The evolutionary history of bees in time and space

A new whip scorpion (Arachnida: Thelyphonida) with a phoretic mite (Acariformes: Trochometridiidae) from Mid-Cretaceous Kachin amber

BACKGROUND

Mid-Cretaceous Kachin amber is a valuable resource for studying the diversity, evolution, and ecology of microarthropods, including arachnids. Its exceptional preservation offers a unique opportunity to uncover biological associations between organisms with high fidelity. Whip scorpions (Thelyphonida) are rare in the fossil record, with a few known from the Paleozoic era and Cretaceous period. However, the ecological interactions of these fossils with other organisms remain largely unexplored.

RESULTS

Here, we describe a new whip scorpion species, Mesothelyphonus xiaoae sp. nov., from Kachin amber. This species is diagnosed by its relatively small body size, an accessory tooth on the pedipalp coxal apophysis, six teeth on the pedipalpal trochanter, and an unmodified abdominal sternite III. Notably, the fossil includes a heterostigmatic mite (Acariformes: Prostigmata: Heterostigmata: Trochometridiidae) attached to the first leg of the whip scorpion. The mite appears to have selected a densely setose area on a limb primarily used for sensory purposes by the host, which may have made it more difficult to dislodge. This association likely represents an instance of phoresy, where the mite benefits from transportation and protection provided by the whip scorpion.

CONCLUSION

Based on modern knowledge of Trochometridium biology and host associations, we suggest that while the whip scorpion served as an incidental host, the primary host was likely a ground-nesting Apoidea (bees or wasps). This hypothesis implies that Mid-Cretaceous ecosystems included early apoids exhibiting nesting behavior, providing an essential niche for the development of this ancient symbiosis.

The radiation of Hymenoptera illuminated by Bayesian inferences from the fossil record

Determining when lineages originated provides fundamental insights into the timing and pace of their diversification, improving our understanding of transformative paleoevents such as the Angiosperm Terrestrial Revolution (ATR)1 and Mid-Mesozoic Parasitoid Revolution (MMPR).2 As the MMPR overlaps with the ATR, improved age estimates help to disentangle the dynamics and temporal succession of these events that shaped modern ecosystems. Hymenoptera (ants, bees, and wasps) played an important role in the MMPR and ATR through their parasitoid and pollinating lineages. Parasitoids impact trophic networks, whereas pollinators interact with flowering plants.3,4 However, our understanding of Hymenoptera diversification remains limited by a lack of fossil-based studies and uncertainties in phylogenetic reconstructions. Combining fossil occurrences and macroevolutionary models, we estimated the origin and diversification of Hymenoptera lineages, considering changes in preservation over time and across taxa.5,6,7 Our results indicate that Hymenoptera diversification is multifaceted and lineage-specific. Sawflies diversified during the Paleozoic and Mesozoic in four episodes (middle Permian, Late Triassic to Middle Jurassic, Early Cretaceous, and the beginning of the Cenozoic) and experienced three extinction episodes (Middle Triassic, Late Jurassic, and mid-Cretaceous). The superfamily Xyeloidea originated during the middle Permian. Apocrita and parasitoid superfamilies emerged during the Early to Middle Triassic, diversified during the Late Jurassic and Early Cretaceous, and declined during the Late Cretaceous. We demonstrate that Hymenoptera experienced successive replacements during the MMPR-likely beginning in the Triassic-and synchronously with changes in floral assemblages of the ATR. We conclude with future directions to refine dating estimates from the fossil record.

Localized Tissue-Specific Gene Expression and Gene Duplications are Important Sources of Social Morph Differences in a Social Bumblebee

Understanding the expression of multiple behaviorally and morphologically distinct phenotypes from a single genome represents a fundamental topic in evolutionary biology. Central to the complication of expressing phenotypes, which may differ in their optima, is the sharing of largely the same genome, which is predicted to manifest in conflict at the genomic level. This is particularly true for social insects where molecular mechanisms, such as differential gene expression, contribute to observed phenotypic differences between reproductive and nonreproductive morphs. In comparison, other mechanisms, such as tissue-specific expression and gene duplications, have been posited as contributing to social morph differences yet formal investigations are limited. Here, using a combination of transcriptomics for multiple tissues and comparative genomics, we show that in a social bumblebee, the strongest differences in gene expression are found in reproductive tissues, such as the spermatheca, an organ previously believed as vestigial in workers but recently shown as functional. In comparison, we find modest expression differences in genes between queens and workers for the brain, fat body, and ovary, which are traditionally investigated in social evolution. Interestingly, morph-biased genes in these three tissues display higher tissue-specificity suggesting that while social morphs may express a shared core transcriptome, localized expression profiles may contribute to phenotypic differences. We also find evidence of differential usage of duplicated genes by queens and workers, highlighting structural variants as a contributing factor to morph differences. Collectively, our findings highlight how social insects can utilize tissue-specific gene regulation and structural variants to contribute to phenotypic differences.

