3,258 research outputs found
Ecological and evolutionary dynamics in "Daphnia" metapopulations
No full textMany animal and plant species occur in
metapopulations that are assemblages of spatially
delimited local populations coupled by some degree
of migration. The occupation of habitat patches may
change over time due to local extinction and
colonisation. Migrants colonise empty habitat
patches, which often leads to founder effects. They
may also invade existing populations, and thereby
increase local genetic diversity. Besides gene flow in
the focal species, migrants may evade parasites or
parasites may co-migrate with their hosts. Often,
migrants are not a random subset of their population
of origin, and populations may differ in their
contribution of migrants. Due to the evolutionary and
ecological significance of migrants, it is important to
know their number and populations of origin.
Parasites may drive the evolution within host
populations. But they also influence the success of
migrants and thereby gene flow between populations.
I studied ecological and evolutionary
dynamics in Daphnia metapopulations. Three
interacting species of Daphnia C namely D. magna,
D. longispina and D. pulex C occur sympatrically
along the coast of southwest Finland. They live in
ephemeral freshwater rock pools of various size and
reproduce asexually during most of the summer. The
sexually produced migration stage, the so-called
ephippium, is essential to survive harsh
environmental conditions such as desiccation during
summer or the freezing during winter. There exist
two different hypotheses on the origin of migrants in
this metapopulation. One hypothesis assumes a
Levins' type metapopulation, with no differences
between the patches, while the other suggests a
mainland-island model, where long-lived populations
in large patches are the source of migrants. In a first
step, I quantified the ephippium production of
populations in various sized natural rock pools and in
containers under outdoor conditions. Populations in
larger habitats produced more ephippia but the
increase was much smaller than the increase in
habitat size, and the numerical dominant populations
in small rock pools produced a substantial number of
ephippia.
In the next chapter, I show that desiccation,
which is a common phenomenon in the natural rock
pools, is not detrimental for the populations. This
year's ephippia are sufficient to survive a desiccation
event and an ephippium bank from previous years is
not required. I developed a mathematical model to
predict desiccation for more than 500 individual
pools over 25 years. During warm and dry periods,
evaporation is high and especially shallow pools with
a small surface area and vegetation tend to dry up.
Mevertheless, also these pools with a high risk of
desiccation are often inhabited by Daphnia
populations. Populations in these ephemeral pools are
usually short-lived, but ephippia are especially
exposed to passive dispersal by wind or birds in the
sediments of desiccated pools. I showed that
populations in small pools (less than about 300 l
volume) produced about 50 % of all ephippia.
Rowever, 90 % of all exposed ephippia originated
from these populations. Exposition of ephippia on dry
sediments is practically non-existing in pools larger
than 1000 l. This analysis suggests that populations in
small ephemeral pools are most relevant for the
metapopulation dynamics.
Consistent with the predictions and the
functional understanding of the production of
migrants, I found increased colonisation rates after
warm and dry summers. The weather in southwest
Finland changed in accordance to global climate
change predictions, and this led to increased
dynamics in the metapopulations of the three
Daphnia species. It is the first time that an influence
of climate change on metapopulation dynamics has
been shown. Furthermore, I also found changes in the
whole metacommunity composition, as the three
species reacted differently to climate change.
In the final chapters, I did not focus on
migrants themselves, but investigated proximate
effects of migration. All ephippia can migrate, but the
successful invasion and establishment depends on
fitness components of the hatchlings. For example,
migrants infected with the microsporidium
1ctosporea bayeri are less successful than uninfected
migrants. I was interested in further correlations
between the fitness of a host and its natural infection
status and compared the cost of resistance hypothesis
with the inbreeding-infection hypothesis. I
experimentally confirmed that the naturally observed
infection status has a genetic basis. However, I did
not find a difference in competitive abilities between
naturally uninfected and cured but former infected
genotypes. This suggests that resistance genes
segregate independently of other fitness associated
genes in this system.
A consequence of migration and dispersal is
either the establishment of low-diversity and
potentially parasite-free populations in newly
colonized habitat patches or the introduction of new
host genotypes into already existing populations,
which increases local genetic diversity. Parasites may
either co-migrate with their hosts or arrive
independently in D. magna populations. I thus
compared the epidemiology of O. bayeri in host
populations of low and high genetic diversity.
