Cyclomorphosis in Daphnia lumholtzi induced by temperature

• 1 Cyclomorphosis is a well known phenomenon in Daphnia that involves a regular, seasonal, or induced change in body allometry. Long helmets and tail spines were induced in laboratory cultures of Daphnia lumholtzi with temperature of 31 °C as the proximal cue (temperature of locally occurring peak abundance in Kentucky Lake). The effect was greater in embryos than juveniles or adults exposed to the temperature cue.
• 2 The temperature cue appears to have a threshold value (animals cultured at 25 or 28 °C did not develop elongated helmets or spines). The helmet and spine length receded both with D. lumholtzi kept at a constant 31 °C temperature and when water temperature was decreased.
• 3 The induced helmet in this experiment (0.66 mm, 1.0 mm animal) was significantly longer than values reported in the literature for induction by planktivorous fish kairomones (0.25 mm, 1.2 mm animal). The strong response to a proximal cue of temperature may require the second weaker chemical cue for maintenance. It is suggested that a synergistic explanation with two cues may be more appropriate for cyclomorphosis induction and maintenance in Daphnia lumholtzi that could be tested with further studies.

     

Cyclomorphosis in Daphnia: an adaptation to avoid invertebrate predation

Seasonal morphological changes in three Daphnia species were followed over a two-year period in two lakes that differ in invertebrate and fish pressure. Whereas the morphology of D. hyalina, the biggest of the three species, varied little from season to season, D. cucullata, the smallest, exhibited the most pronounced seasonal changes in head height/carapace length ratio. The pattern of seasonal changes of body proportions was similar in all size classes and isometric growth of the head was reported for D. cucullata. Unlike the head, tail spine length/carapace length ratio almost did not vary seasonally. Strong negative allometry of tail spine growth was observed. These results are consistent with the hypothesis that helmets and tail spines provide protection against invertebrates in the two smallest, thus most endangered species.     

Cyclomorphosis in natural populations of Daphnia cephalata King

SUMMARY. Daphnia cephalata King is a major component of the zooplankton in freshwater ponds of eastern Australia. Phenotypic variation in head shape both within and between populations is striking. Some of this variation is ontogenetic, but among individuals of similar size head length still varies by a factor of two. The causes of this variation have been investigated by analysing head size in thirty-four populations of D. cephalata over a 17-month period. An annual cycle in crest size, apparently related to seasonal changes in mean temperature, was evident when the data were pooled. Large differences in head size were commonly observed, however, among samples collected simultaneously. Some of this variation was correlated with local differences in resource availability, which were due, in part, to local variation in Daphnia density. Other consistent interpopulation differences in head size were related to stable environmental differences among sites and probably also to genetic differences among populations.     

Population dynamics and production of Daphnia hyalina Leydig and Daphnia cucullata Sars in Tjeukemeer

As part of a research programme on the food chains in Tjeukemeer, the Daphnia hyalina and Daphnia cucullata populations were studied for three successive years. To analyse the factors regulating the production of these two species, their population parameters (density, size distribution, fecundity) and population dynamics (birth rate, mortality rate) were studied and related to environmental factors. Since Daphnia in Tjeukemeer shows continuous recruitment, the population dynamics model INSTAR was developed and used to integrate field data with laboratory data on development rates and length-weight relationships. The dynamics of the Daphnia species are mainly regulated by temperature and fish predation, the latter affects both birth rate and mortality. Total annual Daphnia production was 3.1–6.9 g org. dry wt M^–2, and annual P/B ratio ranged from 25 to 40 for D. cucullata and from 45 to 49 for D. hyalina.     

Resource depression in Daphnia galeata, Daphnia cucullata and their interspecific hybrid: life history consequences

In this study, the effect of the food concentration on severallife history parameters of Daphnia galeata, Daphnia cucullataand the hybrid between these two species was examined. The foodconcentration had a significant effect on both growth and reproductionof all three taxa. Mortality was highly species specific, andto a lesser extent dependent on the food concentration. Changesin life history parameters resulted in changes in the intrinsicrate of population increase (r). The food dependence of thedifferent species was found to be different. Daphnia cucullatahad a relatively constant and low r. The r values for the hybridvaried most, with low values compared with the parental specieswhen grown at low food levels and relatively high values athigher food concentrations. Our results suggest that when foodconcentrations are high the hybrid will have a selective advantageover both parental species, but that at lower food levels D.galeatawill have an advantage over both D.cucullata and the hybrid.     

