Biomedical subjects

Laurent Keller

Publications and source records attributed to Laurent Keller.

At least 19 recordsLinked to original sources

PubMed2007

Uncovering the biodiversity of genetic and reproductive systems: time for a more open approach. American Society of Naturalists E. O. Wilson Award winner address.

Important scientific findings frequently arise from serendipitous findings. Unfortunately, many scientists are not prepared to take advantage of unexpected results and to question established paradigms, and this prevents them from capitalizing on their good fortune. In this essay, I first explain how pure serendipity led us to discover unusual modes of reproduction such as clonal reproduction by males and a green-beard gene. Next, I argue that the reproductive systems of ants and other organisms are probably much more diverse than is generally appreciated. This leads me to advocate for a new "molecular naturalist" approach to reproductive systems and a more "naturalistic" approach in population and evolutionary genetics. Finally, I make two further points. The first is that our current funding and education systems tend to hinder originality and curiosity. The other is that the field of ecology and evolution, and more generally all of science, would benefit from a shift in values from scientific productivity to scientific creativity. A few suggestions are made to this effect.

Animals↗

PubMed2007

Characterization and distribution of Pogonomyrmex harvester ant lineages with genetic caste determination.

Genetic caste determination has been described in two populations of Pogonomyrmex harvester ants, each comprising a pair of interbreeding lineages. Queens mate with males of their own and of the alternate lineage and produce two types of diploid offspring, those fertilized by males of the queens' lineage which develop into queens and those fertilized by males of the other lineage which develop into workers. Each of the lineages has been shown to be itself of hybrid origin between the species Pogonomyrmex barbatus and Pogonomyrmex rugosus, which both have typical, environmentally determined caste differentiation. In a large scale genetic survey across 35 sites in Arizona, New Mexico and Texas, we found that genetic caste determination associated with pairs of interbreeding lineages occurred frequently (in 26 out of the 35 sites). Overall, we identified eight lineages with genetic caste determination that always co-occurred in the same complementary lineage pairs. Three of the four lineage pairs appear to have a common origin while their relationship with the fourth remains unclear. The level of genetic differentiation among these eight lineages was significantly higher than the differentiation between P. rugosus and P. barbatus, which questions the appropriate taxonomic status of these genetic lineages. In addition to being genetically isolated from one another, all lineages with genetic caste determination were genetically distinct from P. rugosus and P. barbatus, even when colonies of interbreeding lineages co-occurred with colonies of either putative parent at the same site. Such nearly complete reproductive isolation between the lineages and the species with environmental caste determination might prevent the genetic caste determination system to be swept away by gene flow.

Animals↗

PubMed2007

An annotated cDNA library and microarray for large-scale gene-expression studies in the ant Solenopsis invicta.

Ants display a range of fascinating behaviors, a remarkable level of intra-species phenotypic plasticity and many other interesting characteristics. Here we present a new tool to study the molecular mechanisms underlying these traits: a tentatively annotated expressed sequence tag (EST) resource for the fire ant Solenopsis invicta. From a normalized cDNA library we obtained 21,715 ESTs, which represent 11,864 putatively different transcripts with very diverse molecular functions. All ESTs were used to construct a cDNA microarray.

Animals↗

PubMed2006

Sham nepotism as a result of intrinsic differences in brood viability in ants.

In animal societies, cooperation for the common wealth and latent conflicts due to the selfish interests of individuals are in delicate balance. In many ant species, colonies contain multiple breeders and workers interact with nestmates of varying degrees of relatedness. Therefore, workers could increase their inclusive fitness by preferentially caring for their closest relatives, yet evidence for nepotism in insect societies remains scarce and controversial. We experimentally demonstrate that workers of the ant Formica exsecta do not discriminate between highly related and unrelated brood, but that brood viability differs between queens. We further show that differences in brood viability are sufficient to explain a relatedness pattern that has previously been interpreted as evidence for nepotism. Hence, our findings support the view that nepotism remains elusive in social insects and emphasize the need for further controlled experiments.

