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

Elevated rates of dietary generalization in eusocial lineages of the secondarily herbivorous bees

BACKGROUND

Within the Hymenoptera, bees are notable for their relationship with flowering plants, being almost entirely dependent on plant pollen and nectar. Though functionally herbivorous, as a result of their role as pollinators, bees have received comparatively little attention as models for insect herbivory. Bees often display dietary specialization, but quantitative comparison against other herbivorous insects has not previously been conducted.

RESULTS

In the most comprehensive analysis to date for 860 bee species, dietary specialization amounted to 50.1% of studied species collecting pollen from between 1 and 2 botanical families with a relatively long tail of dietary generalists, with 11.1% of species collecting from more than 10 botanical families. This distribution deviated from the truncated Pareto distribution of dietary breadth seen in other herbivorous insect lineages. However, this deviation was predominantly due to eusocial bee lineages, which show a range of dietary breadths that conformed to a normal distribution, while solitary bees show a typical truncated distribution not strongly different from other herbivorous insects. We hypothesize that the relatively low level of dietary specialization in bees as a whole reflects the relaxation of the constraints typically observed in herbivorous insects with a comparatively reduced importance of plant chemistry and comparatively increased importance of phenology and foraging efficiency. The long flight periods of eusocial bees that are necessary to allow overlapping generations both allows and necessitates the use of multiple flowering resources, whereas solitary bees with short flight periods have more limited access to varied resources within a constrained activity period.

CONCLUSIONS

Collectively, solitary bees show slightly lower specialization compared to other herbivorous insects, possibly due to their balanced relationship with plants, rather than direct antagonism such as seen in the direct consumption of plant tissues. An additional factor may be the mediocre diversity of bees at low latitudes combined with low levels of dietary specialization, whereas these areas typically display a high rate of specialization by herbivorous insects in general. Though the most important factors structuring dietary specialization in bees appear to differ from many other herbivorous insects, solitary bees show a surprisingly similar overall pattern of dietary specialization.

Diversity of Cladosporium (Cladosporiales, Cladosporiaceae) species in marine environments and report on five new species

Cladosporium species are cosmopolitan fungi, characterized by olivaceous or dark colonies with coronate conidiogenous loci and conidial hila with a central convex dome surrounded by a raised periclinal rim. Cladosporium species have also been discovered in marine environments. Although many studies have been performed on the application of marine originated Cladosporium species, taxonomic studies on these species are scarce. We isolated Cladosporium species from three under-studied habitats (sediment, seawater, and seaweed) in two districts including an intertidal zone in the Republic of Korea and the open sea in the Western Pacific Ocean. Based on multigenetic marker analyses (for the internal transcribed spacer, actin, and translation elongation factor 1), we identified fourteen species, of which five were found to represent new species. These five species were C.lagenariiformesp. nov., C.maltirimosumsp. nov., C.marinumsp. nov. in the C.cladosporioides species complex, C.snafimbriatumsp. nov. in the C.herbarum species complex, and C.marinisedimentumsp. nov. in the C.sphaerospermum species complex. Morphological characteristics of the new species and aspects of differences with the already known species are described herein together with molecular data.

Host shifts in economically significant fruit flies (Diptera: Tephritidae) with high degree of polyphagy

Insects tend to feed on related hosts. Coevolution tends to be dominated by interactions resulting from plant chemistry in defense strategies, and evolution of secondary metabolisms being in response to insect herbivory remains a classic explanation of coevolution. The present study examines whether evolutionary constraints existing in host associations of economically important fruit flies in the species-rich tribe Dacini (Diptera: Tephritidae) and to what extent these species have evolved specialized dietary patterns. We found a strong effect of host phylogeny on associations on the 37 fruit flies tested, although the fruit fly species feeding on ripe commercially grown fruits that lost the toxic compounds after long-term domestication are mostly polyphagous. We assessed the phylogenetic signal of host breadth across the fruit fly species, showing that the results were substantially different depending on partition levels. Further, we mapped main host family associations onto the fruit fly phylogeny and Cucurbitaceae has been inferred as the most likely ancestral host family for Dacini based on ancestral state reconstruction.