Environmental drivers of wild bee reproductive performance across a South American dryland ecoregion

The reproductive performance of wild bees is a key determinant of their population persistence. However, few studies have directly examined the environmental drivers of demographic processes using a geographically broad approach. In this study, we explored how biotic and abiotic factors influence the reproduction of solitary, cavity-nesting bees across the Monte Desert ecoregion in Argentina. Using artificial nests and a standardized sampling spanning 2000 km and 20° of latitude, we related key reproductive metrics-nest establishment, offspring production and offspring survival-to latitude, climate and biotic factors (flower abundance, vegetation cover and brood parasitism). Climate was the strongest predictor of bee reproductive performance: warm, humid conditions during the nesting period were associated with reduced nest establishment and offspring survival. Brood parasitism further reduced offspring survival. Across the Monte Desert's latitudinal gradient, nest establishment peaked at mid-latitudes, while offspring production and survival increased towards higher latitudes. These general patterns matched those of M. leucografa, the most abundant bee species. These findings highlight the sensitivity of wild bee reproduction to climatic conditions, particularly during the nesting period. Our study advances our understanding of the potential impacts of climate change on Neotropical wild bees, where extensive areas are experiencing dramatic land-use changes.

[genus]_[species]; Presenting phylogenies to facilitate synthesis

Each published phylogeny is a potential contribution to the synthesis of the Tree of Life and countless downstream projects. Steps are needed for fully synthesizable science, but only a minority of studies achieve these. We here review the range of phylogenetic presentation and note aspects that hinder further analysis. We provide simple suggestions on publication that would greatly enhance utilizability, and propose a formal grammar for phylogeny terminal format. We suggest that each published phylogeny should be accompanied by at minimum the single preferred result in machine readable tree (e.g. Newick) form in the supplement, a simple task fulfilled by fewer than half of studies. Further, the tree should be clear from the file name and extension; the orientation (rooted or unrooted) should match the figures; terminals labels should include genus and species IDs; underscores should separate strings within-field (instead of white spaces); and if other informational fields are added these should be separated by a unique delimiting character (we suggest multiple underscores or the vertical pipe character, |) and ordered consistently. These requirements are largely independent of phylogenetic study aims, while we note other requirements for synthesis (e.g. removal of species repeats and uninformative terminals) that are not necessarily the responsibility of authors. Machine readable trees show greater variation in terminal formatting than typical phylogeny images (owing presumably to greater scrutiny of the latter), and thus are complex and laborious to parse. Since the majority of existing studies have provided only images, we additionally review typical variation in plotting style, information that will be necessary for developing the automated phylogeny transcription tools needed for their eventual inclusion in the Tree of Life.

Host specificity of gut microbiota associated with social bees: patterns and processes

SUMMARYGut microbes provide benefits to some animals, but their distribution and effects across diverse hosts are still poorly described. There is accumulating evidence for host specificity (i.e., a pattern where different microbes tend to associate with distinct host lineages), but the causes and consequences of this pattern are unclear. Combining experimental tests in the laboratory with broad surveys in the wild is a promising approach to gaining a comprehensive and mechanistic understanding of host specificity prevalence, origin, and importance. Social bees represent an ideal testbed for this endeavor because they are phylogenetically and functionally diverse, with host-specific, stable, and tractable gut microbiota. Furthermore, the western honeybee (Apis mellifera) is an emerging experimental model system for studying microbiota-host interactions. In this review, we summarize data on the prevalence and strength of host specificity of the social bee gut microbiota (bumblebees, stingless bees, and honeybees), as well as the potential and proven ecological and molecular mechanisms that maintain host specificity. Overall, we found that host specificity in bees is relatively strong and likely results from several processes, including host filtering mediated by the immune system and priority effects. However, more research is needed across multiple social bee species to confirm these findings. To help future research, we summarize emerging hypotheses in the field and propose several experimental and comparative tests. Finally, we conclude this review by highlighting the need to understand how host specificity can influence host health.

