Sex ratios under asymmetrical local mate competition in the parasitoid wasp Nasonia vitripennis

Sex ratio theory has proved remarkably useful in testing theadaptive nature of animal behavior. A particularly productivearea in this respect is Hamilton's theory of local mate competition(LMC), which has been extended in numerous directions to includegreater biological realism, allowing more detailed tests inspecific organisms. We have presented one such extension, termedasymmetrical LMC, which occurs when egg laying by females ona patch is asynchronous, and emerging males do not disperse,resulting in the extent of LMC on a patch varying over time.Our aim here is to test whether the parasitoid wasp Nasoniavitripennis responds to variation in the degree of asymmetricalLMC. Specifically, we show that females adjust their offspringsex ratios in response to (1) variation in the amount of asynchronyin emergence between broods on a patch and (2) the number andproportion of previously parasitized hosts on the patch. Ourresults provide qualitative support for the predictions of theory,suggesting new levels of complexity in the sex ratio behaviorof this much-studied organism. However, our results do not alwaysprovide quantitative support for theory, suggesting furthercomplexities that must be clarified.     

HTLV-I protease cleavage of P19/24 substrates is not dependent on NaCl concentration

Understanding the factors that affect the activity of Human T-cell Leukemia Virus type I (HTLV-I) protease is essential for the discovery of inhibitors to be used for the treatment of HTLV-I infection, but little has been reported on the protease to date. Here we report the production of HTLV-I protease in purified yields greater than 150 mg/L, determination of its extinction coefficient, and determination of the optimum conditions for cleavage of the p19/24 substrates (DABCYL)-(GABA)-PQVL-Nph-VMH-(EDANS), (DABSYL)-(GABA)-PQVL-Nph-VMH-(EDANS), and (DABSYL)-(GABA)-PQVLPVMH-(EDANS). The highest activity was found at pH 5.2-5.3 and 37 degrees C. There was no effect on activity upon change in sodium chloride concentration from 0 to 1500 mM. The values of K(m) and k(cat) for cleavage of these substrates by the protease with and without the histidine tag were determined.     

Genetic structure of natural Nasonia vitripennis populations: validating assumptions of sex-ratio theory

The parasitic wasp Nasonia vitripennis has been used extensively in sex allocation research. Although laboratory experiments have largely confirmed predictions of local mate competition (LMC) theory, the underlying assumptions of LMC models have hardly been explored in nature. We genotyped over 3500 individuals from two distant locations (in the Netherlands and Germany) at four polymorphic microsatellite loci to validate key assumptions of LMC theory, in terms of both the original models and more recent extensions to them. We estimated the number of females contributing eggs to patches of hosts and the clutch sizes as well as sex ratios produced by individual foundresses. In addition, we evaluated the level of inbreeding and population differentiation. Foundress numbers ranged from 1 to 7 (average 3.0 ± 0.46 SE). Foundresses were randomly distributed across the patches and across hosts within patches, with few parasitizing more than one patch. Of the hosts, 40% were parasitized by more than one foundress. Clutch sizes of individual foundresses (average 9.99 ± 0.51 SE) varied considerably between hosts. The time period during which offspring continued to emerge from a patch or host correlated strongly with foundress number, indicating that sequential rather than simultaneous parasitism is the more common. Genetic differentiation at the regional level between Germany and the Netherlands, as estimated by Slatkin's private allele method (0.11) and Hedrick's corrected G' _LT (0.23), indicates significant substructuring between regions. The level of population inbreeding for the two localities ( F _IL = 0.168) fitted the expectation based on the average foundress number per patch.     

The quantitative genetic basis of polyandry in the parasitoid wasp, Nasonia vitripennis

Understanding the evolution of female multiple mating (polyandry) is crucial for understanding sexual selection and sexual conflict. Despite this interest, little is known about its genetic basis or whether genetics influences the evolutionary origin or maintenance of polyandry. Here, we explore the quantitative genetic basis of polyandry in the parasitoid wasp Nasonia vitripennis, a species in which female re-mating has been observed to evolve in the laboratory. We performed a quantitative genetic experiment on a recently collected population of wasps. We found low heritabilities of female polyandry (re-mating frequency after 18 h), low heritability of courtship duration and a slightly higher heritability of copulation duration. However, the coefficients of additive genetic variance for these traits were all reasonably large (CV(A)>7.0). We also found considerable dam effects for all traits after controlling for common environment, suggesting either dominance or maternal effects. Our work adds to the evidence that nonadditive genetic effects may influence the evolution of mating behaviour in Nasonia vitripennis, and the evolution of polyandry more generally.     

