From Russia with lobe: genetic differentiation in trilobed uncus Ostrinia spp. follows food plant, not hairy legs.

Trilobed uncus taxa of the genus Ostrinia (Lepidoptera, Crambidae) illustrate the complex relationship, at early stages of speciation, between reproductive isolation and differentiation in morphology, resource use and genetic variation. On the basis of behaviour and ecology, we recently hypothesized that individuals with small mid-tibiae belong to two distinct species depending on host plant--O. nubilalis and O. scapulalis sensu Frolov et al. (2007) feeding on maize and on a number of dicotyledons, respectively. Individuals with small, medium or massive mid-tibiae would all belong to O. scapulalis as long as they feed on these dicotyledons. This contrasts with previous taxonomy, which distinguished three species by male mid-tibia morphology, regardless of host plant. Here, we test our hypothesis by examining the genetic structure of Ostrinia populations from regions with mid-tibia polymorphism--Western Russia and Kazakhstan--and comparing it with that of French populations where only small mid-tibiae occur. Results support two predictions: (1) maize- and dicotyledon-collected populations are genetically differentiated from each other like in France, and (2) dicotyledon-collected populations show no genetic evidence of consisting of more than one species. Between-species differentiation was unrelated to geographic distance, despite significant isolation by distance within species. The distinction between two and only two species differing by host plant thus holds at continental scale. Interestingly, one microsatellite locus contributed ∼10 times more than the others to differentiation between both taxa. This deserves further investigation, as it might reveal a linkage between this outlier and loci involved in host-plant adaptation and/or reproductive isolation.

Can deuterium stable isotope values be used to assign the geographic origin of an auxiliary hoverfly in south-western France?

Deuterium δD isotopic analysis is increasingly being used to trace wildlife movement, and undoubtedly has much to offer in this respect, but questions still remain as to the feasibility and practicality of the method in ecology. Here we report our attempt to determine the geographic origin of an auxiliary hoverfly, Episyrphus balteatus, in south-western France. We used quantile regression to calculate the minimum separation distance, based on the International Atomic Energy Agency/World Meteorological Organization (IAEA/WMO) data, at which two insects could be said to originate from different latitudes with a given degree of confidence. We collected larvae in spring 2007 and 2009 to obtain the δD signal of indigenous hoverflies and we trapped adults during one complete year (from Dec. 2006 to Nov. 2007). The smallest separation distance calculated was about 1400 km in western Europe. Our results revealed greater variability in δD of adults in autumn than in spring. From this we infer an autumnal migration. Because of the presence of mountains and the Mediterranean Sea, the δD gradient in precipitation in western Europe is less clear than on the American continent, where it has been used successfully to infer geographical origins of animals under certain conditions. Despite the complications encountered in Europe, the minimum separation distance model proved a useful first step to obtain a first range of possible origins of E. balteatus and the application of the model to other arthropod species in Europe warrants investigation.

Did the introduction of maize into Europe provide enemy-free space to Ostrinia nubilalis? Parasitism differences between two sibling species of the genus Ostrinia.

We examined whether maize offers enemy-free space (EFS) to its pest Ostrinia nubilalis, and may thereby have contributed to its divergence from the sibling species, Ostrinia scapulalis, feeding mainly on mugwort, when introduced into Europe five centuries ago. We collected Ostrinia larvae on maize (70 populations, 8425 individuals) and mugwort (10 populations, 1184 individuals) and recorded parasitism using both traditional (counting emerging parasitoids) and molecular methods (detection by specific polymerase chain reaction). The main parasitoid was Macrocentrus cingulum (Braconidae). On mugwort, parasitism was twice that on maize, and parasitoid-related mortality was 8 times higher. This suggests that maize affords substantial EFS to Ostrinia feeding on it. The lower Mortality:Infestation ratio in maize suggests that O. nubilalis' immune response might be stronger than that of O. scapulalis. If so, adapting to maize and diverging from O. scapulalis would decrease the impact of parasitism on O. nubilalis at both ecological and evolutionary levels.

Differences in oviposition behaviour of two sympatric sibling species of the genus Ostrinia.

Changes in host preferences are thought to be a major source of genetic divergence between phytophagous insect taxa. In western Europe, two sympatric taxa, O. nubilalis (the European corn borer) and O. scapulalis, feed mainly on maize and hop or mugwort, respectively. These two species may have diverged without geographic isolation after a host shift of ancestral populations onto maize or another cultivated species (e.g. sorghum). A previous study using inbred laboratory strains revealed that the two species differ in their oviposition choices in maize-mugwort tests. We sampled four natural populations in France (two of each taxon) and tested their oviposition behaviour toward four of their main host plant species: maize, sorghum, mugwort and hop. O. nubilalis females showed a very high preference for laying their eggmasses on maize, whereas O. scapulalis females displayed a more balanced range of preferences. O. nubilalis females were attracted slightly to sorghum, suggesting that this plant is an accidental, rather than a regular and ancestral host plant of O. nubilalis. One important result arising from this study is the significant proportion of eggs laid by both Ostrinia species on hop. This may explain why some stands of hop are sometimes not only infested by O. scapulalis but also by O. nubilalis larvae, a situation preventing assortative mating based on microallopatry. Hence, further studies must be conducted to see whether the host preference in the genus Ostrinia might be linked to assortative mating by a mechanism that is not mediated by the host plant.

