitness of hybrids and therefore contribute to the maintenance of coexistence of hybrids and parentals in this lake.

During their lifetime most organisms are exposed to various enemies influencing their victims in multiple direct and indirect ways. Most studies concentrate on the effects of one enemy at a time, thereby not taking into account that in nature organisms are often simultaneously exposed to more than one enemy. We conducted a life-history experiment to investigate the simultaneous effects of predators (fish, Leuciscus idus) and parasites (microparasite, Caullerya mesnili) on their victim (Daphnia galeata). D. galeata were exposed to predator kairomones, parasites or both. D. galeata are able to sense the presence of fish predators via chemical cues (= kairomones). Both fish predator kairomones and microparasite infections influence the life history of Daphnia. Some of the effects of fish predator kairomones are directly opposed to microparasite effects; fecundity, for example, is increased in the presence of fish kairomones and decreased in Daphnia parasitised with C. mesnili. We investigated the influence of both threats on age at maturity, body size at different adult instars, fecundity and survival of one D. galeata clone. In the presence of fish kairomones, all D. galeata matured significantly earlier and increased the number of eggs in the second brood significantly. Parasitised D. galeata matured significantly earlier than non-parasitised ones in the absence and presence of fish kairomones. An infection with the microparasite C. mesnili led to significantly lower clutch sizes at the second adult instar, to significantly smaller body sizes from adult instar three onwards and to significantly reduced survival. No significant interaction effect between the responses to fish presence and to parasite infection was found for any of the investigated life-history traits. The lack of interaction effects between the exposure to predator kairomones and parasite infection was most likely due to the different timing of the effects. Fish kairomones affected D. galeata early in its life history whereas C. mesnili increased in its effects over time. Our results show that parasitised D. galeata are able to exhibit life-history responses to fish predator presence early in their lives. Thus, D. galeata parasitised with C. mesnili have a similar chance as non-parasitised D. galeata to escape from fish predation via life-history changes. Since older parasitised D. galeata are smaller, they may have an even better chance to escape visual predators under actual predation.

The head of Austroperipatus aequabilis bears five types of sensilla. which were examined by electron microscopy. They differ from each other in position, shape of outer sensory elements and cuticular socket structures. Thus, we distinguish sensilla with sensory hairs, sensilla with sensory bulbs, cone‐shaped sensilla. sensilla with sensory bristles, and sensilla of the lips. They are composed of up to 15 cells, which can he separated into four cell types. The most frequent cell type is the bipolar receptor cell that occurs in all sensilla. The apical surface of this primary receptor cell is characterized by one or two partly branched cilia with a basal 9 × 2 + 0 pattern of microtubules. A modified bipolar receptor cell was found in all sensilla bearing a sensory peg except for the sensilla equipped with sensory bristles. The apical dendrite extends to a long pale process which exclusively contains mitochondria and single microtubules. In all sensilla examined in this study at least one supporting cell occurs which is characterized by parallel microvilli. An additional function of this cell type as a part of the stimulus‐conducting system is possible. In the sensillum with a sensory bulb two kinds of supporting cells occur. A unique cell type with an upside down position has regularly been found in all sensilla bearing a sensory peg. Apart from the sensilla they also occur within the labial epidermis. Since most sensilla contain several different receptor cells, they can be considered as complex sense organs. © 1998 The Royal Swedish Academy of Sciences. Published by Elsevier Science Ltd. All rights reserved