Phylogeny, antiquity, and niche occupancy of Trinomia (Hymenoptera: Halictidae), an Afrotropical endemic genus of Nomiinae

The Afrotropical region is home to many endemic bee groups, yet almost none have been studied from an integrated, holistic perspective. Among them, the halictid subfamily Nomiinae contains exceptional African diversity with variable distributions and life histories. Here, we combine phylogenomics, molecular dating, and distributional modelling to explore the evolutionary ecology of the genus Trinomia. We analyzed a matrix of 59 species of Nomiinae using ultraconserved element (UCE) and whole genome data, including all six species of Trinomia, and estimated divergence times for the subfamily. We then generated distribution models for all six species of Trinomia using Maximum Entropy models (MaxEnt) and 671 spatial data points. From these methods, we discovered a monophyletic Trinomia with an unexpected sister group relationship to the Asian-endemic genus Gnathonomia, as well as a recent origin of Trinomia in the late Miocene (∼5.8 million years ago). From our results, we found hints of phylogenetic conservatism in distribution among sister-groups of Trinomia, however, our results also highlight the need for additional efforts inventorying, identifying, and sharing data on African bees. This study represents an exemplary first step into studying bee spatial phylogenomics of African endemic bees.

Gondwanan relic or recent arrival? The biogeographic origins and systematics of Australian tarantulas

The composition of Australia's fauna and flora has been largely assembled by two biogeographic processes, vicariance and long-distance dispersal and establishment. These patterns can be observed today through the survival of Gondwanan lineages contrasted with relatively recent colonization from south-east Asia, respectively. In general, the post-Gondwanan immigrant lineages from south-east Asia are taxa with traits that facilitate dispersal. Consequently, taxa like tarantulas (Araneae, Theraphosidae) that are largely pan-tropical but also have a low propensity for dispersal, are thought to be Gondwanan in origin. However, the Australian tarantulas are unsampled for phylogenomic studies and, as such, their classification and biogeographic origins have been long debated and are unresolved. Here we test if their current, morphology-based classification in Selenocosmiinae is accurate and assess whether the Australian tarantulas were present in Australia while it was part of Gondwana. We sample 369 tarantula specimens from across Australia, greatly expanding the geographic sampling of previous studies, to develop the first continent-wide phylogeny of the Australian tarantulas. To resolve the 'back bone' of the Australian tarantula phylogeny we generate 20 new transcriptomes for species of Australian tarantulas representing distinct lineages uncovered using mitochondrial sequence data and combine these new transcriptomes with published transcriptomic data. Through the recovery of ultra-conserved element (UCE) loci from transcriptomes and testing multiple data occupancy matrices, we find that the Australian clade is monophyletic and nested inside the largely Asian Selenocosmiinae. We find the Australian fauna are a relatively young radiation with a crown age of 18.8-8.3 Ma and we therefore reject the hypothesis of a Gondwanan origin for these animals and, instead, infer a recent dispersal from south-east Asia. Our findings indicate that they underwent a rapid radiation, possibly coinciding with their arrival into Australia. Our findings refute the monophyly of Selenocosmia and Coremiocnemis as currently recognised, and we remove Selenocosmia stalkeri from synonymy with Selenocosmia stirlingi.

Bees remain heat tolerant after acute exposure to desiccation and starvation

Organisms may simultaneously face thermal, desiccation and nutritional stress under climate change. Understanding the effects arising from the interactions among these stressors is relevant for predicting organisms' responses to climate change and for developing effective conservation strategies. Using both dynamic and static protocols, we assessed for the first time how sublethal desiccation exposure (at 16.7%, 50.0% and 83.3% of LD50) impacts the heat tolerance of foragers from two social bee species found on the Greek island of Lesbos: the managed European honey bee, Apis mellifera, and the wild, ground-nesting sweat bee Lasioglossum malachurum. In addition, we explored how a short-term starvation period (24 h), followed by a moderate sublethal desiccation exposure (50% of LD50), influences honey bee heat tolerance. We found that neither the critical thermal maximum (CTmax) nor the time to heat stupor was significantly impacted by sublethal desiccation exposure in either species. Similarly, starvation followed by moderate sublethal desiccation did not affect the average CTmax estimate, but it did increase its variance. Our results suggest that sublethal exposure to these environmental stressors may not always lead to significant changes in bees' heat tolerance or increase vulnerability to rapid temperature changes during extreme weather events, such as heat waves. However, the increase in CTmax variance suggests greater variability in individual responses to temperature stress under climate change, which may impact colony-level performance. The ability to withstand desiccation may be impacted by unmeasured hypoxic conditions and the overall effect of these stressors on solitary species remains to be assessed.

