GENERALISTS,SPECIALISTS, AND THE EVOLUTION OF PHENOTYPIC PLASTICITY IN SYMPATRIC POPULATIONS OF DISTINCT SPECIES

The evolution of phenotypic plasticity is studied in a model with two reproductively isolated “species” in a coarse-grained environment, consisting of two types of habitats. A quantitative genetic model for selection was constructed, in which habitats differ in the optimal value for a focal trait, and with random dispersal among habitats. The main interest was to study the effects of different selection regimes. Three cases were investigated: (1) without any limits to plasticity; (2) without genetic variation for plasticity; and (3) with a fitness cost for phenotypically plastic reactions. In almost all cases a generalist strategy to exploit both habitats emerged. Without any limits to plasticity, optimal adaptive reactions evolved. Without any genetic variation for plasticity, a compromise strategy with an intermediate, fixed phenotype evolved, whereas in the presence of costs a plastic compromise between the demands of the habitats and the costs associated with plasticity was found. Specialization and phenotypic differentiation was only found when selection within habitats was severe and optimal phenotypes for different habitats were widely different. Under soft selection (local regulation of population numbers in each habitat) the specialists coexisted; under hard selection (global regulation of population numbers) one specialist outcompeted the other. The prevalent evolutionary outcome of compromises rather than specialization implies that costs or constraints are not necessarily detectable as local adaptation in transplantation or translocation experiments.     

POPULATION DIFFERENTIATION FOR PHENOTYPIC PLASTICITY IN THE STELLARIA LONGIPES COMPLEX

Population differentiation for phenotypic plasticity of 12 morphological and reproductive traits was investigated in five populations of the Stellaria longipes complex including a population of the sand dune endemic S. arenicola. Population differentiation was detected for the mean (genotypic) value, amount of plasticity, and pattern of plasticity of traits. Average amount of plasticity was not related to degree of isozyme variability in the populations. Differentiation for pattern of plasticity was much more common than for amount. The direction and extent of divergence among populations was dependent on which of the three trait aspects was under consideration (mean, amount of plasticity, pattern of plasticity) and did not reflect their similarity as revealed by enzyme electrophoretic data. It was concluded that trait means, amounts of plasticity, and patterns of plasticity are independent of one another during evolutionary divergence and may be influenced by mosaic selection.     

THE DETECTION OF PLASTICITY GENES IN HETEROGENEOUS ENVIRONMENTS

The molecular genetic mechanisms for phenotypic plasticity across heterogeneous macro- and microenvironments were examined using the Populus genomic map constructed by DNA-based markers. Three hypotheses have been suggested to explain genetic variation in phenotypic response to varying environments (i.e., reaction norm): Lerner's homeostasis, allelic sensitivity, and gene regulation. The homeostasis hypothesis, which predicts that heterozygotes are less sensitive to the environment than homozygotes, was supported for phenotypic plasticity to unpredictable environments (microenvironmental plasticity) at the whole-genome level, but for phenotypic plasticity to predictable environments (macroenvironmental plasticity) the hypothesis was supported only at functioning quantitative trait loci (QTLs). For all growth traits studied, gene regulation was suggested to play a prevailing role in determining the norms of reaction to environments. Indirect evidence for gene regulation is that there tend to be more QTLs with larger effects on the phenotype in optimal growing conditions than suboptimal growing conditions because the expression of these QTLs identified is mediated by regulatory genes. Direct evidence for gene regulation is the identification of some loci that differ from QTLs for trait values within environments and exert an environmentally dependent control over structural gene expression. In this study, fewer environmentally sensitive QTLs were detected that display unparalleled allelic effects across environments. For stem height, there were more regulatory loci and more structural loci (whose expression is determined by gene regulation) affecting phenotypic plasticity than for basal area. It was found that microenvironmental plasticity was likely controlled by different genetic systems than those for macroenvironmental plasticity.     

