Genomic changes underlying host specialization in the bee gut symbiont Lactobacillus Firm5

Bacteria that engage in long‐standing associations with particular hosts are expected to evolve host‐specific adaptations that limit their capacity to thrive in other environments. Consistent with this, many gut symbionts seem to have a limited host range, based on community profiling and phylogenomics. However, few studies have experimentally investigated host specialization of gut symbionts and the underlying mechanisms have largely remained elusive. Here, we studied host specialization of a dominant gut symbiont of social bees, Lactobacillus Firm5. We show that Firm5 strains isolated from honey bees and bumble bees separate into deep‐branching host‐specific phylogenetic lineages. Despite their divergent evolution, colonization experiments show that bumble bee strains are capable of colonizing the honey bee gut. However, they were less successful than honey bee strains, and competition with honey bee strains completely abolished their colonization. In contrast, honey bee strains of divergent phylogenetic lineages were able to coexist within individual bees. This suggests that both host selection and interbacterial competition play important roles in host specialization. Using comparative genomics of 27 Firm5 isolates, we found that the genomes of honey bee strains harbour more carbohydrate‐related functions than bumble bee strains, possibly providing a competitive advantage in the honey bee gut. Remarkably, most of the genes encoding carbohydrate‐related functions were not conserved among the honey bee strains, which suggests that honey bees can support a metabolically more diverse community of Firm5 strains than bumble bees. These findings advance our understanding of the genomic changes underlying host specialization.     

Allatotropin, leucokinin and AKH in honey bees and other Hymenoptera

In the honey bee no allatotropin gene has been found, even though allatotropin stimulates the synthesis of juvenile hormone in this species. We report here that honey bees and other Hymenoptera do have a typical allatotropin gene, although the peptides predicted have a somewhat different structure from that of other insect allatotropins. Polyclonal antisera to honey bee allatotropin reacted with material in the neurohemal organs of the segmental nerves of abdominal ganglia. We were unable to find the allatotropin peptide using mass spectrometry in extracts from these tissues. Thus the expression of this gene in honey bees is less important than in other insect species. We also characterized the leucokinin gene which similarly appears to be very weakly expressed in worker honey bees. Unlike the allatotropin gene, which is conserved within Hymenoptera, the leucokinin gene is much more variable in structure and was not found in ants nor the parasitic wasp Nasonia vitripennis. The absence of significant expression of adipokinetic hormone (AKH) in the honey bee may be due to the existence of a second TATA box in the promotor region of the gene, which explains the production of an mRNA encoding a putative peptide precursor from which no AKH should be released. Such a second TATA box was not found in other Hymenoptera, and may therefore be specific for the two Apis species. It is suggested that functional disintegration of this important metabolic gene became possible in Apis because of the highly evolved social nature of the species.     

Cuticle differentiation during Drosophila embryogenesis

The constitutive criterion for the evolutionary successful clade of ecdysozoans is a protective exoskeleton. In insects the exoskeleton, the so-called cuticle consists of three functional layers, the waterproof envelope, the proteinaceous epicuticle and the chitinous procuticle that are produced as an extracellular matrix by the underlying epidermal cells. Here, we present our electron-microscopic study of cuticle differentiation during embryogenesis in the fruit fly Drosophila melanogaster. We conclude that cuticle differentiation in the Drosophila embryo occurs in three phases. In the first phase, the layers are established. Interestingly, we find that establishment of the layers occurs partially simultaneously rather than in a strict sequential manner as previously proposed. In the second phase the cuticle thickens. Finally, in the third phase, when secretion of cuticle material has ceased, the chitin laminae acquire their typical orientation, and the epicuticle of the denticles and the head skeleton darken. Our work will help to understand the phenotypes of embryos mutant for genes encoding essential cuticle factors, in turn revealing mechanisms of cuticle differentiation.     

Honey bee (Apis mellifera) gut microbiota promotes host endogenous detoxification capability via regulation of P450 gene expression in the digestive tract

There is growing number of studies demonstrating a close relationship between insect gut microbiota and insecticide resistance. However, the contribution of the honey bee gut microbiota to host detoxification ability has yet to be investigated. In order to address this question, we compared the expression of cytochrome P450s (P450s) genes between gut microbiota deficient (GD) workers and conventional gut community (CV) workers and compared the mortality rates and the pesticide residue levels of GD and CV workers treated with thiacloprid or tau-fluvalinate. Our results showed that gut microbiota promotes the expression of P450 enzymes in the midgut, and the mortality rate and pesticide residue levels of GD workers are significantly higher than those of CV workers. Further comparisons between tetracycline-treated workers and untreated workers demonstrated that antibiotic-induced gut dysbiosis leads to attenuated expression of P450s in the midgut. The co-treatment of antibiotics and pesticides leads to reduced survival rate and a significantly higher amount of pesticide residues in honey bees. Taken together, our results demonstrated that honey bee gut symbiont could contribute to bee health through the modification of the host xenobiotics detoxification pathways and revealed a potential negative impact of antibiotics to honey bee detoxification ability and health.     