Following parasite prevalence over two years, I
showed that the parasite spread less successful in host
populations of higher genetic diversity. In the long-
term, this may influence coevolution and hamper
local adaptation of the parasite
Desempleo juvenil en Chile : desafıós y respuestas a la luz de la evolución en los años 90
No full textIgnacio Larraechea L. ; Andrea Guzmán G.Literaturverz. S. 41 - 42 - Electronic ed.: Bonn : FES Library, 200
Some aspects of the analysis of offshore structures
No full textIn this thesis, a study is made of the effect of random wave forces on self-supporting steel and concrete oil drilling platforms. Various methods of estimating the forces on the structure, and various ways of idealising both the forces and the structure itself, are compared, the - objective being a realistic and safe design.The sea is here represented by a wave amplitude spectrum, from which spectra for the forces on the structure are derived using a linear wave theory, in two ways. Firstly, using the well-known Morison equation, which requires experimental drag and inertia coefficients; and secondly by considering wave diffraction from the structural members. A quantitative comparison is made of the two methods. Using the diffraction theory, it is possible to gauge the effect of sheltering - i.e. the effect on the forces on one member due to the presence of another.The principal structures considered here are idealised as plane framed structures (though the theory is applicable for structures with, say, plate elements also), and in this connection wave forces on inclined frame members are considered. This is particularly useful for steel structures. A comparison is made between the results obtained by evaluating the forces 'consistently' and by 'lumping' them at element nodal points.In addition, a comparison is made of solution methods which ignore certain cross-correlation terms in the equations of motion for the response with one that includes such terms, in an attempt to show that a fuller analysis is no more difficult, and is likely to be safer, than the more approximate methods
Intragenic tandem repeats in Daphnia magna: structure, function and distribution
No full textAbstract Background Expressed sequence tag (EST) databases provide a valuable source of genetic data in organisms whose genome sequence information is not yet compiled. We used a published EST database for the waterflea Daphnia magna (Crustacea:Cladocera) to isolate variable number of tandem repeat (VNTR) markers for linkage mapping, Quantitative Trait Loci (QTL), and functional studies. Findings Seventy-four polymorphic markers were isolated and characterised. Analyses of repeat structure, putative gene function and polymorphism indicated that intragenic tandem repeats are not distributed randomly in the mRNA sequences; instead, dinucleotides are more frequent in non-coding regions, whereas trinucleotides (and longer motifs involving multiple-of-three nucleotide repeats) are preferentially situated in coding regions. We also observed differential distribution of repeat motifs across putative genetic functions. This indicates differential selective constraints and possible functional significance of VNTR polymorphism in at least some genes. Conclusion Databases of VNTR markers situated in genes whose putative function can be inferred from homology searches will be a valuable resource for the genetic study of functional variation and selection.</p
Resolving the infection process reveals striking differences in the contribution of environment, genetics and phylogeny to host-parasite interactions
No full textAbstract Background Infection processes consist of a sequence of steps, each critical for the interaction between host and parasite. Studies of host-parasite interactions rarely take into account the fact that different steps might be influenced by different factors and might, therefore, make different contributions to shaping coevolution. We designed a new method using the Daphnia magna - Pasteuria ramosa system, one of the rare examples where coevolution has been documented, in order to resolve the steps of the infection and analyse the factors that influence each of them. Results Using the transparent Daphnia hosts and fluorescently-labelled spores of the bacterium P. ramosa, we identified a sequence of infection steps: encounter between parasite and host; activation of parasite dormant spores; attachment of spores to the host; and parasite proliferation inside the host. The chances of encounter had been shown to depend on host genotype and environment. We tested the role of genetic and environmental factors in the newly described activation and attachment steps. Hosts of different genotypes, gender and species were all able to activate endospores of all parasite clones tested in different environments; suggesting that the activation cue is phylogenetically conserved. We next established that parasite attachment occurs onto the host oesophagus independently of host species, gender and environmental conditions. In contrast to spore activation, attachment depended strongly on the combination of host and parasite genotypes. Conclusions Our results show that different steps are influenced by different factors. Host-type-independent spore activation suggests that this step can be ruled out as a major factor in Daphnia-Pasteuria coevolution. On the other hand, we show that the attachment step is crucial for the pronounced genetic specificities of this system. We suggest that this one step can explain host population structure and could be a key force behind coevolutionary cycles. We discuss how different steps can explain different aspects of the coevolutionary dynamics of the system: the properties of the attachment step, explaining the rapid evolution of infectivity and the properties of later parasite proliferation explaining the evolution of virulence. Our study underlines the importance of resolving the infection process in order to better understand host-parasite interactions.</p