Spatial and temporal genetic heterogeneity in a natural Daphnia population

Temporal and spatial genetic changes in a Daphnia pulex populationinhabiting a permanent pond were examined for 2     

Cyclomorphosis as a factor explaining success of a Daphnia hybrid in Tjeukemeer

Recent studies have shown that Daphnia hybrids are intermediate to their parental species for several life history traits, and that they combine advantageous traits of the parental species, thereby decreasing their risk of being preyed upon by planktivorous fish. In this study D. galeata, D. cucullata and their interspecific hybrid are compared with respect to cyclomorphosis and juvenile growth as a potential mechanism to avoid invertebrate predation. Helmets and tail spines are known to reduce predation risk, whereas fast juvenile growth reduces the period that Daphnia are vulnerable to predation. In laboratory cultures, I tested the hypothesis that, with regard to cyclomorphosis, juvenile hybrids are more comparable to the juveniles of D. cucullata, and with respect to growth rate, they are more comparable to D. galeata. The hybrid did not differ significantly from D. galeata in the time needed to reach 0.9 mm body length. In contrast, the relative spine and helmet length of juvenile hybrids were more comparable with those of D. cucullata, whereas in adult instars these characters were more similar to D. galeata. Thus during the first juvenile instars, the hybrids combine the relative long helmet and spine of D. cucullata, with the faster growth of D. galeata. This combination of characteristics might reduce hybrid vulnerability to invertebrate predation in field situations.     

Strong intraspecific variation in genetic diversity and genetic differentiation in Daphnia magna: the effects of population turnover and population size

Theory predicts that genetic diversity and genetic differentiation may strongly vary among populations of the same species depending on population turnover and local population sizes. Yet, despite the importance of these predictions for evolutionary and conservation issues, empirical studies comparing high‐turnover and low‐turnover populations of the same species are scarce. In this study, we used Daphnia magna, a freshwater crustacean, as a model organism for such a comparison. In the southern/central part of its range, D. magna inhabits medium‐sized, stable ponds, whereas in the north, it occurs in small rock pools with strong population turnover. We found that these northern populations have a significantly lower genetic diversity and higher genetic differentiation compared to the southern/central populations. Total genetic diversity across populations was only about half and average within‐population diversity only about a third of that in southern/central populations. Moreover, an average southern population contains more genetic diversity than the whole metapopulation system in the north. We based our analyses both on silent sites and microsatellites. The similarity of our results despite the contrasting mutation rates of these markers suggests that the differences are caused by contemporary rather than by historical processes. Our findings show that variation in population turnover and population size may have a major impact on the genetic diversity and differentiation of populations, and hence may lead to differences in evolutionary processes like local adaptation, hybrid vigour and breeding system evolution in different parts of a species range.     

Extreme helmet formation in Daphnia cucullata induced by small-scale turbulence

Although Daphnia cucullata is used as a textbook example forcyclomorphosis, distinct helmet development, as shown in thefield, has not been demonstrated in the laboratory until now.We show for the first time that small-scale turbulence is ableto induce the maximum response of morphological plasticity inD. cucullata. Helmet elongation reached magnitudes as extremeas observed in the field. Different modes of generating small-scaleturbulence caused different levels of helmet formation. Small-scaleturbulence alone may not be responsible for cyclomorphosis innature because field data show that two nearby lakes with similarmorphometry differ in cyclomorphosis patterns, while laboratoryexperiments show that there is no difference in the abilityto form helmets in the clones of each lake. Although helmetformation in D. cucullata is inducible with predator kairomones,helmet elongation is not as strong as that induced by turbulence.We discuss the possible role of helmets under turbulent conditions.     

Cyclomorphosis in Daphnia galeata mendotae Birge and D. retrocurva Forbes as a predator-induced response

SUMMARY. 1. Populations of Daphnia galeata mendotae and D. retrocurva were exposed for one generation to predators enclosed in mesh bags. The exposed Daphnia showed adaptive morphological changes that mimic cyclomorphosis in nature. The invertebrate predators Chaoborus and Notonecta induced longer helmets, while the fish Lepomis induced shorter bodies but had no effect on helmet length. Longer tail spines (relative to body length) were induced by Notonecta and Lepomis.
2. The responses of D. retrocurva were influenced by algae concentration, with the more extreme responses occurring at a higher food concentration and higher lipid index.     

