Piet Spaak

Chemically induced anti-predator defences in plankton: a review

Planktonic organisms exhibit diverse morphological, behavioural and life-history responses to the chemical presence of potential predators. Prey organisms have been found to sense such predators via predator-derived kairomones. The induced reactions are assumed to reduce predation risk and thus to be adaptive. Numerous studies have investigated various aspects of inducible defences in different crustaceans, in rotifers, planktonic ciliates and algae. As a first step, we summarise recent work on chemically induced anti-predator defences in morphology, life history and behaviour. Morphological defences have been found in a wide range of different plankton organisms and recent studies on predator-induced morphologies mainly addressed the question of costs for these changes. Life-history responses were mainly studied in cladocerans and several studies have recently addressed some novel topics, such as diapause induction and the influence of predator kairomones on hatching of resting stages. Behavioural anti-predator defences also have been found for several plankton species and are characterised by relatively fast induction times. We further identified four research directions in which substantial progress has been made recently: (I) The effects of simultaneous exposure to infochemicals from different predators and the consequences of a complex chemical environment. Some environmental contaminants, such as synthetic chemicals or heavy metals, have been found to potentially disturb natural chemical communication in aquatic predator-prey systems. (II) The influence of genetic variation on the reaction to infochemicals and its implications. Clonal differences have not only been found for the presence or absence of a certain trait but also with respect to the type of response. (III) The degree to which different types of responses to a specific kairomone are coupled. Recent studies underline the uncoupling of different anti-predator responses of which some have been considered to be coupled. (IV) Studies on the chemical properties and on the metabolic origin of predator kairomones. Substantial progress has been made recently, especially with respect to the identification of predator kairomones that are important for planktonic ciliates. The identification and isolation of kairomones are an important step towards studies addressing the consequences of predator-induced defences on the level of populations, communities and ecosystems. So far most studies have considered effects and consequences on the level of individual prey organisms and studies taking the consequences at higher ecological levels into account are rare.

Predator-Mediated Plasticity in Morphology, Life History, and Behavior of Daphnia: The Uncoupling of Responses

We studied the way 12 traits responded to fish kairomones in a set of 16 Daphnia magna clones derived from four different habitats—two where daphnids co‐occur with fish and two without fish. These clones differed widely in their response to predator kairomones, with none of the clones showing a significant response in all traits and all clones showing a response for at least one trait. Most of the clones showed a significant response in one to four traits, with no evidence for an association between different traits. Clones from fish habitats were slightly more responsive to the presence of fish kairomones than clones from fishless locations. We conclude that most clones show an induced response to the presence of their predators (fish) but that there is a large genetic variability with respect to the traits for which clones show a response. Our results indicate that the major distinction is not between inducible and noninducible genotypes but rather that the genotypes differ in the combination of traits for which they show inducible responses.

Genetic markers, genealogies and biogeographic patterns in the cladocera

Cladoceran crustaceans are an important component of zooplankton in a wide range of freshwater habitats. Although the ecological characteristics of several cladoceran species have been well studied, biogeographical studies have been hampered by problematic taxonomic affiliations. However, recently developed molecular techniques, provide a powerful tool to subject aquatic taxa to comparative analyses. Here we highlight recent molecular approaches in aquatic ecology by presenting a simple method of DNA preparation and PCR amplification of the mitochondrial DNA (16S rDNA) in species from nine different families within the cladocera. On a broad taxonomic scale, sequence analysis of this mtDNA fragment has been used to produce the first molecular based phylogeny of the cladocera. This analysis clustered the cladoceran families in a fashion similar to that suggested by previous systematic classifications. In a more detailed analysis of the family Daphniidae, nuclear randomly amplified polymorphic DNA (RAPD), mitochondrial restriction fragment length polymorphism (RFLP) and morphological analyses were combined to identify species and interspecific hybrids within the Daphnia galeata species complex across 50 lakes in 13 European countries and one lake in Africa. The study revealed interspecific hybridization and backcrossing between some taxa (D. cucullata and D. galeata) to be widespread, and species and hybrids to frequently occur in sympatry. Genetic, as well as morphological information, suggests the occurrence of D. hyalina outside the Holarctic.

