Correlation between Olfactory Responses, Dispersal Tendencies, and Life-history Traits of the Predatory Mite Neoseiulus Womersleyi (Acari: Phytoseiidae) of Eight Local Populations

To investigate the relationship between foraging behavior and life-history traits of the predatory mite Neoseiulus womersleyi, the olfactory responses, dispersal ratios from a prey patch, predation rates, fecundity, and developmental times in eight local populations of N. womersleyi were investigated. Significant differences among local populations were found in all these traits except fecundity. None of the life-history traits correlated with foraging behavior. A significant positive correlation was found only between the olfactory response and the dispersal ratio. These results suggested that predatory mites with low olfactory responses would stay in a prey patch longer than predatory mites with high olfactory responses.     

Selection on olfactory response to semiochemicals from a plant-host complex in a parasitic wasp

Parasitic wasps (parasitoids) use volatiles from the plants infested by phytophagous insects to locate host herbivores, but their behavioural response to such semiochemicals is highly variable. Bi-directional selection on Cotesia glomerata (Hymenoptera: Braconidae) was conducted to investigate the importance of genetic variation in the olfactory response of parasitoids. Female wasps were assessed for flight orientation and landing success in response to the hexane extract from a plant-host complex in a wind tunnel. After the first generation of selection, two strains significantly differentiated in both flight orientation and landing success, and their divergence continued with further selection. The two selected strains genetically differentiated in olfactory perception rather than upwind flight ability. The realized heritability was estimated as 0.248 for flight orientation and 0.216 for landing success. The selection experiment further demonstrated that a prior exposure to the semiochemicals significantly enhanced the subsequent response of female wasps, independent of genetic differences. These results suggest that both genetic component and environmental conditioning have played an important role in the evolution of host selection and utilization by the parasitoid in a tritrophic system.     

Immature performance linked with exaggeration of a sexually selected trait in an armed beetle

Exaggerated traits can be costly and are often trade-off against other characters, such as life-history traits. Thus, the evolution of an exaggerated trait is predicted to affect male life-history strategies. However, there has been very little experimental evidence of the impact of the evolution of sexually selected traits on life-history traits. This study investigated whether increased investment in exaggerated traits can generate evolutionary changes in the life-history strategy for armed males. Male flour beetles, Gnatocerus cornutus, have enlarged mandibles that are used in male-male competition, but females lack this character exaggeration completely. We subjected these weapons to 11 generations of bidirectional selection and found a correlated response in pupal survival but not in larval survival or adult longevity in the male. That is, selecting for male mandibles negatively impacted survival during the production of mandibles. There is no correlated response in the life-history traits of the female.     

Response of a complex foraging phenotype to artificial selection on its component traits

In nature, where predators must often track dynamic and dispersed prey populations, predator consumption rate, conversion efficiency, dispersal, and prey finding are likely to be important links between foraging and predator–prey population dynamics. Small differences in predator foraging caused by variation in any of the abovementioned traits might lead to significant differences in predator success as well as population dynamics. We used artificial selection to create lines of the predatory mite, Phytoseiulus persimilis in order to determine the potential for or constraints on the evolution of predator foraging behaviors. All four foraging traits demonstrated considerable phenotypic variation. They also exhibited significant realized heritabilities after artificial selection, except that prey finding did not respond to downward selection. Lines that responded to selection did so rapidly, and high-consumption, high-conversion efficiency, and high- and low-dispersal were stable for at least four generations after artificial selection was relaxed. There were some indirect responses to selection among the foraging traits. For example, there was positive correlation between consumption and dispersal. However, none of the correlated responses were of the magnitude of the direct responses we measured on the same trait. We also observed some correlations between foraging traits and life-history traits such as low-consumption and development time (negative), high-consumption and fecundity (positive), and high-conversion efficiency and fecundity (positive), but these were more likely to represent non-genetic constraints. Intrinsic rates of increase in low-consumption and low-conversion efficiency lines were lower than in their respective high lines and the unselected control, whereas rates of increase in dispersal and olfactory response lines did not differ from the unselected control. Thus, traits that make up foraging share partially overlapping genetic architectures with highly heritable phenotypic components, suggesting that each foraging trait will be able to respond rapidly to changes in the density and distribution of resources.     