Asymmetric gene introgression in two closely related Orchis species: evidence from morphometric and genetic analyses

ABSTRACT: BACKGROUND: In food-deceptive orchids of the genera Anacamptis, Neotinea and Orchis floral isolation has been shown to be weak, whereas late-acting reproductive barriers are mostly strong, often restricting hybridization to the F1 generation. Only in a few species hybridization extends beyond the F1 generation, giving rise to hybrid swarms. However, little is known about the abundance of later-generation hybrids and what factors drive their occurrence in hybrid populations. In this study, molecular analyses were combined with detailed morphological measurements in a hybrid population of two closely related Orchis species (Orchis militaris and O. purpurea) to investigate the hypothesis that the abundance of later-generation hybrids is driven by changes in floral characters after hybridization that exert selective pressures that in turn affect hybridization. RESULTS: Both the molecular and morphological data point to extensive genetic and morphological homogenization and asymmetric introgression. Estimating genomic clines from the multi-locus genotype data and testing for deviation from neutrality revealed that 30 out of 113 (27%) AFLP markers significantly deviated from neutral expectations. Plants with large floral displays or plant with flowers that resembled more O. purpurea had higher female fitness than plants with small floral displays or plants with flowers resembling more O. militaris, suggesting that directional selection may have contributed to the observed patterns of introgression. CONCLUSIONS: These results indicate that in closely related orchid species hybridization and gene introgression may be partly driven by selection for floral traits of one of the parental types. However, because some pure individuals were still present in the studied population, the parental species appeared to be sufficiently isolated to survive the challenge of sympatry.     

Immunogenetic and population genetic analyses of Iberian cattle

Blood samples were collected from more than 100 animals in each of 2 Spanish cattle breeds (Retinto and De Lidia), 2 Portuguese breeds (Alentejana and Mertolenga), and American Longhorn cattle. All samples for the 4 Iberian breeds were tested for 20 polymorphic systems; American Longhorn were tested for 19 of the 20. For each breed an average inbreeding coefficient was estimated by a comparison of the observed and expected heterozygosity at 7 or 8 codominant systems tested. All breeds had positive values but only 3 breeds had estimates of inbreeding that were statistically significantly different from 0: De Lidia with = 0.17, Retinto with = 0.08 and Mertolenga with f = 0.05. The De Lidia breed especially may be suffering from inbreeding depression since this high value is greater than expected if all of the animals were progeny of half-sib matings. Genetic distances were calculated from the gene frequency data on these 5 breeds plus 9 other European breeds. Analyses of these distances show a closely related group of the 4 Iberian breeds and American Longhorn, confirming the close relationships among the Iberian breeds and the Iberian, probably Portuguese, origin of American Longhorn cattle.     

Reproducibility of short-term and reproduction toxicity experiments with Daphnia magna and comparison of the sensitivity of Daphnia magna with Daphnia pulex and Daphnia cucullata in short-term experiments

Short-term toxicity tests with Daphnia magna are reasonably reproducible. The sensitivity of Daphnia pulex, Daphnia cucullata and Daphnia magna proved to be about the same in short-term tests with 15 different chemical compounds.Reproduction toxicity experiments with Daphnia magna can easily be carried out within three weeks. At least duplicate experiments are necessary to arrive at an approximate no-effect level.     

Fundamentals of fungal molecular population genetic analyses

The last two decades have seen tremendous growth in the development and application of molecular methods in the analyses of fungal species and populations. In this paper, I provide an overview of the molecular techniques and the basic analytical tools used to address various fundamental population and evolutionary genetic questions in fungi. With increasing availability and decreasing cost, DNA sequencing is becoming a mainstream data acquisition method in fungal evolutionary genetic studies. However, other methods, especially those based on the polymerase chain reaction, remain powerful in addressing specific questions for certain groups of taxa. These developments are bringing fungal population and evolutionary genetics into mainstream ecology and evolutionary biology.     