The Mitochondrial Genome of Melipona fasciculata (Apidae, Meliponini): Genome Organization and Comparative Analyses, Phylogenetic Implications and Divergence Time Estimations

The native stingless bee Melipona fasciculata is economically and ecologically important to the Brazilian Northeast, providing a sustainable source of income to family farmers and being considered an effective pollinator in most ecosystems and crops. This study describes, for the first time, the mitogenome of the species and its phylogenetic position. The mitochondrial genome was sequenced using a MiSeq Sequencer (Illumina Inc.) and compared with other GenBank bee mitogenomes. The length of the mitochondrial DNA, excluding most of the control region, is 14,753 bp, and contains 13 protein-coding genes (PCGs), 21 transfer RNAs, 2 ribosomal RNAs (16S and 12S), and 1 AT-rich region. The GC-content of the M. fasciculata mitogenome was 13.4%. Of the 36 coding regions, 12 tRNAs and 9 PCGs were encoded in the heavy strand, and 9 tRNAs, 4 PCGs and 2 rRNAs were encoded in the light strand. The relative orientation and gene order was the same as other stingless bee mitogenomes. Phylogenetic inference produced well-resolved relationships with high statistical support for concordant branch topologies, under different optimization schemes and model parameters, within and among Melipona, Bombus, Apis, and related clades of Hymenoptera. In general, our divergence time estimates, which were based on the concatenated gene sequences (PCGs + rRNAs) from various groups, overlapped estimations captured by Bayesian analysis from different studies. The divergence time among Melipona species was estimated to occur during the Oligocene, approximately 24 Mya (95% HPD 14-36 Mya). Our results represent a valuable addition to help understanding not only the taxonomy and evolution of Brazilian stingless bee species, but also to uncover historical dispersal and isolation patterns in Meliponinae.

Molecular Basis of Eusocial Complexity: The Case of Worker Reproductivity in Bees

In eusocial insects, the molecular basis of worker reproductivity, including how it changes with eusocial complexity, remains relatively poorly understood. To address this, we used mRNA-seq to isolate genes differentially expressed between ovary-active and ovary-inactive workers in the intermediately eusocial bumblebee Bombus terrestris. By comparisons with data from the advanced eusocial honeybee Apis mellifera, which shows reduced worker reproductivity, we characterized gene expression differences associated with change in worker reproductivity as a function of eusocial complexity. By comparisons with genes associated with queen-worker caste development in B. terrestris larvae, we tested the behavioral-morphological caste homology hypothesis, which proposes co-option of genes influencing reproductive division of labor in adults in morphological caste evolution. We conducted comparisons having isolated genes expressed in B. terrestris worker-laid eggs to remove the potential confound caused by gene expression in eggs. Gene expression differences between the B. terrestris worker phenotypes were mainly in fat body and ovary, not brain. Many genes (86%) more highly expressed in ovary of ovary-active workers were also expressed in worker-laid eggs, confirming egg-expressed genes were potentially confounding. Comparisons across B. terrestris and A. mellifera, and with B. terrestris larvae, returned significant percentage overlaps in differentially expressed genes and/or enriched Gene Ontology terms, suggesting conserved gene functions underpin worker reproductivity as it declines with increasing eusocial complexity and providing support for the behavioral-morphological caste homology hypothesis. Therefore, within bees, both a degree of conserved gene use and gene co-option appear to underlie the molecular basis of worker reproductivity and morphological caste evolution.

Phylogenomic insights into the worldwide evolutionary relationships of the stingless bees (Apidae, Meliponini)

