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Stuart A West

Publications and source records attributed to Stuart A West.

36 records · Page 2Linked to original sources
PubMed2004

Bacteriocins, spite and virulence.

There has been much interest in using social evolution theory to predict the damage to a host from parasite infection, termed parasite virulence. Most of this work has focused on how high kinship between the parasites infecting a host can select for more prudent exploitation of the host, leading to a negative relationship between virulence and parasite kinship. However, it has also been shown that if parasites can cooperate to overcome the host, then high parasite kinship within hosts can select for greater cooperation and higher growth rates, hence leading to a positive relationship between virulence and parasite kinship. We examine the impact of a spiteful behaviour, chemical (bacteriocin) warfare between microbes, on the evolution of virulence, and find a new relationship: virulence is maximized when the frequency of kin among parasites' social partners is low or high, and is minimized at intermediate values. This emphasizes how biological details can fundamentally alter the qualitative nature of theoretical predictions made by models of parasite virulence.

Bacteria↗
PubMed2004

Information constraints and the precision of adaptation: sex ratio manipulation in wasps.

Sex allocation theory offers excellent opportunities for studying the precision of adaptation. One of the best-supported areas in the field of sex allocation is Hamilton's theory of local mate competition, which predicts female-biased offspring sex ratios when populations are structured such that mating takes place locally before females disperse. As predicted by local mate competition theory, females of numerous species, especially parasitoid wasps, have been shown to lay a less female-biased sex ratio as the number of females simultaneously laying eggs on a patch increases. It has usually been assumed that this sex ratio adjustment comes through individuals adjusting their behavior directly in response to the presence of other females. Here we show that in the parasitoid wasp Nasonia vitripennis, this shift in offspring sex ratios is primarily caused by the presence of eggs laid by other females and to a lesser extent by the presence of other females. We confirm that females are behaving as predicted by theory, but the way in which they do so is not as straightforward as is often assumed. Instead, even when there are multiple females on a patch, individuals still use the cues that are more commonly associated with sex ratio adjustment in response to sequential visits to a patch by females. This result provides a possible explanation for the observed variation in N. vitripennis sex ratios. More generally, it confirms the need to consider the mechanistic basis of a behavior to understand fully its adaptive value.

Adaptation, Physiological↗
PubMed2004

Testing small clutch size models with Daphnia.

Life-history theory predicts that for small clutches, variance in egg size (between individuals) should decrease in a predictable invariant manner as clutch size increases. To test this, we studied Daphnia magna at 350 different food treatments and recorded the number of eggs and the volume of each egg for their first clutch. As predicted, we found that the relationship between clutch size and resources devoted to reproduction was linear, variance in egg volume decreased with increasing clutch size, and resources were shared relatively equally between the eggs in a clutch. However, we found that the rate at which the range of egg volumes decreased with clutch size was slower than predicted. We discuss possible explanations for this discrepancy, including a lower limit on the volume of eggs that are produced and selection for smaller eggs when food is abundant. Consistent with this, we found that mean egg volume decreased with increasing clutch size.

Animals↗
PubMed2004

Sex-ratio evolution in sex changing animals.

Sex allocation theory is often able to make clear predictions about when individuals should facultatively adjust their offspring sex ratio (proportion male) in response to local conditions, but not the consequences for the overall population sex ratio. A notable exception to this is in sex changing organisms, where theory predicts that: (1) organisms should have a sex ratio biased toward the "first" sex: (2) the bias should be less extreme in partially sex changing organisms, where a proportion of the "second" sex matures directly from the juvenile stage; and (3) the sex ratio should be more biased in protogynous (female first) than in protandrous (male first) species. We tested these predictions with a comparative study using data from 121 sex changing animal species spanning five phyla, covering fish, arthropods, echinoderms, molluscs, and annelid worms. We found support for the first and third predictions across all species. The second prediction was supported within the protogynous species (mainly fish), but not the protandrous species (mainly invertebrates).

Animals↗
PubMed2004

Learning, odour preference and flower foraging in moths.