Animals↗

PubMed2006

Division of labour and colony efficiency in social insects: effects of interactions between genetic architecture, colony kin structure and rate of perturbations.

The efficiency of social insect colonies critically depends on their ability to efficiently allocate workers to the various tasks which need to be performed. While numerous models have investigated the mechanisms allowing an efficient colony response to external changes in the environment and internal perturbations, little attention has been devoted to the genetic architecture underlying task specialization. We used artificial evolution to compare the performances of three simple genetic architectures underlying within-colony variation in response thresholds of workers to five tasks. In the 'deterministic mapping' system, the thresholds of individuals for each of the five tasks is strictly genetically determined. In the second genetic architecture ('probabilistic mapping'), the genes only influence the probability of engaging in one of the tasks. Finally, in the 'dynamic mapping' system, the propensity of workers to engage in one of the five tasks depends not only on their own genotype, but also on the behavioural phenotypes of other colony members. We found that the deterministic mapping system performed well only when colonies consisted of unrelated individuals and were not subjected to perturbations in task allocation. The probabilistic mapping system performed well for colonies of related and unrelated individuals when there were no perturbations. Finally, the dynamic mapping system performed well under all conditions and was much more efficient than the two other mapping systems when there were perturbations. Overall, our simulations reveal that the type of mapping between genotype and individual behaviour greatly influences the dynamics of task specialization and colony productivity. Our simulations also reveal complex interactions between the mode of mapping, level of within-colony relatedness and risk of colony perturbations.

Animals↗

PubMed2006

Social insects as a model to study the molecular basis of ageing.

One major gap in the current knowledge of the molecular bases of ageing is that most of the work has been done using short-lived model organisms such as fruitflies, nematodes, yeast and mice. Here, we argue that ants and social bee species provide an excellent complementary system to study ageing, and this for two reasons: first, in contrast to model organisms, ant and bee queens are extraordinarily long-lived, and second, there is a tremendous variation in lifespan among the genetically identical queens, workers (non-reproductive females) and males, with queens living up to 500 times longer than males and 10 times longer than workers. We review recent experimental work aimed at testing the role of antioxidant genes within the conceptual framework of the free radical theory of ageing, as well as studies investigating the role of juvenile hormone, vitellogenin and telomeres as mediators of ageing in social insects.

Aging↗

PubMed2006

Strong association between a single gene and fertilization efficiency of males and fecundity of their mates in the bulb mite.

Although variation in male fertilization efficiency has been shown to have a genetic basis in several species, the genes responsible for the effect are generally unknown. Here, we show a strong association between the fertilization success of males and their phosphogluconate dehydrogenase (Pgdh) genotype in the bulb mite Rhizoglyphus robini. Males homozygous for the slow (S) allele fathered a significantly greater proportion of offspring when competing with males homozygous for the fast (F) allele. There was no evidence that female fecundity was influenced by their Pgdh genotype. The fecundity of FF females did not differ significantly from the fecundity of SS females but female fecundity was significantly influenced by the genotype of their mate. Females paired with SS males laid significantly fewer eggs than females paired with FF males. Altogether these data show a trade-off, with the male SS genotype associated with their higher fertilization efficiency but at the cost of a negative impact on the fecundity of females mating with them.

Animals↗

PubMed2006

Communication in bacteria: an ecological and evolutionary perspective.

Individual bacteria can alter their behaviour through chemical interactions between organisms in microbial communities - this is generally referred to as quorum sensing. Frequently, these interactions are interpreted in terms of communication to mediate coordinated, multicellular behaviour. We show that the nature of interactions through quorum-sensing chemicals does not simply involve cooperative signals, but entails other interactions such as cues and chemical manipulations. These signals might have a role in conflicts within and between species. The nature of the chemical interaction is important to take into account when studying why and how bacteria react to the chemical substances that are produced by other bacteria.

