Extremely low effective population sizes, genetic structuring and reduced genetic diversity in a threatened bumblebee species, Bombus sylvarum (Hymenoptera: Apidae)

Biology, Genetic Diversity, and Conservation of Wild Bees in Tree Fruit Orchards

Different species of bees provide essential ecosystem services by pollinating various agricultural crops, including tree fruits. Many fruits and nuts depend on insect pollination, primarily by wild and managed bees. In different geographical regions where orchard crops are grown, fruit growers rely on wild bees in the farmscape and use orchard bees as alternative pollinators. Orchard crops such as apples, pears, plums, apricots, etc., are mass-flowering crops and attract many different bee species during their bloom period. Many bee species found in orchards emerge from overwintering as the fruit trees start flowering in spring, and the active duration of these bees aligns very closely with the blooming time of fruit trees. In addition, most of the bees in orchards are short-range foragers and tend to stay close to the fruit crops. However, the importance of orchard bee communities is not well understood, and many challenges in maintaining their populations remain. This comprehensive review paper summarizes the different types of bees commonly found in tree fruit orchards in the fruit-growing regions of the United States, their bio-ecology, and genetic diversity. Additionally, recommendations for the management of orchard bees, different strategies for protecting them from multiple stressors, and providing suitable on-farm nesting and floral resource habitats for propagation and conservation are discussed.

Distinct Colony Types Caused by Diploid Male Production in the Buff-Tailed Bumblebee Bombus terrestris

The buff-tailed bumblebee Bombus terrestris presents two distinct colony phenotypes in which some colonies already start producing males very early in the season, while others switch to producing sexuals much later in the season, and specialize mainly in the production of virgin queens. Despite having been extensively investigated in the past, the mechanisms underlying these two distinct phenotypes remain poorly understood. Here we use a combination of behavioral and genetic data to demonstrate that early switchers are in fact diploid male producing colonies, while late switchers produced normal, haploid males. Hence, at a proximate level, early switching to male production was caused by queens mating with males that by chance carried the same sex allele. These patterns were not the result of strong inbreeding within the study population, as the average inbreeding coefficient was very low and in fact slightly negative (−0.11). Moreover, an effect of inbreeding was further excluded because hybrids produced by crossing two distinct outbred populations also produced diploid males in early switching colonies. Finally, we found that diploid males only emerged after a first cohort of workers was produced, while we would have expected diploid males to be produced simultaneously with workers in a 50:50 ratio. We demonstrate that adult diploid males had a distinct cuticular hydrocarbon profile and propose that queens likely cull diploid males during early colony development to reduce the cost of diploid male production.

Maine's Bumble Bees (Hymenoptera: Apidae)-Part 2: Comparisons of a Common (Bombus ternarius) and a Rare (Bombus terricola) Species

As part of a quantitative survey of Maine's bumble bee fauna (Butler et al. 2021), we compared and contrasted genetic diversity, parasite and pathogen burdens, and pesticide exposure of the relatively common Bombus ternarius Say, 1937 and the spatially rare Bombus terricola Kirby, 1837. We recorded 11 Bombus species at 40 survey sites across three Maine ecoregions, and B. ternarius was the most common species, while B. terricola was spatially rare. Nonmetric multidimensional scaling indicated that B. terricola was associated with higher elevation sites in Maine, while B. ternarius was more broadly distributed in the state. Pollinator networks constructed for each bee indicated B. ternarius foraged on more plant species than B. terricola, but that there was considerable overlap (73%) in plant species visited. Genetic diversity was greater in the spatially restricted B. terricola, whereas the widely distributed B. ternarius was characterized by greater genetic differentiation among regions. Bombus terricola had higher molecular marker levels of the microsporidian fungi Nosema spp. and the trypanosome Crithidia spp., and both species had high levels of Trypanosoma spp. exposure. No Western Honey Bee (Apis mellifera, Linnaeus, 1758) viruses were detected in either species. Pesticides were not detected in pollen samples collected from workers of either species, and B. ternarius worker tissue samples exhibited only trace levels of diflubenzuron.

A genomic and morphometric analysis of alpine bumblebees: Ongoing reductions in tongue length but no clear genetic component

Over the last six decades, populations of the bumblebees Bombus sylvicola and Bombus balteatus in Colorado have experienced decreases in tongue length, a trait important for plant-pollinator mutualisms. It has been hypothesized that this observation reflects selection resulting from shifts in floral composition under climate change. Here we used morphometrics and population genomics to determine whether morphological change is ongoing, investigate the genetic basis of morphological variation, and analyse population structure in these populations. We generated a genome assembly of B. balteatus. We then analysed whole-genome sequencing data and morphometric measurements of 580 samples of both species from seven high-altitude localities. Out of 281 samples originally identified as B. sylvicola, 67 formed a separate genetic cluster comprising a newly-discovered cryptic species ("incognitus"). However, an absence of genetic structure within species suggests that gene flow is common between mountains. We found a significant decrease in tongue length between bees collected between 2012-2014 and in 2017, indicating that morphological shifts are ongoing. We did not discover any genetic associations with tongue length, but a SNP related to production of a proteolytic digestive enzyme was implicated in body size variation. We identified evidence of covariance between kinship and both tongue length and body size, which is suggestive of a genetic component of these traits, although it is possible that shared environmental effects between colonies are responsible. Our results provide evidence for ongoing modification of a morphological trait important for pollination and indicate that this trait probably has a complex genetic and environmental basis.

