Gerard Driessen

Single locus complementary sex determination in Hymenoptera: an "unintelligent" design?

The haplodiploid sex determining mechanism in Hymenoptera (males are haploid, females are diploid) has played an important role in the evolution of this insect order. In Hymenoptera sex is usually determined by a single locus, heterozygotes are female and hemizygotes are male. Under inbreeding, homozygous diploid and sterile males occur which form a genetic burden for a population. We review life history and genetical traits that may overcome the disadvantages of single locus complementary sex determination (sl-CSD). Behavioural adaptations to avoid matings between relatives include active dispersal from natal patches and mating preferences for non-relatives. In non-social species, temporal and spatial segregation of male and female offspring reduces the burden of sl-CSD. In social species, diploid males are produced at the expense of workers and female reproductives. In some social species, diploid males and diploid male producing queens are killed by workers. Diploid male production may have played a role in the evolution or maintenance of polygyny (multiple queens) and polyandry (multiple mating). Some forms of thelytoky (parthenogenetic female production) increase homozygosity and are therefore incompatible with sl-CSD. We discuss a number of hypothetical adaptations to sl-CSD which should be considered in future studies of this insect order.

Information acquisition and time allocation in insect parasitoids

Abstract All animals face the problem of finding resources for growth, maintenance and reproduction. Foraging in a heterogeneous (i.e. patchy) environment requires seemingly complex decisions, such as where to forage, and for how long. To make such decisions, animals need to acquire relevant information from their environment. Recent studies of how parasitoids acquire information and allocate their time to the exploitation of host patches use a combination of functional (evolutionary) and causal (mechanistic) approaches. They show that parasitoids can allocate foraging time to patches in an adaptive way and that members of the same species can respond differently to the same environmental cues, depending on their physiological state and previous experiences or on genetic differences. Functional models now help to explain these contrasting responses.

Egg Load Evolution in Parasitoids

Parasitic wasps lay their eggs in or on other insects. Allocation of resources to reproduction (eggs) and survival (life span) should maximize reproductive success, but stochasticity in the number of hosts encountered precludes an exact match of allocation with reproductive opportunity. We study optimal egg loads using a general model for pro‐ovigenic parasitoids (which only mature eggs before adult life) and a dynamic programming model for synovigenic parasitoids (which can mature additional eggs during adult life). We find that stochasticity has a major effect on optimal egg loads and that the patchy distribution of hosts is the major source of stochasticity in reproductive opportunities. In a stochastic world, the further “relative habitat quality” (expected number of oviposition opportunities gained by reducing reproductive investment by one egg) is below unity (as we argue it generally is), the more the optimal egg load exceeds the expected number of hosts. In unusually rich environments, however, stochasticity decreases optimal egg loads. Although synovigenic parasitoids can adjust to the conditions they encounter, they too have to invest for an uncertain future. Because they can replenish their egg supply throughout life, the incidence of egg limitation is even lower than in pro‐ovigenic parasitoids.

AN EVOLUTIONARY ARGUMENT FOR TIME LIMITATION

Shine et al. (1996) argue that the Shine and Schwarzkopf (1992) model demonstrates that energy allocation measures will provide little insight into evolution of RE in squamate reptiles. Our analysis shows why that interpretation is premature if not incorrect. But the claim that energy allocation measures should be abandoned also stems from confusion about the meaning of CR and RE. Contrary to what Shine and Schwarzkopf (1992) and Shine et al. (1996) state, CR is not the same as RE. In fact, CR is a function of RE. CR is measured in units of fitness and RE is measured in units of energy. The relationship between CR and RE can be derived from Williams (1966): E (reproductive effort), is the ratio of current reproduction to expected future reproduction (Fig. 5). A quantitative expression for CR involves an assumption and a mathematical expression, neither of which were provided by Williams. In the context of Williamss construct, an expression for CR must specify the relationship between the expected future reproduction and current reproduction. For example, we could specify an expression for CR as the magnitude of reduced future expectation of reproduction, Z, attendant upon changing current reproduction (Fo) from F0 1 to F02 (Fig. 5). From this formalism, it is clear that CR and RE are not the same, and that the assertion that the Shine and Schwarzkopf (1992) model demonstrates that energy allocation measures will provide little insight into the evolution of RE is unfounded.

Effects of intra-patch experiences on patch time, search time and searching efficiency of the parasitoid Leptopilina clavipes (Hartig).

1. This paper considers the effects of intra-patch experiences, such as contact with kairomone, ovipositions and rejections on the searching behaviour of individual female parasitoids of the species Leptopilina clavipes. 2. Behavioural records were analysed by means of the proportional hazards model (Cox 1972) taking effects of fixed as well as time-varying covariates into account. 3. Analyses were carried out at three levels of resolution: (i) patch leaving and return tendencies; (ii) tendencies to stop and start searching while on the patch; and (iii) the encounter rate during search bouts

The time and egg budget of Leptopilina clavipes, a parasitoid of larval Drosophila

