Thomas Lecocq

Born in an Alien Nest : How Do Social Parasite Male Offspring Escape from Host Aggression?

Social parasites exploit the colony resources of social insects. Some of them exploit the host colony as a food resource or as a shelter whereas other species also exploit the brood care behavior of their social host. Some of these species have even lost the worker caste and rely completely on the hosts worker force to rear their offspring. To avoid host defenses and bypass their recognition code, these social parasites have developed several sophisticated chemical infiltration strategies. These infiltration strategies have been highly studied in several hymenopterans. Once a social parasite has successfully entered a host nest and integrated its social system, its emerging offspring still face the same challenge of avoiding host recognition. However, the strategy used by the offspring to survive within the host nest without being killed is still poorly documented. In cuckoo bumblebees, the parasite males completely lack the morphological and chemical adaptations to social parasitism that the females possess. Moreover, young parasite males exhibit an early production of species-specific cephalic secretions, used as sexual pheromones. Host workers might thus be able to recognize them. Here we used a bumblebee host-social parasite system to test the hypothesis that social parasite male offspring exhibit a chemical defense strategy to escape from host aggression during their intranidal life. Using behavioral assays, we showed that extracts from the heads of young cuckoo bumblebee males contain a repellent odor that prevents parasite males from being attacked by host workers. We also show that social parasitism reduces host worker aggressiveness and helps parasite offspring acceptance.

Patterns of genetic and reproductive traits differentiation in Mainland vs. Corsican populations of bumblebees.

Populations on islands often exhibit lower levels of genetic variation and ecomorphological divergence compared to their mainland relatives. While phenotypic differentiation in characters, such as size or shape among insular organisms, has been well studied, insular differentiation in quantitative reproductive traits involved in chemical communication has received very little attention to date. Here, we investigated the impact of insularity on two syntopic bumblebee species pairs: one including species that are phylogenetically related (Bombus terrestris and B. lucorum), and the other including species that interact ecologically (B. terrestris and its specific nest inquiline B. vestalis). For each bumblebee species, we characterized the patterns of variation and differentiation of insular (Corsican) vs. mainland (European) populations (i) with four genes (nuclear and mitochondrial, 3781 bp) and (ii) in the chemical composition of male marking secretions (MMS), a key trait for mate attraction in bumblebees, by gas chromatography-mass spectrometry (GC-MS). Our results provide evidence for genetic differentiation in Corsican bumblebees and show that, contrary to theoretical expectations, island populations of bumblebees exhibit levels of genetic variation similar to the mainland populations. Likewise, our comparative chemical analyses of MMS indicate that Corsican populations of bumblebees are significantly differentiated from the mainland yet they hold comparative levels of within-population MMS variability compared to the mainland. Therefore, insularity has led Corsican populations to diverge both genetically and chemically from their mainland relatives, presumably through genetic drift, but without a decrease of genetic diversity in island populations. We hypothesize that MMS divergence in Corsican bumblebees was driven by a persistent lack of gene flow with mainland populations and reinforced by the preference of Corsican females for sympatric (Corsican) MMS. The impoverished Corsican bumblebee fauna has not led to relaxation of stabilizing selection on MMS but to consistent differentiation chemical reproductive traits on the island.

Scent of a break-up: phylogeography and reproductive trait divergences in the red-tailed bumblebee (Bombus lapidarius)

BackgroundThe Pleistocene climatic oscillations are considered as a major driving force of intraspecific divergence and speciation. During Ice Ages, populations isolated in allopatric glacial refugia can experience differentiation in reproductive traits through divergence in selection regimes. This phenomenon may lead to reproductive isolation and dramatically accentuates the consequences of the climatic oscillations on species. Alternatively, when reproductive isolation is incomplete and populations are expanding again, further mating between the formerly isolated populations can result in the formation of a hybrid zone, genetic introgression or reinforcement speciation through reproductive trait displacements. Therefore changes in reproductive traits driven by population movements during climatic oscillations can act as an important force in promoting pre-zygotic isolation. Notwithstanding, divergence of reproductive traits has not been approached in the context of climatic oscillations. Here we investigate the impact of population movements driven by climatic oscillations on a reproductive trait of a bumblebee species (Bombus lapidarius). We characterise the pattern of variation and differentiation across the species distribution (i) with five genes (nuclear and mitochondrial), and (ii) in the chemical composition of male marking secretions (MMS), a key trait for mate attraction in bumblebees.ResultsOur results provide evidence that populations have experienced a genetic allopatric differentiation, in at least three main refugia (the Balkans, Centre-Eastern Europe, and Southern Italy) during Quaternary glaciations. The comparative chemical analyses show that populations from the Southern Italian refugium have experienced MMS differentiation and an incipient speciation process from another refugium. The meeting of Southern Italian populations with other populations as a result of range expansion at a secondary contact zone seems to have led to a reinforcement process on local MMS patterns.ConclusionsThis study suggests that population movement during Quaternary climatic oscillations can lead to divergence in reproductive traits by allopatric differentiation during Ice Ages and by reinforcement during post-glacial recolonization.