Stingless bees (tribe Meliponini) are remarkable for their characteristically large social colonies, their capacity to produce honey and other useful products, and their morphological and behavioral diversity. They have a disjunct pan-tropical distribution, primarily occurring in warm and humid environments in the Neotropical, Afrotropical, and Indo-Australasian regions. Even though phylogenetic hypotheses have been proposed for Meliponini based on morphology and molecular data, many questions are still unsolved regarding the evolutionary relationships and systematics of the tribe. In this contribution, we present a large phylogenomic dataset comprising over 2500 ultra-conserved element (UCE) loci sequenced for 153 species of Meliponini, representing all known genera of stingless bees. The genera Camargoia, Paratrigonoides, Plectoplebeia, Cleptotrigona, Ebaiotrigona, Papuatrigona, Pariotrigona, Platytrigona, and Sahulotrigona were included in molecular phylogenetic analyses for the first time. Concatenated and species-tree analyses were performed using different partitioning strategies and summary methods. We performed gene-genealogy interrogation (GGI) on several recalcitrant nodes to resolve discordances among recovered tree topologies. Results were mostly consistent among analyses, recovering three main lineages of Meliponini congruent with the biogeographic domains to which they are associated. Within major clades, discordances were found in relation to previous works. The genus Frieseomelitta was recovered as paraphyletic in relation to Trichotrigona, and the genus Lepidotrigona was revealed to be composed of two independent lineages. Even though concatenated and weighted ASTRAL analyses were mostly effective in recovering the relationships favored by GGI, they retrieved different results in relation to the phylogenetic placements of Oxytrigona and Cephalotrigona. The most favored hypothesis in GGI analyses was not found in any other analyses, being more congruent with morphological evidence and highlighting the relevance of exploring the support given to alternative hypotheses through topological tests. Recent advances in our capacity to generate molecular sequences from old specimens using modern sequencing methods allowed for unparalleled sampling across genera, yielding a backbone for the phylogenetic relationships of stingless bees, which will further investigations into their systematics and evolution.

Revealing the Baja California Peninsula's Hidden Treasures: An Annotated checklist of the native bees (Hymenoptera: Apoidea: Anthophila)

To date, the knowledge of bee diversity in the Baja California Peninsula has primarily relied on large, sporadic expeditions from the first half of the 20th century. To address the knowledge gaps, we conducted extensive fieldwork from 2019 to 2023, visited entomological collections in Mexico and USA, and accessed digital databases and community science platforms to compile records. As a result of our field surveys, we identified 521 morphospecies, with 350 recognized as valid species, including 96 new records for the Baja California Peninsula and 68 new findings for Mexico, including the rediscovery of Megachile seducta Mitchell, 1934, ranked as possibly extinct. Additionally, museum visits added 24 new species records for the peninsula, including 12 new to Mexico. Integrating the new and existing records results in a comprehensive checklist that documents 728 species for the peninsula, 613 for Baja California, and 300 for Baja California Sur. Notably, 62 species are endemic to the peninsula, of which 22 are only found in Baja California, and 23 in Baja California Sur. Our findings show a greater bee diversity in northern latitudes, with a sharp decrease to the central and southern peninsula, which corresponds to the geographic distribution of the records. This supports the premise that the Baja California peninsula remains an unexplored area and highlights the importance of conducting studies like the one presented here.

Phylogenomics and biogeography of sawflies and woodwasps (Hymenoptera, Symphyta)

Phylogenomic approaches have recently helped elucidate various insect relationships, but large-scale comprehensive analyses on relationships within sawflies and woodwasps are still lacking. Here, we infer the relationships and long-term biogeographic history of these hymenopteran groups using a large dataset of 354 UCE loci collected from 385 species that represent all major lineages. Early Hymenoptera started diversifying during the Early Triassic ∼249 Ma and spread all over the ancient supercontinent Pangaea. We recovered Xyeloidea as a monophyletic sister group to other Hymenoptera and Pamphilioidea as sister to Unicalcarida. Within the diverse family Tenthredinidae, our taxonomically and geographically expanded taxon sampling highlights the non-monophyly of several traditionally defined subfamilies. In addition, the recent removal of Athalia and related genera from the Tenthredinidae into the separate family Athaliidae is supported. The deep historical biogeography of the group is characterised by independent dispersals and re-colonisations between the northern (Laurasia) and southern (Gondwana) palaeocontinents. The breakup of these landmasses led to ancient vicariance in several Gondwanan lineages, while interchange across the Northern Hemisphere has continued until the Recent. The little-studied African sawfly fauna is likewise a diverse mixture of groups with varying routes of colonization. Our results reveal interesting parallels in the evolution and biogeography of early hymenopterans and other ancient insect groups.

