CLONAL-DIVERSITY PATTERNS AND BREEDING-SYSTEM VARIATION IN DAPHNIA PULEX,AN ASEXUAL-SEXUAL COMPLEX

Some individuals of the cladoceran crustacean, Daphnia pulex, reproduce by cyclic parthenogenesis, while others are obligate parthenogens. Cyclic parthenogenesis is the primitive breeding system; the transition to obligate parthenogenesis has been linked to sex-limited meiosis-suppression. Detailed study of patterns of breeding-system distribution and clonal diversity is justified because D. pulex is the first species in which the loss of sex has been related to this mechanism. The present study investigated the genotypic characteristics of 10 D. pulex populations from each of 22 sites in the Great Lakes watershed. This analysis revealed that populations reproducing by cyclic parthenogenesis were uncommon and restricted to southern sites. Most populations reproduced by obligate parthenogenesis, with the electrophoretic survey revealing an average of three clones per pond and 145 unique clones over the watershed. A combinatorial analysis was used to examine the relationships between clone discovery in the asexual populations and both sample size and genetic-sampling intensity. This analysis showed that the few clones found in individual ponds were readily discriminated, while diversity on a regional scale was underestimated. These methods provide a quantitative basis for assessing the level of clonal diversity in asexual populations and in asexually transmitted segments of the genome.     

Predator-induced defences in <Emphasis Type="Italic">Daphnia pulex</Emphasis>: Selection and evaluation of internal reference genes for gene expression studies with real-time PCR

Background  

The planktonic microcrustacean Daphnia pulex is among the best-studied animals in ecological, toxicological and evolutionary research. One aspect that has sustained interest in the study system is the ability of D. pulex to develop inducible defence structures when exposed to predators, such as the phantom midge larvae Chaoborus. The available draft genome sequence for D. pulex is accelerating research to identify genes that confer plastic phenotypes that are regularly cued by environmental stimuli. Yet for quantifying gene expression levels, no experimentally validated set of internal control genes exists for the accurate normalization of qRT-PCR data.     

Mitochondrial DNA diversity in an apomictic Daphnia complex from the Canadian High Arctic

Cyclic parthenogenesis is the ancestral mode of reproduction in the cladoceran crustacean, Daphnia pulex, but some populations have made the transition to obligate parthenogenesis and this is the only mode of reproduction known to occur in arctic populations. Melanism and polyploidy are also common in arctic populations of this species. Prior allozyme studies of arctic D. pulex revealed substantial levels of clonal diversity on a regional scale. Clonal groupings based on cluster analysis of allozyme genotypes do not conform to groupings based on the presence/absence of melanin or on ploidy level. In order to further elucidate genetic relationships among arctic D. pulex clones, mitochondrial DNA (mtDNA) variation was examined in 31 populations from two Canadian high-arctic sites. The data were also compared to a previous study of mtDNA variation in populations from a Canadian low-arctic site. Cladistic analysis of restriction site variation of the entire mitochondrial genome and nucleotide sequence variation of the mitochondrial control region was used to construct genetic relationships among mitochondrial genotypes. Three distinct mitochondrial lineages were detected. One lineage was associated with diploid, nonmelanic clones and is the same as the lineage that is found in temperate populations of D. pulex. The other two lineages (A & B) were associated with polyploid, melanic clones. Sequence divergence between the A and B lineages was 2.4%. Sequence divergence between D. pulex and either of these two lineages exceeded 3%. It is suggested that the melanic, polyploid clones are hybrids between males of D. pulex (and/or a closely related congener, D. pulicaria) and females of either of two ancestral melanic species that have mitochondrial lineages A and B. Geographic patterns of mitochondrial diversity in ‘melanic’ lineage B support the hypothesis of an high-arctic refuge for the ancestral species during the last glacial period.     