Bacteria↗

PubMed2006

Reproductive isolation between Pogonomyrmex rugosus and two lineages with genetic caste determination.

Hybrid speciation occurs when combination of two interspecific genomes results in individuals that are of high fitness but reproductively incompatible with the parental species. Although hybrid speciation is a relatively common source of new species in plants, it appears to be a much rarer occurrence in animal taxa. Here we report on reproductive isolation and range overlap between the rough harvester ant Pogonomyrmex rugosus and two lineages with hybrid genotypes (H 1 and H2). Both lineages obligately interbreed and produce genetically distinct queen and worker offspring, a phenomenon referred to as genetic caste determination (GCD). Diploid offspring produced by gametes of the same lineage develop only into queens, whereas diploid offspring derived from gametes of distinct lineages develop into workers. We investigated small-scale patterns of gene flow between the parent and the two H lineages by sampling along an 80-km transect between a pure P. rugosus population and a two-lineage population. Microsatellite and mitochondrial markers both indicated virtually no gene flow between the parent species and either lineage even at sites where parental and H-lineage colonies co-occurred. The geographic ranges of the parental species and the two-lineage population were essentially parapatric, with a surprisingly narrow band of overlap and evidence of spatial structuring even at microgeographic scales within the transition zone. This suggests that ecological competition with the parent species plays a significant role in determining the evolutionary persistence and current distribution of the hybrid lineages and the genetic caste system.

Animals↗

PubMed2006

Native supercolonies of unrelated individuals in the invasive Argentine ant.

Kinship among group members has long been recognized as a main factor promoting the evolution of sociality and reproductive altruism, yet some ants have an extraordinary social organization, called unicoloniality, whereby individuals mix freely among physically separated nests. This type of social organization is not only a key attribute responsible for the ecological dominance of these ants, but also an evolutionary paradox because relatedness between nestmates is effectively zero. Recently, it has been proposed that, in the Argentine ant, unicoloniality is a derived trait that evolved after its introduction into new habitats. Here we test this basic assumption by conducting a detailed genetic analysis of four native and six introduced populations with five to 15 microsatellite loci and one mitochondrial gene. In contrast to the assumption that native populations consist of family-based colonies with related individuals who are aggressive toward members of other colonies, we found that native populations also form supercolonies, and are effectively unicolonial. Moreover, just as in introduced populations, the relatedness between nestmates is not distinguishable from zero in these native range supercolonies. Genetic differentiation between native supercolonies was very high for both nuclear and mitochondrial markers, indicating extremely limited gene flow between supercolonies. The only important difference between the native and introduced populations was that supercolonies were several orders of magnitude smaller in the native range (25-500 m). This size difference has important consequences for our understanding of the evolution and stability of unicolonial structures because the relatively small size of supercolonies in the native range implies that competition can occur between supercolonies, which can act as a break on the spread of selfish mutants by eliminating supercolonies harboring them.

Animals↗

PubMed2005

Experimental manipulation of queen number affects colony sex ratio investment in the highly polygynous ant Formica exsecta.

In polygynous (multiple queens per nest) ants, queen dispersal is often limited with young queens being recruited within the parental colony. This mode of dispersal leads to local resource competition between nestmate queens and is frequently associated with extremely male-biased sex ratios at the population level. The queen-replenishment hypothesis has been recently proposed to explain colony sex ratio investment under such conditions. It predicts that colonies containing many queens (subject to high local resource competition) should only produce males, whereas colonies hosting few queens (reduced or no local resource competition) should produce new queens in addition to males. We experimentally tested this hypothesis in the ant Formica exsecta by manipulating queen number over three consecutive years in 120 colonies of a highly polygynous population. Queens were transferred from 40 colonies into another 40 colonies while queen number was not manipulated in 40 control colonies. Genetic analyses of worker offspring revealed that our treatment significantly changed the number of reproductive queens. The sex ratio of colonies was significantly different between treatments in the third breeding season following the experiment initiation. We found that, as predicted by the queen-replenishment hypothesis, queen removal resulted in a significant increase in the proportion of colonies that produced new queens. These results provide the first experimental evidence for the queen-replenishment hypothesis, which might account for sex ratio specialization in many highly polygynous ant species.