Coevolution between parasite virulence and host life-history traits

Epidemiological models generally explore the evolution of parasite life-history traits, namely, virulence and transmission, against a background of constant host life-history traits. However, life-history models have predicted the evolution of host traits in response to parasitism. The coevolution of host and parasite life-history traits remains largely unexplored. We present an epidemiological model, based on resource allocation theory, that provides an analysis of the coevolution between host reproductive effort and parasite virulence. This model allows for hosts with either a fixed (i.e., genetic) or conditional (i.e., a phenotypically plastic) response to parasitism. It also considers superinfections. We show that parasitism always favors increased allocation to host reproduction, but because of epidemiological feedbacks, the evolutionarily stable host reproductive effort does not always increase with parasite virulence. Superinfection drives the evolution of parasite virulence and acts on the evolution of the host through parasite evolution, generally leading to higher host reproductive effort. Coevolution, as opposed to cases where only one of the antagonists evolves, may generate correlations between host and parasite life-history traits across environmental gradients affecting the fecundity or the survival of the host. Our results provide a theoretical framework against which experimental coevolution outcomes or field observations can be contrasted.     

Sugar in moderation: sugar diets affect short‐term parasitoid behaviour

The biological control potential of parasitic wasps in the field is expected to increase with the provisioning of sugar sources, which increase longevity and replenish carbohydrate reserves. Apanteles aristoteliae is an important parasitoid of Argyrotaenia franciscana, the orange tortrix, an economic pest in fruit crops. In the present study, the effect of sugar diet on the physiological status of A. aristoteliae is investigated in the laboratory, as well as the effects of nutritional status on short‐term olfactory orientation and parasitism behaviour, and the association between olfactory orientation and immediate parasitism activity. Levels of glycogen, fructose, total sugars, proportional weight gain and volume consumed are higher among females fed 25% sucrose solution than 10% sucrose solution, and lowest for those fed water. Sugar feeding also affects behaviour: wasps with higher levels of fructose or weight gain have a lower probability of making a choice in the olfactometer. Wasps with intermediate fructose levels or weight gain have a higher probability of orienting towards hosts than wasps with low or high levels. Among wasps that make a choice in the olfactometer, wasps fed 10% or 25% sucrose significantly prefer host versus food cues, whereas starved wasps are just as likely to select food cues as host cues.     

RESPONSES TO SELECTION AMONG LIFE-HISTORY TRAITS IN A NONMIGRATORY POPULATION OF MILKWEED BUGS (ONCOPELTUS FASCIATUS)

Genetic parameters were assessed in the nonmigratory Puerto Rico population of the milkweed bug, Oncopeltus fasciatus, and compared with parameters estimated in a migratory population from Iowa (Palmer and Dingle, 1986). Offspring-parent regression analysis provided initial estimates of heritabilities and phenotypic and genetic correlations among wing length, head-capsule width, female age at first reproduction, fecundity for the first and second five days of reproduction by females, and clutch size for the first and second five days of reproduction by females. Replicated bidirectional selection for wing length was then imposed, with a direct response to selection revealing substantial additive genetic variance for this trait, as was also the case with the Iowa population. Assays for correlated response to selection yielded two further similarities to Iowa: a positive response in head-capsule width and no consistent response in age at first reproduction. In contrast to the results with Iowa bugs, neither regression analysis nor selection revealed statistically significant genetic correlations between fecundity measures and those of other traits. In both populations the potential exists for body-size characters to evolve together independently of age at first reproduction; but in the nonmigratory Puerto Rico bugs, fecundity does not contribute to a life-history syndrome involving genetic correlations among these traits.     

