Biomedical subjects

Bernard Godelle

Publications and source records attributed to Bernard Godelle.

11 recordsLinked to original sources

PubMed2006

Geographic and within-population variation in the globeflower-globeflower fly interaction: the costs and benefits of rearing pollinators' larvae.

Interspecific interactions can vary within and among populations and geographic locations, and this variation can influence the nature of the interaction (e.g. mutualistic vs. antagonistic) and its evolutionary stability. Globeflowers are exclusively pollinated by flies, whose larvae feed only on their seeds. Here we document geographic variability in costs and benefits in globeflowers in sustaining their pollinating flies throughout the range of this arctic-alpine European plant over several years. A total of 1,710 flower heads from 38 populations were analysed for their carpel, egg and seed contents. Individual and population analyses control for the confounding influences of variation in both: (1) population traits, such as fly density and egg distribution among flower heads; and (2) individuals traits, such as carpel and egg numbers per flower head. Despite considerable variation in ecological conditions and pollinator densities across populations, large proportions (range 33-58%) of seeds were released after predation, with a benefit-to-cost ratio of 3, indicating that the mutualism is stable over the whole globeflower geographical range. The stability of the mutualistic interaction relies on density-dependent competition among larvae co-developing in a flower head. This competition is revealed by a sharp decrease in the number of seeds eaten per larva with increasing larval number, and is intensified by non-uniform egg distribution among globeflowers within a population. Carpel number is highly variable across globeflowers (range 10-69), and flies lay more eggs in large flowers. Most plants within a population contribute to the rearing of pollinators, but the costs are greater for some than for others. Large globeflowers lose more seed to pollinator larvae, but also release more seed than smaller plants. The apparent alignment of interests between fly and plants (positive relationship between numbers of seed released and destroyed) is shown to hide a conflict of interest found when flower size is controlled for.

Analysis of Variance↗

PubMed2006

History of the Crested Lark in the Mediterranean region as revealed by mtDNA sequences and morphology.

The Crested Lark has a very complex taxonomy, partly as a result of a strong variation in plumage ground color seemingly linked with environmental factors. However, large variations in body size and bill shape further complicate the situation in the Maghreb. In this paper, we first present a set of hypotheses to explain patterns of morphological variation around the Mediterranean Sea. A phylogeographical analysis covering all major biogeographical areas in the species' range is then performed to test these scenarios. Three mtDNA groups with distinct geographical distribution were identified. The randonii clade (= G. (c.) randonii) is endemic from central Maghreb and is phylogenetically basal relative to cristata and senegallensis. These two latter groups are much more widespread. The cristata clade is found in NW Morocco, throughout Europe and W Asia and in NE Africa, while senegallensis regroups the populations sampled in the Western Sub-Saharan Africa and in NE Maghreb (E Algeria, Tunisia). A combination of genetic and paleoenvironmental evidences supports a scenario of allopatric differentiation of these two lineages outside the Maghreb, with subsequent range expansion leading to their secondary presence in the Maghreb. However, the alternative hypothesis of differentiation in two, or even three separate Maghreb refuges cannot be completely dismissed with the present data. Interestingly, the Sahara desert and the Gibraltar Strait did not act as permanent barriers to dispersal in this species. In addition, the populations in the Maghreb are consistently longer-billed than their closest relatives, suggesting a role for natural selection or phenotypic plasticity.

Animals↗

PubMed2006

Within-host evolution and virulence in microparasites.

An important aspect of microparasite biology is the fact that infections are populations of thousands of microbes. As any population, infections are hence subject to two main types of changes: demographic and evolutionary. Here we analyse the consequences of within-host evolutionary changes. We build an epidemiological model where infections are regularly invaded by locally favored mutations affecting various infectious traits (virulence, transmissibility and clearance). Our results are the following. In durable infections, where within-host evolution is an important matter, a drop of transmissibility is only slightly deleterious to the infection, while a reduction of infection lifespan is very costly. In consequence, locally favored mutations reducing transmissibility reach a larger frequency, or even the complete fixation, and the suboptimality accumulated in infections owing to within-host evolution affects more their transmission than their duration. Conversely, taking an infection at random and observing the events of within-host evolution, one is more likely to observe reductions of infection length than reductions of transmissibility, because the mutations affecting transmissibility are often already present in infections. We then discuss the interpretation of these results in terms of deleterious mutations, and we also emphasize that the management of within-host evolution could be used as a novel therapeutic approach to the treatment of infection.

