Regula Schmid-Hempel

Trans-generational immune priming in a social insect.

Detecting functional homology between invertebrate and vertebrate immunity is of interest in terms of understanding the dynamics and evolution of immune systems. Trans-generational effects on immunity are well known from vertebrates, but their existence in invertebrates remains controversial. Earlier work on invertebrates has interpreted increased offspring resistance to pathogens as trans-generational immune priming. However, interpretation of these earlier studies involves some caveats and thus full evidence for a direct effect of maternal immune experience on offspring immunity is still lacking in invertebrates. Here we show that induced levels of antibacterial activity are higher in the worker offspring of the bumblebee, Bombus terrestris L., when their mother queen received a corresponding immune challenge prior to colony founding. This shows trans-generational immune priming in an insect, with ramifications for the evolution of sociality.

Female mating frequencies in Bombus spp. from Central Europe.

Summary: The mating frequency of females in social insects is particularly interesting, because polyandry reduces colony relatedness and increases within-colony genetic variance. It thereby affects a complex balance of benefits and costs that determine the degree of reproductive skew, sex allocation to offspring, or the opportunities for nepotism and policing strategies. Few systematic surveys of female mating frequencies exist and many are based on unreliable behavioural observation or sperm counts. Here, we report the results of a survey of mating frequencies in eight European Bombus spp. by means of highly polymorphic microsatellite loci. Only B. hypnorum was found to be multiply mated, while the pattern found in B. terrestris, B. lucorum, B. pratorum, B. lapidarius, B. sicheli, B. hortorum, and B. pascuorum was compatible with single mating. The findings are compatible with recent claims that, with some exceptions, mating frequencies of social insect females are generally low.

Transmission of a pathogen in Bombus terrestris, with a note on division of labour in social insects

SummaryParasites of social insect workers can be transmitted within the colony to other, related host individuals or, alternatively, to unrelated workers of other colonies. Division of labour affects the probability of transmission, as young individuals often work inside the nest whereas older ones often leave the nest to forage. Therefore, the relative probabilities of transmission within-vs. between-nests is also affected by the delay between host infection and the shedding of propagules, i.e. the latent period of the parasite strain. We therefore hypothesized that strains of the flagellate parasite Crithidia bombi (Trypanosomatidae, Zoomastigophorea) infecting workers of the bumble bee Bombus terrestris (Hymenoptera, Apidae) could differ in their delays and coexist in a population. This would be the case if strains that are shed after a short time delay were more efficiently transmitted to other colony members, whereas strains with long delays were more efficiently transmitted to non-related workers in the population. We tested this hypothesis by experimentally varying time delay and by allowing transmission to either sister workers from the same nest or unrelated workers from other nests. Transmission of C. bombi was measured as the number of parasitic cells shed by the exposed workers after a standard period. The results showed that relatedness as such had no effect, but that delay and nest identity were highly significant effects to explain variation in transmission success. There was a significant interaction between nest identity and delay, such that bees of some colonies acted as efficient transmitters for C. bombi under short delays and vice versa. We discuss how division of labour may affect parasitism in social insects and, vice versa, how division of labour may be under selection from the effects of parasitism, using available evidence from the literature.

DYNAMIC AND GENETIC CONSEQUENCES OF VARIATION IN HORIZONTAL TRANSMISSION FOR A MICROPARASITIC INFECTION

Transmission to a new host is a critical step in the life cycle of a parasite. Variation in the characteristics of the transmission process, for example, due to host demography, is assumed to select for different variants of the parasite. We have experimentally tested how variation in the time to transmission (early or late after infection) and exposure to adverse conditions outside the host (immediate or delayed contact with new host) interact to determine the success of the infection in the next host, using the trypanosome Crithidia bombi infecting its bumblebee host, Bombus terrestris. These two experimentally manageable steps mimic the processes of within‐ and among‐host selection for the parasite. We found that early transmission led to higher infection success in the next host as did immediate contact with the new host. However, there was no interaction between the two parameters as would be expected if early‐transmitted variants, resulting from rapid multiplication within the host, would be less adapted to the conditions encountered during the between‐host transfer or infection of the next host. Furthermore, typing the genetic variability of the parasites with microsatellites showed that the four different transmission routes of our experiment selected for different degrees of allelic diversity of the infecting parasite populations. The results support the idea that variation in the transmission process selects for different genotypic variants of the parasite. At the same time, the relationship of allelic diversity with infection intensity suggested that the coinfection model of May and Nowak (1995) may be appropriate, where each parasite is able to infect and multiply independent of others within the same host.