Phylogenomics and biogeography of the small carpenter bees (Apidae: Xylocopinae: Ceratina)

Small carpenter bees in the genus Ceratina are behaviourally diverse, species-rich, and cosmopolitan, with over 370 species and a range including all continents except Antarctica. Here, we present the first comprehensive phylogeny of the genus based on ultraconserved element (UCE) phylogenomic data, covering a total of 185 ingroup specimens representing 22 of the 25 current subgenera. Our results support most recognized subgenera as natural groups, but we also highlight several groups in need of taxonomic revision - particularly the nominate subgenus Ceratina sensu stricto - and several clades that likely need to be described as new subgenera. In addition to phylogeny, we explore the evolutionary history of Ceratina through divergence time estimation and biogeographic reconstruction. Our findings suggest that Ceratinini split from its sister tribe Allodapini about 72 million years ago. The common ancestor of Ceratina emerged in the Afrotropical realm approximately 42 million years ago, near the Middle Eocene Climatic Optimum. Multiple subsequent dispersal events led to the present cosmopolitan distribution of Ceratina, with the majority of transitions occurring between the Afrotropics, Indomalaya, and the Palearctic. Additional movements also led to the arrival of Ceratina in Madagascar, Australasia, and a single colonization of the Americas. Dispersal events were asymmetrical overall, with temperate regions primarily acting as destinations for migrations from tropical source regions.

Host specificity and cophylogeny in the "animal-gut bacteria-phage" tripartite system

Cophylogeny has been identified between gut bacteria and their animal host and is highly relevant to host health, but little research has extended to gut bacteriophages. Here we use bee model to investigate host specificity and cophylogeny in the "animal-gut bacteria-phage" tripartite system. Through metagenomic sequencing upon different bee species, the gut phageome revealed a more variable composition than the gut bacteriome. Nevertheless, the bacteriome and the phageome showed a significant association of their dissimilarity matrices, indicating a reciprocal interaction between the two kinds of communities. Most of the gut phages were host generalist at the viral cluster level but host specialist at the viral OTU level. While the dominant gut bacteria Gilliamella and Snodgrassella exhibited matched phylogeny with bee hosts, most of their phages showed a diminished level of cophylogeny. The evolutionary rates of the bee, the gut bacteria and the gut phages showed a remarkably increasing trend, including synonymous and non-synonymous substitution and gene content variation. For all of the three codiversified tripartite members, however, their genes under positive selection and genes involving gain/loss during evolution simultaneously enriched the functions into metabolism of nutrients, therefore highlighting the tripartite coevolution that results in an enhanced ecological fitness for the whole holobiont.

Bee morphology: A skeletomuscular anatomy of Thyreus (Hymenoptera: Apidae)

Although the knowledge of the skeletal morphology of bees has progressed enormously, a corresponding advance has not happened for the muscular system. Most of the knowledge about bee musculature was generated over 50 years ago, well before the digital revolution for anatomical imaging, including the application of microcomputed tomography. This technique, in particular, has made it possible to dissect small insects digitally, document anatomy efficiently and in detail, and visualize these data three dimensionally. In this study, we document the skeletomuscular system of a cuckoo bee, Thyreus albomaculatus and, with that, we provide a 3D atlas of bee skeletomuscular anatomy. The results obtained for Thyreus are compared with representatives of two other bee families (Andrenidae and Halictidae), to evaluate the generality of our morphological conclusions. Besides documenting 199 specific muscles in terms of origin, insertion, and structure, we update the interpretation of complex homologies in the maxillolabial complex of bee mouthparts. We also clarify the complicated 3D structure of the cephalic endoskeleton, identifying the tentorial, hypostomal, and postgenal structures and their connecting regions. We describe the anatomy of the medial elevator muscles of the head, precisely identifying their origins and insertions as well as their homologs in other groups of Hymenoptera. We reject the hypothesis that the synapomorphic propodeal triangle of Apoidea is homologous with the metapostnotum, and instead recognize that this is a modification of the third phragma. We recognize two previously undocumented metasomal muscle groups in bees, clarifying the serial skeletomusculature of the metasoma and revealing shortcomings of Snodgrass' "internal-external" terminological system for the abdomen. Finally, we elucidate the muscular structure of the sting apparatus, resolving previously unclear interpretations. The work conducted herein not only provides new insights into bee morphology but also represents a source for future phenomic research on Hymenoptera.