Analysis of Variance↗

PubMed2005

Clonal reproduction by males and females in the little fire ant.

Sexual reproduction can lead to major conflicts between sexes and within genomes. Here we report an extreme case of such conflicts in the little fire ant Wasmannia auropunctata. We found that sterile workers are produced by normal sexual reproduction, whereas daughter queens are invariably clonally produced. Because males usually develop from unfertilized maternal eggs in ants and other haplodiploid species, they normally achieve direct fitness only through diploid female offspring. Hence, although the clonal production of queens increases the queen's relatedness to reproductive daughters, it potentially reduces male reproductive success to zero. In an apparent response to this conflict between sexes, genetic analyses reveal that males reproduce clonally, most likely by eliminating the maternal half of the genome in diploid eggs. As a result, all sons have nuclear genomes identical to those of their father. The obligate clonal production of males and queens from individuals of the same sex effectively results in a complete separation of the male and female gene pools. These findings show that the haplodiploid sex-determination system provides grounds for the evolution of extraordinary genetic systems and new types of sexual conflict.

Alleles↗

PubMed2005

What are the effects of maternal and pre-adult environments on ageing in humans, and are there lessons from animal models?

An open issue in research on ageing is the extent to which responses to the environment during development can influence variability in life span in animals, and the health profile of the elderly in human populations. Both affluence and adversity in human societies have profound impacts on survivorship curves, and some of this effect may be traceable to effects in utero or in infancy. The Barker Hypothesis that links caloric restriction in very early life to disruptions of glucose-insulin metabolism in later life has attracted much attention, as well as some controversy, in medical circles. It is only rarely considered by evolutionary biologists working on phenotypic plasticity, or by biogerontologists studying model organisms such as C. elegans or Drosophila. One crucial mechanism by which animals can respond in an adaptive manner to adverse conditions, for example in nutrition or infection, during development is phenotypic plasticity. Here we begin with a discussion of adaptive plasticity in animals before asking what such phenomena may reveal of relevance to rates of ageing in animals, and in humans. We survey the evidence for effects on adult ageing of environmental conditions during development across mammalian and invertebrate model organisms, and ask whether evolutionary conserved mechanisms might be involved. We conclude that the Barker Hypothesis is poorly supported and argue that more work in human populations should be integrated with multi-disciplinary studies of ageing-related phenomena in experimental populations of different model species that are subjected to nutritional challenges or infections during pre-adult development.

Adaptation, Physiological↗

PubMed2005

Sex-ratio conflict between queens and workers in eusocial Hymenoptera: mechanisms, costs, and the evolution of split colony sex ratios.

Because workers in the eusocial Hymenoptera are more closely related to sisters than to brothers, theory predicts that natural selection should act on them to bias (change) sex allocation to favor reproductive females over males. However, selection should also act on queens to prevent worker bias. We use a simulation approach to analyze the coevolution of this conflict in colonies with single, once-mated queens. We assume that queens bias the primary (egg) sex ratio and workers bias the secondary (adult) sex ratio, both at some cost to colony productivity. Workers can bias either by eliminating males or by directly increasing female caste determination. Although variation among colonies in kin structure is absent, simulations often result in bimodal (split) colony sex ratios. This occurs because of the evolution of two alternative queen or two alternative worker biasing strategies, one that biases strongly and another that does not bias at all. Alternative strategies evolve because the mechanisms of biasing result in accelerating benefits per unit cost with increasing bias, resulting in greater fitness for strategies that bias more and bias less than the population equilibrium. Strategies biasing more gain from increased biasing efficiency whereas strategies biasing less gain from decreased biasing cost. Our study predicts that whether queens or workers evolve alternative strategies depends upon the mechanisms that workers use to bias the sex ratio, the relative cost of queen and worker biasing, and the rates at which queen and worker strategies evolve. Our study also predicts that population and colony level sex allocation, as well as colony productivity, will differ diagnostically according to whether queens or workers evolve alternative biasing strategies and according to what mechanism workers use to bias sex allocation.