The invasion of southern South America by imported bumblebees and associated parasites

The Palaearctic Bombus ruderatus (in 1982/1983) and Bombus terrestris (1998) have both been introduced into South America (Chile) for pollination purposes. We here report on the results of sampling campaigns in 2004, and 2010-2012 showing that both species have established and massively expanded their range. Bombus terrestris, in particular, has spread by some 200 km year(-1) and had reached the Atlantic coast in Argentina by the end of 2011. Both species, and especially B. terrestris, are infected by protozoan parasites that seem to spread along with the imported hosts and spillover to native species. Genetic analyses by polymorphic microsatellite loci suggest that the host population of B. terrestris is genetically diverse, as expected from a large invading founder population, and structured through isolation by distance. Genetically, the populations of the trypanosomatid parasite, Crithidia bombi, sampled in 2004 are less diverse, and distinct from the ones sampled later. Current C. bombi populations are highly heterozygous and also structured through isolation by distance correlating with the genetic distances of B. terrestris, suggesting the latters expansion to be a main structuring factor for the parasite. Remarkably, wherever B. terrestris spreads, the native Bombus dahlbomii disappears although the reasons remain unclear. Our ecological and genetic data suggest a major invasion event that is currently unfolding in southern South America with disastrous consequences for the native bumblebee species.

A depauperate immune repertoire precedes evolution of sociality in bees

BackgroundSociality has many rewards, but can also be dangerous, as high population density and low genetic diversity, common in social insects, is ideal for parasite transmission. Despite this risk, honeybees and other sequenced social insects have far fewer canonical immune genes relative to solitary insects. Social protection from infection, including behavioral responses, may explain this depauperate immune repertoire. Here, based on full genome sequences, we describe the immune repertoire of two ecologically and commercially important bumblebee species that diverged approximately 18 million years ago, the North American Bombus impatiens and European Bombus terrestris.ResultsWe find that the immune systems of these bumblebees, two species of honeybee, and a solitary leafcutting bee, are strikingly similar. Transcriptional assays confirm the expression of many of these genes in an immunological context and more strongly in young queens than males, affirming Bateman’s principle of greater investment in female immunity. We find evidence of positive selection in genes encoding antiviral responses, components of the Toll and JAK/STAT pathways, and serine protease inhibitors in both social and solitary bees. Finally, we detect many genes across pathways that differ in selection between bumblebees and honeybees, or between the social and solitary clades.ConclusionsThe similarity in immune complement across a gradient of sociality suggests that a reduced immune repertoire predates the evolution of sociality in bees. The differences in selection on immune genes likely reflect divergent pressures exerted by parasites across social contexts.

Frequency and ecological correlates of parasitism by conopid flies (Conopidae, Diptera) in populations of bumblebees