Floral volatiles play a major role in plant-insect communication. We examined the influence of two volatiles, phenylacetaldehyde and alpha-pinene, on the innate and learnt foraging behaviour of the moth Helicoverpa armigera. In dual-choice wind tunnel tests, adult moths flew upwind towards both volatiles, with a preference for phenylacetaldehyde. When exposure to either of these volatiles was paired with a feeding stimulus (sucrose), all moths preferred the learnt odour in the preference test. This change in preference was not seen when moths were exposed to the odour without a feeding stimulus. The learnt preference for the odour was reduced when moths were left unfed for 24 h before the preference test. We tested whether moths could discriminate between flowers that differed in a single volatile component. Moths were trained to feed on flowers that were odour-enhanced using either phenylacetaldehyde or alpha-pinene. Choice tests were then carried out in an outdoor flight cage, using flowers enhanced with either volatile. Moths showed a significant preference for the flower type on which they were trained. Moths that were conditioned on flowers that were not odour-enhanced showed no preference for either of the odour-enhanced flower types. The results imply that moths may be discriminating among odour profiles of individual flowers from the same species. We discuss this behaviour within the context of nectar foraging in moths and odour signalling by flowering plants.

Acetaldehyde↗
PubMed2003

Kin discrimination and the benefit of helping in cooperatively breeding vertebrates.

In many cooperatively breeding vertebrates, a dominant breeding pair is assisted in offspring care by nonbreeding helpers. A leading explanation for this altruistic behavior is Hamilton's idea that helpers gain indirect fitness benefits by rearing relatives (kin selection). Many studies have shown that helpers typically provide care for relatives, but relatively few have shown that helpers provide closer kin with preferential care (kin discrimination), fueling the suggestion that kin selection only poorly accounts for the evolution of cooperative breeding in vertebrates. We used meta-analysis to show that (i) individuals consistently discriminate between kin, and (ii) stronger discrimination occurs in species where the benefits of helping are greater. These results suggest a general role for kin selection and that the relative importance of kin selection varies across species, as predicted by Hamilton's rule.

Altruism↗
PubMed2003

Host sanctions and the legume-rhizobium mutualism.

Explaining mutualistic cooperation between species remains one of the greatest problems for evolutionary biology. Why do symbionts provide costly services to a host, indirectly benefiting competitors sharing the same individual host? Host monitoring of symbiont performance and the imposition of sanctions on 'cheats' could stabilize mutualism. Here we show that soybeans penalize rhizobia that fail to fix N(2) inside their root nodules. We prevented a normally mutualistic rhizobium strain from cooperating (fixing N(2)) by replacing air with an N(2)-free atmosphere (Ar:O(2)). A series of experiments at three spatial scales (whole plants, half root systems and individual nodules) demonstrated that forcing non-cooperation (analogous to cheating) decreased the reproductive success of rhizobia by about 50%. Non-invasive monitoring implicated decreased O(2) supply as a possible mechanism for sanctions against cheating rhizobia. More generally, such sanctions by one or both partners may be important in stabilizing a wide range of mutualistic symbioses.

Argon↗
PubMed2003

Cooperation, virulence and siderophore production in bacterial parasites.

Kin selection theory predicts that the damage to a host resulting from parasite infection (parasite virulence) will be negatively correlated to the relatedness between parasites within the host. This occurs because a lower relatedness leads to greater competition for host resources, which favours rapid growth to achieve greater relative success within the host, and that higher parasite growth rate leads to higher virulence. We show that a biological feature of bacterial infections can lead to the opposite prediction: a positive correlation between relatedness and virulence. This occurs because a high relatedness can favour greater (cooperative) production of molecules that scavenge iron (siderophores), which results in higher growth rates and virulence. More generally, the same underlying idea can predict a positive relationship between relatedness and virulence in any case where parasites can cooperate to increase their growth rate; other examples include immune suppression and the production of biofilms to aid colonization.