Bee flowers drive macroevolutionary diversification in long-horned bees

The role of plant–pollinator interactions in the rapid radiation of the angiosperms have long fascinated evolutionary biologists. Studies have brought evidence for pollinator-driven diversification of various plant lineages, particularly plants with specialized flowers and concealed rewards. By contrast, little is known about how this crucial interaction has shaped macroevolutionary patterns of floral visitors. In particular, there is currently no empirical evidence that floral host association has increased diversification in bees, the most prominent group of floral visitors that essentially rely on angiosperm pollen. In this study, we examine how floral host preference influenced diversification in eucerine bees (Apidae, Eucerini), which exhibit large variations in their floral associations. We combine quantitative pollen analyses with a recently proposed phylogenetic hypothesis, and use a state speciation and extinction probabilistic approach. Using this framework, we provide the first evidence that multiple evolutionary transitions from host plants with accessible pollen to restricted pollen from ‘bee-flowers’ have significantly increased the diversification of a bee clade. We suggest that exploiting host plants with restricted pollen has allowed the exploitation of a new ecological niche for eucerine bees and contributed both to their colonization of vast regions of the world and their rapid diversification.

Expanding insect pollinators in the Anthropocene

Global changes are severely affecting pollinator insect communities worldwide, resulting in repeated patterns of species extirpations and extinctions. Whilst negative population trends within this functional group have understandably received much attention in recent decades, another facet of global changes has been overshadowed: species undergoing expansion. Here, we review the factors and traits that have allowed a fraction of the pollinating entomofauna to take advantage of global environmental change. Sufficient mobility, high resistance to acute heat stress, and inherent adaptation to warmer climates appear to be key traits that allow pollinators to persist and even expand in the face of climate change. An overall flexibility in dietary and nesting requirements is common in expanding species, although niche specialization can also drive expansion under specific contexts. The numerous consequences of wild and domesticated pollinator expansions, including competition for resources, pathogen spread, and hybridization with native wildlife, are also discussed. Overall, we show that the traits and factors involved in the success stories of expanding pollinators are mostly species specific and context dependent, rendering generalizations of 'winning traits' complicated. This work illustrates the increasing need to consider expansion and its numerous consequences as significant facets of global changes and encourages efforts to monitor the impacts of expanding insect pollinators, particularly exotic species, on natural ecosystems.

Global patterns in bumble bee pollen collection show phylogenetic conservation of diet

Bumble bees (Bombus) are a group of eusocial bees with a strongly generalised feeding pattern, collecting pollen from many different botanical families. Though predominantly generalists, some bumble bee species seem to have restricted dietary choices. It is unclear whether restricted diets in bumble bees are inherent or a function of local conditions due to a lack of data for many species across different regions. The objective of this study was to determine whether bumble bee species displayed specific patterns of pollen collection, and whether patterns were influenced by phylogenetic relatedness or tongue length, a trait known to be associated with structuring floral visitation. Bumble bee pollen collection patterns were quantified from 4,132 pollen loads taken from 58 bumble bee species, representing 24% of the pollen-collecting diversity of this genus. Phylogenetic trait mapping showed a conserved pattern of dietary dissimilarity across species, but not for dietary breadth. Dietary dissimilarity was driven by collection of Fabaceae, with the most similar species collecting around 50%-60% of their diet from this botanical family. The proportion of the diet collected from Fabaceae also showed a conserved phylogenetic signal. Greater collection of Fabaceae was associated with longer tongue lengths, with shorter tongued species focusing on alternative botanical families. However, this result was largely driven by phylogenetic relatedness, not tongue length per se. These results demonstrate that, though generalists, bumble bees are still subject to dietary restrictions that constrain their foraging choices. These dietary constraints have implications for their persistence should their core resources decline in abundance.

Comparative Mitogenomic Analysis of Two Cuckoo Bees (Apoidea: Anthophila: Megachilidae) with Phylogenetic Implications

Bees (Hymenoptera, Apoidea and Anthophila) are distributed worldwide and considered the primary pollinators of angiosperm. Megachilidae is one of the largest families of Anthophila. In this study, two complete mitogenomes of cuckoo bees in Megachilidae, namely Coelioxys fenestrata and Euaspis polynesia, were amplified and sequenced, with a length of 17,004 bp (C. fenestrata) and 17,682 bp (E. polynesia). The obtained results show that 37 mitogenomic genes and one putative control region were conserved within Hymenoptera. Truncated stop codon T was found in the cox3 gene of E. polynesia. The secondary structure of small (rrnS) and large (rrnL) rRNA subunits contained three domains (28 helices) and five domains (44 helices) conserved within Hymenoptera, respectively. Compared with ancestral gene order, gene rearrangement events included local inversion and gene shuffling. In order to reveal the phylogenetic position of cuckoo bees, we performed phylogenetic analysis. The results supported that all families of Anthophila were monophyletic, the tribe-level relationship of Megachilidae was Osmiini + (Anthidiini + Megachilini) and Coelioxys fenestrata was clustered to the Megachile genus, which was more closely related to Megachile sculpturalis and Megachile strupigera than Euaspis polynesia.