Ambulatory dispersal in <Emphasis Type="Italic">Tetranychus urticae</Emphasis>: an artificial selection experiment on propensity to disperse yields no response

Dispersal to new hosts is an important process for an invasive herbivore, such as the two-spotted spider mite. A recent study, using artificial selection experiments, has suggested that genetic variation and genetic trade-offs are present for propensity to disperse in this species. However, due to the experimental setup alternative explanations for the response to selection could not be ruled out. Using an altered setup, we investigated whether the propensity for ambulatory dispersal differs genetically between individuals and whether genetic correlations with life-history traits exist. Upward and downward selection on propensity to leave the colony was performed for seven generations in four replicate artificial selection experiments and the results were compared to control lines. No consistent responses to selection were found and no significant effect on life-history traits (oviposition rate, juvenile survival, development rate and number of adult offspring) or sex ratio was present across the replicates. The data suggest that our base population of spider mites harbours at best a low amount of additive genetic variation for this behaviour.     

Intra-Population Genetic Variation in the Temporal Pattern of Egg Maturation in a Parasitoid Wasp

Parasitoid wasps are taxonomically and biologically extremely diverse. A conceptual framework has recently been developed for understanding life-history evolution and diversification in these animals, and it has confirmed that each of two linked life-history traits – the mode of larval development and the temporal pattern of egg maturation – acts as an organiser of life-history. The framework has been predicated on the assumption that there exists sufficient genetic variation in the latter trait to allow it to be shaped by natural selection. Focusing on the parasitoid wasp Trichogramma brassicae, our aim was to test the validity of that assumption, using established quantitative genetic methods. We demonstrate the existence of a statistically significant degree of intra-population polygenic variation in the temporal pattern of egg production within the wasp population we studied. Furthermore, our results, together with published data on clinal variation in the egg maturation pattern of another species, suggest that intra-specific evolutionary shifts in the temporal pattern of egg maturation of parasitoid wasps can result from a change in allocation to egg production either before, or very shortly after adult emergence, without there being an accompanying change in lifetime fecundity. As well as opening new avenues of research into the reproductive strategies, behaviour, community organisation and biological control potential of parasitoid wasps, this discovery also has implications for studies of life-history evolution and diversification in insects generally.     

Quantitative Genetic Variation of Odor-Guided Behavior in a Natural Population of Drosophila Melanogaster

Quantitative genetic variation in behavioral response to the odorant, benzaldehyde, was assessed among a sample of 43 X and 35 third chromosomes extracted from a natural population and substituted into a common inbred background. Significant genetic variation among chromosome lines was detected. Heritability estimates for olfactory response, however, were low, as is typical for traits under natural selection. Furthermore, the loci affecting naturally occurring variation in olfactory response to benzaldehyde were not the same in males and females, since the genetic correlation between the sexes was low and not significantly different from zero for the chromosome 3 lines. Competitive fitness, viability and fertility of the chromosome 3 lines were estimated using the balancer equilibrium technique. Genetic correlations between fitness and odor-guided behavior were not significantly different from zero, suggesting the number of loci causing variation in olfactory response is small relative to the number of loci causing variation in fitness. Since different genes affect variation in olfactory response in males and females, genetic variation for olfactory response could be maintained by genotype X sex environment interaction. This unusual genetic architecture implies that divergent evolutionary trajectories for olfactory behavior may occur in males and females.     