Animals↗
PubMed2003

Toxoplasma gondii, sex and premature rejection.

Adaptive sex ratio theory explains why gametocyte sex ratios are female-biased in many populations of apicomplexan parasites such as Plasmodium and Toxoplasma. Recently, Ferguson has criticized this framework and proposed two alternative explanations--one for vector-borne parasites (e.g. Plasmodium) and one for Toxoplasma. Ferguson raises some interesting issues that certainly deserve more empirical attention. However, it should be pointed out that: (1) there are theoretical and empirical problems for his alternative hypotheses; and (2) existing empirical data support the application of sex ratio theory to these parasites, not its rejection.

Animals↗
PubMed2003

Darwinian agriculture: when can humans find solutions beyond the reach of natural selection?

Progress in genetic improvement of crop yield potential has slowed since 1985. Simultaneously, more sustainable management of agricultural ecosystems is needed. A better understanding of natural selection can help solve both problems. We illustrate this point with two specific examples. First, the genetic legacy of crop plants has been refined by millions of years of natural selection, often driven by competition among plants. We therefore suggest that most simple, tradeoff-free options to increase competitiveness (e.g., increased gene expression, or minor modifications of existing plant genes) have already been tested by natural selection. Further genetic improvement of crop yield potential over the next decade will mainly involve tradeoffs, either between fitness in past versus present environments, or between individual competitiveness and the collective performance of plant communities. Eventually, we may develop the ability to predict the consequences of genetic alterations so radical that they have not yet been tested by natural selection. Second, natural selection acts mainly at the level of genes, individuals, and family groups, rather than ecosystems as a whole. Consequently, there is no reason to expect the structure of natural ecosystems (diversity, spatial, or temporal patterns) to be a reliable blueprint for agricultural ecosystems. Natural ecosystems are nonetheless an important source of information that could be used to improve agriculture.

Agriculture↗
PubMed2003

Testing for epistasis between deleterious mutations in a parasitoid wasp.

Determining the way in which deleterious mutations interact to effect fitness is crucial to numerous areas in evolutionary biology. For example, if each additional mutation leads to a greater decrease in log fitness than the last, termed synergistic epistasis, then sex and recombination provide an advantage because they enable deleterious mutations to be eliminated more efficiently. However, there is a severe shortage of relevant empirical data, especially of the form that can help test mutational explanations for the widespread occurrence of sex. Here, we test for epistasis in the parasitic wasp Nasonia vitripennis, examining the fitness consequences of chemically induced deleterious mutations. We examine two components of fitness, both of which are thought to be important in natural populations of parasitic wasps: longevity and egg production. Our results show synergistic epistasis for longevity, but not for egg production.

Animals↗
PubMed2002

Sanctions and mutualism stability: why do rhizobia fix nitrogen?

Why do rhizobia expend resources on fixing N(2) for the benefit of their host plant, when they could use those resources for their own reproduction? We present a series of theoretical models which counter the hypotheses that N(2) fixation is favoured because it (i) increases the exudation of useful resources to related rhizobia in the nearby soil, or (ii) increases plant growth and therefore the resources available for rhizobia growth. Instead, we suggest that appreciable levels of N(2) fixation are only favoured when plants preferentially supply more resources to (or are less likely to senesce) nodules that are fixing more N(2) (termed plant sanctions). The implications for different agricultural practices and mutualism stability in general are discussed.

Fabaceae↗
PubMed2002

Inbreeding and parasite sex ratios.