RAPID LOSS OF BEHAVIORAL PLASTICITY AND IMMUNOCOMPETENCE UNDER INTENSE SEXUAL SELECTION

Phenotypic plasticity allows animals to maximize fitness by conditionally expressing the phenotype best adapted to their environment. Although evidence for such adjustment in reproductive tactics is common, little is known about how phenotypic plasticity evolves in response to sexual selection. We examined the effect of sexual selection intensity on phenotypic plasticity in mating behavior using the beetle Callosobruchus maculatus. Male genital spines harm females during mating and females exhibit copulatory kicking, an apparent resistance trait aimed to dislodge mating males. After exposing individuals from male‐ and female‐biased experimental evolution lines to male‐ and female‐biased sociosexual environments, we examined behavioral plasticity in matings with standard partners. While females from female‐biased lines kicked sooner after exposure to male‐biased sociosexual contexts, in male‐biased lines this plasticity was lost. Ejaculate size did not diverge in response to selection history, but males from both treatments exhibited plasticity consistent with sperm competition intensity models, reducing size as the number of competitors increased. Analysis of immunocompetence revealed reduced immunity in both sexes in male‐biased lines, pointing to increased reproductive costs under high sexual selection. These results highlight how male and female reproductive strategies are shaped by interactions between phenotypically plastic and genetic mechanisms of sexual trait expression.     

Predator-induced phenotypic plasticity in Daphnia pulex: uncoupling morphological defenses and life history shifts

Chemical cues from a predator Chaoborus sp. induce morphological defense (neck spine) and life history shifts (later reproduction, decreased fecundity but larger juvenile size) in the waterflea Daphnia pulex. These shifts have been interpreted either as costs of defense or as separate adaptation. In order to investigate if the life history shifts can be separated from the morphological defense, Daphnia pulex individuals were exposed to chemical cues from Chaoborus at different stages of life for variable periods. The daphnids that were exposed to Chaoborus started their reproduction later than the controls, although the differences were not statistically significant. Neck spine was induced only if daphnids were exposed to Chaoborus in an early stage of their life. Numbers of eggs produced were not affected by the different treatments, but egg mortality was higher in mothers exposed to Chaoborus. With these treatments it was possible to see neck spine induction without measurable life history changes or costs. On the other hand, irrespective of neck spine presence, the Chaoborus chemical(s) had an effect on Daphnia pulex mothers.Publication no 2159. Netherlands Institute of Ecology, Centre for LimnologyPublication no 2159. Netherlands Institute of Ecology, Centre for Limnology     

QUANTITATIVE GENETICS OF BRYOZOAN PHENOTYPIC EVOLUTION. III. PHENOTYPIC PLASTICITY AND THE MAINTENANCE OF GENETIC VARIATION

Cheilostome bryozoan species show long-term morphologic stasis, implying stabilizing selection sustained for millions of years, but nevertheless retain significant heritable variation in traits of skeletal morphology. The possible role of within-genotype (within-colony) phenotypic variability in preserving genetic diversity was analyzed using breeding data for two species of Stylopoma from sites along 110 km of the Caribbean coast of Panama. Variation among zooids within colonies accounts for nearly two-thirds of the phenotypic variance on average, increases with environmental heterogeneity, and includes significant genotype-environment interaction. Thus, within-colony variability apparently represents phenotypic plasticity, at least some of which is heritable, rather than random “developmental noise.” Almost all of the among-colonies component of phenotypic variance is accounted for by additive genetic differences in trait means, suggesting that within-colony plasticity includes virtually all of the environmental component of phenotypic variance in these populations of Stylopoma. Thus, heritable within-colony plasticity could play a significant part in maintaining genetic diversity in cheilostomes, but it is also possible that rates of polygenic mutation alone are sufficient to balance the effects of selection.     

Temperature-dependent energetics of Chaoborus populations: hypothesis for anomalous distributions in the great lakes of East Africa

Chaoborus, the phantom midge (Insecta, Diptera, Chaoboridae), has a widespread distribution, commonly occurring in lakes and ponds all over the world. In the great lakes region of East Africa Chaoborus is present in Lakes Victoria, Albert, Edward, Malawi and George, but absent from Lakes Tanganyika, Kivu and Turkana. Tropical lakes typically have water temperatures in the range of 22–26 °C year round. Lakes Tanganyika and Kivu have only 20% of their bottom sediments oxygenated during full circulation, contrary to 95–100% in all of the other lakes, excluding Lake Malawi (45%) (Hecky & Kling, 1987). Planktivorous fish are present in all lakes (Lehman, 1995). We hypothesized that the absence of Chaoborus larvae from some lakes of East Africa may be the result of interaction among high temperatures, low oxygen levels, and fish predation.We developed a model to estimate energetic costs for Chaoborus larvae at temperatures greater than 14 °C. We hoped to shed light on the bioenergetics of Chaoborus populations, and the possibility that extant distributions of Chaoborus larvae are the result of energetic constraints.We found that relative respiratory and growth costs of Chaoborus larvae are highest in the early stages of development. We estimated that non-feeding instar I larvae living in 25 °C water will starve to death in less than one day. It is possible that Chaoborus populations are prevented from establishing in certain areas because high energetic costs condemn young larvae to death by either predation or starvation.     