Spectral Libraries for SWATH‐MS Assays for Drosophila melanogaster and Solanum lycopersicum

Quantitative proteomics methods have emerged as powerful tools for measuring protein expression changes at the proteome level. Using MS‐based approaches, it is now possible to routinely quantify thousands of proteins. However, prefractionation of the samples at the protein or peptide level is usually necessary to go deep into the proteome, increasing both MS analysis time and technical variability. Recently, a new MS acquisition method named SWATH is introduced with the potential to provide good coverage of the proteome as well as a good measurement precision without prior sample fractionation. In contrast to shotgun‐based MS however, a library containing experimental acquired spectra is necessary for the bioinformatics analysis of SWATH data. In this study, spectral libraries for two widely used models are built to study crop ripening or animal embryogenesis, Solanum lycopersicum (tomato) and Drosophila melanogaster, respectively. The spectral libraries comprise fragments for 5197 and 6040 proteins for S. lycopersicum and D. melanogaster, respectively, and allow reproducible quantification for thousands of peptides per MS analysis. The spectral libraries and all MS data are available in the MassIVE repository with the dataset identifiers MSV000081074 and MSV000081075 and the PRIDE repository with the dataset identifiers PXD006493 and PXD006495.     

Population genetic structure of three pond-inhabiting Daphnia species on a regional scale (Flanders, Belgium)

1. Only a few studies have compared patterns of genetic variation among populations of different Daphnia species on a regional scale. The present study addresses this gap and examines the relationship between diversity as revealed by allozyme variation and habitat size for populations of Daphnia pulex, D. obtusa and D. curvirostris in Flanders (Belgium). In addition, we examined whether patterns of isolation‐by‐distance could be observed in each of these three Daphnia species. 2. The relationship between genetic diversity and habitat size varied among Daphnia species that occur in the same region. In D. pulex and D. obtusa populations, a positive relationship between local genetic diversity and habitat size was found, whereas the relationship was negative in D. curvirostris populations. 3. Regional genetic diversity was lower than expected from patterns of local genetic diversity in D. pulex and D. obtusa populations in Flanders. This suggests that the subdivision of local Daphnia populations in a region did not obviously increase genetic diversity. 4. Genetic differentiation among populations of these three species in Flanders was moderate and comparable with values observed in other Daphnia species. Patterns of isolation‐by‐distance could be observed, but the scatter was high (D. pulex) or the slope was very low (D. obtusa).     

In this issue: Proteomics 8/2008

In this issue of Proteomics you will find the following highlighted articles: Have a heart (mitochondrial) proteome Is a rose always a rose? How clean is clean? Is a proteome always a proteome? Such deep questions to ponder. Zhang et al. don't just ponder, they attack the last two questions. Taking meticulous care to prepare clean mouse cardiac mitochondria, they identify almost a thousand proteins from the functionally and morphologically validated organelle. Half of the proteins had not been previously identified. Functional clusters include the expected and the “under‐appreciated” – proteolysis, protein folding, apoptosis and redox signaling. A close association with rough ER could not be disrupted without damage to the outer mitochondrial membrane. Immunocytological localization of many of the proteins revealed roles in other sites as well, including ER, cytoplasm, and Golgi. Comparative analysis of published mitochondrial proteomes from different tissues suggests that the proteomes are functionally adapted to their particular milieu. A mitochondrion (heart) is not a mitochondrion (liver). Zhang, J. et al., Proteomics 2008, 8, 1564–1575. Ibuprofen: split personality complicates proteome analyses Ibuprofen is one of those two‐fisted drugs that comes in an S form and an R form. The S form of this nonsteroidal anti‐inflammatory drug (NSAID) is the only active one, in this case. Normally sold over the counter for general aches and pains in the US, statistical analysis of its regular users has found it associated with a reduced incidence of Alzheimer's disease. Following up on this lead, Zhang et al. performed proteomic analysis of the effect of the R and S forms and their mixture on neuroblastoma cells. From three replicates, 167 proteins were identified as being quantitatively shifted. A total of 13 were unique. Functionally, they included representatives from metabolic enzymes (5), signaling (6), and cytoskeleton (2). Of interest for the Alzheimer's association was the reduced levels of reactive oxygen species (ROS), probably linked to levels of peroxiredoxins 2 and 6 in ibuprofen S‐treated cells. Zhang, J. et al., Proteomics 2008, 8, 1595–1607. Not your usual marine bacterium Rhodopirellula baltica is a member of the Planctomycetes phylum. These bacteria exhibit a proteinaceous cell wall, budding cell division, and intracellular compartments. From genome sequencing, it has >7300 ORFs. Analyzing the soluble proteins over the range of pH 3–10 by 2‐D PAGE, using narrow range pH gradient gels, nHPLC‐MS, and 1‐D SDS‐PAGE, Hieu et al. added 709 proteins to the proteins identified previously to bring the total identified to 1267, 17% of the predicted total ORFs. Gel‐free analysis (multiple dimension LC‐MS) yielded 145 proteins not seen in gel‐based methods. Both 1‐D and gel‐free methods were used for identification of cell wall and ribosomal proteins. Ninety three proteins were identified in the cell wall proteome and 13 extracellular proteins. No support was found for the hypothesis that R. baltica fed on sinking dead “marine snow” organisms by secreting proteases. Hieu, C. X. et al., Proteomics 2008, 8, 1608–1623.     