The breeding system of parasitic protozoa affects the evolution of drug resistance and virulence, and is relevant to disease diagnosis and the development of chemo- and immunotherapy. A major group of protozoan parasites, the phylum Apicomplexa, that includes the aetiological agents of malaria, toxoplasmosis and coccidiosis, all have dimorphic sexual stages. The sex ratio (proportion of males produced by parasites) is predicted to depend upon the inbreeding rate, and it has been suggested that sex-ratio data offer a relatively cheap and easy method for indirectly estimating inbreeding rates. Here, we exploit a new theoretical machinery to show that there are generally valid relationships between f, Wright's coefficient of inbreeding, and sex ratio, z(*), the generality being with respect to population structure. To focus the discussion, we concentrate on malaria and show that the previously derived result, f = 1 - 2z(*), does not depend on the artificial assumptions about population structure that were previously made. Not only does this justify the use of sex ratio as an indirect measure of f, but also we argue that it may actually be preferable to measure f by measuring sex ratios, rather than by measuring departures from Hardy-Weinberg genotypic proportions both in malaria and parasites more generally.

Animals↗
PubMed2002

Cooperation and competition between relatives.

Individuals are predicted to behave more altruistically and less competitively toward their relatives, because they share a relatively high proportion of their genes (e.g., one-half for siblings and one-eighth for cousins). Consequently, by helping a relative reproduce, an individual passes its genes to the next generation, increasing their Darwinian fitness. This idea, termed kin selection, has been applied to a wide range of phenomena in systems ranging from replicating molecules to humans. Nevertheless, competition between relatives can reduce, and even totally negate, the kin-selected benefits of altruism toward relatives. Recent theoretical work has clarified the processes and selective forces underlying this effect and has demonstrated the generality of the effect of competition between relatives.

Altruism↗
PubMed2002

A general model for host plant selection in phytophagous insects.

We develop a general theoretical framework for exploring the host plant selection behaviour of herbivorous insects. This model can be used to address a number of questions, including the evolution of specialists, generalists, preference hierarchies, and learning. We use our model to: (i) demonstrate the consequences of the extent to which the reproductive success of a foraging female is limited by the rate at which they find host plants (host limitation) or the number of eggs they carry (egg limitation); (ii) emphasize the different consequences of variation in behaviour before and after landing on (locating) a host (termed pre- and post-alighting, respectively); (iii) show that, in contrast to previous predictions, learning can be favoured in post-alighting behaviour--in particular, individuals can be selected to concentrate oviposition on an abundant low-quality host, whilst ignoring a rare higher-quality host; (iv) emphasize the importance of interactions between mechanisms in favouring specialization or learning.

Animals↗
PubMed2002

Constraints in the evolution of sex ratio adjustment.

When the relative fitness of male and female offspring varies with environmental conditions, evolutionary theory predicts that parents should adjust the sex of their offspring accordingly. Qualitative and even quantitative support for this prediction is striking in some taxa but much less convincing in others. Explaining such variation across taxa in the fit of sex ratio theory remains a major challenge. We use meta-analysis to test the role of two constraints in the evolution of sex ratios. Based on analysis of sex ratio skews in birds and wasps, we show that (i) mechanisms of sex determination do not necessarily constrain the evolution of sex ratio adjustment, and (ii) parental ability to predict their offsprings' environment influences the evolution of sex ratio patterns across taxa. More generally, our results show that multiple constraints may determine the precision of adaptation.

Adaptation, Physiological↗
PubMed2002

Fertility insurance and the sex ratios of malaria and related hemospororin blood parasites.

The sex ratio (z*; proportion of gametocytes that are male) of malaria and related hemospororin blood parasites has been predicted to be related to the inbreeding rate (f) by the simple equation z* = (1 - f)/2. Although there is some empirical support for this prediction, there are several cases where the sex ratio is less female biased or more variable than expected. Here, we present a theoretical model that may be able to explain some of these discrepancies. We show that if low gametocyte densities lead to a danger that female gametes may not encounter any male gametes, then natural selection favors a less female-biased sex ratio as a form of 'fertility insurance' to ensure that female gametes are mated. This model can be applied to a number of situations. In particular, (1) empirical data suggest that the number of gametocyes per blood meal can be low enough to favor fertility insurance in some Plasmodium infections in humans and (2) our model predicts facultative shifting toward less-biased sex ratios in response to immune pressure that reduces gametocyte or gamete survival or mobility, consistent with some recent experimental data from Plasmodium species of birds and mice.

Animals↗