Sex allocation theory reveals a hidden cost of neonicotinoid exposure in a parasitoid wasp

Sex allocation theory has proved to be one the most successful theories in evolutionary ecology. However, its role in more applied aspects of ecology has been limited. Here we show how sex allocation theory helps uncover an otherwise hidden cost of neonicotinoid exposure in the parasitoid wasp Nasonia vitripennis. Female N. vitripennis allocate the sex of their offspring in line with Local Mate Competition (LMC) theory. Neonicotinoids are an economically important class of insecticides, but their deployment remains controversial, with evidence linking them to the decline of beneficial species. We demonstrate for the first time to our knowledge, that neonicotinoids disrupt the crucial reproductive behaviour of facultative sex allocation at sub-lethal, field-relevant doses in N. vitripennis. The quantitative predictions we can make from LMC theory show that females exposed to neonicotinoids are less able to allocate sex optimally and that this failure imposes a significant fitness cost. Our work highlights that understanding the ecological consequences of neonicotinoid deployment requires not just measures of mortality or even fecundity reduction among non-target species, but also measures that capture broader fitness costs, in this case offspring sex allocation. Our work also highlights new avenues for exploring how females obtain information when allocating sex under LMC.     

The extent and costs of reproductive interference among four species of true bug

Reproductive interference arises when individuals of one species engage in reproductive activities with individuals of another, leading to fitness costs in one or both species. Reproductive interference (RI) therefore has two components. First, there must be mis-directed mating interactions. Second, there must be costs associated with these mis-directed interactions. Here we consider RI between four species of true bug in the family Lygaeidae, focusing in particular on the fitness consequences to Lygaeus equestris. The species we consider vary in their relationships with each other, including species in the same or different genus, and with or without natural overlap in their geographic ranges. First we show that inter-specific mating interactions, although not a certain outcome, are common enough to perhaps influence mating behaviour in these species (arising in up to 10 % of inter-specific pairings). Second, we show that reproductive interference can seriously reduce female fitness in L. equestris. Importantly, different species impose different costs of RI on L. equestris, with interactions with male Spilostethus pandurus inflicting fitness costs of similar magnitude to the costs of mating with con-specifics. On the other hand, mating interactions with male Oncopeltus fasciatus appear to have no effect on female fitness. In a follow-up experiment, when we allowed competition amongst just females of S. pandurus and L. equestris, the fitness of the latter was not reduced, arguing more strongly for the role of reproductive interference. However, in our final experiments under mass mating conditions with extended ecological interactions (including scope for competition for resources and cannibalism), the costs of RI were less apparent. Our data therefore suggest that the costs of RI will be context-specific and may act in concert with, or be swamped by, other ecological effects. We suggest that comparative studies of this sort that both mimic naturally occurring reproductive interference events, and also artificially generate new ones, will be necessary if we are to better understand the ecological and evolutionary significance of reproductive interference.     

The evolutionary ecology of the Lygaeidae

The Lygaeidae (sensu lato) are a highly successful family of true bugs found worldwide, yet many aspects of their ecology and evolution remain obscure or unknown. While a few species have attracted considerable attention as model species for the study of insect physiology, it is only relatively recently that biologists have begun to explore aspects of their behavior, life history evolution, and patterns of intra‐ and interspecific ecological interactions across more species. As a result though, a range of new phenotypes and opportunities for addressing current questions in evolutionary ecology has been uncovered. For example, researchers have revealed hitherto unexpectedly rich patterns of bacterial symbiosis, begun to explore the evolutionary function of the family's complex genitalia, and also found evidence of parthenogenesis. Here we review our current understanding of the biology and ecology of the group as a whole, focusing on several of the best‐studied characteristics of the group, including aposematism (i.e., the evolution of warning coloration), chemical communication, sexual selection (especially, postcopulatory sexual selection), sexual conflict, and patterns of host‐endosymbiont coevolution. Importantly, many of these aspects of lygaeid biology are likely to interact, offering new avenues for research, for instance into how the evolution of aposematism influences sexual selection. With the growing availability of genomic tools for previously “non‐model” organisms, combined with the relative ease of keeping many of the polyphagous species in the laboratory, we argue that these bugs offer many opportunities for behavioral and evolutionary ecologists.     