The allometry of proboscis length in Melittidae (Hymenoptera: Apoidae) and an estimate of their foraging distance using museum collections

An appreciation of body size allometry is central for understanding insect pollination ecology. A recent model utilises allometric coefficients for five of the seven extant bee families (Apoidea: Anthophila) to include crucial but difficult-to-measure traits, such as proboscis length, in ecological and evolutionary studies. Melittidae were not included although they are important pollinators in South Africa where they comprise an especially rich and morphologically diverse fauna. We measured intertegular distance (correlated with body size) and proboscis length of 179 specimens of 11 species from three genera of Melittidae. With the inclusion of Melittidae, we tested the between family differences in the allometric scaling coefficients. AIC model selection was used to establish which factors provide the best estimate of proboscis length. We explored a hypothesis that has been proposed in the literature, but which has not been tested, whereby body and range sizes of bees are correlated with rainfall regions. We tested this by using body size measurements of 2109 museum specimens from 56 species of Melittidae and applied the model coefficients to estimate proboscis length and foraging distance. Our results from testing differences across bee families show that with the addition of Melittidae, we retained the overall pattern of significant differences in the scaling coefficient among Apoidea, with our model explaining 98% of the variance in species-level means for proboscis length. When testing the relationship between body size and rainfall region we found no relationship for South African Melittidae. Overall, this study has added allometric scaling coefficients for an important bee family and shown the applicability of using these coefficients when linked with museum specimens to test ecological hypothesis.

Phylogenetic analysis of the mitochondrial genomes in bees (Hymenoptera: Apoidea: Anthophila)

In this study, the first complete mitogenome of Andrenidae, namely Andrena camellia, is newly sequenced. It includes 13 protein-coding (PCG) genes, 22 transfer RNA (rRNA) genes, two ribosomal RNA (tRNA) genes, and a control region. Among PCGs, high conservation is observed in cytochrome oxidase genes with cox1 exhibits the highest conservation. Conversely, NADH dehydrogenase and ATPase subunit genes are more variable with atp8 presents the maximal variation. Comparison of the gene order indicates complex rearrangement in bees. Most of the rearranged events are located in the tRNA clusters of trnI-trnQ-trnM, trnW-trnC-trnY, and trnA-trnR-trnN-trnS1-trnE-trnF. Furthermore, we present the most comprehensive mitochondrial phylogeny of bee families. The monophyly of each family and the long-tongued bees is highly supported. However, short-tongued bees are inferred as paraphyletic relative to the sister relationship between Melittidae and other bee families. Furthermore, to improve the resolution of phylogeny, various datasets and analytical approaches are performed. It is indicated that datasets including third codons of PCGs facilitate to produce identical topology and higher nodal support. The tRNA genes that have typical cloverleaf secondary structures also exhibit similar positive effects. However, rRNAs present poor sequence alignment and distinct substitution saturation, which result in negative effects on both tree topology and nodal support. In addition, Gblocks treatment can increase the congruence of topologies, but has opposite effects on nodal support between the two inference methods of maximum likelihood and Bayesian inference.

Playing with extremes: Origins and evolution of exaggerated female forelegs in South African Rediviva bees

Despite close ecological interactions between plants and their pollinators, only some highly specialised pollinators adapt to a specific host plant trait by evolving a bizarre morphology. Here we investigated the evolution of extremely elongated forelegs in females of the South African bee genus Rediviva (Hymenoptera: Melittidae), in which long forelegs are hypothesised to be an adaptation for collecting oils from the extended spurs of their Diascia host flowers. We first reconstructed the phylogeny of the genus Rediviva using seven genes and inferred an origin of Rediviva at around 29MYA (95% HPD=19.2-40.5), concurrent with the origin and radiation of the Succulent Karoo flora. The common ancestor of Rediviva was inferred to be a short-legged species that did not visit Diascia. Interestingly, all our analyses strongly supported at least two independent origins of long legs within Rediviva. Leg length was not correlated with any variable we tested (ecological specialisation, Diascia visitation, geographic distribution, pilosity type) but seems to have evolved very rapidly. Overall, our results indicate that foreleg length is an evolutionary highly labile, rapidly evolving trait that might enable Rediviva bees to respond quickly to changing floral resource availability.