Cover Picture: Proteomics 8/2008

In this issue of Proteomics you will find the following highlighted articles: Have a heart (mitochondrial) proteome Is a rose always a rose? How clean is clean? Is a proteome always a proteome? Such deep questions to ponder. Zhang et al. don't just ponder, they attack the last two questions. Taking meticulous care to prepare clean mouse cardiac mitochondria, they identify almost a thousand proteins from the functionally and morphologically validated organelle. Half of the proteins had not been previously identified. Functional clusters include the expected and the “under‐appreciated” – proteolysis, protein folding, apoptosis and redox signaling. A close association with rough ER could not be disrupted without damage to the outer mitochondrial membrane. Immunocytological localization of many of the proteins revealed roles in other sites as well, including ER, cytoplasm, and Golgi. Comparative analysis of published mitochondrial proteomes from different tissues suggests that the proteomes are functionally adapted to their particular milieu. A mitochondrion (heart) is not a mitochondrion (liver). Zhang, J. et al., Proteomics 2008, 8, 1564–1575. Ibuprofen: split personality complicates proteome analyses Ibuprofen is one of those two‐fisted drugs that comes in an S form and an R form. The S form of this nonsteroidal anti‐inflammatory drug (NSAID) is the only active one, in this case. Normally sold over the counter for general aches and pains in the US, statistical analysis of its regular users has found it associated with a reduced incidence of Alzheimer's disease. Following up on this lead, Zhang et al. performed proteomic analysis of the effect of the R and S forms and their mixture on neuroblastoma cells. From three replicates, 167 proteins were identified as being quantitatively shifted. A total of 13 were unique. Functionally, they included representatives from metabolic enzymes (5), signaling (6), and cytoskeleton (2). Of interest for the Alzheimer's association was the reduced levels of reactive oxygen species (ROS), probably linked to levels of peroxiredoxins 2 and 6 in ibuprofen S‐treated cells. Zhang, J. et al., Proteomics 2008, 8, 1595–1607. Not your usual marine bacterium Rhodopirellula baltica is a member of the Planctomycetes phylum. These bacteria exhibit a proteinaceous cell wall, budding cell division, and intracellular compartments. From genome sequencing, it has >7300 ORFs. Analyzing the soluble proteins over the range of pH 3–10 by 2‐D PAGE, using narrow range pH gradient gels, nHPLC‐MS, and 1‐D SDS‐PAGE, Hieu et al. added 709 proteins to the proteins identified previously to bring the total identified to 1267, 17% of the predicted total ORFs. Gel‐free analysis (multiple dimension LC‐MS) yielded 145 proteins not seen in gel‐based methods. Both 1‐D and gel‐free methods were used for identification of cell wall and ribosomal proteins. Ninety three proteins were identified in the cell wall proteome and 13 extracellular proteins. No support was found for the hypothesis that R. baltica fed on sinking dead “marine snow” organisms by secreting proteases. Hieu, C. X. et al., Proteomics 2008, 8, 1608–1623.     

Meiosis genes in Daphnia pulex and the role of parthenogenesis in genome evolution

Background  

Thousands of parthenogenetic animal species have been described and cytogenetic manifestations of this reproductive mode are well known. However, little is understood about the molecular determinants of parthenogenesis. The Daphnia pulex genome must contain the molecular machinery for different reproductive modes: sexual (both male and female meiosis) and parthenogenetic (which is either cyclical or obligate). This feature makes D. pulex an ideal model to investigate the genetic basis of parthenogenesis and its consequences for gene and genome evolution. Here we describe the inventory of meiotic genes and their expression patterns during meiotic and parthenogenetic reproduction to help address whether parthenogenesis uses existing meiotic and mitotic machinery, or whether novel processes may be involved.     