SummaryWe present field data on the ecology of a host-parasite system, consisting of several species of parasitoid flies (Conopidae, Diptera) and their bumblebee hosts (Bombini, Apoidea, Hymenoptera). Host animals were systematically sampled from different study sites throughout a season and checked for successful infestation in the form of puparia of these endoparasitic flies. Such dissection of the bees revealed that infestation occurs primarily during the summer months (June to September), with an observed maximum frequency of parasitization of 46.7% in workers in one of our study sites. On average, 13.2% of all workers (range 0–46.7%) and 7.1% of all males (range 0–28.6%) contained the puparium of a conopid. Two conopid generaSicus (64% of cases) andPhysocephala (36%) accounted for the infestation, with the latter being more abundant later in the year. A multivariate analysis identified host species, sex (male or worker), and study area as the most important factors accounting for the observed variance in the probability of being parasitized during the summer months. On average, males were less affected than workers. The marked seasonal appearance of conopids seems to account for differences among species, in particular for low levels of infestation among species completing their life cycles early (e.g.B. pratorum) and among the early flying, hibernated quens ofBombus andPsithyrus species. The results are discussd with respect to the impact of conopids on host ecology and evolution. Additional observations on the occurrence of further endoparasites (Sphaerularia bombi andSyntretus sp.) are reported.ZusammenfassungErgebnisse aus einer Feldstudie zur Ökologie eines Wirt-Parasit Systems, bestehend aus mehreren Arten von parasitoiden Fliegen (Conopidae, Diptera) und ihren Hummel-Wirten (Bombini, Apoidea, Hymenoptera), werden dargestellt. Wirtstiere wurden systematisch in verschiedenen Untersuchungsgebieten über eine ganze Saison gesammelt und auf erfolgreiche Parasitierung, erkennbar am Puparium der endoparasitischen Fliege, geprüft. Parasitierung erfolgt vor allem in den Sommermonaten (Juli–September), wobei die beobachtete maximale Häufigkeit des Befalls 46.7% (für Arbeiterinnen) betrug. Durchschnittlich sind 13.2% aller Arbeiterinnen (Spannweite: 0–46,7%) und 7.1% aller Männchen (0–28.6%) befallen, d.h. enthielten ein Puparium wenn die Tiere nach ihrem Tod im Labor eröffnet wurden. Zwei Conopiden-Gattungen,Sicus (64% der Beobachtungen) undPhysocephala (36%) waren zu finden, wobei die letztere später im Jahr häufiger wird. Eine multivariate Analyse zeigte, dass Wirtsart, Geschlecht (Arbeiterin, Männchen) und Undersuchungsgebiet die wichtigsten Faktoren sind, welche die Wahrscheinlichkeit der Parasitierung beeinflussen. Im Durchschnitt waren Männchen weniger befallen als Arbeiterinnen. Das ausgeprägte saisonale Auftreten der Conopiden scheint in erster Linie für die Unterschiede im Befall zwischen Wirtsarten verantwortlich zu sein. So sind frühe Arten (z.B.B. pratorum) und früh fliegende Königinnen vonBombus undPsithyrus im Frühjahr kaum befallen. Die Ergebnisse werden im Zusammenhang mit dem möglichen Einfluss der Conopiden auf die Ökologie und Evolution des Wirts diskutiert. Zusätzliche Beobachtungen über das Vorkommen weiterer wichtiger Endoparasiten (Sphaerularia bombi undSyntretus sp.) werden berichtet.

The Bee Microbiome: Impact on Bee Health and Model for Evolution and Ecology of Host-Microbe Interactions

ABSTRACT As pollinators, bees are cornerstones for terrestrial ecosystem stability and key components in agricultural productivity. All animals, including bees, are associated with a diverse community of microbes, commonly referred to as the microbiome. The bee microbiome is likely to be a crucial factor affecting host health. However, with the exception of a few pathogens, the impacts of most members of the bee microbiome on host health are poorly understood. Further, the evolutionary and ecological forces that shape and change the microbiome are unclear. Here, we discuss recent progress in our understanding of the bee microbiome, and we present challenges associated with its investigation. We conclude that global coordination of research efforts is needed to fully understand the complex and highly dynamic nature of the interplay between the bee microbiome, its host, and the environment. High-throughput sequencing technologies are ideal for exploring complex biological systems, including host-microbe interactions. To maximize their value and to improve assessment of the factors affecting bee health, sequence data should be archived, curated, and analyzed in ways that promote the synthesis of different studies. To this end, the BeeBiome consortium aims to develop an online database which would provide reference sequences, archive metadata, and host analytical resources. The goal would be to support applied and fundamental research on bees and their associated microbes and to provide a collaborative framework for sharing primary data from different research programs, thus furthering our understanding of the bee microbiome and its impact on pollinator health.