Abstract. 1 For the understanding of the influence of natural selection on the persistence of host selection behaviour in populations of Drosophila parasitoids it is important to know whether parasitoids will become time‐ or egg‐limited. We investigated whether the Drosophila parasitoid Leptopilina clavipes (Hartig) meets egg‐ or time‐limited conditions in the field. 2 To this end the following aspects of the parasitoids life were studied: egg load at emergence, travelling velocity between patches, patch residence times, oviposition rates and life expectancy. Together with the results from earlier studies on host and patch distributions, this formed the input of a ‘Monte Carlo’ simulation model, in which the life history of an individual parasitoid can be traced. 3 The simulations revealed that under the conditions found in the field 12.9% of the parasitoid population is egg‐limited. The model was also run for a number of scenarios which reflect ‘good’ or ‘bad’ circumstances. In most cases a significant proportion of the parasitoid population proved to be egg‐limited. 4 For the measurement of travelling velocities and patch residence times a marking method, especially applicable to small‐sized parasitoids such as L.clavipes, is described. Marking did not affect survival, host habitat location or host detection rate. Parasitoids were found to be attracted to the odour of fruit‐bodies of Phallus impudicus, the most important breeding substrate of their hosts.

Energy dynamics in a parasitoid foraging in the wild

Although parasitoids are used widely as a biological models for understanding the evolution of animal behaviour, most studies have been constrained to the laboratory. The dearth of field studies has been compounded by the almost complete ignorance of the physiological parameters involved in foraging and dispersal, in particular of the energetic constraints imposed by resource limitation. We estimated the dynamics of carbohydrates and lipids reserves of Venturia canescens (Gravenhorst) females by releasing individuals of known nutritional status in a natural environment and recapturing them using host-containing traps. The recapture rate was around 30%. These results were compared with the reserves of caged animals kept under different experimental conditions (freshly emerged, starved to death, fed ad libitum and partially starved). Wild animals were also sampled in order to estimate the resource levels of the local population. The results show that: (i) wasps are able to maintain a nearly constant level of energy over an extended foraging period; (ii) V. canescens takes sugars in the field; and (iii) the lipid reserves accumulated during the larval life may be limiting as lipogenesis does not take place in adults even under conditions of high sugar availability. These results demonstrate that wasps can forage for hosts and food and disperse in this habitat for hours and days without running into a severe risk of energy limitation.

Host and food searching in a parasitic wasp Venturia canescens : a trade-off between current and future reproduction?

Whether to invest in current or future reproduction is an important trade-off in life history evolution. For insect parasitoids, this trade-off is determined, among other factors, by the decision whether to search for hosts (immediate gain of fitness) or food (delayed fitness gains). Although host searching has been well studied, food sources, cues that parasitoids use to search for food and how insects modify their feeding behaviour have not. To address these questions, we investigated the food- and host-searching decisions made by the parasitoid Venturia canescens in both laboratory (olfactometer experiments) and field conditions (choice experiments). The wasps detected chemical cues associated with food, as well as those associated with hosts, and moved towards one or the other according to their nutritional state. Females used as food sources the same fruits that harboured hosts and detected the olfactory cues that would direct them to these fruits. Field results were consistent with those obtained in the laboratory. By integrating responses to infochemicals related to host and food, V. canescens might reduce the costs associated with food searching, so that more time and energy can be allocated to foraging for hosts. This behaviour should result in an adaptive advantage over parasitoids that incur additional costs by searching for hosts and food in different locations.

The impact of parasitoids on natural populations of temperate woodland Drosophila

The species complex of temperate woodland Drosophila and its hymenopterous parasitoids was studied to elucidate the importance of parasitization as a mortality factor of Drosophila populations. A total of 5841 Drosophila and 801 parasitoids emerged from the collected field samples, indicating an overall parasitization rate of 12.1 %. Parasitization rates during a period of 9 weeks in midsummer were considerably higher, with a mean rate of 32.5 % . Parasitization rates in some substrate types, i.e. sap fluxes of wounded trees, ranged from 70% to 100%, indicating that risks of parasitization for Drosophila species that specialize on this substrate are very high, whereas parasitoids specialized on this microhabitat may experience high intra- and interspecific competition for hosts. It is concluded that parasitization is an important mortality factor during a considerable part of the Drosophila breeding season.

Geographical distribution and genetic relatedness of sympatrical thelytokous and arrhenotokous populations of the parasitoid Venturia canescens (Hymenoptera)

Theory predicts that asexual reproduction has a competitive advantage over sexual reproduction because of the twofold cost of producing males. Few systems are suitable for directly testing this prediction. In the solitary parasitoid wasp Venturia canescens both arrhenotokously (sexual) and thelytokously (asexual) reproducing individuals occur sympatrically. We sampled 922 wasps from 22 localities along the coast of south‐eastern France. Thelytokous wasps were less abundant (23%) than arrhenotokous wasps and were almost always found in sympatry with arrhenotokous ones. An analysis of genetic relatedness using amplified fragment length polymorphism (AFLP) markers showed the existence of a widespread thelytokous clone. In addition, a few thelytokous individuals were found to be closely related to arrhenotokous ones and vice versa. These data suggest the occurrence of occasional gene flow between both reproductive modes and/or recurrent origin of thelytokous clones from coexisting arrhenotokous populations in the area. The results are discussed in the context of the paradox of sex.