The role of hybridization in the origin and spread of asexuality in Daphnia

The molecular mechanisms leading to asexuality remain little understood despite their substantial bearing on why sexual reproduction is dominant in nature. Here, we examine the role of hybridization in the origin and spread of obligate asexuality in Daphnia pulex, arguably the best‐documented case of contagious asexuality. Obligately parthenogenetic (OP) clones of D. pulex have traditionally been separated into ‘hybrid’ (Ldh SF) and ‘nonhybrid’ (Ldh SS) forms because the lactase dehydrogenase (Ldh) locus distinguishes the cyclically parthenogenetic (CP) lake dwelling Daphnia pulicaria (Ldh FF) from its ephemeral pond dwelling sister species D. pulex (Ldh SS). The results of our population genetic analyses based on microsatellite loci suggest that both Ldh SS and SF OP individuals can originate from the crossing of CP female F₁ (D. pulex × D. pulicaria) and backcross with males from OP lineages carrying genes that suppress meiosis specifically in female offspring. In previous studies, a suite of diagnostic markers was found to be associated with OP in Ldh SS D. pulex lineages. Our association mapping supports a similar genetic mechanism for the spread of obligate parthenogenesis in Ldh SF OP individuals. Interestingly, our study shows that CP D. pulicaria carry many of the diagnostic microsatellite alleles associated with obligate parthenogenesis. We argue that the assemblage of mutations that suppress meiosis and underlie obligate parthenogenesis in D. pulex originated due to a unique historical hybridization and introgression event between D. pulex and D. pulicaria.     

Competitive abilities and distribution ranges of four asexual Daphnia pulex lineages and Daphnia mitsukuri in Eurasian continental islands

Daphnia pulex and D. mitsukuri are morphologically similar and distributed in small lowland Japanese lakes. Daphnia pulex in Japan reproduces by obligate parthenogenesis and is composed of four different lineages (JPN1–JPN4) with North American origins. Although this species is found throughout Japan, JPN1 has the largest distribution range, followed by JPN2. Daphnia mitsukuri is a putatively endemic species that is now rarely found in Japan. Since Daphnia share the same algal food, the difference in distribution ranges among the four asexual D. pulex lineages and D. mitsukuri may be caused by exploitative competition for algal food. To examine this possibility, we measured the threshold food concentration (TFC) at zero-net growth rate and the food concentration required for 50% of the individuals to survive (FC50). In addition, we examined the effects of temperature on the somatic growth rate (SGR) and the mortality rates of four asexual D. pulex lineages and D. mitsukuri. The experiments showed that TFC was lowest in D. pulex JPN1 and highest in D. mitsukuri, supporting the idea that the largest distribution range of JPN1 is due to its superiority in inter-lineage competition, and that the distribution area of D. mitsukuri is reduced through exploitative competition with D. pulex. However, although JPN2 was found in relatively large areas, it had higher TFC than JPN3 and JPN4, which appear in very limited areas. These results clearly showed that the difference in distribution ranges among D. pulex lineages are not necessarily determined by their competitive superiority alone.     

Updating the proteome of the uncultivable hemotrophic Mycoplasma suis in experimentally infected pigs

Mycoplasma suis belongs to the hemotrophic mycoplasmas that are associated with acute and chronic anemia in a wide range of livestock and wild animals. The inability to culture M. suis in vitro has hindered its characterization at the molecular level. Since the publication of M. suis genome sequences in 2011 only one proteome study has been published. Aim of the presented study was to significantly extend the proteome coverage of M. suis strain KI_3806 during acute infection by applying three different protein extraction methods followed by 1D SDS‐PAGE and LC‐MS/MS. A total of 404 of 795 M. suis KI_3806 proteins (50.8%) were identified. Data analysis revealed the expression of 83.7% of the predicted ORFs with assigned functions but also highlights the expression of 179 of 523 (34.2%) hypothetical proteins with unknown functions. Computational analyses identified expressed membrane‐associated hypothetical proteins that might be involved in adhesion or host–pathogen interaction. Furthermore, analyses of the expressed proteins indicated the existence of a hexose‐6‐phosphate‐transporter and an ECF transporter. In conclusion, our proteome study provides a further step toward the elucidation of the unique life cycle of M. suis and the establishment of an in vitro culture. All MS data have been deposited in the ProteomeXchange with identifier PXD002294 ( http://proteomecentral.proteomexchange.org/dataset/PXD002294 ).     