Animals↗

PubMed2004

Loss of phenotypic plasticity generates genotype-caste association in harvester ants.

Caste differentiation and reproductive division of labor are the hallmarks of insect societies. In ants and other social Hymenoptera, development of female larvae into queens or workers generally results from environmentally induced differences in gene expression. However, several cases in which certain gene combinations may determine reproductive status have been described in bees and ants. We investigated experimentally whether genotype directly influences caste determination in two populations of Pogonomyrmex harvester ants in which genotype-caste associations have been observed. Each population contains two genetic lineages. Queens are polyandrous and mate with males of both lineages , but in mature colonies, over 95% of daughter queens have a pure-lineage genome, whereas all workers are of F1 interlineage ancestry. We found that this pattern is maintained throughout the colony life cycle, even when only a single caste is being produced. Through controlled crosses, we demonstrate that pure-lineage eggs fail to develop into workers even when interlineage brood are not present. Thus, environmental caste determination in these individuals appears to have been lost in favor of a hardwired genetic mechanism. Our results reveal that genetic control of reproductive fate can persist without loss of the eusocial caste structure.

Animals↗

PubMed2004

Conditional use of sex and parthenogenesis for worker and queen production in ants.

The near-ubiquity of sexual reproduction in animal species has long been considered a paradox because sexually reproducing individuals transmit only half of their genome to their progeny. Here, we show that the ant Cataglyphis cursor circumvents this cost by using alternative modes of reproduction for the production of reproductive and nonreproductive offspring. New queens are almost exclusively produced by parthenogenesis, whereas workers are produced by normal sexual reproduction. By selectively using sex for somatic growth and parthenogenesis for germline production, C. cursor has taken advantage of the ant caste system to benefit from the advantages of both sexual and asexual reproduction.

Alleles↗

PubMed2004

Evolution: informational constraints on adaptation.

Many species adaptively alter offspring sex ratios, yet little is known about how they obtain relevant environmental information. New work on the parasitoid wasp Nasonia vitripennis reveals that the presence of eggs on hosts provides a reliable cue allowing females to determine the degree of local mate competition and adaptively adjust offspring sex ratios.

Adaptation, Physiological↗

PubMed2004

Conflict over male parentage in social insects.

Mutual policing is an important mechanism that maintains social harmony in group-living organisms by suppressing the selfish behavior of individuals. In social insects, workers police one another (worker-policing) by preventing individual workers from laying eggs that would otherwise develop into males. Within the framework of Hamilton's rule there are two explanations for worker-policing behavior. First, if worker reproduction is cost-free, worker-policing should occur only where workers are more closely related to queen- than to worker-produced male eggs (relatedness hypothesis). Second, if there are substantial costs to unchecked worker reproduction, worker-policing may occur to counteract these costs and increase colony efficiency (efficiency hypothesis). The first explanation predicts that patterns of the parentage of males (male parentage) are associated with relatedness, whereas the latter does not. We have investigated how male parentage varies with colony kin structure and colony size in 50 species of ants, bees, and wasps in a phylogenetically controlled comparative analysis. Our survey revealed that queens produced the majority of males in most of the species and that workers produced more than half of the males in less than 10% of species. Moreover, we show that male parentage does not vary with relatedness as predicted by the relatedness hypothesis. This indicates that intra- and interspecific variation in male parentage cannot be accounted for by the relatedness hypothesis alone and that increased colony efficiency is an important factor responsible for the evolution of worker-policing. Our study reveals greater harmony and more complex regulation of reproduction in social insect colonies than that expected from simple theoretical expectations based on relatedness only.

Aggression↗