Distribution and predictors of wing shape and size variability in three sister species of solitary bees

Morphological traits can be highly variable over time in a particular geographical area. Different selective pressures shape those traits, which is crucial in evolutionary biology. Among these traits, insect wing morphometry has already been widely used to describe phenotypic variability at the inter-specific level. On the contrary, fewer studies have focused on intra-specific wing morphometric variability. Yet, such investigations are relevant to study potential convergences of variation that could highlight micro-evolutionary processes. The recent sampling and sequencing of three solitary bees of the genus Melitta across their entire species range provides an excellent opportunity to jointly analyse genetic and morphometric variability. In the present study, we first aim to analyse the spatial distribution of the wing shape and centroid size (used as a proxy for body size) variability. Secondly, we aim to test different potential predictors of this variability at both the intra- and inter-population levels, which includes genetic variability, but also geographic locations and distances, elevation, annual mean temperature and precipitation. The comparison of spatial distribution of intra-population morphometric diversity does not reveal any convergent pattern between species, thus undermining the assumption of a potential local and selective adaptation at the population level. Regarding intra-specific wing shape differentiation, our results reveal that some tested predictors, such as geographic and genetic distances, are associated with a significant correlation for some species. However, none of these predictors are systematically identified for the three species as an important factor that could explain the intra-specific morphometric variability. As a conclusion, for the three solitary bee species and at the scale of this study, our results clearly tend to discard the assumption of the existence of a common pattern of intra-specific signal/structure within the intra-specific wing shape and body size variability.

The importance of pollen chemistry in evolutionary host shifts of bees

Although bee-plant associations are generally maintained through speciation processes, host shifts have occurred during evolution. Understanding shifts between both phylogenetically and morphologically unrelated plants (i.e., host-saltation) is especially important since they could have been key processes in the origin and radiation of bees. Probably far from being a random process, such host-saltation might be driven by hidden constraints associated with plant traits. We selected two clades of oligolectic bees (i.e., Colletes succinctus group and Melitta leporina group) foraging on co-flowering but unrelated host-plants to test this hypothesis. We analyzed floral scent, floral color and chemical composition of pollen from host and non-host plants of these two clades. We did not find evidence for host-plant evolution in the Melitta leporina group driven by one of the assayed floral traits. On the contrary, hosts of the C. succinctus group display similar primary nutritive content of pollen (i.e., amino acids and sterols) but not similar floral scent or color, suggesting that shared pollen chemistry probably mediates saltation in this clade. Our study revealed that constraints shaping floral associations are diverse and clearly depend on species life-history traits, but evidence suggests that pollen chemistry may act as a major floral filter and guide evolutionary host-shifts.

Impact of past climatic changes and resource availability on the population demography of three food-specialist bees

Past climate change is known to have strongly impacted current patterns of genetic variation of animals and plants in Europe. However, ecological factors also have the potential to influence demographic history and thus patterns of genetic variation. In this study, we investigated the impact of past climate, and also the potential impact of host plant species abundance, on intraspecific genetic variation in three codistributed and related specialized solitary bees of the genus Melitta with very similar life history traits and dispersal capacities. We sequenced five independent loci in samples collected from the three species. Our analyses revealed that the species associated with the most abundant host plant species (Melitta leporina) displays unusually high genetic variation, to an extent that is seldom reported in phylogeographic studies of animals and plants. This suggests a potential role of food resource abundance in determining current patterns of genetic variation in specialized herbivorous insects. Patterns of genetic variation in the two other species indicated lower overall levels of diversity, and that M. nigricans could have experienced a recent range expansion. Ecological niche modelling of the three Melitta species and their main host plant species suggested a strong reduction in range size during the last glacial maximum. Comparing observed sequence data with data simulated using spatially explicit models of coalescence suggests that M. leporina recovered a range and population size close to their current levels at the end of the last glaciation, and confirms recent range expansion as the most likely scenario for M. nigricans. Overall, this study illustrates that both demographic history and ecological factors may have contributed to shape current phylogeographic patterns.

A new interpretation of the bee fossil Melitta willardi Cockerell (Hymenoptera, Melittidae) based on geometric morphometrics of the wing

Although bees are one of the major lineages of pollinators and are today quite diverse, few well-preserved fossils are available from which to establish the tempo of their diversification/extinction since the Early Cretaceous. Here we present a reassessment of the taxonomic affinities of Melitta willardiCockerell 1909, preserved as a compression fossil from the Florissant shales of Colorado, USA. Based on geometric morphometric wing shape analyses M. willardi cannot be confidently assigned to the genus Melitta Kirby (Anthophila, Melittidae). Instead, the species exhibits phenotypic affinity with the subfamily Andreninae (Anthophila, Andrenidae), but does not appear to belong to any of the known genera therein. Accordingly, we describe a new genus, Andrenopteryxgen. n., based on wing shape as well as additional morphological features and to accommodate M. willardi. The new combination Andrenopteryx willardi (Cockerell) is established.