Animals↗

PubMed2005

The evolution of mutation rate in finite asexual populations.

In this article, we model analytically the evolution of mutation rate in asexual organisms. Three selective forces are present. First, everything else being equal, individuals with higher mutation rate have a larger fitness, thanks to the energy and time saved by not replicating DNA accurately. Second, as a flip side, the genome of these individuals is replicated with errors that may negatively affect fitness. Third, and conversely, replication errors have a potential benefit if beneficial mutations are to be generated. Our model describes the fate of modifiers of mutation rate under the three forces and allows us to predict the long-term evolutionary trajectory of mutation rate. We obtain three major results. First, in asexuals, the needs for both adaptation and genome preservation are not evolutionary forces that can stabilize mutation rate at an intermediate optimum. When adaptation has a significant role, it primarily destabilizes mutation rate and yields the emergence of strong-effect mutators. Second, in contrast to what is usually believed, the appearance of modifiers with large mutation rate is more likely when the fitness cost of each deleterious mutation is weak, because the cost of replication errors is then paid after a delay. Third, in small populations, and even if adaptations are needed, mutation rate is always blocked at the minimum attainable level, because the rate of adaptation is too slow to play a significant role. Only populations whose size is above a critical mass see their mutation rate affected by the need for adaptation.

Adaptation, Physiological↗

PubMed2005

Diversification of transmission modes and the evolution of mutualism.

In order for mutualism to evolve, some force must align the interests of the two interacting partners. Vertical transmission can fill this role, but it is still unknown whether mutualism can be stable when vertically transmitted symbionts can evolve toward horizontal transmission. In this article, we investigate how symbionts' transmission mode and virulence should evolve, depending on the relationship between these two traits. We show that pathogens that reduce their host's fecundity can have more complex evolutionary dynamics than those that increase mortality. In some cases, runaway evolution of virulence can drive the host population extinct. In most cases, evolutionary branching results in the differentiation of avirulent, vertically transmitted symbionts from virulent, contagious pathogens. The population of symbionts then becomes polymorphic, and because the least virulent symbionts are the most frequent, the average virulence of symbionts is much lower than it would be in a monomorphic population. When the link between transmission and virulence results from correlated mutational changes and not from fixed constraints, vertically transmitted symbionts do not simply lose virulence; they evolve toward mutualism. We show that the force that stabilizes mutualism in such situations is the competition for transmission between symbionts.

Evolution, Molecular↗

PubMed2004

Molecular adaptation in plant hemoglobin, a duplicated gene involved in plant-bacteria symbiosis.

The evolutionary history of the hemoglobin gene family in angiosperms is unusual in that it involves two mechanisms known for potentially generating molecular adaptation: gene duplication and among-species interaction. In plants able to achieve symbiosis with nitrogen-fixing bacteria, class 2 hemoglobin is expressed at high concentrations in nodules and appears to be a key factor for the achievement and regulation of the symbiotic exchange. In this study, we make use of codon models of DNA sequence evolution with the goal of determining the nature of the selective forces which have driven the evolution of this gene. Our results suggest that adaptive evolution occurred during the period of time following the duplication event (functional divergence) and that a change in the selective pressures arose in class 2 hemoglobin in relation to the acquisition of a symbiotic function.

Adaptation, Biological↗

PubMed2003

Effect of native Xenorhabdus on the fitness of their Steinernema hosts: contrasting types of interaction.

Steinernema species are entomopathogenic nematodes. They are symbiotically associated with Enterobacteriaceae of the genus Xenorhabdus. These nematode-bacteria symbioses are extremely diversified and constitute an important new model in ecology and evolution to investigate symbioses between microbes and invertebrates. However, no study has so far adequately evaluated either the outcome of the interactions or the obligate nature of interactions in different Steinernema species in the same way. Studying three different species of Steinernema, we showed that symbiotic nematodes are always fitter than aposymbiotic ones. Nevertheless, we revealed contrasting types of interaction in terms of outcome and obligate nature of the interaction. Bacterial analyses showed that nematode species differed dramatically in the number of symbiotic Xenorhabdus they carried. We suggested that when the interaction appeared more facultative for a nematode species, the nematodes carried fewer Xenorhabdus cells than strongly dependent worm species. Thus, the symbiont transmission appeared to become more efficient as the relationship between the nematode and the bacteria became tighter.