Genetic analysis of life-history constraint and evolution in a wild ungulate population

Trade-offs among life-history traits are central to evolutionary theory. In quantitative genetic terms, trade-offs may be manifested as negative genetic covariances relative to the direction of selection on phenotypic traits. Although the expression and selection of ecologically important phenotypic variation are fundamentally multivariate phenomena, the in situ quantification of genetic covariances is challenging. Even for life-history traits, where well-developed theory exists with which to relate phenotypic variation to fitness variation, little evidence exists from in situ studies that negative genetic covariances are an important aspect of the genetic architecture of life-history traits. In fact, the majority of reported estimates of genetic covariances among life-history traits are positive. Here we apply theory of the genetics and selection of life histories in organisms with complex life cycles to provide a framework for quantifying the contribution of multivariate genetically based relationships among traits to evolutionary constraint. We use a Bayesian framework to link pedigree-based inference of the genetic basis of variation in life-history traits to evolutionary demography theory regarding how life histories are selected. Our results suggest that genetic covariances may be acting to constrain the evolution of female life-history traits in a wild population of red deer Cervus elaphus: genetic covariances are estimated to reduce the rate of adaptation by about 40%, relative to predicted evolutionary change in the absence of genetic covariances. Furthermore, multivariate phenotypic (rather than genetic) relationships among female life-history traits do not reveal this constraint.     

Heritability of life-history tactics and genetic correlation with body size in a natural population of brook charr (Salvelinus fontinalis)

A common dimorphism in life-history tactic in salmonids is the presence of an anadromous pathway involving a migration to sea followed by a freshwater reproduction, along with an entirely freshwater resident tactic. Although common, the genetic and environmental influence on the adoption of a particular life-history tactic has rarely been studied under natural conditions. Here, we used sibship-reconstruction based on microsatellite data and an 'animal model' approach to estimate the additive genetic basis of the life-history tactic adopted (anadromy vs. residency) in a natural population of brook charr, Salvelinus fontinalis. We also assess its genetic correlation with phenotypic correlated traits, body size and body shape. Significant heritability was observed for life-history tactic (varying from 0.52 to 0.56 depending on the pedigree scenario adopted) as well as for body size (from 0.44 to 0.50). There was also a significant genetic correlation between these two traits, whereby anadromous fish were genetically associated with bigger size at age 1 (r(G) = -0.52 and -0.61). Our findings thus indicate that life-history tactics in this population have the potential to evolve in response to selection acting on the tactic itself or indirectly via selection on body size. This study is one of the very few to have successfully used sibship-reconstruction to estimate quantitative genetic parameters under wild conditions.     

The genetic covariance among clinal environments after adaptation to an environmental gradient in Drosophila serrata

We examined the genetic basis of clinal adaptation by determining the evolutionary response of life-history traits to laboratory natural selection along a gradient of thermal stress in Drosophila serrata. A gradient of heat stress was created by exposing larvae to a heat stress of 36 degrees for 4 hr for 0, 1, 2, 3, 4, or 5 days of larval development, with the remainder of development taking place at 25 degrees. Replicated lines were exposed to each level of this stress every second generation for 30 generations. At the end of selection, we conducted a complete reciprocal transfer experiment where all populations were raised in all environments, to estimate the realized additive genetic covariance matrix among clinal environments in three life-history traits. Visualization of the genetic covariance functions of the life-history traits revealed that the genetic correlation between environments generally declined as environments became more different and even became negative between the most different environments in some cases. One exception to this general pattern was a life-history trait representing the classic trade-off between development time and body size, which responded to selection in a similar genetic fashion across all environments. Adaptation to clinal environments may involve a number of distinct genetic effects along the length of the cline, the complexity of which may not be fully revealed by focusing primarily on populations at the ends of the cline.     

Life-history traits in closely related secondary parasitoids sharing the same primary parasitoid host: evolutionary opportunities and constraints