THE COSTS AND BENEFITS OF TOLERANCE TO COMPETITION IN IPOMOEA PURPUREA,THE COMMON MORNING GLORY

Tolerance to competition has been hypothesized to reduce the negative impact of plant–plant competition on fitness. Although competitive interactions are a strong selective force, an analysis of net selection on tolerance to competition is absent in the literature. Using 55 full/half‐sibling families from 18 maternal lines in the crop weed Ipomoea purpurea, we measured fitness and putative tolerance traits when grown with and without competition in an agricultural field. We tested for the presence of genetic variation for tolerance to competition and determined if there were costs and benefits of this trait. We also assessed correlations between tolerance and potential tolerance traits. We uncovered a fitness benefit of tolerance in the presence of competition and a cost in its absence. We failed to detect evidence of additive genetic variation underlying tolerance, but did uncover the presence of a significant maternal‐line effect for tolerance, which suggests its evolutionary trajectory is not easily predicted. The cost of tolerance is likely due to later initiation of flowering of tolerant individuals in the absence of competition, whereas relative growth rate was found to positively covary with tolerance in the presence of competition, and can thus be considered a tolerance trait.     

DEVELOPMENTAL INSTABILITY IS GENETICALLY CORRELATED WITH PHENOTYPIC PLASTICITY,CONSTRAINING HERITABILITY,AND FITNESS

Although adaptive plasticity would seem always to be favored by selection, it occurs less often than expected. This lack of ubiquity suggests that there must be trade‐offs, costs, or limitations associated with plasticity. Yet, few costs have been found. We explore one type of limitation, a correlation between plasticity and developmental instability, and use quantitative genetic theory to show why one should expect a genetic correlation. We test that hypothesis using the Landsberg erecta × Cape Verde Islands recombinant inbred lines (RILs) of Arabidopsis thaliana. RILs were grown at four different nitrogen (N) supply levels that span the range of N availabilities previously documented in North American field populations. We found a significant multivariate relationship between the cross‐environment trait plasticity and the within‐environment, within‐RIL developmental instability across 13 traits. This genetic covariation between plasticity and developmental instability has two costs. First, theory predicts diminished fitness for highly plastic lines under stabilizing selection, because their developmental instability and variance around the optimum phenotype will be greater compared to nonplastic genotypes. Second, empirically the most plastic traits exhibited heritabilities reduced by 57% on average compared to nonplastic traits. This demonstration of potential costs in inclusive fitness and heritability provoke a rethinking of the evolutionary role of plasticity.     

EVOLUTION OF PHENOTYPIC PLASTICITY IN THE STELLARIA LONGIPES COMPLEX: COMPARISONS AMONG CYTOTYPES AND HABITATS

Variation in the amount and pattern of plasticity was studied in three cytotypes (4x, 6x, and 8x) of Stellaria longipes and diploids of its suspected progenitor S. longifolia. All 13 traits considered showed plasticity. There were significant differences among cytotypes and habitats in plasticity for many traits. Overall, the diploids, S. longifolia, were most plastic, and the three cytotypes of S. longipes did not differ in amount of plasticity. Stellaria longifolia showed divergence from S. longipes in the pattern of plasticity as well. In general, cytotypes with more similar chromosome numbers had the same pattern of plasticity for more traits. Individuals from tundra populations differed in their pattern of plasticity from those of montane, boreal, and prairie origin, which were more similar to one another. Differences in plasticity among cytotypes were due primarily to divergence in amount, while differences among habitats were most often accounted for by divergent patterns of plasticity. We conclude that both polyploidy and natural selection have affected the evolution of plastic responses in this species complex. Analysis of the correlation between pairs of traits provided evidence that the pattern and amount of plasticity operate independently of one another and may be evolving separately.     

Oxygen consumption of a Chaoborus species from a neotropical reservoir (socuy reservoir,Venezuela)

Oxygen consumption of all four larval instars of a Chaoborus species from Socuy Reservoir in western Venezuela was studied under laboratory conditions at 25°C. On a logarithmic basis, oxygen consumption per organism increased as a smooth, linear function of body weight (0.49 power of body weight). Observed rates were lower than other Chaoborus species with comparable body weight at the same temperature. Oxygen consumption per unit weight decreased from smaller to large instar larvae. With the exception of instar I, values of oxygen consumption per unit weight were lower than values reported for other Chaoborus species. The results suggest that our species have low energy demands for maintenance, and are consistent with suggestions about high efficiency in use of energy in genus Chaoborus.     