Genetic structure and gene flow in French populations of two Ostrinia taxa: host races or sibling species?

Most models of ecological speciation concern phytophagous insects in which speciation is thought to be driven by host shifts and subsequent adaptations of populations. Despite the ever-increasing number of studies, the current evolutionary status of most models remains incompletely resolved, as estimates of gene flow between taxa remain extremely rare. We studied the population genetics of two taxa of the Ostrinia genus--one feeding mainly on maize and the other on mugwort and hop--occurring in sympatry throughout France. The actual level of divergence of these taxa was unknown because the genetic structure of populations had been investigated over a limited geographical area and the magnitude of gene flow between populations had not been estimated. We used 11 microsatellite markers to investigate the genetic structure of populations throughout France and the extent of gene flow between the two Ostrinia taxa at several sites at which they are sympatric. We observed clear genetic differentiation between most populations collected on the typical respective hosts of each taxon. However, populations displaying intermediate allelic frequencies were found on hop plants in southern France. Individual assignments revealed that this result could be accounted for by the presence of both taxa on the same host. Gene flow, estimated by determining the proportion of hybrids detected, was low: probably<1% per generation, regardless of site. This indicates that the two Ostrinia taxa have reached a high level of genetic divergence and should be considered sibling species rather than host races.

Using stable isotopes to trace origin and host plants of an African polyphagous pest and an European beneficial insect.

Crop protection against insect pests requires first a good knowledge of the biology and ecology of the different pest species and the associated beneficials, in particular the spatial distribution of the populations. But the movement of insect populations in the landscape remains often poorly known and in some cases does not make it possible to know the role of the various cultivated and wild habitats in the dynamics of pest and useful insects. Stable isotopes are a tool contributing to the knowledge of host plants (13carbon/12carbon) as well as geographical origin of insects (1hydrogen/2hydrogen). The analysis of stable isotopic ratios has been performed in south-western France on populations of the hoverfly Episyrphus balteatus, one of the most important predators of the cereal aphids in Europe and on West African populations of the bollworm Helicoverpa armigera, an important polyphagous pest attacking cotton and vegetables in the Old World. Methodology, preliminary results and perspectives given by stable isotopes are presented here.

Should growing and adult animals fed on the same diet show different delta 15N values?

Animals usually show a delta 15N value that is slightly higher than that of their food. The value of this enrichment appears to be fairly constant among species (approximately 3/1000). This phenomenon is more and more widely used in ecological research to study individual diets or the trophic structure of whole communities. However, very little is known about the mechanisms that actually explain how this trophic isotopic enrichment occurs. Most ideas about these mechanisms are only expressed verbally, so that it is difficult to get a clear picture of what is actually known, and how these pieces of knowledge interact. We propose a very simple model that describes mathematically what is currently known on the trophic isotopic enrichment phenomenon in animals. This model cannot replace actual measurements and investigations about the mechanisms explaining the phenomenon. However, it allows the clarification of some ideas such as what conditions have to be fulfilled in order for the trophic isotopic enrichment to occur. Our formalization accounts for all the known features of the trophic isotopic enrichment phenomenon that we consider in this paper in both a qualitative and quantitative manner (at least for orders of magnitude). A prediction of the theory, that can be tested, is that growing animals should show the same delta 15N values as those of adults fed the same diet if the total amount of nitrogen they assimilate during their growth is large compared to the total nitrogen content of their adult body. It seems likely that this condition is fulfilled in most cases.

Effects of several preservation methods on the isotopic content of Drosophila samples.

Researchers working with stable isotopes are faced with the problem of preserving animal samples without altering their isotope ratios. We evaluated the effects of several preservation treatments on the isotopic content of Drosophila samples. Results show that, when there is a danger of rotting, preservation treatment is indispensable to preserve intact isotope ratios, but that not all treatments are equally appropriate. The best is freezing without liquid nitrogen. Ethanol or ethylene glycol preserve the delta 15N, but change the delta 13C. Formalin should be avoided. Storage in a NaCl solution may be a good short-term alternative to freezing for both elements. In the control males and females, the trophic isotopic shift was, respectively, +2.9 and +2.6/1000 for delta 15N, and +1.1/1000 for delta 13C (no difference between sexes). Despite a standardised rearing protocol, there were small but significant inter-tube differences, caused by microenvironmental rather than by genetic factors.