Evidence for widespread cryptic sexual generations in apparently purely asexual Andricus gallwasps

Oak gallwasps (Hymenoptera, Cynipidae, Cynipini) are one of seven major animal taxa that commonly reproduce by cyclical parthenogenesis (CP). A major question in research on CP taxa is the frequency with which lineages lose their sexual generations, and diversify as purely asexual radiations. Most oak gallwasp species are only known from an asexual generation, and secondary loss of sex has been conclusively demonstrated in several species, particularly members of the holarctic genus Andricus. This raises the possibility of widespread secondary loss of sex in the Cynipini, and of diversification within purely parthenogenetic lineages. We use two approaches based on analyses of allele frequency data to test for cryptic sexual generations in eight apparently asexual European species distributed through a major western palaearctic lineage of the gallwasp genus Andricus. All species showing adequate levels of polymorphism (7/8) showed signatures of sex compatible with cyclical parthenogenesis. We also use DNA sequence data to test the hypothesis that ignorance of these sexual generations (despite extensive study on this group) results from failure to discriminate among known but morphologically indistinguishable sexual generations. This hypothesis is supported: 35 sequences attributed by leading cynipid taxonomists to a single sexual adult morphospecies, Andricus burgundus, were found to represent the sexual generations of at least six Andricus species. We confirm cryptic sexual generations in a total of 11 Andricus species, suggesting that secondary loss of sex is rare in Andricus.     

Isozyme differences between the developmental stages of the parthenogenetic gall wasp Diplolepis rosae

The value of electrophoretic analysis of enzymes as an aid in connecting a morphologically deviating form to one of several possible species is dependent on the proportion of shared enzyme loci between the forms. In order to determine this proportion for the different instars of the parthenogenetic gall wasp Diplolepis rosae, 16 different enzyme systems were analysed. Out of the 37 loci detected, 25 (68%) were active in all instars. The larvae, pupae, and imagines had two, one and five unique loci, respectively. Larvae and pupae shared four loci not expressed in the imagines. In the ten enzymes analysed for comparison of males and females a total of 22 loci were detected, 21 found in both sexes and one unique to the females. The difference in isozyme pattern was found to be much more pronounced between pupa and imago than between larva and pupa. The chance to find a locus in an instar when already detected in another was calculated to 87%. If this estimate is valid for other forms within species, differential activity should not present a problem when electrophoretic analysis is used in order to connect such forms.     

Biology and host selection behaviour of the aphid hyperparasitoid Alloxysta victrix in association with the primary parasitoid Aphidius colemani and the host aphid Myzus persicae

The biology of the aphid hyperparasitoid Alloxysta victrix was investigated with respect to its interaction with the primary parasitoid Aphidius colemani and the host aphid Myzus persicae. Laboratory investigations of host selection behaviour indicated that naive A. victrix females could rapidly distinguish between unparasitized aphids and those parasitized by A. colemani, but showed little tendency to discriminate between aphids parasitized by the primary parasitoid three, five, seven, or ten days earlier (even though the latter were already mummified). Host suitability studies, in contrast, indicated that aphids parasitized by A. colemani became progressively less suitable as the primary parasitoid developed. When aphids were exposed to A. victrix more than seven days after being parasitized by A. colemani, survivorship of both primary and secondary parasitoid species decreased, with a significantly higher proportion of mummies producing no viable adult parasitoid of either species. The developmental period (egg to adult eclosion) of surviving hyperparasitoids was also longer in older host stages, and females developed significantly more slowly than males (14.2 days versus 13.6 days, respectively). The sex ratio of hyperparasitoid broods emerging from all host stages was consistently male-biased (31–46% female), but adult longevity was higher for females than for males (37.1 days versus 23.6 days).     

Influence of galls of Phanacis taraxaci on carbon partitioning within common dandelion, Taraxacum officinale

We examined how leaf galls, induced by the cynipid wasp Phanacis taraxaci, influence the partitioning of photoassimilates within the host, the common dandelion, Taraxacum officinale. Galled and ungalled plants were exposed to ¹⁴CO₂ and the labelled photoassimilates accumulating within galls and other parts of the host were measured. During the growth phase of the gall they were physiological sinks for photoassimilates, accumulating 9% to 70% of total carbon produced by the host, depending upon the number of galls per plant. High levels of ¹⁴C assimilation in the leaves of galled plants compared to controls, suggest that galls actively redirect carbon resources from unattacked leaves of their host plant. This represents a significant drain on the carbon resources of the host, which increases with the number and size of galls per plant. Active assimilation of ¹⁴C by the gall is greatest in the growth phase and is several orders of magnitude lower in the maturation phase. This finding is consistent with physiological and anatomical changes that occur during the two phases of gall development and represents a key developmental strategy by cynipids to ensure adequate food resources before larval growth begins.     

How a complex life cycle can improve a parasite's sex life

How complex life cycles of parasites are maintained is still a fascinating and unresolved topic. Complex life cycles using three host species, free-living stages, asexual and sexual reproduction are widespread in parasitic helminths. For such life cycles, we propose here that maintaining a second intermediate host in the life cycle can be advantageous for the individual parasite to increase the intermixture of different clones and therefore decrease the risk of matings between genetically identical individuals in the definitive host. Using microsatellite markers, we show that clone mixing occurs from the first to the second intermediate host in natural populations of the eye-fluke Diplostomum pseudospathaceum. Most individuals released by the first intermediate host belonged to one clone. In contrast, the second intermediate host was infected with a diverse array of mostly unique parasite genotypes. The proposed advantage of increased parasite clone intermixture may be a novel selection pressure favouring the maintenance of complex life cycles.     

