J. J. M. van Alphen

Evolution of colour patterns in East African cichlid fish

African cichlid fishes have undergone outbursts of explosive speciation in several lakes, accompanied by rapid radiations in coloration and ecology. Little is known about the evolutionary forces that triggered these events but a hypothesis, published by Wallace Dominey in 1984, has figured prominently. It states that the evolution of colour patterns is driven by sexual selection and that these colour patterns are important in interspecific mate choice, a combination which holds the potential for rapid speciation. Here we present phylogenetic analyses that describe major events in colour evolution and test predictions yielded by Domineys hypothesis. We assembled information on stripe patterns and the presence or absence of nuptial coloration from more than 700 cichlid species representing more than 90 taxa for which molecular phylogenetic hypotheses were available. We show that sexual selection is most likely the selection force that made male nuptial coloration arise and evolve quickly. In contrast, stripe patterns, though phylogenetically not conserved either, are constrained ecologically. The evolution of vertical bar patterns is associated with structurally complex habitats, such as rocky substrates or vegetation. The evolution of a horizontal stripe is associated with a piscivorous feeding mode. Horizontal stripes are also associated with shoaling behaviour. Strength of sexual selection, measured in terms of the mating system (weak in monogamous, strong in promiscuous species), has no detectable effects on stripe pattern evolution. In promiscuous species the frequency of difference between sister species in nuptial hue is higher than in pair bonding and harem forming species, but the frequency of difference in stripe pattern is lower. We argue that differences between the two components of coloration in their exposure to natural selection explain their very different evolutionary behaviour. Finally, we suggest that habitat‐mediated selection upon chromomotor flexibility, a special form of phenotypic plasticity found in the river‐dwelling outgroups of the lake‐dwelling cichlids, explains the rapid and recurrent ecology‐associated radiation of stripe patterns in lake environments, a new hypothesis that yields experimentally testable predictions.

Life history evolution in Asobara tabida: plasticity in allocation of fat reserves to survival and reproduction

Life history variation can be genetically based, or it may be due to environmental effects on the phenotype. In this paper we examine life history variation in relation to differences in habitat in the parasitoid Asobara tabida. Differences in the spatial distribution of host patches, the length of the season, host suitability, and competition between parasitoids all contribute to the selection for differences in life history between strains from northern and southern Europe.

The function of host discrimination and superparasitization in parasitoids

SummaryHost discrimination, i.e. the ability to distinguish unparasitized hosts from parasitized ones, and to reject the latter for egg laying is present in many parasitic wasp species. This property is classically considered as an example of contest competition, and is supposed to have a number of functions. However, different species do not react to each others marks and lay eggs in hosts parasitized by the other species. Apparently the marks used for recognition are specific.Multiparasitization is the best strategy when hosts are scarce and the egg supplies of the parasitoids are not limited. Interspecific host discrimination is not an ESS.Superparasitization within one species would have selective advantage if the number of unparasitized hosts is small and the wasp has a reasonable chance to lay her egg in a host that is not parasitized by herself, and if the chance for her offspring to survive the competitive battle with the first parasitoid larva is not too small. This is shown to be the case.However, marks are not individual and wasps cannot distinguish hosts parasitized by themselves from those parasitized by others. The hypothesis is tested that the egg laying strategy (i.e. the decision to superparasitize) of wasps is dependent on the number of conspecifics that is searching simultaneously for hosts, since this determines the chance that a parasitized host encountered by a wasp is parasitized by herself.It is shown that host discrimination cannot be regarded as a case of contest competition. Other aspects of superparasitization, related to interference and population regulation, sex allocation and encapsulation are briefly discussed.

Superparasitism and Host Discrimination By Asobara Tabida Nees )Braconidae: Alysiinae), a Larval Parasitoid of Drosophilidae

This paper presents the results of a study of superparasitism and host discrimination by Asobara tabida Nees. We found that: (1) A. tabida females are able to distinguish unparasitized hosts from those previously parasitized by themselves or by a conspecific; (2) There is no evidence that A. tabida females are able to distinguish hosts in which they laid an egg themselves from hosts parasitized by conspecifics; (3) A. tabida females, unlike those of Leptopilina heterotoma cannot discriminate between hosts with different numbers of eggs; (4) Superparasitism may occur because: (a) inexperienced females of A. tabida may initially lay two eggs during one oviposition. (b) a female A. tabida may re-attack a host after oviposition within the period needed for building up the factor which causes avoidance of superparasitism. (c) the restraint to oviposit in parasitized hosts breaks down when a female A. tabida only meets parasitized hosts and does not lay eggs during a period of at least 8 hours. (d) females that have never oviposited in unparasitized hosts do not refrain from oviposition in parasitized hosts. We discuss whether superparasitism by insect parasitoids can be adaptive under particular circumstances.

Adaptive superparasitism and patch time allocation in solitary parasitoids: an ESS model.