AN INTERPRETATION OF PHENOTYPIC PLASTICITY IN AGROPYRON REPENS (GRAMINAE)

Phenotypic plasticity and genotypic variation were studied in Agropyron repens L. (Beauv.) collected from populations in two grassland communities which differed in the length of time since the last major disturbance. Twenty genotypes were collected from each population. Each genotype was vegetatively propagated, and subjected to six different treatments in a greenhouse. Phenotypic plasticity and genotypic variability were measured as across- and within-treatment standardized variances respectively. Patterns of plasticity were measured by genotype correlations across treatments. The results were presented graphically by the regression method of Garbutt and Zangerl (1983). Analysis of variance revealed significant population, genotype and treatment effects. Significant positive correlations between magnitude and variability of performance were found for all characters. Phenotypic plasticity and magnitude of performance were generally greater in plants collected from the older established field. Evidence for greater specialization in the older population was suggested by negative correlations between performance in the most favorable and least favorable treatments and by greater dissimilarity of genotype response across treatments. A more variable phenotypic response across treatments (i.e., higher plasticity) for plants from the older population may therefore be a consequence of specialization and not an adaptive trait per se.     

Costs of predator‐induced morphological defences in Daphnia

1. Inducible defences are advantageous because they protect the prey while limiting associated fitness costs. The presence of these costs is an essential component of this conditional strategy, since their absence would favour constitutive (fixed) defences. In some cases, however, these costs have been difficult to measure because of complex interactions between the defences themselves, resultant life history changes and the organism’s environment. 2. The pond‐dwelling water flea, Daphnia pulex, forms defensive neck spines in response to kairomones released by predatory larvae of the phantom midge, Chaoborus. This predator–prey interaction and the formation of these inducible defences have been well studied, but costs associated with the development of neck spines remain unclear. In this study, I address this problem by analysing the effect of Chaoborus kairomones on the life history responses (and fitness costs associated with these responses) of two clones of D. pulex that are from the same pond population, but differ greatly in their degree of neck spine development. 3. Both D. pulex clones exhibited the same predator‐induced shifts in life history: larger size at birth, reduced juvenile growth rate (producing a smaller size at maturity), delayed reproduction and a reduction in the number of neonates produced after the first clutch. Relative fitness decreased significantly and to the same degree (c. 10% reduction in r) in each clone. This observed fitness cost was not directly related to the neck spines per se since the cost was the same in both clones, despite their considerable differences in neck spine development. Rather, it appears to be indirectly related to this antipredator morphology via a combination of delayed reproduction and a set of life history trade‐offs (decreased growth rate, decreased reproduction after the first clutch) for increased neonate body size, which is necessary for neck spines to be effective defences. This suite of induced responses is probably a result of local adaptation of these two D. pulex clones to their common pond environment. 4. Costs of inducible defences do not always entail direct allocation costs associated with forming and maintaining a defence, but may also involve indirect life history responses that are specific to particular environmental situations. This local adaptation would explain the highly variable life history responses observed among D. pulex clones from different pond environments.     

COSTS AND LIMITS OF PHENOTYPIC PLASTICITY IN ISLAND POPULATIONS OF THE COMMON FROG RANA TEMPORARIA UNDER DIVERGENT SELECTION PRESSURES

Costs and limits are assumed to be the major constraints on the evolution of phenotypic plasticity. However, despite their expected importance, they have been surprisingly hard to find in natural populations. It has therefore been argued that natural selection might have removed high-cost genotypes in all populations. However, if costs of plasticity are linked to the degree of plasticity expressed, then high costs of plasticity would only be present in populations where increased plasticity is under selection. We tested this hypothesis by investigating costs and limits of adaptive phenotypic plasticity in development time in a common garden study of island populations of the common frog Rana temporaria , which have varying levels of development time and phenotypic plasticity. Costs of plasticity were only found in populations with high-plastic genotypes, whereas the populations with the most canalized genotypes instead had a cost of canalization. Moreover, individuals displaying the most extreme phenotypes also were the most plastic ones, which mean we found no limits of plasticity. This suggests that costs of plasticity increase with increased level of plasticity in the populations, and therefore costs of plasticity might be more commonly found in high-plastic populations.     