Global patterns and drivers of buzzing bees and poricidal plants

Foraging behavior frequently plays a major role in driving the geographic distribution of animals. Buzzing to extract protein-rich pollen from flowers is a key foraging behavior used by bee species across at least 83 genera (these genera comprise ∼58% of all bee species). Although buzzing is widely recognized to affect the ecology and evolution of bees and flowering plants (e.g., buzz-pollinated flowers), global patterns and drivers of buzzing bee biogeography remain unexplored. Here, we investigate the global species distribution patterns within each bee family and how patterns and drivers differ with respect to buzzing bee species. We found that both distributional patterns and drivers of richness typically differed for buzzing species compared with hotspots for all bee species and when grouped by family. A major predictor of the distribution, but not species richness overall for buzzing members of four of the five major bee families included in analyses (Andrenidae, Halictidae, Colletidae, and to a lesser extent, Apidae), was the richness of poricidal flowering plant species, which depend on buzzing bees for pollination. Because poricidal plant richness was highest in areas with low wind and high aridity, we discuss how global hotspots of buzzing bee biodiversity are likely influenced by both biogeographic factors and plant host availability. Although we explored global patterns with state-level data, higher-resolution work is needed to explore local-level drivers of patterns. From a global perspective, buzz-pollinated plants clearly play a greater role in the ecology and evolution of buzzing bees than previously predicted.

Biomechanical properties of non-flight vibrations produced by bees

Bees use thoracic vibrations produced by their indirect flight muscles for powering wingbeats in flight, but also during mating, pollination, defence and nest building. Previous work on non-flight vibrations has mostly focused on acoustic (airborne vibrations) and spectral properties (frequency domain). However, mechanical properties such as the vibration's acceleration amplitude are important in some behaviours, e.g. during buzz pollination, where higher amplitude vibrations remove more pollen from flowers. Bee vibrations have been studied in only a handful of species and we know very little about how they vary among species. In this study, we conducted the largest survey to date of the biomechanical properties of non-flight bee buzzes. We focused on defence buzzes as they can be induced experimentally and provide a common currency to compare among taxa. We analysed 15,000 buzzes produced by 306 individuals in 65 species and six families from Mexico, Scotland and Australia. We found a strong association between body size and the acceleration amplitude of bee buzzes. Comparison of genera that buzz-pollinate and those that do not suggests that buzz-pollinating bees produce vibrations with higher acceleration amplitude. We found no relationship between bee size and the fundamental frequency of defence buzzes. Although our results suggest that body size is a major determinant of the amplitude of non-flight vibrations, we also observed considerable variation in vibration properties among bees of equivalent size and even within individuals. Both morphology and behaviour thus affect the biomechanical properties of non-flight buzzes.

The earliest large carpenter bee (Xylocopa) and its adhering pollen (Araliaceae, Theaceae)

The association of pollinators with their host plants is a critical element of ecosystem functioning and one that is usually determined indirectly in the fossil record from specific morphological traits of flowers or putative pollinating animals. The exceptionally fine preservation at Messel, Germany, offers an excellent source of data on pollen from fossil flowers as well as preserved adhering to insects as direct evidence of their association with specific floral lineages. Here, we report on pollen recovered from the body and legs of a large carpenter bee (Apidae: Xylocopinae: Xylocopini) from the Eocene of Messel. The fossil is the earliest occurrence of the tribe Xylocopini and represents an extinct subgenus and species, described as Xylocopa (Apocolyx) primigenia subgen. et sp. nov. Two eudicot pollen types were recovered from the bee, one of the family Theaceae (Asterids: Ericales) and another of Araliaceae (Euasterids: Apiales). The pollen grains are compared with various extinct and extant pollen types, and data on floral visitors to modern theaceous and araliaceous flowers are explored in relation to understanding the association of the fossil carpenter with these floral types in the paratropical Eocene biota of Messel.

Gene regulation supporting sociality shared across lineages and variation in complexity

Across evolutionary lineages, insects vary in social complexity, from those that exhibit extended parental care to those with elaborate divisions of labor. Here, we synthesize the sociogenomic resources from hundreds of species to describe common gene regulatory mechanisms in insects that regulate social organization across phylogeny and levels of social complexity. Different social phenotypes expressed by insects can be linked to the organization of co-expressing gene networks and features of the epigenetic landscape. Insect sociality also stems from processes like the emergence of parental care and the decoupling of ancestral genetic programs. One underexplored avenue is how variation in a group's social environment affects the gene expression of individuals. Additionally, an experimental reduction of gene expression would demonstrate how the activity of specific genes contributes to insect social phenotypes. While tissue specificity provides greater localization of the gene expression underlying social complexity, emerging transcriptomic analysis of insect brains at the cellular level provides even greater resolution to understand the molecular basis of social insect evolution.