Animals↗

PubMed2003

Amplification of Nicotiana sylvestris mitochondrial subgenomes is under nuclear control and is associated with phenotypic changes.

We have previously shown the presence in a Nicotiana sylvestris protoplast-derived plant of both a nuclear mutation conferring male sterility (ms4) and a mtDNA reorganisation, named U, characterised by the amplification of substoichiometric mtDNA fragments generated by recombination in the parent T mtDNA. Here we show by physical mapping that the recombining repeats are in direct orientation, thus generating two subgenomes both of which are amplified in the U organisation to the detriment of the parent molecule, and are maintained through sexual reproduction. The nuclear ms4 mutation is likely to have play a role in the shift in mitochondrial molecule equilibrium, as higher levels of recombinant fragments were present in protoplast-derived T calli carrying the ms4 allele than in wild type calli or leaves. The MS4 gene could then lead to conflictual situation. However, subgenomic molecules were counter-selected during the regeneration process, suggesting the existence of different selective pressures in differentiated and non-differentiated cells. The U organisation is associated with higher stem height and late flowering, characters that may not be neutral from a selection point of view. The U equilibrium is an unusual example of sudden mtDNA reorganisation, without obvious differences in genetic information and with only a limited phenotypic impact.

Cell Nucleus↗

PubMed2003

Within-host parasite dynamics, emerging trade-off, and evolution of virulence with immune system.

Virulence is an evolutionary paradox because parasites never benefit from their host's death. The adaptive explanation of virulence is classically based upon the existence of physiological constraints that create a trade-off between parasites' epidemiological traits (virulence, transmissibility, and clearance). Here we develop an epidemiological model where infections are dynamic processes and we demonstrate how these dynamics generate a trade-off between emerging epidemiological parameters. We then study how host's immune strength modifies this trade-off and hence influences virulence evolution. We found that in acute infections, where parasites are engaged in a race with immune cells, immunity restrains more the duration of the infection than its intensity. As a consequence parasites evolve to provoke more virulent but shorter infections in strongly immunized hosts.

Biological Evolution↗

PubMed2002

Aperture pattern ontogeny in angiosperms.

Pollen grains display a wide range of variation in aperture number and arrangement (pattern) in angiosperms. Apertures are well-defined areas of the pollen wall surface that permit pollen tube germination. For low aperture numbers, aperture patterns are characteristic of the major taxonomic divisions of angiosperms. This paper presents a developmental model that explains most of the aperture patterns that are recorded in angiosperms. It is based on the analysis of the different events that occur during meiosis and lead to microspore differentiation. It demonstrates that variation occurring during meiosis in angiosperms is sufficient to produce the core morphological set of the most commonly observed pollen morphologies.

Cell Division↗

PubMed2002

Evolution of mutualism between globeflowers and their pollinating flies.

Plant/seed-eater pollinators mutualisms involve a plant pollinated by an insect whose larvae develop by eating a fraction of host-plant seeds. The outcome of the interaction therefore depends on the number of ovules fertilized by adult visits and the number of seeds destroyed by larvae. Among the very few cases of such mutualisms reported so far is the globeflower-globeflower flies mutualism, which is unique in that it involves several congeneric fly species (Chiastocheta genus) coexisting within a single host-plant species, Trollius europaeus. These species exhibit contrasted oviposition behaviors resulting in a more or less beneficial outcome for the plant. We designed an adaptive dynamics model to investigate how morphological traits of globeflower could affect the evolution of oviposition in its pollinating flies. Three fly traits (flower age at oviposition, clutch size and the level of avoidance of already parasitized flowers) and one plant trait (closed or open corolla) were examined. Whatever the shape of the flower, evolutionary branching occurs between early and late ovipositing flies, driven by strong competition among larvae within a fruit. Once this branching occurred, the closed shape of the corolla is likely to offer a better protection to eggs of early but not of late ovipositing flies. The difference in egg survival results in higher competition among early larvae and thus selects for decreased clutch size in early flies. This can be seen as a first step in the evolution of a mutualistic behavior. The prediction of our model fits field observations of fly behavior, giving theoretical support to the hypothesis of fly sympatric speciation within its host plant. Moreover, flower closed globe shape can be positively selected in globeflowers as it results in a reduction of parasitism strength. This last evolution therefore leads to a stable mutualism between globeflowers and globeflower flies.

Animals↗