Optimising survival under predation: chemical cues modify curvature in Daphnia galeata

Morphological responses to the presence of predator info-chemicals havebeen described for many Daphnia (Cladocera) species, butD. galeata is generally considered to exhibit almost nomorphological changes that could increase its fitness under predation.Therefore, the aim of our study was to examine the nature and magnitude ofmorphological responses of D. galeata to their predatorsindetail and assess their potential role in decreasing the predation threat. Twoclones of Daphnia were exposed to predator info-chemicals(kairomones) from perch, a fish (Perca), and a phantommidge larvae (Chaoborus) an invertebrate, and a kairomone mixture fromboththese organisms. Laboratory life-table experiments were carried out and fiveparameters characterising the body shape of the daphnids were measured: helmetlength, head- and carapace width, eye diameter and body size. The last-namedthree parameters did not differ significantly between the clones or thetreatments. The differences found between the clones were significant for headwidth and helmet length, but only in combination with the treatment effects.Ourresults on genotype-dependent phenotypic plasticity indicated that, althoughphenotypic plasticity is present, the clonal composition of aDaphnia population can be altered by selection on themorphotype. This potential for a change in clonal frequencies is given by thedifferences measured between the two clones in head width and helmet length,altering the curvature of the Daphina body in response tokairomone presence.     

THE EVOLUTIONARY GENETICS OF AN ADAPTIVE MATERNAL EFFECT: EGG SIZE PLASTICITY IN A SEED BEETLE

In many organisms, a female's environment provides a reliable indicator of the environmental conditions that her progeny will encounter. In such cases, maternal effects may evolve as mechanisms for transgenerational phenotypic plasticity whereby, in response to a predictive environmental cue, a mother can change the type of eggs that she makes or can program a developmental switch in her offspring, which produces offspring prepared for the environmental conditions predicted by the cue. One potentially common mechanism by which females manipulate the phenotype of their progeny is egg size plasticity, in which females vary egg size in response to environmental cues. We describe an experiment in which we quantify genetic variation in egg size and egg size plasticity in a seed beetle, Stator limbatus, and measure the genetic constraints on the evolution of egg size plasticity, quantified as the genetic correlation between the size of eggs laid across host plants. We found that genetic variation is present within populations for the size of eggs laid on seeds of two host plants (Acacia greggii and Cercidium floridum; h² ranged between 0.217 and 0.908), and that the heritability of egg size differed between populations and hosts (higher on A. greggii than on C. floridum). We also found that the evolution of egg size plasticity (the maternal effect) is in part constrained by a high genetic correlation across host plants (r_G > 0.6). However, the cross-environment genetic correlation is less than 1.0, which indicates that the size of eggs laid on these two hosts can diverge in response to natural selection and that egg size plasticity is thus capable of evolving in response to natural selection.     

DEVELOPMENTAL PLASTICITY,GENETIC VARIATION,AND THE EVOLUTION OF ANDROMONOECY IN SOLANUM HIRTUM (SOLANACEAE)

Studies of andromonoecious species have shown that sex expression (proportions of hermaphrodite and staminate flowers) is quite variable. It is not known, however, whether this variation is due to variation among individuals for genetically fixed patterns of allocation to staminate and hermaphrodite flowers (population level variation) and/or to developmental plasticity of individuals in a heterogeneous environment (organismal level variation). Distinguishing between these two levels of variation is important for understanding the evolution of andromonoecy. This study investigates levels of variation in sex expression in the andromonoecious Solanum hirlum. Sex expression in this species is shown to be plastic among individuals of the same genotype (organismal level variation) and determined, in part, by the resource status of the individual. Among the genotypes examined there is also genetic variation for developmental plasticity. Thus, developmental plasticity can potentially respond to selection, and the evolution of this developmental system may have been instrumental in the establishment and maintenance of andromonoecy in S. hirtum.     

THE GENETIC ARCHITECTURE OF GROWTH RATE IN JUVENILE TAKIFUGU SPECIES

Closely related species have often evolved dramatic differences in body size. Takifugu rubripes (fugu) is a large marine pufferfish whose genome has been sequenced, whereas T. niphobles is the smallest species among Takifugu. We show that, unsurprisingly, the juvenile growth rate of T. rubripes is higher than that of T. niphobles in a laboratory setting. We produced F₂ progenies of their F₁ hybrids and found one quantitative trait locus (QTL) significantly associated with variation in juvenile body size. This QTL region (3.5 Mb) contains no known genes directly related to growth phenotype (such as IGFs) except Fgf21, which inhibits growth hormone signaling in mouse. The QTL in Takifugu spp. is distinct from the region previously known to control body size variations in stickleback or tilapia. Our results suggest that in the fish tested herein, genomic regions underlying body size evolution might have different genetic origins. They also suggest that many diverse traits in Takifugu spp. are amenable to genetic mapping.     

