Water fleas of the genus Daphnia have been a model system for hundreds of years and is among the best studied ecological model organisms to date. Daphnia are planktonic crustaceans with a cyclic ...parthenogenetic life-cycle. They have a nearly worldwide distribution, inhabiting standing fresh- and brackish water bodies, from small temporary pools to large lakes. Their predominantly asexual reproduction allows for the study of phenotypes excluding genetic variation, enabling us to separate genetic from non-genetic effects. Daphnia are often used in studies related to ecotoxicology, predator-induced defence, host-parasite interactions, phenotypic plasticity and, increasingly, in evolutionary genomics. The most commonly studied species are Daphnia magna and D. pulex, for which a rapidly increasing number of genetic and genomic tools are available. Here, I review current research topics, where the Daphnia model system plays a critical role.
Daphnia and its parasites have become recognized as a model system for studying the epidemiological, evolutionary and genetic interactions between hosts and parasites. The key advantages of the ...Daphnia –parasite system are the propagation of the host as iso-female lines, that is clonal, but at the same time the possibility to cross lines. Furthermore, Daphnia have diverse parasites, including bacteria, fungi, microsporidia and helminths, which can be kept in culture with the hosts. For two parasites of Daphnia magna , coevolution has been demonstrated phenotypically. Coevolution in D. magna and the bacterium Pasteuria ramosa is consistent with model predictions of coevolution by negative frequency dependent selection, the Red Queen hypothesis. The genetic mechanisms have not yet been elucidated.
A Genome for the Environment Ebert, Dieter
Science (American Association for the Advancement of Science),
02/2011, Letnik:
331, Številka:
6017
Journal Article
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Gene duplication might explain the phenotypic adaptability of water fleas.
Water fleas of the genus
Daphnia
are among the oldest model systems in biological research. Today, we know more about their ...natural history and ecology than of any other taxon. The
Daphnia
model also has left a notable mark on other fields. élie Metchnikoff (
1
) used
Daphnia
to test his 1908 Nobel prize–winning idea that macrophages attack invading parasites as part of cellular immunity. August Weismann's studies of water fleas were instrumental in developing his theory that only germ cells transmit heritable information in animals (
2
). Richard Woltereck (
3
) used
Daphnia
to develop the notion of phenotypic plasticity—that an organism can change its characteristics in response to the environment—an idea that still guides experiments with many organisms that distinguish genetic from environmental effects. With all of these historical achievements, why did the National Institutes of Health (NIH) only recently add
Daphnia
to its list of model organisms (
4
) for biomedical research? Moreover, why has
Daphnia
, at this point in time, become NIH's 13th model system?
Although the idea of coevolution was first presented 150 years ago, we still only vaguely understand the genetic basis of its workings. Identifying the genes responsible for coevolutionary ...interactions would enable us to distinguish between fundamentally different models of coevolution and would represent a milestone in population genetics and genomics.
Studies of local adaptation provide important insights into the power of natural selection relative to gene flow and other evolutionary forces. They are a paradigm for testing evolutionary hypotheses ...about traits favoured by particular environmental factors. This paper is an attempt to summarize the conceptual framework for local adaptation studies. We first review theoretical work relevant for local adaptation. Then we discuss reciprocal transplant and common garden experiments designed to detect local adaptation in the pattern of deme × habitat interaction for fitness. Finally, we review research questions and approaches to studying the processes of local adaptation – divergent natural selection, dispersal and gene flow, and other processes affecting adaptive differentiation of local demes. We advocate multifaceted approaches to the study of local adaptation, and stress the need for experiments explicitly addressing hypotheses about the role of particular ecological and genetic factors that promote or hinder local adaptation. Experimental evolution of replicated populations in controlled spatially heterogeneous environments allow direct tests of such hypotheses, and thus would be a valuable way to complement research on natural populations.
In species with separate sexes, parasite prevalence and disease expression is often different between males and females. This effect has mainly been attributed to sex differences in host traits, such ...as immune response. Here, we make the case for how properties of the parasites themselves can also matter. Specifically, we suggest that differences between host sexes in many different traits, such as morphology and hormone levels, can impose selection on parasites. This selection can eventually lead to parasite adaptations specific to the host sex more commonly encountered, or to differential expression of parasite traits depending on which host sex they find themselves in. Parasites adapted to the sex of the host in this way can contribute to differences between males and females in disease prevalence and expression. Considering those possibilities can help shed light on host-parasite interactions, and impact epidemiological and medical science.
