Jacques J. M. van Alphen

The effect of male coloration on female mate choice in closely related Lake Victoria cichlids (Haplochromis nyererei complex)

Abstract We studied the effect of male coloration on interspecific female mate choice in two closely related species of haplochromine cichlids from Lake Victoria. The species differ primarily in male coloration. Males of one species are red, those of the other are blue. We recorded the behavioral responses of females to males of both species in paired male trials under white light and under monochromatic light, under which the interspecific differences in coloration were masked. Females of both species exhibited species-assortative mate choice when colour differences were visible, but chose non-assortatively when colour differences were masked by light conditions. Neither male behaviour nor overall female response frequencies differed between light treatments. That female preferences could be altered by manipulating the perceived colour pattern implies that the colour itself is used in interspecific mate choice, rather than other characters. Hence, male coloration in haplochromine cichlids does underlie sexual selection by direct mate choice, involving the capacity for individual assessment of potential mates by the female. Females of both species responded more frequently to blue males under monochromatic light. Blue males were larger and displayed more than red males. This implies a hierarchy of choice criteria. Females may use male display rates, size, or both when colour is unavailable. Where available, colour has gained dominance over other criteria. This may explain rapid speciation by sexual selection on male coloration, as proposed in a recent mathematical model.

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.

Intraspecific sexual selection on a speciation trait, male coloration, in the Lake Victoria cichlid Pundamilia nyererei

The haplochromine cichlids of Lake Victoria constitute a classical example of explosive speciation. Extensive intra– and interspecific variation in male nuptial coloration and female mating preferences, in the absence of postzygotic isolation between species, has inspired the hypothesis that sexual selection has been a driving force in the origin of this species flock. This hypothesis rests on the premise that the phenotypic traits that underlie behavioural reproductive isolation between sister species diverged under sexual selection within a species. We test this premise in a Lake Victoria cichlid, by using laboratory experiments and field observations. We report that a male colour trait, which has previously been shown to be important for behavioural reproductive isolation between this species and a close relative, is under directional sexual selection by female mate choice within this species. This is consistent with the hypothesis that female choice has driven the divergence in male coloration between the two species. We also find that male territoriality is vital for male reproductive success and that multiple mating by females is common.

Nuclear markers reveal unexpected genetic variation and a Congolese-Nilotic origin of the Lake Victoria cichlid species flock

Phylogenetic analyses based on mitochondrial (mt) DNA have indicated that the cichlid species flock of the Lake Victoria region is derived from a single ancestral species found in East African rivers, closely related to the ancestor of the Lake Malawi cichlid species flock. The Lake Victoria flock contains ten times less mtDNA variation than the Lake Malawi radiation, consistent with current estimates of the ages of the lakes. We present results of a phylogenetic investigation using nuclear (amplified fragment length polymorphism) markers and a wider coverage of riverine haplochromines. We demonstrate that the Lake Victoria–Edward flock is derived from the morphologically and ecologically diverse cichlid genus Thoracochromis from the Congo and Nile, rather than from the phenotypically conservative East African Astatotilapia. This implies that the ability to express much of the morphological diversity found in the species flock may by far pre–date the origin of the flock. Our data indicate that the nuclear diversity of the Lake Victoria–Edward species flock is similar to that of the Lake Malawi flock, indicating that the genetic diversity is considerably older than the 15 000 years that have passed since the lake began to refill. Most of this variation is manifested in trans–species polymorphisms, indicating very recent cladogenesis from a genetically very diverse founder stock. Our data do not confirm strict monophyly of either of the species flocks, but raise the possibility that these flocks have arisen from hybrid swarms.

ADAPTIVE SUPERPARASITISM AND PATCH TIME ALLOCATION IN SOLITARY PARASITOIDS: THE INFLUENCE OF THE NUMBER OF PARASITOIDS DEPLETING A PATCH

An ESS model that predicts more superparasitism and longer patch times with an increasing number of searching parasitoids in a patch, was tested in experiments with Leptopilina heterotoma, a solitary larval parasitoid of Drosophila. The observed egg distributions and patch times were in quantitative agreement with the predictions of the model; oviposition and patch time decisions are clearly influenced by the number of conspecifics in the patch. Both in the model and in the experiment patch quality was kept constant (the number of hosts and the patch area per parasitoid were kept constant). The model predicted and the experiments showed that parasitoids gain less offspring per unit of time when searching a patch together: superparasitism leads to mutual interference. No self-superparasitism should have occurred when parasitoids searched alone. This prediction was only met with females that had been kept in isolation in the days before the experiment; when stored in groups of four, self-superparasitism did occur. This indicates an ability of the parasitoids to assess the probability of future superparasitism by conspecifics.