Intraspecific trait variation and colonization sequence alter community assembly and disease epidemics

When individuals from multiple populations colonize a new habitat patch, intraspecific trait variation can make the arrival order of colonists an important factor for subsequent population and community dynamics. In particular, intraspecific priority effects (IPEs) allow early arrivers to limit the growth or establishment of later arrivers, even when competitively inferior on a per‐capita basis. Through their effects on genes and traits, IPEs can alter short‐term growth and long‐term evolutionary change in single species metapopulations. Given their importance for intraspecific interactions, IPEs in a dominant species have the potential to affect the composition of entire communities. We conducted an experiment to determine whether and how arrival order and IPEs in the zooplankter Daphnia pulex affected its interactions with both competitors (the cladoceran Simocephalus vetulus) and parasites (the virulent fungus Metschnikowia bicuspidata). We found strong evidence for IPEs in Daphnia, as early arrivers inhibited late arrivers even when competitively inferior. These IPEs in Daphnia altered both the establishment success of interspecific competitors and the size of disease epidemics: early colonization by fast‐growing D. pulex led to large Daphnia populations and low competitor establishment, but large disease epidemics. Early colonization by slow‐growing D. pulex, on the other hand, resulted in small Daphnia populations with high competitor establishment, but smaller disease epidemics. Overall, our results demonstrate the importance of intraspecific variation and arrival order for community dynamics, and highlight IPEs as a general mechanism driving variation in natural communities.     

Is competition important to arctic zooplankton community structure?

1. Daphnia pulex and Daphnia middendorffiana are commonly found in the Toolik Lake region of arctic Alaska. These two species are very similar morphologically, although their natural distributions differ markedly: D. pulex is restricted to shallow ponds, while D. middendorffiana is widely distributed and found in a variety of ponds and lakes. We compared the reproductive capabilities of D. pulex and D. middendorffiana grown under similar resource conditions and in the absence of the invertebrate predator Heterocope septentrionalis. In situ life table and mesocosm experiments were conducted in Toolik Lake and Dam Pond, habitats that have historically contained natural populations of D. middendorffiana, but never D. pulex. 2. Daphnia pulex exhibited a significantly higher net growth rate than D. middendorffiana in both life table and mesocosm experiments although D. pulex has never been found in either Toolik Lake or Dam Pond. Daphnia middendorffiana exhibited a negative net growth rate in Dam Pond, which had lower resource levels then Toolik Lake. Therefore, the smaller D. pulex appears to have a lower food threshold concentration than the larger D. middendorffiana. 3. Our results indicate that D. pulex is a superior resource competitor in the Toolik Lake region. These results combined with distributional patterns suggest that the restricted distribution of D. pulex in these arctic lakes and ponds cannot be explained by resource competition alone. We suggest that in the presence of H. septentrionalis, predation is an important factor structuring arctic zooplankton communities in the Toolik Lake region.     

Proteomic analysis of the bacterial pathogen Bartonella henselae and identification of immunogenic proteins for serodiagnosis

Bartonella henselae is a slow growing, fastidious and facultative intracellular pathogen causing cat scratch disease and vasculoproliferative disorders. To date, knowledge about the pathogenicity of this human pathogenic bacterium is limited and, additionally, serodiagnosis still needs further improvement. Here, we investigated the proteome of B. henselae using 2‐D SDS‐PAGE and MALDI‐TOF‐MS. We provide a comprehensive 2‐D proteome reference map of the whole cell lysate of B. henselae with 431 identified protein spots representing 191 different proteins of which 16 were formerly assigned as hypothetical proteins. To unravel immunoreactive antigens, we applied 2‐D SDS‐PAGE and subsequent immunoblotting using 33 sera of patients suffering from B. henselae infections. The analysis revealed 79 immunoreactive proteins of which 71 were identified. Setting a threshold of 20% seroreactivity, 11 proteins turned out to be immunodominant antigens potentially useful for an improved Bartonella‐specific serodiagnosis. Therefore, we provide for the first time (i) a comprehensive 2‐D proteome map of B. henselae for further proteome‐based studies focussed on the pathogenicity of B. henselae and (ii) an integrated view into the humoral immune responses targeted against this newly emerged human pathogenic bacterium.     