Molecular and chemical characters to evaluate species status of two cuckoo bumblebees: Bombus barbutellus and Bombus maxillosus (Hymenoptera, Apidae, Bombini)

Many methods, based on morphological, molecular or chemical characters, have been used to address the question of species taxonomic status. Integrative taxonomy aims to define stronger supported taxonomic hypotheses by considering complementary datasets from different characters. By following an integrative approach, the present study includes molecular, chemical and morphological criteria to establish the taxonomic status of two rare and doubtful cuckoo bumblebee taxa: Bombus (Psithyrus) barbutellus and Bombus (Psithyrus) maxillosus. These two sympatric taxa are discriminated by few morphological criteria (mainly wing darkness and hair length). We used these morphological character diagnoses to establish an a priori status of our samples (23 specimens). We developed a combined molecular dataset from one nuclear gene, elongation factor 1α (EF‐1α), and one mitochondrial gene, cytochrome c oxidase subunit I (COI), spanning 1623 bp, and a chemical dataset of sexual marking pheromones (73 compounds). The molecular data were subjected to maximum‐likelihood and Bayesian phylogenetic inference under partitioned model and maximum parsimony. The chemical data were analysed by clustering and the two‐group k‐means method to test divergences between the two species. The resulting phylogenetic trees show no consistent divergence between the two taxa. Moreover, we found no divergence in the sexual marking pheromones in the clustering and two‐group k‐means analyses. These converging results support the conspecificity of both taxa. Nonetheless, our determinations using the traditional morphological criteria separated our samples into two taxa. We conclude that the morphological criteria seem to relate to intraspecific variations: B. maxillosus is regarded as a syn.n. of B. barbutellus.

Methods for species delimitation in bumblebees (Hymenoptera, Apidae, Bombus): towards an integrative approach

Delimitation of closely related species is often hindered by the lack of discrete diagnostic morphological characters. This is exemplified in bumblebees (genus Bombus). There have been many attempts to clarify bumblebee taxonomy by using alternative features to discrete morphological characters such as wing shape, DNA, or eco‐chemical traits. Nevertheless each approach has its own limitations. Recent studies have used a multisource approach to gather different lines of speciation evidence in order to draw a strongly supported taxonomic hypothesis in bumblebees. Yet, the resulting taxonomic status is not independent of selected evidence and of consensus methodology (i.e. unanimous procedure, majority, different weighting of evidence). In this article, we compare taxonomic conclusions for a group of taxonomically doubtful species (the Bombus lapidarius‐group) obtained from the four commonly used lines of evidence for species delimitation in bumblebees (geometric morphometric of wing shape, genetic differentiation assessment, sequence‐based species delimitation methods and differentiation of cephalic labial gland secretions). We ultimately aim to assess the usefulness of these lines of evidence as components of an integrative decision framework to delimit bumblebee species. Our results show that analyses based on wing shape do not delineate any obvious cluster. In contrast, nuclear/mitochondrial, sequence‐based species delimitation methods, and analyses based on cephalic labial gland secretions are congruent with each other. This allows setting up an integrative decision framework to establish strongly supported species and subspecies status within bumblebees.

GCALIGNER 1.0: An alignment program to compute a multiple sample comparison data matrix from large eco‐chemical datasets obtained by GC

GCALIGNER 1.0 is a computer program designed to perform a preliminary data comparison matrix of chemical data obtained by GC without MS information. The alignment algorithm is based on the comparison between the retention times of each detected compound in a sample. In this paper, we test the GCALIGNER efficiency on three datasets of the chemical secretions of bumble bees. The algorithm performs the alignment with a low error rate (<3%). GCALIGNER 1.0 is a useful, simple and free program based on an algorithm that enables the alignment of table-type data from GC.