Thus far, few studies have compared life-history traits amongst secondary parasitoids attacking and developing in cocoons of their primary parasitoid hosts. This study examines development and reproduction in Lysibia nana Gravenhorst and Acrolyta nens Hartig (both Hymenoptera: Ichneumonidae), two related and morphologically similar secondary parasitoids that attack pupae of the gregarious endoparasitoid, Cotesia glomerata L. (Hymenoptera: Braconidae). On black mustard, Brassica nigra L. (Brassicaceae) plants in a field plot, adults of L. nana and A. nens frequently emerged from the same cocoon broods of C. glomerata . Based on similarities in their phylogeny and morphology, it was hypothesized that both species would exhibit considerable overlap in other life-history traits. In both L. nana and A. nens , adult wasp size increased with host cocoon mass at parasitism, although L. nana wasps were slightly larger than A. nens wasps, and completed their development earlier. Adult females of both species emerged with no eggs but matured eggs at similar rates over the following days. When provided with 20 host cocoons daily, fecundity in female L. nana was slightly more skewed towards early life than in A. nens , although lifetime fecundity did not differ between the two species. Longevity was significantly reduced in females of both species that were provided with hosts. Both parasitoids were found to exhibit strong similarities in life-history and development traits and in their ecological niche, thereby supporting our general hypothesis. Competition between L. nana and A. nens is presumably diffused because their preferred host ( C. glomerata ) is relatively abundant in open habitats.     

Genetic correlations do not constrain the evolution of sexual dimorphism in the moss Ceratodon purpureus

The trajectory of phenotypic evolution is constrained in the short term by genetic correlations among traits. However, the extent to which genetic correlations impose a lasting constraint is generally unknown. Here, I examine the genetic architecture of life-history variation in male and female gametophytes from two populations of the moss Ceratodon purpureus, focusing on genetic correlations within and between the sexes. A significant negative correlation between allocation to vegetative and reproductive tissue was evident in males of both populations, but not females. All traits showed between-sex correlations of significantly less than one, indicating additive genetic variance for sexual dimorphism. The degree of dimorphism for traits was significantly negatively associated with the strength of the between-sex correlation. The structure of genetic correlations among life-history traits was more divergent between the two populations in females than in males. Collectively, these results suggest that genetic correlations do not impose a lasting constraint on the evolution of life-history variation in the species.     

THE POTENTIAL FOR COEVOLUTION IN A HOST-PARASITOID SYSTEM. I. GENETIC VARIATION WITHIN AN APHID POPULATION IN SUSCEPTIBILITY TO A PARASITIC WASP

For coevolution to occur, there must be genetic variation in each species for traits relevant to their interaction. Here, statistically significant variation in susceptibility to a parasitic wasp was found among pea-aphid clones collected from a single population. In a subset of clones that was tested further, wasps were found to oviposit in aphids from both resistant and susceptible lines, but eggs failed to develop in resistant hosts. Significant genetic variance in susceptibility provides evidence that this aphid population has the potential to evolve resistance in response to selection by one of its major natural enemies. Predictions of an expected response to selection based on the experimental measures of variation and field parasitism rates suggested that there should be a detectable change in susceptibility over the course of a season. However, an experimental comparison of mean susceptibility of clones collected early and late in the summer, a period of several generations, revealed no response to selection by the wasps. Aphids collected late in the season were as susceptible, on the average, as those collected early in the summer. Possible constraints on the response of the aphids to selection by this natural enemy are considered.     

Differential fitness effects of immunocompetence and neighbourhood density in alternative female lizard morphs

1. A growing number of studies demonstrate that natural selection acts on traits important in whole animal performance and physiology. 2. Here we describe a heritable polymorphism in female dorsal pattern in the lizard Anolis sagrei (Dumeril & Bibron 1837). 3. Morphs did not differ in body size or habitat use (perch diameter), however, we show that the social environment, estimated by the number of female neighbours, had different selective effects on alternative morphs in nature. 4. We show that morphs displayed a significantly different immune response to phytohaemagglutinin. Furthermore, natural selection differentially acted on combinations of female morph and immunocompetence, favouring high levels of immune function in one morph and low levels of immune function in the other. 5. We discuss the possibility that morph-specific investments in life-history traits may lead to correlational selection between traits, even when those traits are likely to be determined by different genetic loci.     