Intercomparison of simulations using 4 WRF microphysical schemes with dual-polarization data for a German squall line
No full textSimulations of a squall line system which occurred on 12 August 2004 near Munich, Germany are performed using a fine grid version of the Weather Research and Forecasting (WRF) model with five different microphysical schemes. Synthetic dual polarization observations are created from the model output and compared with detailed observations gathered by the DLR polarimetric radar POLDIRAD located near Munich. Synthetic polarimetric radar scans are derived from the model forecasts employing the polarimetric radar forward operator SynPolRad. Evaluations of the microphysical parameterization schemes are carried out comparing Plan Position Indicator (PPI) and Range Height Indicator (RHI) scans of reflectivity and the spatial distribution of hydrometeor types. The hydrometeor types are derived applying a hydrometeor classification scheme to the observed and simulated polarimetric radar quantities. Furthermore, the Ebert-McBride contiguous rain area method of verification is tested in a variety of ways on the reflectivity output from the simulations. It is found that all five schemes overestimate reflectivity in the domain, particularly in the stratiform region of the convective system. All four schemes including graupel as a hydrometeor type produce too much of it. Differences are seen among the schemes in their depiction of reflectivity in the convective line and their representation of radar bright bands
Singer Line Orbits in P G(3; q)
No full textAbstract A complete characterization of reguli that are contained in Singer line orbits is given. The characterization is field theoretic and depends upon modeling P G(3; q) by the finite field GF (q4), viewed as a 4-dimensional vector space over GF (q). As applications of this characterization one is able to construct various balanced incomplete block designs and group divisible designs, the most interesting one having the parameters of a (q + 1)-fold cover of an inversive plane. A robust method for constructing large families of mutually inequivalent unembeddable translation nets of order q2 and deficiency q is also given. 1 Introduction In [2] it is shown that spreads of P G(3; q) yielding nondesarguesian flagtransitive planes of odd order are regulus-free. One key idea in that argument is to show that any regulus which is completely contained in a Singer line orbit is either in the unique regular spread fixed by the Singer cycle or else has a rather elegant description in terms of the field multiplication of GF (q4), if this field is identified in the usual way with P G(3; q). In the present paper we show that such descriptions always generate reguli, thereby completely characterizing the reguli that lie in a full Singer line orbit
Single- and Multi-carrier Quadrature Amplitude Modulation: Principles and Applications for Personal Communications, WATM and Broadcasting: 2nd
No full textSingle- and Multi-carrier Quadrature Amplitude Modulation Principles and Applications for Personal Communications, WLANs and Broadcasting L. Hanzo Department of Electronics and Computer Science, University of Southampton, UK W. Webb Motorola, Arlington Heights, USA formerly at Multiple Access Communications Ltd, Southampton, UK T. Keller Ubinetics, Cambridge Technology Centre, Melbourn, UK formerly at Department of Electronics and Computer Science, University of Southampton, UK Motivated by the rapid evolution of wireless communication systems, this expanded second edition provides an overview of most major single- and multi-carrier Quadrature Amplitude Modulation (QAM) techniques commencing with simple QAM schemes for the uninitiated through to complex, rapidly-evolving areas, such as arrangements for wide-band mobile channels. Targeted at the more advanced reader, the multi-carrier modulation based second half of the book presents a research-orientated outlook using a variety of novel QAM-based arrangements. * Features six new chapters dealing with the complexities of multi-carrier modulation which has found applications ranging from Wireless Local Area Networks (WLAN) to Digital Video Broadcasting (DVB) * Provides a rudimentary introduction for readers requiring a background in the field of modulation and radio wave propagation * Discusses classic QAM transmission issues relevant to Gaussian channels * Examines QAM-based transmissions over mobile radio channels * Incorporates QAM-related orthogonal techniques, considers the spectral efficiency of QAM in cellular frequency re-use structures and presents a QAM-based speech communications system design study * Introduces Orthogonal Frequency Division Multiplexing (OFDM) over both Gaussian and wideband fading channels By providing an all-encompassing self-contained treatment of single- and multi- carrier QAM based communications, a wide range of readers including senior undergraduate and postgraduate students, practising engineers and researchers alike will all find the coverage of this book attractive
Caps on classical varieties and their projections
No full textAbstractA family of caps constructed by G. L. Ebert, K. Metsch and T. Szönyi results from projecting a Veronesian or a Grassmannian to a suitable lower-dimensional space. We improve on this construction by projecting to a space of much smaller dimension. More precisely, we partition PG(3 r− 1, q) into a (2 r− 1)-space, an (r− 1)-space andqr− 1 cyclic caps, each of size (q2r− 1)/(q− 1). We also decide when one of our caps can be extended by a point from the (2 r− 1)-space or the (r− 1)-space. The proof of the results uses several ingredients, most notably hyperelliptic curves
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