Evolutionary and Ecological Consequences of Interspecific Hybridization in Cladocerans

The evolutionary process of interspecific hybridization in cladocerans is reviewed based on ecological and population genetic data. The evolutionary consequences of hybridization, biogeographic patterns and fitness comparisons are analyzed within the conceptual framework of theories on hybridization. Among species of theD. longispina complex no interpopulational transition zones (hybrid zones) have been detected, but rather patchy distributions of hybrids and parentals have been found. Hybrids occur across broad geographic ranges and can be more abundant than parental species. Due to asexual reproduction (ameiotic parthenogenesis), hybrid breakdown can be avoided, and hybrids can even (temporarily) combine advantageous traits of both parental species. Evolutionary consequences may arise from repeated backcrossing, which in some cases results in introgression and patterns of reticulate evolution.

Population genetic structure of 3 alpine stream insects: influences of gene flow, demographics, and habitat fragmentation

Estimating scales of dispersal for benthic macroinvertebrates using neutral genetic markers requires consideration of genetic, demographic, and historical influences on population genetic structure. We used allozyme electrophoresis to investigate the population genetic structure of 3 species of alpine stream insects among major drainages of the Swiss Alps (Rhine, Inn, and Ticino rivers), among streams within each drainage, and within single streams. Within streams we examined reaches that were fragmented by lakes or resevoirs and unfragmented reaches. Rhithrogena loyolaea (Heptageniidae) exhibited little genetic differentiation (θ) within (θ = 0.01-0.03) and among (θ = 0.02-0.03) streams but significant differentiation among drainages (θ = 0.08), suggesting that dispersal occurs among stream fragments and among stream valleys. Allogamus auricollis (Limnephilidae) did not exhibit genetic differentiation at any scale, suggesting that dispersal occurs throughout the geographical range of the study. In contrast, Baetis alpinus (Baetidae) showed moderate to substantial differentiation both within (θ = 0.08-0.39) and among (θ = 0.06-0.09) streams. However, a distinct lack of genetic differentiation for B. alpinus among major drainages of the Alps (θ = 0.01) suggests that low θ values reflect historical rather than present-day levels of gene flow. We suggest that genetic population structure reflects a lack of equilibrium between gene flow and genetic drift, resulting from historical gene flow that continues to mask reduced dispersal and from recurring processes of recruitment that lead to random changes in genetic signatures. We conclude that demographic processes affect small-scale patterns and historical processes affect large-scale patterns. The simultaneous study of multiple spatial scales helps determine the relative importance of each. A synthesis of our results and data from published studies indicated that 4 consistent patterns of genetic differentiation emerged when multiple spatial scales were investigated. These patterns are indicative of taxon-specific dispersal ability within and among streams and whether taxa are in gene flow-genetic drift equilibrium.

Life History Variation and the Coexistence of a Daphnia Hybrid With Its Parental Species

Life history variation and genotype composition were studied in two species of the Daphnia longispina group: the relatively large D. galeata (1.4 mm) and the smaller D. cucullata (0.9 mm). Several multi-locus genotypes of these species were compared with genotypes of their interspecific hybrid, D. cucullata X D. galeata. Both species and the hybrid co-occur in the shallow eutrophic Tjeukemeer. For two successive years, laboratory life table experiments were conducted at three temperatures (12.5?, 17.5?, and 22.5eC) and two food levels (carbon concentrations 0.62 and 1.64 pg/mL) to study the mechanisms that permit coexistence of taxa in this Daphnia hybrid species complex. Three-way analyses of variance showed significant main effects (e.g., temperature, food level, taxon) for most reproductive traits (e.g., number of newborns, age at first reproduction, number of juvenile instars, intrinsic rate of increase) among the Daphnia taxa. For size-related traits (e.g., size at maturity and offspring size) only one main effect, taxon, was found. Hybrids were intermediate to the parental species with respect to size at maturity, offspring size, and number of newborns. For developmental traits (number of juvenile instars and age at maturity) the hybrids tended to have the lowest values for all conditions. The intrinsic rate of increase (r) of the hybrid did not differ significantly from that of D. galeata, but was significantly higher than that of D. cucullata. We argue that essentially r will reach higher values for the hybrids than for D. galeata, because of the strong influence of size-selective fish predation. Our results are discussed in reference to different theories about the main- tenance of hybrid species complexes in nature.