Temporal variation in sex allocation in the mealybug Planococcus citri: adaptation, constraint, or both?

Sex ratio theory has been very successful in predicting under which circumstances parents should bias their investment towards a particular offspring sex. However, most examples of adaptive sex ratio bias come from species with well-defined mating systems and sex determining mechanisms, while in many other groups there is still an on-going debate about the adaptive nature of sex allocation. Here we study the sex allocation in the mealybug Planococcus citri, a species in which it is currently unclear how females adjust their sex ratio, even though experiments have shown support for facultative sex ratio adjustment. Previous work has shown that the sex ratio females produce changes over the oviposition period, with males being overproduced early and late in the laying sequence. Here we investigate this complex pattern further, examining both the robustness of the pattern and possible explanations for it. We first show that this sex allocation behaviour is indeed consistent across lines from three geographical regions. Second, we test whether females produce sons first in order to synchronize reproductive maturation of her offspring, although our data provide little evidence for this adaptive explanation. Finally we test the age at which females are able to mate successfully and show that females are able to mate and store sperm before adult eclosion. Whilst early-male production may still function in promoting protandry in mealybugs, we discuss whether mechanistic constraints limit how female allocate sex across their lifetime.     

Sex-ratio adjustment when relatives interact: a test of constraints on adaptation

Studies of sex allocation offer excellent opportunities for examining the constraints and limits on adaptation. A major topic of debate within this field concerns the extent to which the ability of individuals to adaptively manipulate their offspring sex ratio is determined by constraints such as the method of sex determination. We address this problem by comparing the extent of sex-ratio adjustment across taxa with different methods of sex determination, under the common selective scenario of interactions between relatives. These interactions comprise the following: local resource competition (LRC), local mate competition (LMC), and local resource enhancement (LRE). We found that: (1) species with supposedly constraining methods of sex determination showed consistent sex-ratio adjustment in the predicted direction; (2) vertebrates with chromosomal sex determination (CSD) showed less adjustment then haplodiploid invertebrates; (3) invertebrates with possibly constraining sex-determination mechanisms (CSD and pseudo-arrhenotoky) did not show less adjustment then haplodiploid invertebrates; (4) greater sex-ratio adjustment was seen in response to LRC and LMC than LRE; (5) greater sex-ratio adjustment was seen in response to interactions between relatives (LRC, LMC, and LRE) compared to responses to other environmental factors. Our results also illustrate the problem that sex-determination mechanism and selective pressure are confounded across taxa because vertebrates with CSD are influenced primarily by LRE whereas invertebrates are influenced by LRC and LMC. Overall, our analyses suggest that sex-allocation theory needs to consider simultaneously the influence of variable selection pressures and variable constraints when applying general theory to specific cases.     

Mate-guarding in a promiscuous insect: species discrimination influences context-dependent behaviour

Mating strategy is often informed by social context. However, information on social environment may be sensitive to interference by nearby heterospecifics, a process known as reproductive interference (RI). When heterospecific individuals are present in the environment, failures in species discrimination can lead to sub-optimal mating behaviours, such as misplaced courtship, misplaced rivalry behaviours, or heterospecific copulation attempts. All aspects of mating behaviour that are influenced by social context may be prone to RI, including copulatory behaviours associated with mate-guarding in the presence of possible competitors. Here we investigate the effect of three heterospecifics on the mate-guarding behaviour of male Lygaeus equestris seed bugs. We find that, despite previously reported heterospecific mating harassment amongst these species of lygaeid bug, male L. equestris are able to effectively distinguish rival conspecific males from heterospecifics. Thus, heterospecific mating attempts in this group may reflect selection on males to mate opportunistically, rather than a failure of species discrimination.