While examples of bacteria benefiting eukaryotes are increasingly documented, studies examining effects of eukaryote hosts on microbial fitness are rare. Beneficial bacteria are often called ...“mutualistic” even if mutual reciprocity of benefits has not been demonstrated and despite the plausibility of other explanations for these microbes' beneficial effects on host fitness. Furthermore, beneficial bacteria often occur in diverse communities, making mutualism both empirically and conceptually difficult to demonstrate. We suggest reserving the terms “mutualism” and “parasitism” for pairwise interactions where the relationship is largely independent of other species and can be verified by measuring the fitness effect experienced by both partners. In hosts with diverse microbial communities, we propose re‐formulating some of the essential questions of symbiosis research – e.g. concerning specificity, transmission mode, and common evolutionary fates – as questions of community ecology and ecosystem function, allowing important biological interactions to be investigated without making assumptions about reciprocity. Understanding the fitness of host‐associated bacteria is a crucial component of investigations into the role of microbes in eukaryote evolution.
Abundance and specificity are two key characteristics of species distribution and biodiversity. Theories of species assembly aim to reproduce the empirical joint patterns of specificity and ...abundance, with the goal to explain patterns of biodiversity across habitats. The specialist‐generalist paradigm predicts that specialists should have a local advantage over generalists and thus be more abundant. We developed a specificity index to analyse abundance–specificity relationships in microbial ecosystems. By analysing microbiota spanning 23 habitats from three very different data sets covering a wide range of sequencing depths and environmental conditions, we find that habitats are consistently dominated by specialist taxa, resulting in a strong, positive correlation between abundance and specificity. This finding is consistent over several levels of taxonomic aggregation and robust to errors in abundance measures. The relationship explains why shallow sequencing captures similar β‐diversity as deep sequencing, and can be sufficient to capture the habitat‐specific functions of microbial communities.
Molecular and cellular studies reveal that the resistance of hosts to parasites and pathogens is a cascade-like process with multiple steps required to be passed for successful infection. By ...contrast, much of evolutionary reasoning is based on strongly simplified, one- or two-step infection processes with simple genetics or on resistance being a quantitative trait. Here we attempt a conceptual unification of these two perspectives with the aim of cross-fostering research and filling some of the gaps in our concepts of the ecology and evolution of disease. This conceptual unification has a profound impact on the way we understand the genetics and evolution of host resistance, ecological immunity, evolution of virulence, defence portfolios, and host–pathogen coevolution.
Many biological traits are determined by the progression of stepwise events. Dissecting host–parasite interactions into steps offers great potential for understanding infectious disease biology and evolution.
The steps of infection are typically governed by unequal contributions of genetic (G), environmental (E), and G×E effects, allowing unique evolutionary trajectories at each step.
Variation at each step has different consequences for hosts and pathogens. A pathogen must pass through all steps until transmission starts or else its fitness is zero. For the host, the profitability of resistance at a given step declines with increasing virulence experienced by the host.
Red Queen coevolution driven by negative frequency-dependent selection can occur only at steps with host genotype–pathogen genotype interactions. By contrast, selective sweeps may occur at any step.
The diversity-disease hypothesis states that decreased genetic diversity in host populations increases the incidence of diseases caused by pathogens (= monoculture effect) and eventually influences ...ecosystem functioning. The monoculture effect is well-known from crop studies and may be partially specific to the artificial situation in agriculture. The effect received little attention in animal populations of different diversities. Compared with plants, animals are mobile and exhibiting social interactions. We followed the spread of a microsporidian parasite in semi-natural outdoor Daphnia magna populations of low and high genetic diversity. We used randomly selected, naturally occurring host genotypes. Host populations of low diversity were initially monoclonal, while the host populations of high diversity started with 10 genotypes per replicate. We found that the parasite spread significantly better in host populations of low diversity compared with host populations of high diversity, independent of parasite diversity. The difference was visible over a 3-year period. Host genotypic diversity did not affect host population density. Our experiment demonstrated a monoculture effect in independently replicated semi-natural zooplankton populations, indicating that the monoculture effect may be relevant beyond agriculture.
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