The impact of human-made ecological changes on the genetic architecture of Daphnia species

The overenrichment (eutrophication) of aquatic ecosystems with nutrients leading to algal blooms and anoxic conditions has been a persistent and widespread environmental problem. Although there are many studies on the ecological impact of elevated phosphorus (P) levels (e.g., decrease in biodiversity and water quality), little is known about the evolutionary consequences for animal species. We reconstructed the genetic architecture of a Daphnia species complex in 2 European lakes using diapausing eggs that were isolated from sediment layers covering the past 100 years. Changes in total P were clearly associated with a shift in species composition and the population structure of evolutionary lineages. Although environmental conditions were largely re-established after peak eutrophication during the 1970s and 1980s, original species composition and the genetic architecture of species were not restored but evolved along new evolutionary trajectories. Our data demonstrate that anthropogenically induced temporal alterations of habitats are associated with long-lasting changes in communities and species via interspecific hybridization and introgression.

The Cost of Being Common: Evidence from Natural Daphnia Populations

The Red Queen coevolutionary hypothesis predicts that parasites drive oscillations in host genotype frequencies due to frequencydependent selection where common hosts are at disadvantage. However, examples of this phenomenon in natural populations are scarce. To examine if the Red Queen theory operates in the wild, we studied the genetic structure of populations of the crustacean waterflea (Daphnia), in relation to their infection levels, for which we collected multiple samples from a variety of lakes. The most common clone in a given population was often underinfected. This advantage, however, did not remain stable over time. Instead, the most common clone decreased in frequency over subsequent generations, indicating that parasites can track common clones. Such decreases were not observed in uninfected populations. Moreover, host clonal evenness was higher across the set of infected lakes compared to uninfected lakes; suggesting that any common clone is selected against when parasites are present. These results strongly suggest that Red Queen dynamics do operate in the wild.

Genetic differentiation of Baetis alpinus Pictet (Ephemeroptera: Baetidae) in fragmented alpine streams.

The interpretation of low FST values as evidence of high levels of gene flow among habitat fragments may be confounded by population genetic structures that are indicative of historical rather than present-day levels of gene flow. We examined the genetic structure of 23 populations of Baetis alpinus (Insecta: Ephemeroptera) living in alpine streams fragmented by lakes (≈10 000 years old), reservoirs (≈100 years old), and in nonfragmented streams, to examine if lakes act as barriers to gene flow and to investigate the temporal resolution of allozyme markers. Estimates of gene flow indicated little or no genetic divergence along four nonfragmented reference streams and across two lakes and two reservoirs (FST=0.004–0.041), but marked differentiation across four lakes (FST=0.092–0.362) and across one reservoir that was a lake enlarged by a dam (FST=0.075). Differentiation was unrelated to distance between fragments, but occurred only in lakes found in valleys that have been ice-free throughout the Holocene. We suggest that standing water bodies act as barriers to gene flow in B. alpinus and that low FST values observed between fragments separated by reservoirs do not indicate high levels of gene flow but rather show that genetic differentiation was not detectable within the first 100–1000 years of habitat fragmentation.

Genetic structure of cyclic parthenogenetic zooplankton populations - : a conceptual framework

The genetic structure of cyclic parthenogenetic zooplankton populations is strongly determined by the consequences of combining sexual and asexual reproduction in the same life cycle. Since the pioneering population genetic studies on freshwater zooplankton in the 1970s, a distinction has been made between the genetic structure of permanent and intermittent populations. However, the results of many studies do not fit the expectations of this dichotomous model, for example when large lake populations are considered. In this paper, we present a unifying framework for understanding the genetic structure of cyclic parthenogenetic zooplankton populations, focusing on three factors that determine their degree of clonality and within-population genetic diversity as well as their among-population genetic differentiation: the size of the dormant egg bank, length of the growing season, and strength of clonal selection. We illustrate the importance of each of these factors, and show that our broader concept better explains the variation in genetic structure observed in natural populations of cyclic parthenogens than the earlier implicitly dichotomous model.