OPTIMIZATION MODELS,QUANTITATIVE GENETICS,AND MUTATION

The process of selection on a multivariate set of characters subject to functional constraints is considered from the points of view of both evolutionary optimization theory and quantitative genetics. Special attention is given to life-history characteristics. It is shown that, under suitable conditions (including weak selection), useful approximate formulas for the relations between the functional constraints and the additive genetic variance-covariance matrix can be derived. These can be used to show that the conditions for equilibrium under selection according to the two different approaches are approximately equivalent. Although large negative genetic correlations are to be expected between some pairs of life-history traits in populations at equilibrium under selection, in general some small negative genetic correlations and some positive genetic correlations will also be present. Thus, the observation of a positive genetic correlation between a pair of life-history traits does not necessarily refute the possibility of trade-offs among a multivariate set of traits that contains the pair in question. The relation between the pattern of functional constraints and the genetic correlations is often complex, and little insight into the former can be derived from the latter. The effects of mutations that lower the overall efficiency of resource utilization, thereby creating a positive component to the genetic covariances among life-history traits, are also considered for a specific model. Although such mutations can have a substantial effect on the form of the life history, extreme conditions seem to be needed for them to produce a large effect on the pattern of genetic correlations in a random-mating population. They can, however, cause the appearance of positive correlations following inbreeding, due to the exposure of deleterious recessive or partially recessive mutations. The analysis also suggests that the population means of individual components of a constrained multivariate system may often equilibrate at values that are far from the optima that would be attained if they were selected in isolation from the other members of the system.     

ON THE LOW HERITABILITY OF LIFE-HISTORY TRAITS

Life-history traits such as longevity and fecundity often show low heritability. This is usually interpreted in terms of Fisher's fundamental theorem to mean that populations are near evolutionary equilibrium and genetic variance in total fitness is low. We develop the causal relationship between metric traits and life-history traits to show that a life-history trait is expected to have a low heritability whether or not the population is at equilibrium. This is because it is subject to all the environmental variation in the metric traits that affect it plus additional environmental variation. There is no simple prediction regarding levels of additive genetic variance in life-history traits, which may be high at equilibrium. Several other patterns in the inheritance of life-history traits are readily predicted from the causal model. These include the strength of genetic correlations between life-history traits, levels of nonadditive genetic variance, and the inevitability of genotype-environment interaction.     

SEX RATIO VARIATION IN A PARASITIC WASP II. DIALLEL CROSS

Sex ratio has been studied from many theoretical and empirical perspectives, but a general assumption in sex ratio research is that changes in sex ratio occur because of selection on sex ratio itself. I carried out a quantitative genetic experiment—a diallel cross among three strains—on a parasitic wasp, Muscidifurax raptor (Hymenoptera: Pteromalidae), to measure genetic variation for sex ratio. I also tested whether sex ratio may change as a consequence of selection on other life-history traits by estimating genetic covariances between sex ratio, fecundity, longevity, and development time. Most of the variation among strains could be accounted for by a maternal effect, likely caused by a microsporidian parasite that was transmitted through the West Germany (WG) strain. Genetic variation was small by comparison, but almost all traits were affected by dominance. The only significant additive genetic effect was for fecundity early in life. Upon crossing, all traits displayed heterosis: more female-biased sex ratio, greater fecundity, longer life, and faster development time. All life-history traits were correlated phenotypically, but the correlations were mainly the result of decreased performance in crosses with the WG strain that carried the microsporidian parasite. Dominance genetic correlations were also found between sex ratio, fecundity, and longevity. How the correlation between sex ratio and other life-history traits would affect sex ratio evolution depends upon the frequencies of sex-ratio genotypes within a population as well as the signs of the correlations, because sex ratio is under frequency-dependent selection whereas other traits are generally under directional selection. Although the results from crosses among laboratory populations should be approached with caution, the inbreeding depression (the difference between inbred and outcrossed progeny) found in M. raptor implies that the evolution of a female-biased sex ratio could be affected by selection for inbreeding avoidance.