Molecular phylogeny, biogeography, and host plant shifts in the bee genus Melitta (Hymenoptera: Anthophila)

New molecular studies suggested that the family Melittidae is either a paraphyletic group from which all the other bees are derived, or the sister clade to all other existing bees. Studying the historical biogeography and evolution of each major lineage within this group is a key step to understand the origin and early radiation of bees. Melitta is the largest genus of melittid bees, for which a robust molecular phylogeny and a biogeographic analysis are still lacking. Here, we derive a phylogenetic hypothesis from the sequences of seven independent DNA fragments of mitochondrial and nuclear origin. This phylogenetic hypothesis is then used to infer the evolution of the species range and of the host-plant shifts in Melitta. Our results confirmed the monophyly of Melitta, but did not recover all previously defined clades within the genus. We propose new taxa by splitting the genus in three subgenera (including two new subgenera described in the Appendix: Afromelitta subgen. nov., Plesiomelitta subgen. nov.) and describe two new species: Melitta avontuurensis sp. n. and M. richtersveldensis sp. n. Regarding the evolution of host-plant use, our analysis suggests that all species currently specialized on one plant family originated from an ancestor that was specialized on Fabaceae plants. The inferred biogeographic history for the genus supported an African origin. In concordance with previous studies identifying Africa as the geographic origin for many clades of bees, our data bring new evidence for an African origin of melittid bees.

A method for year-round rearing of cuckoo bumblebees (Hymenoptera: Apoidea: Bombus subgenus Psithyrus)

Summary. The study of the social interactions between host bumblebees and their inquiline species requires rearing them. Here we provide a simple method for rearing of cuckoo bumblebees (Bombus subgenus Psithyrus) in the laboratory. Two bumblebee cuckoo-host systems are used: Bombus (Psithyrus) vestalis hosted by B. (Bombus) terrestris and B. (Psithyrus) sylvestris hosted by B. (Pyrobombus) pratorum. First, the invasion of the Psithyrus female is performed in species-specific host colonies containing approximately 10 young workers. On average, a parasitized colony of B. terrestris produced 90 ± 9 young males and 21 ± 3 young females of B. vestalis, whereas a parasitized colony of B. pratorum produced 16 ± 2 young males and 5 ± 1 young females of B. sylvestris. One week after emergence, Psithyrus virgin females are placed in a flight cage exposed to natural light, with Psithyrus males of other colonies. After successful mating, the females are isolated and overwintered in a cold room at 4°C for 2–4 months. This method enables a year-round and mass breeding of Psithyrus species to facilitate studies of these rare species.

Highly polytypic taxon complex: interspecific and intraspecific integrative taxonomic assessment of the widespread pollinator Bombus pascuorum Scopoli 1763 (Hymenoptera: Apidae)

The recent development of the integrative taxonomic approach in bumblebees has led to unexpected merging or splitting of several taxa. Here we investigate the taxonomic statuses of one of the most abundant, widespread and polytypic Palearctic bumblebees, Bombus pascuorum. The latest review of this species includes 24 subspecies. We used an integrative approach based on genetic markers and male chemical reproductive traits and compared our results with the former classifications. Our results show that all B. pascuorum taxa are conspecific and share the same male chemical reproductive traits. The genetic structure observed in one mitochondrial and two nuclear markers poorly reflects the current subspecific classification. Indeed, the concordance of population genetic differentiation, population geographic distribution, and population colour pattern similarity suggests a different meaningful prospective classification with four taxon complexes: (i) the B. pascuorum dusmeti group, including all taxa from the Iberian Peninsula and south‐west France; (ii) B. pascuorum rehbinderi; (iii) B. pascuorum siciliensis; and (iv) B. pascuorum floralis groups (including all other taxa studied here).

Subspecific differentiation in male reproductive traits and virgin queen preferences, in Bombus terrestris

Many species display local variations in pre-mating signals and in mating preferences. This may lead to discrimination against potential foreign mates that may ultimately lead to reproductive isolation. However, the extent to which population differentiation in mating cues affects the species recognition has received little empirical support. Here, we investigate the consequence of geographic differentiation in male reproductive traits on female preferences to these traits in Bombus terrestris. We characterise (1) the geographic differentiation in male cephalic labial gland secretions (CLGS), a key trait for mate attraction, and (2) the preference of virgin females to the CLGS of different subspecies. Our results show geographic CLGS differences parallel with divergences in female preferences for these secretions. This geographic CLGS differentiation in males, along with female preference for sympatric males, could lead to or reflect a pre-mating isolation among subspecies.