When is dispersal for dispersal? Unifying marine and terrestrial perspectives

Recent syntheses on the evolutionary causes of dispersal have focused on dispersal as a direct adaptation, but many traits that influence dispersal have other functions, raising the question: when is dispersal ‘for’ dispersal? We review and critically evaluate the ecological causes of selection on traits that give rise to dispersal in marine and terrestrial organisms. In the sea, passive dispersal is relatively easy and specific morphological, behavioural, and physiological adaptations for dispersal are rare. Instead, there may often be selection to limit dispersal. On land, dispersal is relatively difficult without specific adaptations, which are relatively common. Although selection for dispersal is expected in both systems and traits leading to dispersal are often linked to fitness, systems may differ in the extent to which dispersal in nature arises from direct selection for dispersal or as a by‐product of selection on traits with other functions. Our analysis highlights incompleteness of theories that assume a simple and direct relationship between dispersal and fitness, not just insofar as they ignore a vast array of taxa in the marine realm, but also because they may be missing critically important effects of traits influencing dispersal in all realms.     

Microgeographic variation in the effects of larval temperature environment on juvenile morphology and locomotion in the pool frog

In animals with complex life cycles, the environment experienced early during the development may have strong effects on later performance and fitness. We investigated the intraspecific variation in the effects of larval temperature environment on the morphology and locomotory performance of juvenile pool frogs Rana lessonae originating from three closely located populations of the northern fringe metapopulation in central Sweden. Tadpoles were raised individually at two temperatures (20 and 25 °C) until metamorphosis. We measured the morphology of the metamorphs and tested the jumping performance of the froglets after complete tail absorption. We found that early temperature environment affected juvenile morphology, metamorphs from high-temperature environments having relatively longer hindlimbs (tibiofibulas) and longer tails when weight at metamorphosis was accounted for. In absolute terms, froglets from low temperature jumped significantly longer; however, after correcting for size differences the relationship was reversed, individuals raised at high temperature performing better. In both temperatures, relative jumping performance was positively associated with tibiofibula and body length. Populations differed both in metamorphic traits and in jumping capacity, especially at low temperature, suggesting microgeographical variation in temperature sensitivity within the metapopulation. Our results indicate that the temperature environment experienced during the early aquatic stages can influence the morphology and performance of juvenile frogs, and that these effects can be population specific.     

Reproductive value in a complex life cycle: heat tolerance of the pitcher-plant mosquito, Wyeomyia smithii

Because mortality accumulates with age, Fisher proposed that the strength of selection acting on survival should increase from birth up to the age of first reproduction. Hamilton later theorized that the strength of selection acting on survival should not change from birth to age at first reproduction. As organisms in nature do not live in uniform environments but, rather, experience periodic stress, we hypothesized that resistance to environmental stress should increase (Fisher) or remain constant (Hamilton) from birth to age at first reproduction. Using the pitcher-plant mosquito, Wyeomyia smithii, we imposed heat stress by simulating the passage of a warm-weather front at different pre-adult and adult stages. Contrary to either Fisher or Hamilton, stress tolerance declined from embryos to larvae to pupae to adults. Consequently, reproductive value appears to have been of little consequence in the evolution of stage-specific tolerance of heat stress in W. smithii.     

The shape of things to come: linking developmental plasticity to post‐metamorphic morphology in anurans

Development consists of growth and differentiation, which can be partially decoupled and can be affected by environmental factors to different extents. In amphibians, variation in the larval environment influences development and causes changes in post‐metamorphic shape. We examined post‐metamorphic consequences, both morphological and locomotory, of alterations in growth and development. We reared tadpoles of two phylogenetically and ecologically distant frog species (the red‐eyed treefrog Agalychnis callidryas and the African clawed frog Xenopus laevis) under different temperatures with ad libitum food supply and under different food levels at a constant temperature. Low temperature and low food levels both resulted in similarly extended larval periods. However, low temperature yielded relatively long‐legged frogs with a lower degree of ossification than warm temperature, whereas low food yielded relatively short‐legged frogs with a higher degree of ossification than high food levels. Such allometric differences had no effect on locomotor performance of juveniles. Our results provide a basis for understanding the relationship between growth, differentiation and post‐metamorphic shape in anurans and help explain many of the discrepancies reported in previous studies.     

Density reductions by predatory trout increase adult size and fecundity of surviving caddisfly larvae in a detritus-based stream food web

1. In some situations, individuals surviving in environments where predation is intense can grow faster because the benefits of release from intraspecific competition outweigh costs associated with anti-predator responses. Whether these 'thinning' effects of predation occur in detritus-based food webs where resource renewal occurs independently of consumption by consumers was studied. We investigated how effects of predatory brown trout ( Salmo trutta ) on the larvae of the detritivorous stream caddisfly, Zelandopsyche ingens , influenced the size and fecundity of the caddisfly adults.
2. Trout substantially reduced the abundance of Z. ingens larvae, but adult male and female Z. ingens were significantly larger in trout streams compared to fishless streams. Females in trout streams had 33% more eggs than fishless stream females, and egg sizes were not significantly different. In mesocosms, Z. ingens larvae in low density treatments reflecting trout stream abundances grew significantly faster than larvae in high density treatments that were characteristic of fishless stream abundances. Non-lethal trout presence did not influence case building behaviour, feeding rates or growth or Z. ingens larvae, indicating non-lethal effects of predators were negligible.
3. Increased adult size and fecundity associated with trout stream individuals were probably a result of predator thinning of larval density indirectly releasing surviving Z. ingens from intraspecific competition. Thus, predator thinning did influence interactions between larvae in this detritus-based food web as larval growth was strongly density-dependent. However, extrapolating the total number of eggs potentially produced indicates the increased fecundity of females in trout streams would not compensate for losses of larvae to trout predation.