1. The adaptive value of superparasitism (oviposition in a previously parasitized host) when solitary parasitoids deplete a patch simultaneously is investigated, using a deterministic game theoretical model in which encounter rates with host types, which differ in the number of parasitoid eggs they contain, change over time due to depletion. 2. Parasitoids should never superparasitize when they search a patch alone. 3. When parasitoids search in the presence of conspecifics, the evolutionarily stable strategy (ESS) is to reject parasitized hosts at first but later, after a threshold has been reached, to accept them. Different thresholds exist for hosts containing different numbers of eggs. 4. The thresholds are expressed in gross rate of offspring gain. 5. With an increasing number of parasitoids per patch, females should begin to superparasitize at higher rates of gain. 6. The gross rate of gain at which a parasitoid leaves the patch (the leaving threshold) influences the threshold rate at which they should begin to superparasitize. Patch time allocation and host acceptance decisions are interdependent. 7. Both the handling time and the pay-off from an egg laid in a parasitized host strongly influence the threshold rate at which parasitoids should begin to superparasitize. This threshold is only weakly influenced by the number of hosts in the patch. 8. The model predicts that patch time per female and degree of superparasitism are influenced by the number of females in a patch. 9. The total number of offspring gained per unit time in the patch decreases with the number of females, even when the patch area per female and the number of hosts per female are kept constant; thus, superparasitism leads to interference. 10. Experimental evidence is in qualitative agreement with the predictions of the model.

The coevolution of host resistance and parasitoid virulence

Host-parasitoid interactions are abundant in nature and offer great scope for the study of coevolution. A particularly fertile area is the interaction between internal feeding parasitoids and their hosts. Hosts have evolved a variety of means of combating parasitoids, in particular cellular encapsulation, while parasitoids have evolved a wide range of countermeasures. Studies of the evolution of host resistance and parasitoid virulence are reviewed, with an emphasis on work involving Drosophila and its parasitoids. Genetic variation in both traits has been demonstrated using isofemale line and artificial selection techniques. Recent studies have investigated the fitness costs of maintaining the ability to resist parasitoids, the comparative fitness of flies that have successfully defended themselves against parasitoids, and the degree to which resistance and virulence act against one or more species of host or parasitoid. A number of studies have examined geographical patterns, and sought to look for local adaptation; or have compared the traits across a range of species. Finally, the physiological and genetic basis of change in resistance and virulence is being investigated. While concentrating on Drosophila, the limited amount of work on different systems is reviewed, and other possible areas of coevolution in host-parasitoid interactions are briefly discussed.

Host Selection By Asobara Tabida Nees (Braconidae;Alysiinae) a Larval Parasitoid of Fruit Inhabiting Drosophila Species

We studied the host selection behaviour of Asobara tabida Nees in experiments in which this parasitoid could choose between equal numbers of larvae of Drosophila melanogaster and larvae of one of the following 4 species: D. busckii, D. funebris, D. immigrans and D. subobscura. We found that: 1) the percentage parasitism in a host species increased with the survival probability of A. tabida in larvae of that species; 2) though the percentage parasitism was often the result of active rejection by the parasitoid of larvae of the less profitable species, the parasitoid often gave up before it had inserted its ovipositor in the host; this occurred: a) when a larva, after initial detection by the parasitoid, kept motionless and could not be located. b) when after detection of a larva the wasp failed to hit it with the ovipositor; the number of encounters in which the parasitoid gave up after missing stabs with the ovipositor was negatively correlated with the size of the host larvae c) when the wasp hit the larva but failed to pierce the hosts skin with the ovipositor; the number of encounters in which the parasitoid gave up after it failed to pierce the hosts skin was positively correlated with skin thickness d) after a combination of the events mentioned under b) and c) e) when the wasp was frightened off by vigorous wriggling of the host larva; 3) the host selection behaviour of A. tabida was affected by previous experience of the wasps with a host species; 4) A. tabida prefers D. subobscura over the 4 other species and attacks this species more efficiently than the other 4 species. D. subobscura is the only native species used in our study, D. busckii, D. funebris, D. immigrans and D. melanogaster are alien in western Europe, but may dominate the Drosophila fauna during the second part of the summer. Oviposition by A. tabida in larvae of alien Drosophila species with a low survival probability for the parasitoids eggs may therefore only occur when native host species are scarce and may thus help the parasitoid to persist during the period when the preferred hosts cannot be found.

PATCH TIME ALLOCATION AND PARASITIZATION EFFICIENCY OF ASOBARA TABIDA, A LARVAL PARASITOID OF DROSOPHILA

SUMMARY (1) Patch time allocation by the parasitoid Asobara tabida Nees was studied on patches with different host densities. (2) The parasitoid increased its searching time and giving up time with increasing host density. These increases were caused by a response of the parasitoid to the number of encounters with unparasitized hosts; the amount of kairomone in a patch may have had an additional incremental effect on patch time. (3) Encounters with parasitized hosts had no effect on searching time and giving up time. (4) The percentage of hosts parasitized first increased with host density and then levelled off at densities of more than four host larvae per patch, thus producing an accelerating functional response. (5) The foraging behaviour of A. tabida is compared to that predicted by optimal foraging models.

A life history trade-off in Drosophila species and community structure in variable environments.

Within taxa at the class or family level, the developmental period is often proportional to adult life span. In Drosophila species, a short developmental period increases larval competitive ability. Species with a long adult life, however, may have a better chance to reach new breeding sites in time and space. In another paper (Sevenster & Van Alphen 1993), we presented a model incorporating this trade-off. It shows that fast larval developers («fast species») are dominant when breeding opportunities are frequent, and that good adult survivors («slow species») are dominant when breeding opportunities are scarce. Moreover, the model demonstrates that a fast and a slow species may coexist in intermediate environments

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.