Conservation insights from wild bee genetic studies: Geographic differences, susceptibility to inbreeding, and signs of local adaptation

Conserving bees are critical both ecologically and economically. Genetic tools are valuable for monitoring these vital pollinators since tracking these small, fast-flying insects by traditional means is difficult. By surveying the current state of the literature, this review discusses how recent advances in landscape genetic and genomic research are elucidating how wild bees respond to anthropogenic threats. Current literature suggests that there may be geographic differences in the vulnerability of bee species to landscape changes. Populations of temperate bee species are becoming more isolated and more genetically depauperate as their landscape becomes more fragmented, but tropical bee species appear unaffected. These differences may be an artifact of historical differences in land-use, or it suggests that different management plans are needed for temperate and tropical bee species. Encouragingly, genetic studies on invasive bee species indicate that low levels of genetic diversity may not lead to rapid extinction in bees as once predicted. Additionally, next-generation sequencing has given researchers the power to identify potential genes under selection, which are likely critical to species' survival in their rapidly changing environment. While genetic studies provide insights into wild bee biology, more studies focusing on a greater phylogenetic and life-history breadth of species are needed. Therefore, caution should be taken when making broad conservation decisions based on the currently few species examined.

Phylogeography of the northernmost distributed Anisocentropus caddisflies and their comparative genetic structures based on habitat preferences

Knowledge of the factors that determine the distribution ranges of organisms is necessary to understand their evolutionary and ecological significance and contribution to biodiversity. A very effective mean of studying such factors is to compare the distribution characteristics and genetic structures of closely related species with differing habitat preferences. Freshwater aquatic insects are relatively easy to observe and the basis of their corresponding niche differentiation easier to identify. Freshwater habitats are categorized lotic or lentic water according to flow regime. In Japanese Islands, the genus Anisocentropus of the calamoceratid caddisfly, the target group in this study, was morphologically reconfirmed that three species, that is, Anisocentropus kawamurai, A. pallidus, and A. magnificus. Among these, A. kawamurai prefers lotic environments and A. pallidus is adapted to lentic water habitats. The distribution range of these sister species overlaps within the Japanese Islands. We estimated the phylogeny and the evolutionary history of Anisocentropus caddisflies worldwide. We estimated divergence periods by two methods, a single locus with various specimens and multiple loci with reduced numbers of the specimens. As a result, we elucidated the phylogenetic position of Japanese species within the cosmopolitan genus Anisocentropus, and also revealed their dual origin. In addition, we demonstrated that the contrasting genetic structures between the sister species distributed in widely overlapping areas were due to differentiation in their respective adapted environmental preferences. Although, in general, it is known that species adapted to lentic water have greater dispersal potential and so are associated with wider distribution areas by means of examining their comparative genetic structures, we revealed a new pattern of genetic locality existing in the genetic structures of the species adapted to lentic water. We then present evidence that suggests the ecological preferences of a species are an important factor in understanding the evolutionary history of that species.

Wide-Scale Gene Flow, Even in Insects that have Lost their Flight Ability: Presence of Dispersion Due to a Unique Parasitic Ecological Strategy of Piggybacking Hosts

We focused on Meloe beetles that have lost all flight ability, and conducted molecular phylogeographic analyses based on their mitochondrial DNA COI and nuclear DNA EF1- α regions. Meloe beetles infiltrate bumblebee nests by attaching to bumblebees as they pollinate flowers and thereafter have a unique and specific life history as they complete their life-cycle within the host nest; flight-based dispersal is achieved by piggybacking on bumblebees. In fact, Meloe beetles, which cannot fly, even inhabit remote islands (i.e., "Oceanic Islands"). Regarding four species, i.e., Meloe coarctatus, Meloe proscarabaeus, Meloe violaceus and Meloe corvinus, the conventional morphological classification system based on morphological characteristics was strongly supported by the molecular markers. On the other hand, for two species, Meloe menoko and Meloe auriculatus, it was found that M. menoko may be evaluated as having a paraphyletic relationship with M. auriculatus. Furthermore, two other cryptic, undescribed species were also discovered in this study. One was collected in the Nikko Highland, and inhabited the area sympatrically with M. coarctatus. The other was collected from Hachijo-jima Island. These cryptic species were highly differentiated, independent lineages in terms of mitochondrial and nuclear gene regions. That is to say, a new level of species diversity was revealed among the Meloe beetle species, known for their unique and strange ecological and ethological characteristics.