Plastic and constant developmental traits contribute to adaptive differences in co-occurring Polygonum species

Adaptive differences among species are often thought to result from developmentally constant trait differences that enhance fitness in alternative environments. Species differences in patterns of individual phenotypic plasticity can also have ecological consequences. Indeed, functionally related constant and plastic traits may interact to determine the phenotype's adaptive value in particular conditions. We compared juvenile shade avoidance traits (height and its components, internode length and node number) across two field density treatments in Polygonumpersicaria and P. hydropiper, annual plant species that co‐occur in pastures comprised of a mosaic of plant densities. We used selection analyses to test trait contributions to fitness in alternative density treatments. Seedlings of both species expressed plasticity for internode elongation in response to density; P. persicaria plants increased internode length and consequently height significantly more in high density than did those of P. hydropiper. As predicted by the shade avoidance hypothesis, increased height was adaptive for both species in high density stands, so P. persicaria plants had higher fitness in this environment. By contrast, node numbers were relatively constant across density treatments in both species: P. hydropiper seedlings consistently produced more nodes than did those of P. persicaria. This constant trait difference contributed to P. hydropiper's greater relative fitness at low density, where more nodes and hence leaves enable plants to better exploit available light. Differences between species in these juvenile shade‐avoidance traits did not result from the evolutionary constraints of lack of heritable variation or costs of plasticity. We discuss how these interspecific trait differences may have been generated by divergent selective histories resulting from differences in herbivore resistance. These results illustrate how adaptive differences in both plastic and constantly expressed traits may jointly contribute to ecological distribution, including coexistence in patchy habitats.     

Morphological defenses induced in situ by the invertebrate predator Chaoborus : comparison of responses between Daphnia pulex and D. rosea

The presence of plankton predators may induce altered morphology in their potential prey. To date, the mechanism of induction and adaptive value of such defensive responses have been examined in the laboratory. This study investigated the morphological defense structures induced by the invertebrate predator Chaoborus in two coexisting Daphnia species, D. pulex and D. rosea, in the field. In Piscivore Lake (Gr?fenhain, Germany), continuous and intense biomanipulation had led to near elimination of planktivorous fish and greatly increased abundances of Chaoborus (up to >10 larvae l^–1). Here, the density of Chaoborus was manipulated within the lake by an enclosure/exclosure setup and resulting morphological responses of Daphnia spp. were investigated in situ. Three replicate enclosures (4.6 m³) contained no Chaoborus (predator exclusion bags), whereas Chaoborus entered three others at ambient densities (predator enclosures). In both species of Daphnia, formation of neckteeth and elongation of the tail spine were recorded in the predator enclosures, but not in the predator exclusion treatments. Additionally, D. rosea responded to predator inclusion with an increase of the size at first reproduction. Despite the induced defense structures, the presence of Chaoborus caused increased mortality of both Daphnia species. In addition, Chaoborus affected the coexistence of the two populations of Daphnia by causing higher relative mortality in D. rosea. Neckteeth formation was always more pronounced in D. pulex than in D. rosea of the same size. Neckteeth were induced specifically in vulnerably sized juvenile instars of D. pulex, but were not found in all vulnerable instars of D. rosea. In D. rosea, neckteeth were few or absent in the ephippial hatchlings, and neckteeth formation ceased before juveniles reached a body size outside the range that larger larval stages of Chaoborus could ingest. This study provides the first experimental demonstration in the field of the inducibility of morphological defense structures in Daphnia at ambient densities of Chaoborus larvae, and quantifies these in situ responses. This expands on earlier observations of a correlation between predator density in the field and the expression of neckteeth in Daphnia. The term ”maximum size for neckteeth formation” (MSNF) is defined as the limit in body size above which no production of neckteeth was evident. This limit was used to distinguish the size classes of Daphnia that show a sensitive response to Chaoborus kairomone. This new term may be used for further comparisons among species and among different types of predator-induced responses as well as for the evaluation of the adaptive value of defense structures. Received: 10 April 1999 / Accepted: 6 April 2000