蜜蜂肠道菌群结构功能研究进展

在长期的共同进化中,肠道菌群与其宿主形成了紧密的联系,为宿主提供了许多有益的作用。作为一种社会性昆虫,蜜蜂的生活习性为其肠道菌群提供了良好而稳定的传播途径,因此,蜜蜂与其肠道菌群形成了一种紧密的互惠互利共生关系。近年来,随着对蜜蜂肠道菌群了解的不断加深,对蜜蜂肠道菌群功能的研究也不断深入,大量研究表明蜜蜂的肠道菌群在宿主食物的消化代谢、宿主免疫的激活和抵抗致病菌、调节宿主生理等方面都有着重要的作用,同时破坏肠道菌群的稳定对蜜蜂的健康有着明显的负面影响。本文对近年来西方蜜蜂肠道菌群功能研究进行了总结,旨在为进一步深入探索蜜蜂肠道菌群与其宿主的相互作用及在养蜂生产上应用肠道菌群防控疾病提供参考。     

Bacterial probiotics induce an immune response in the honey bee (Hymenoptera: Apidae)

To explore immune system activation in the honey bee, Apis mellifera L., larvae of four ages were exposed through feeding to spores of a natural pathogen, Paenibacillus larvae larvae, to cells of a diverse set of related nonpathogenic bacteria, and to bacterial coat components. These larvae were then assayed for RNA levels of genes encoding two antibacterial peptides, abaecin and defensin. Larvae exposed to either P. l. larvae or a mix of nonpathogenic bacteria showed high RNA levels for the abaecin gene relative to controls. First instars responded significantly to the presence of the nonpathogenic mix within 12 h after exposure, a time when they remain highly susceptible to bacterial invasion. This response was sustained for two successive instars, eventually becoming 21-fold higher in larvae exposed to probiotic spores versus control larvae. The mixture of nonpathogenic bacteria is therefore presented as a potential surrogate for assaying the immune responses of different honey bee lineages. It also is proposed that nonpathogenic bacteria can be used as a probiotic to enhance honey bee immunity, helping bee larvae, and other life stages, survive attacks from pathogens in the field.     

Genetic control of epidermis differentiation in Drosophila

In arthropods, the animal body is isolated from the external environment by a protective exoskeleton called the cuticle. The cuticle of young larvae has certainly been the most scrutinized structure in Drosophila and genetic studies of the pattern of cuticular extensions has provided the main source of our comprehension of the control of embryonic development. However, the complex structure of the cuticle remains poorly understood and analysis of the underlying epidermis has started only recently. Here I review different aspects of epidermis differentiation with the aim of presenting an integrated view of the organisation of the Drosophila integument. Although profound differences in epidermis organisation are observed across species, accumulated results suggest that epidermis formation and differentiation might share an unsuspected number of homologies between Drosophila and vertebrates.     

Pyrosequencing analysis of the bacterial communities in the guts of honey bees Apis cerana and Apis mellifera in Korea

The bacterial communities in the guts of the adults and larvae of the Asian honey bee Apis cerana and the European honey bee Apis mellifera were surveyed by pyrosequencing the 16S rRNA genes. Most of the gut bacterial 16S rRNA gene sequences were highly similar to the known honey bee-specific ones and affiliated with Pasteurellaceae or lactic acid bacteria (LAB). The numbers of operational taxonomic units (OTUs, defined at 97% similarity) were lower in the larval guts (6 or 9) than in the adult guts (18 or 20), and the frequencies of Pasteurellaceae-related OTUs were higher in the larval guts while those of LAB-related OTUs in the adult guts. The frequencies of Lactococcus, Bartonella, Spiroplasma, Enterobacteriaceae, and Flavobacteriaceae-related OTUs were much higher in A. cerana guts while Bifidobacterium and Lachnospiraceae-related OTUs were more abundant in A. mellfera guts. The bacterial community structures in the midguts and hindguts of the adult honey bees were not different for A. cerana, but significantly different for A. mellifera. The above results substantiated the previous observation that honey bee guts are dominated by several specific bacterial groups, and also showed that the relative abundances of OTUs could be markedly changed depending on the developmental stage, the location within the gut, and the honey bee species. The possibility of using the gut bacterial community as an indicator of honey bee health was discussed.