Linking biotic interactions and climate change to the success of exotic Daphnia lumholtzi

1. Warmer temperatures may increase cyanobacterial blooms in freshwater ecosystems, yet few ecological studies examine how increases in temperature and cyanobacterial blooms will alter the performance of non‐native species. We evaluated how competitive interactions and interactions between these two drivers of ecological change influence the performance of non‐native species using the native zooplankton Daphnia pulex and the non‐native zooplankton Daphnia lumholtzi as a model system. Based on the literature, we hypothesised that D. lumholtzi would perform best in warmer temperatures and in the presence of cyanobacteria. 2. Laboratory competition experiments showed that in the absence of competitors, growth rates of D. pulex (but not D. lumholtzi) were reduced at higher temperatures and with the cyanobacterial foods Anabaena flos‐aquae and Microcystis aeruginosa. In the presence of competitors, however, D. pulex emerged as the superior resource competitor at both temperatures with cyanobacterial food. We therefore predicted that, if competitive interactions are important to its establishment, D. lumholtzi would perform best in the absence of cyanobacteria in heated environments. 3. As predicted, when both species were introduced at low densities in field mesocosms, D. lumholtzi performed best at high temperatures without added cyanobacteria and worst at ambient temperatures with added cyanobacteria, indicating that competitive interactions are likely to be important for its establishment. 4. Taken together, these studies indicate that, while D. lumholtzi may benefit from increases in temperature, associated increased cyanobacterial blooms may hinder its performance. Thus, our findings underscore the importance of considering biotic interactions such as competition when predicting the future establishment of non‐native species in response to climate warming.     

Shotgun proteome analysis utilising mixed mode (reversed phase‐anion exchange chromatography) in conjunction with reversed phase liquid chromatography mass spectrometry analysis

The 2‐D peptide separations employing mixed mode reversed phase anion exchange (MM (RP‐AX)) HPLC in the first dimension in conjunction with RP chromatography in the second dimension were developed and utilised for shotgun proteome analysis. Compared with strong cation exchange (SCX) typically employed for shotgun proteomic analysis, peptide separations using MM (RP‐AX) revealed improved separation efficiency and increased peptide distribution across the elution gradient. In addition, improved sample handling, with no significant reduction in the orthogonality of the peptide separations was observed. The shotgun proteomic analysis of a mammalian nuclear cell lysate revealed additional proteome coverage (2818 versus 1125 unique peptides and 602 versus 238 proteins) using the MM (RP‐AX) compared with the traditional SCX hyphenated to RP‐LC‐MS/MS. The MM analysis resulted in approximately 90% of the unique peptides identified present in only one fraction, with a heterogeneous peptide distribution across all fractions. No clustering of the predominant peptide charge states was observed during the gradient elution. The application of MM (RP‐AX) for 2‐D LC proteomic studies was also extended in the analysis of iTRAQ‐labelled HeLa and cyanobacterial proteomes using nano‐flow chromatography interfaced to the MS/MS. We demonstrate MM (RP‐AX) HPLC as an alternative approach for shotgun proteomic studies that offers significant advantages over traditional SCX peptide separations.     

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.     

Zooplankton-induced unicell-colony transformation in Scenedesmus acutus and its effect on growth of herbivore Daphnia

The effects of colonial ecomorphs of the green alga Scenedesmus acutus on growth of Daphnia cucullata and D. pulex were examined. In ecologically relevant densities (up to 200 animals l^-1) the relatively small D. cucullata did not induce colony formation in Scenedesmus acutus, whereas the larger congener D. pulex significantly promoted colony formation. Both clearance rate and population growth rate (r) were significantly lower in D. cucullata when fed colonial Scenedesmus then when fed unicellular food. However, for D. pulex no effects of food type were observed. These results show that large Daphnia may influence its food in such a way that smaller congeners and competitors are negatively affected.