Biomedical subjects

T B H Reusch

Publications and source records attributed to T B H Reusch.

9 recordsLinked to original sources

PubMed2006

Genetic variation in MHC class II expression and interactions with MHC sequence polymorphism in three-spined sticklebacks.

Genes of the major histocompatibility complex (MHC) have been studied for several decades because of their pronounced allelic polymorphism. Structural allelic polymorphism is, however, not the only source of variability subjected to natural selection. Genetic variation may also exist in gene expression patterns. Here, we show that in a natural population of three-spined sticklebacks (Gasterosteus aculeatus) the expression of MHC class IIB genes was positively correlated with parasite load, which indicates increased immune activation of the MHC when infections are frequent. To experimentally study MHC expression, we used laboratory-bred sticklebacks that were exposed to three naturally occurring species of parasite. We found strong differences in MHC class IIB expression patterns among fish families, which were consistent over two generations, thus demonstrating a genetic component. The average number of MHC class IIB sequence variants within families was negatively correlated to the MHC expression level suggesting compensatory up-regulation in fish with a low (i.e. suboptimal) MHC sequence variability. The observed differences among families and the negative correlation with individual sequence diversity imply that MHC expression is evolutionary relevant for the onset and control of the immune response in natural populations.

Alleles↗

PubMed2005

Local genetic structure in a clonal dioecious angiosperm.

We used seven microsatellite loci to characterize genetic structure and clonal architecture at three different spatial scales (from meters to centimetres) of a Cymodocea nodosa population. C. nodosa exhibits both sexual reproduction and vegetative propagation by rhizome elongation. Seeds remain buried in the sediment nearby the mother plant in a dormant stage until germination. Seed dispersal potential is therefore expected to be extremely restricted. High clonal diversity (up to 67% of distinct genotypes) and a highly intermingled configuration of genets at different spatial scales were found. No significant differences in genetic structure were found among the three spatial scales, indicating that genetic diversity is evenly distributed along the meadow. Autocorrelation analyses of kinship estimates confirmed the absence of spatial clumping of genets at small spatial scale and the expectations of a very restricted seed dispersal (observed dispersal range 1-21 m) in this species.

DNA, Plant↗

PubMed2005

Evolutionary divergence and possible incipient speciation in post-glacial populations of a cosmopolitan aquatic plant.

Habitat configuration is expected to have a major influence on genetic exchange and evolutionary divergence among populations. Aquatic organisms occur in two fundamentally different habitat types, the sea and freshwater lakes, making them excellent models to study the contrasting effects of continuity vs. isolation on genetic divergence. We compared the divergence in post-glacial populations of a cosmopolitan aquatic plant, the pondweed Potamogeton pectinatus that simultaneously occurs in freshwater lakes and coastal marine sites. Relative levels of gene flow were inferred in 12 lake and 14 Baltic Sea populations in northern Germany using nine highly polymorphic microsatellite markers developed for P. pectinatus. We found highly significant isolation-by-distance in both habitat types (P < 0.001). Genetic differentiation increased approximately 2.5-times faster among freshwater populations compared with those from the Baltic Sea. As different levels of genetic drift or population history cannot explain these differences, higher population connectivity in the sea relative to freshwater populations is the most likely source of contrasting evolutionary divergence. These findings are consistent with the notion that freshwater angiosperms are more conducive to allopatric speciation than their life-history counterparts in the sea, the relative species poor seagrasses. Surprisingly, population pairs from different habitat types revealed almost maximal genetic divergence expected for complete reproductive isolation, regardless of their respective geographical distance. Hence, the barrier to gene flow between lake and sea habitat types cannot be due to dispersal limitation. We may thus have identified a case of rapid incipient speciation in post-glacial populations of a widespread aquatic plant.

Baltic States↗

PubMed2005

How a complex life cycle can improve a parasite's sex life.

How complex life cycles of parasites are maintained is still a fascinating and unresolved topic. Complex life cycles using three host species, free-living stages, asexual and sexual reproduction are widespread in parasitic helminths. For such life cycles, we propose here that maintaining a second intermediate host in the life cycle can be advantageous for the individual parasite to increase the intermixture of different clones and therefore decrease the risk of matings between genetically identical individuals in the definitive host. Using microsatellite markers, we show that clone mixing occurs from the first to the second intermediate host in natural populations of the eye-fluke Diplostomum pseudospathaceum. Most individuals released by the first intermediate host belonged to one clone. In contrast, the second intermediate host was infected with a diverse array of mostly unique parasite genotypes. The proposed advantage of increased parasite clone intermixture may be a novel selection pressure favouring the maintenance of complex life cycles.

Animals↗

PubMed2003

Genetic neighbourhood of clone structures in eelgrass meadows quantified by spatial autocorrelation of microsatellite markers.

Limited dispersal distances in plant populations frequently cause local genetic structure, which can be quantified by spatial autocorrelation. In clonal plants, three levels of spatial organization can contribute to positive autocorrelation; namely, the neighbourhood of (a) ramets, (b) clone fragments and (c) entire clones. Here we use data from an exhaustive sampling scheme on a clonal plant to measure the contribution of the neighbourhoods of each distinct clonal structure to total spatial autocorrelation. Four plots (256 grid points each) within dense meadows of the marine clonal plant Zostera marina (eelgrass) were sampled for clone structure with nine microsatellite markers ( approximately 80 alleles). We found significant coancestry (f(ij)), at all three levels of spatial organization, even when not allowing for joins between samples of identical genets. In addition, absolute values of f(ij) and the maximum distance with significant positive f(ij) decreased with the progressive exclusion of joins between alike genotypes. The neighbourhood of this clonal plant thus consists of three levels of organization, which are reflected in different kinship structures. Each of these kinship structures may affect the level of biparental inbreeding and the physical distance between flowering shoots and their outcrossing neighbourhood. These results also emphasize the notion that spatial autocorrelation crucially depends on the scale and intensity of sampling.