Parasitic flies (Conopidae, Diptera) may be important stress factors for the ergonomics of their bumblebee hosts

Bumblebees harbour a wide range of parasitic organisms that attack all stages of their life cycles (reviews in Postner, 1951; Pouvreau, 1973, 1974; Alford, 1975; Kistner, 1982). Among them, conopid flies (Conopidae, Diptera) are particularly interesting because they attack foraging bumblebees which are handling flowers, or even on the wing (Frison, 1926; Cumber, 1949; Postner, 1951; Howell, 1967; Askew, 1971). A single egg is attached to (Frison, 1926; Plath, 1934; Cumber, 1949) or inserted into (DeMeijre, 1904; Howell, 1967) the hosts abdomen, where the larva hatches and feeds on haemolymph and internal organs. Within 6–10 days the larva passes through three recognizable stages (Pouvreau, 1974) before the fly pupates in situ within the abdomen. The host bee dies shortly before the parasite pupates (Postner, 1951; Smith, 1966) and the parasite overwinters in its puparium; the adult fly then emerges in early summer (Frison, 1926; Townsend, 1935; Cumber, 1949; Postner, 1951). Conopid flies as parasites of bumblebees are known from all major habitats where the hosts occur (e.g. Kröber, 1939; Smith, 1966). However, the effect of parasitism on distribution and abundance of bumblebees is not known. In this preliminary note we have estimated degrees of infestation and concomitant reduction of life span in affected workers. The results are compared with literature reports on infestation levels in Europe.

Do parasitized bumblebees forage for their colony

Bumblebee workers gain most of their fitness indirectly by helping their mother rear her offspring although some may lay their own eggs (e.g. Duchateau & Velthuis 1988). However, fitness in either case depends on the resources available in the nest which ultimately determines productivity through the effects on colony growth and survival. Amount and quality of resources in turn are a function of the activity of the workers that forage for nectar and pollen. It is therefore assumed that foraging patterns of bumblebee workers, and social insects in general, are adaptations solely for obtaining food, and should follow economic principles that allow for maximum rate of colony growth, and hence, maximum reproductive output (Houston et al. 1988). The food-collecting behaviour of bumblebees has therefore been widely studied and factors that influence economic profitability of foraging have been analysed; some of these studies became classical examples of behavioural economics (e.g. optimal foraging: Pyke 1979; resource partitioning: Inouye 1980; Pyke 1982; Harder 1985). However, foraging bumblebee workers not only obtain benefits by collecting resources, they are also at risk of becoming parasitized by parasitic flies (Diptera: Conopidae; Schmid-Hempel et al. 1990). From June to September, when workers are most abundant, female conopids attack bumblebees foraging on flowers or even in flight, and insert an egg into the abdomen of the host. The larva develops through three larval instars in about 12 days. By this time the whole abdomen is filled, the host dies and the parasite pupates in situ. The next generation of conopids emerges in early summer of the next year. Parasitism frequencies of up to 70% have been found in field populations, depending on site and season (Schmid-Hempel & Schmid-Hempel 1989; Schmid-Hempel et al. 1990). Parasitism by conopid flies reduces the lifespan of the host and should therefore also affect colony growth and reproductive output. However, during its remaining lifespan a parasitized worker could still contribute to colony reproduction (and therefore its own) by working as hard as possible. On the other hand, parasitism could lower foraging efficiency by stressing the host physiologically. Schmid-Hempel & Schmid-Hempel (1990) suggested that infested workers change their foraging pattern by choosing flowers with shorter corollas and shorter handling times, factors known to be important in determining economic profitability. These results indicate that with respect to foraging behaviour parasitized bees may behave differently from unparasitized ones. It is difficult to test directly whether the behaviour of parasitized bumblebee foragers is altered, since artificial infestation is not yet feasible. We therefore compared two samples ofBombus lucorum workers: foragers arriving at or departing from nine field colonies (Colony foragers) and another sample of workers foraging in adjacent flower meadows (Field foragers of unknown colony origin). The data were collected within 2 weeks in the second half of July, a time when conopids are most abundant. We assumed that Colony foragers and Field foragers collected resources in the same area and therefore experienced similar parasite pressure. These workers were dissected to check for the presence of parasitoids. Prevalence (frequency of hosts containing at least one larva or egg) for each sample was determined. Additionally, length of wing radial cell and fresh weight were taken to assess the effect of host size. Prevalence among Colony foragers was significantly lower than that among Field foragers (Table I). In addition, parasitized and non-parasitized bees did not differ in mean size (Welchs t = 1.03, df= 162, NS) or fresh weight (t =0.90, df= 164, NS). Our result supports the idea that foraging behaviour of bumblebees is not independent of parasitism by conopid flies. The significantly higher prevalence among Field foragers suggests that infested foragers remain outside the nest longer. They may have abandoned the nest entirely, which is supported by