Transmission mode affects the population genetic structure of Daphnia parasites

Parasite life cycle variation can shape parasite evolution, by predisposing them towards different population genetic structures. We compared the population genetic structure of two co-occurring parasite species of Daphnia, to collect evidence for their expected transmission modes. The ichthyosporean Caullerya mesnili has a direct life cycle, whereas the microsporidian Berwaldia schaefernai is hypothesized to require passage through a secondary host. The parasites were collected from three geographically isolated Daphnia populations. The nucleotide variation in the internal transcribed spacer (ITS) region was assessed at the within-individual, within-population and among-population levels, using amova. We detected significant partitioning at all levels, except for a lack of among-population variation in Berwaldia. This was confirmed by neighbour-joining and principal component analyses; Caullerya populations were distinct from each other, while there was much overlap among parasite isolates representing different populations of Berwaldia. This all implies a higher amount of gene flow for Berwaldia, consistent with the hypothesized transmission mode.     

Food and predation as major factors limiting two natural populations of Daphnia cucullata Sars

A detailed analysis of two euplanktonic populations carried out in the same period, 18 May–11 August, 1978, in two neighboring Mazurian lakes revealed different patterns of changes in population densities and significant differences in fecundity, age structure and mean sizes of individuals despite similar epilimnion temperatures in the lakes. Similar densities were maintained in both populations although food limitation was less important and predation more intense in one than in the other of the two lakes. The relative importance of food limitation and predation is discussed in lakes of low and high fertility. It is also suggested that both low food concentration and high selective predation pressure favors small clutch sizes in cladocerans.     

EST-SSRs as a resource for population genetic analyses

Simple-sequence repeats (SSRs) have increasingly become the marker of choice for population genetic analyses. Unfortunately, the development of traditional 'anonymous' SSRs from genomic DNA is costly and time-consuming. These problems are further compounded by a paucity of resources in taxa that lack clear economic importance. However, the advent of the genomics age has resulted in the production of vast amounts of publicly available DNA sequence data, including large collections of expressed sequence tags (ESTs) from a variety of different taxa. Recent research has revealed that ESTs are a potentially rich source of SSRs that reveal polymorphisms not only within the source taxon, but in related taxa, as well. In this paper, we review what is known about the transferability of EST-SSRs from one taxon to another with particular reference to the potential of these markers to facilitate population genetic studies. As an example of the utility of these resources, we then cross-reference existing EST databases against lists of rare, endangered and invasive plant species and conclude that half of all suitable EST databases could be exploited for the population genetic analysis of species of conservation concern. We then discuss the advantages and disadvantages of EST-SSRs in the context of population genetic applications.     

Differences in life-history and aging in two clonal groups of Daphnia cucullata Sars (Crustacea: Cladocera)

Life-history traits were compared in twenty-two clones of Daphnia cucullata from the pre-alpine lake Klostersee in southern Germany. Clones were kept under standardized conditions (18 °C; 16:8 L/D) both singly and with 25 females per glass. Clones belonged to two genotype groups distinguished by enzyme electrophoresis. 11 of the clones had the genotype FM at the phosphate glucose isomerase locus and 11 the genotype MM. Neonates and adults of the age class 10 to 21 days were larger in one group (FM) than in the other group (MM). The size specific growth rates within any age class depended on age and body size and differed between the genotype groups. The fecundity of young FM animals (< 16 days) was significantly higher than that of MM animals, but for older females fecundity did not differ significantly. Senescence started about 6 days earlier in FM than in MM. The data are consistent with results of a field study on Daphnia cucullata in the Klostersee (Jacobs, 1990).     

Hierarchical analysis of population genetic variation in mitochondrial and nuclear genes of Daphnia pulex

The geographic structure of Daphnia pulex populations from the central United States is analyzed with respect to isozyme and mitochondrial DNA variation. The species complex consists of cyclic and obligate parthenogens. A hierarchical analysis of population structure in the cyclic parthenogens by using a fixation-index approach indicates that this is one of the most extremely subdivided species yet studied. This genetic structure, much of which accrues within 100 km, is certainly due in part to the limited dispersal ability of Daphnia. However, previous work has shown that fluctuating selection can account for the spatial heterogeneity in isozyme frequencies in these populations. This may explain why the population subdivision for the mitochondrial genome increases approximately three times as rapidly with distance as does that for nuclear genes, which is slower than the neutral expectation. The obligate parthenogens are shown to be polyphyletic in origin, evolutionarily young, and, in some cases, geographically widespread.