Lessons from assembling UCEs: A comparison of common methods and the case of Clavinomia (Halictidae)

Sequence data assembly is a foundational step in high-throughput sequencing, with untold consequences for downstream analyses. Despite this, few studies have interrogated the many methods for assembling phylogenomic UCE data for their comparative efficacy, or for how outputs may be impacted. We study this by comparing the most commonly used assembly methods for UCEs in the under-studied bee lineage Nomiinae and a representative sampling of relatives. Data for 63 UCE-only and 75 mixed taxa were assembled with five methods, including ABySS, HybPiper, SPAdes, Trinity and Velvet, and then benchmarked for their relative performance in terms of locus capture parameters and phylogenetic reconstruction. Unexpectedly, Trinity and Velvet trailed the other methods in terms of locus capture and DNA matrix density, whereas SPAdes performed favourably in most assessed metrics. In comparison with SPAdes, the guided-assembly approach HybPiper generally recovered the highest quality loci but in lower numbers. Based on our results, we formally move Clavinomia to Dieunomiini and render Epinomia once more a subgenus of Dieunomia. We strongly advise that future studies more closely examine the influence of assembly approach on their results, or, minimally, use better-performing assembly methods such as SPAdes or HybPiper. In this way, we can move forward with phylogenomic studies in a more standardized, comparable manner.

Integrated phylogenomic approaches in insect systematics

The increased accessibility of genomic and imaging methods, and the improved access to ecological, spatial, and other natural history-related data is allowing for insect systematics to grow and find answers to central evolutionary and taxonomic questions. Today, integrated studies in insect phylogenomics and systematics are combining natural history, behavior, developmental biology, morphology, fossils, geographic range data, and ecological interactions. This integration is contributing to the clarification of evolutionary relationships, and the recognition of the role played by these factors on the evolution of insects. Future work should continue to build on these advances, seeking to further increase open-access databasing and support for natural history research, as well as expand its analytical palettes.

The angiosperm radiation played a dual role in the diversification of insects and insect pollinators

Interactions with angiosperms have been hypothesised to play a crucial role in driving diversification among insects, with a particular emphasis on pollinator insects. However, support for coevolutionary diversification in insect-plant interactions is weak. Macroevolutionary studies of insect and plant diversities support the hypothesis that angiosperms diversified after a peak in insect diversity in the Early Cretaceous. Here, we used the family-level fossil record of insects as a whole, and insect pollinator families in particular, to estimate diversification rates and the role of angiosperms on insect macroevolutionary history using a Bayesian process-based approach. We found that angiosperms played a dual role that changed through time, mitigating insect extinction in the Cretaceous and promoting insect origination in the Cenozoic, which is also recovered for insect pollinator families only. Although insects pollinated gymnosperms before the angiosperm radiation, a radiation of new pollinator lineages began as angiosperm lineages increased, particularly significant after 50 Ma. We also found that global temperature, increases in insect diversity, and spore plants were strongly correlated with origination and extinction rates, suggesting that multiple drivers influenced insect diversification and arguing for the investigation of different explanatory variables in further studies.

A supermatrix phylogeny of the world's bees (Hymenoptera: Anthophila)

The increasing availability of large molecular phylogenies has provided new opportunities to study the evolution of species traits, their origins and diversification, and biogeography; yet there are limited attempts to synthesise existing phylogenetic information for major insect groups. Bees (Hymenoptera: Anthophila) are a large group of insect pollinators that have a worldwide distribution, and a wide variation in ecology, morphology, and life-history traits, including sociality. For these reasons, as well as their major economic importance as pollinators, numerous molecular phylogenetic studies of family and genus-level relationships have been published, providing an opportunity to assemble a bee 'tree-of-life'. We used publicly available genetic sequence data, including phylogenomic data, reconciled to a taxonomic database, to produce a concatenated supermatrix phylogeny for the Anthophila comprising 4,586 bee species, representing 23% of species and 82% of genera. At family, subfamily, and tribe levels, support for expected relationships was robust, but between and within some genera relationships remain uncertain. Within families, sampling of genera ranged from 67 to 100% but species coverage was lower (17-41%). Our phylogeny mostly reproduces the relationships found in recent phylogenomic studies with a few exceptions. We provide a summary of these differences and the current state of molecular data available and its gaps. We discuss the advantages and limitations of this bee supermatrix phylogeny (available online at beetreeoflife.org), which may enable new insights into long standing questions about evolutionary drivers in bees, and potentially insects more generally.

Biogeography: The origin and spread of bee lineages

Where and when bees originated and how they dispersed and diversified across ancient continents has remained ambiguous. A new study that combines phylogenetics with fossil data reconstructs the origin and diversification of bees across geological time and space.