Demography↗

PubMed2003

Habitat differentiation vs. isolation-by-distance: the genetic population structure of Elymus athericus in European salt marshes.

We investigated genetic differentiation among populations of the clonal grass Elymus athericus, a common salt-marsh species occurring along the Wadden Sea coast of Europe. While E. athericus traditionally occurs in the high salt marsh, it recently also invaded lower parts of the marsh. In one of the first analyses of the genetic population structure in salt-marsh species, we were interested in population differentiation through isolation-by-distance, and among strongly divergent habitats (low and high marsh) in this wind- and water-dispersed species. High and low marsh habitats were sampled at six sites throughout the Wadden Sea. Based on reciprocal transplantation experiments conducted earlier revealing lower survival of foreign genotypes we predicted reduced gene flow among habitats. Accordingly, an analysis with polymorphic cross-species microsatellite primers revealed significant genetic differentiation between high and low marsh habitats already on a very small scale (< 100 m), while isolation-by-distance was present only on larger scales (60-443 km). In an analysis of molecular variance we found that 14% of the genetic variance could be explained by the differentiation between habitats, as compared to only 8.9% to geographical (isolation-by-distance) effects among six sites 2.5-443 km distant from each other. This suggests that markedly different selection regimes between these habitats, in particular intraspecific competition and herbivory, result in habitat adaptation and restricted gene flow over distances as small as 80 m. Hence, the genetic population structure of plant species can only be understood when considering geographical and selection-mediated restrictions to gene flow simultaneously.

Ecosystem↗

PubMed2003

Inbreeding depression influences genet size distribution in a marine angiosperm.

Although inbreeding depression is a major genetic phenomena influencing individual fitness, it is difficult to measure in wild populations. An alternative approach is to correlate heterozygosity, measured using highly polymorphic markers, with a fitness-correlated trait. In clonal plants, genet size is predicted to be fitness correlated. Here we test the prediction that the genet size distribution of the marine clonal plant Zostera marina (eelgrass) is influenced by inbreeding depression. We used nine polymorphic microsatellite markers to access the fine scale clonal structure and to measure individual heterozygosity within 4 plots (each corresponds to 256 m2, sampled at 1-m intervals) in two populations along the German Baltic Coast. The same plots were also sampled for flowering and vegetative shoots to obtain estimates for sexual reproductive output at the level of the genetic individual. We found substantial differences in the genet size distribution between the two populations that may be explained by different disturbance frequency. In both populations, clone size was significantly positively correlated with the total number of flowering shoots, indicating that larger clones have a higher reproductive output. Individual heterozygosity was significantly positively associated with clone size. The effect was much stronger in Falkenstein (low disturbance) than in Maasholm (high disturbance). The results indicate that in a low disturbance population the relatively outbred clones occupy a higher proportion of the available space, possibly because they outcompete relatively inbred neighbours.

Alleles↗

PubMed2003

Multiple parasites are driving major histocompatibility complex polymorphism in the wild.

Parasite mediated selection may result in arms races between host defence and parasite virulence. In particular, simultaneous infections from multiple parasite species should cause diversification (i.e. balancing selection) in resistance genes both at the population and the individual level. Here, we tested these ideas in highly polymorphic major histocompatibility complex (MHC) genes from three-spined sticklebacks (Gasterosteus aculeatus L.). In eight natural populations, parasite diversity (15 different species), and MHC class IIB diversity varied strongly between habitat types (lakes vs. rivers vs. estuaries) with lowest values in rivers. Partial correlation analysis revealed an influence of parasite diversity on MHC class IIB variation whereas general genetic diversity assessed at seven microsatellite loci was not significantly correlated with parasite diversity. Within individual fish, intermediate, rather than maximal allele numbers were associated with minimal parasite load, supporting theoretical models of self-reactive T-cell elimination. The optimal individual diversity matched those values female fish try to achieve in their offspring by mate choice. We thus present correlative evidence supporting the 'allele counting' strategy for optimizing the immunocompetence in stickleback offspring.

Animals↗

PubMed2003

Flexible mating: cross-pollination affects sex-expression in a marine clonal plant.

Many functionally hermaphroditic plants have evolved mechanisms to reduce interference between the sex functions and to optimize reproductive output. In addition to physical mechanisms such as the spatial (herkogamy) and temporal (dichogamy) separation of male and female functions, plasticity in sex expression by means of mate-recognition (flexible mating) could be important in plants with variable access to cross-pollen. This applies particularly to clonal plants because of their modular growth form. We experimentally tested for the effects of pollen source and vegetative neighbourhood on instantaneous sex ratio and seed production in the self-compatible clonal marine angiosperm Zostera marina L. To this end, we exposed the (monoecious) flowering shoots to self and cross-pollen and to neighbourhoods of their own and a mix of foreign vegetative shoots. Flowering shoots that had been exposed to cross-pollen showed (1) a significantly lower female/male ratio at peak flowering, evidence for mate-recognition, and (2) a significantly higher seed set by the end of the season. Both effects were independent of the genetic composition of their vegetative neighbourhood. The results suggest that Z. marina maintains a cryptic self-incompatibility system not previously described for angiosperms with sub-aqueous pollination. In Z. marina, and possibly other self-compatible clonal plant species, mate-recognition could be a means of increasing the out-crossing probability for flowering shoots with central positions within their clone.

Biological Evolution↗