Sensory drive in cichlid speciation

The role of selection in speciation is a central yet poorly understood problem in evolutionary biology. The rapid radiations of extremely colorful cichlid fish in African lakes have fueled the hypothesis that sexual selection can drive species divergence without geographical isolation. Here we present experimental evidence for a mechanism by which sexual selection becomes divergent: in two sibling species from Lake Victoria, female mating preferences for red and blue male nuptial coloration coincide with their context‐independent sensitivities to red and blue light, which in turn correspond to a difference in ambient light in the natural habitat of the species. These results suggest that natural selection on visual performance, favoring different visual properties in different spectral environments, may lead to divergent sexual selection on male nuptial coloration. This interplay of ecological and sexual selection along a light gradient may provide a mechanism of rapid speciation through divergent sensory drive.

Mechanisms of rapid sympatric speciation by sex reversal and sexual selection in cichlid fish.

Mechanisms of speciation in cichlid fish were investigated by analyzing population genetic models of sexual selection on sex-determining genes associated with color polymorphisms. The models are based on a combination of laboratory experiments and field observations on the ecology, male and female mating behavior, and inheritance of sex-determination and color polymorphisms. The models explain why sex-reversal genes that change males into females tend to be X-linked and associated with novel colors, using the hypothesis of restricted recombination on the sex chromosomes, as suggested by previous theory on the evolution of recombination. The models reveal multiple pathways for rapid sympatric speciation through the origin of novel color morphs with strong assortative mating that incorporate both sex-reversal and suppressor genes. Despite the lack of geographic isolation or ecological differentiation, the new species coexists with the ancestral species either temporarily or indefinitely. These results may help to explain different patterns and rates of speciation among groups of cichlids, in particular the explosive diversification of rock-dwelling haplochromine cichlids.

Why five fingers? Evolutionary constraints on digit numbers

Evolutionary changes in the number of digits and other limb elements appear to be severely constrained, probably as a result of a low level of modularity during limb development. Reduced limb structures typically develop through a process of construction followed by destruction and amniotes have evolved many digit-like structures rather than actual extra digits. In amniotes, limb development occurs during the crucial phylotypic stage, when many inductive interactions are occurring throughout the body. As a result, changes in limb development usually engender changes in other body parts. Thus, mutations that change the number of limb bones are expected to have many pleiotropic effects, which severely reduces the chance of such mutations being successful. In amphibians with aquatic larvae, limb development occurs after the phylotypic stage and limb development is decoupled from the interactivity of the phylotypic stage. The constraint of pleiotropic effects is, therefore, expected to be weaker. This expectation agrees with the larger variability in the number of hand and foot structures in amphibians, with frogs even occasionally possessing six toes. These facts once again emphasize the importance of pleiotropic effects as constraints to evolutionary change, including their role in the conservation of body plans.

Geographical Variation in Resistance of the Parasitoid Asobara-Tabida against Encapsulation by Drosophila-Melanogaster Larvae - the Mechanism Explored

Abstract. The braconid parasitoid Asobara tabida Nees attacks larvae of several Drosophila species in fermenting substrates. Northwestern and central European populations of the parasitoid attack mainly D.subobscura Collin. Southern European parasitoids attack mainly D.melanogaster Meigen. Larvae of this last species can defend themselves against parasitoids by encapsulating the parasitoid egg. Parasitoids from southern European populations are better able to resist encapsulation of their eggs than their northwestern and central European conspecifics. The eggs of southern European parasitoids appear to have a ‘sticky’ egg chorion. As a result of this ‘stickiness’ the eggs become embedded in host tissue where they are not completely covered by the hosts blood cells. This leads to, at most, partial encapsulation of the egg. Parasitoid larvae can escape from partially closed capsules.

Foraging Behaviour of Rhagoletis pomonella, a Parasite of Hawthorn (Crataegus viridis), in Nature

(4) Flies never or rarely oviposited in non-host and marked fruit respectively, and in both cases emigrated from trees harbouring those fruit soon after examining the fruit. (5) Flies exhibited success-motivated search following discovery of and oviposition in uninfested, unmarked fruit. (6) Flies visited more fruit, oviposited more often and remained longer in trees harbouring high v. low densities of fruit clusters. (7) Flies emigrated sooner after the last egg they laid on trees harbouring high, v. low, densities of fruit clusters (Giving Up Time shorter). (8) Rhagoletis pomonella foraging behaviour is discussed in relation to current foraging theory.