Global Trends in Bumble Bee Health

Bumble bees (Bombus) are unusually important pollinators, with approximately 260 wild species native to all biogeographic regions except sub-Saharan Africa, Australia, and New Zealand. As they are vitally important in natural ecosystems and to agricultural food production globally, the increase in reports of declining distribution and abundance over the past decade has led to an explosion of interest in bumble bee population decline. We summarize data on the threat status of wild bumble bee species across biogeographic regions, underscoring regions lacking assessment data. Focusing on data-rich studies, we also synthesize recent research on potential causes of population declines. There is evidence that habitat loss, changing climate, pathogen transmission, invasion of nonnative species, and pesticides, operating individually and in combination, negatively impact bumble bee health, and that effects may depend on species and locality. We distinguish between correlational and causal results, underscoring the importance of expanding experimental research beyond the study of two commercially available species to identify causal factors affecting the diversity of wild species.

Level of Genetic Diversity in European Bumblebees is Not Determined by Local Species Abundance

Bumblebee species with declining population trends tend to show lower genetic diversity levels than stable species. The observed difference might be explained by abundance differences, with declining bumblebee species having lower genetic diversity levels simply due to their lower local species abundances. However, whether this holds true is not known. Here, we investigated whether bumblebee local abundances determines population genetic diversity levels. Therefore, local species abundances were measured for bumblebee species at four locations in Belgium and two locations in Estonia during bumblebee foraging years 2013–2017. These locations and countries were chosen to ensure the greatest possible variance in both local abundances and population trends for these species. Hence, genetic diversity levels were obtained for seven species by genotyping collected specimens with 16 microsatellites. Our results showed that the observed patterns in genetic diversity did not depend on local species abundance. So, although declining bumblebee species can become locally abundant, they will still show lower genetic diversity levels than stable species. This result implies that measuring bumblebees’ local abundance cannot be used to directly determine the health status of a population. Furthermore, this result has also major impact on future conservation strategies as increasing the genetic diversity levels of declining species will be very difficult, and habitat quality should be high to maintain their populations, otherwise these species are doomed to disappear first.

Pattern of population structuring between Belgian and Estonian bumblebees

Several population genetic studies investigated the extent of gene flow and population connectivity in bumblebees. In general, no restriction in gene flow is considered for mainland populations of common bumblebee species. Whether this assumption holds true for all species is not known. An assessment of bumblebee genetic structure in the context of their geographic distribution is needed to prioritize conservation and management needs. Here, we conducted a genetic study on seven bumblebee species occurring in Belgium and Estonia. Using 16 microsatellite markers, we investigated genetic diversity and population structuring in each species. This is the first study investigating population structuring of both declining and stable bumblebee species on both small and large geographic scales. Our results showed no or only low population structuring between the populations of the restricted and declining bumblebee species on both scales, while significant structuring was found for populations of the common species on the larger scale. The latter result, which may be due to human or environmental changes in the landscape, implies the need for the conservation of also widespread bumblebee species. Conservation strategies to improve gene flow and connectivity of populations could avoid the isolation and future losses of populations of these important species.

Concordance between molecular biogeography of Dipteromimus tipuliformis and geological history in the local fine scale (Ephemeroptera, Dipteromimidae)

Species distribution area is determined by both biotic and abiotic factors. In particular, significant geological events influence the biodiversity and the genetic structures of the organisms inhabiting the area. The establishment of physical barriers (e.g., mountains, rivers), drives species differentiation by their interference with biological dispersal or gene flow. The Japanese Islands have a high degree of biodiversity. This study focused on the Kii Peninsula, which stands out as a region of exhibiting particularly high biodiversity, and also exceptionally high endemism. The Kii Peninsula has experienced active mountain formation ever since the Quaternary period. In this study, we investigate the influence of geological events on the establishment of genetic diversity. We focused on the mayfly, Dipteromimus tipuliformis. Phylogenetic analyses were performed utilizing the mitochondrial DNA 16S rRNA and COI regions, and the nuclear DNA histone H3, PEPCK and 28S rRNA regions. As a result, it was shown that this mayfly exhibits a genetic structure that strongly reflects the geological history of the Kii Peninsula, and detected their dispersal process across the Median Tectonic Line. This is a unique and significant study, in that it clearly shows the relationship between the phylogenetic evolution of this mayfly and the corresponding geological history in surprisingly geographic fine scale.

Genetic Differentiation of Eastern Honey Bee (Apis cerana) Populations Across Qinghai-Tibet Plateau-Valley Landforms

Many species of high-altitude plateaus tend to be narrowly distributed along river valleys at lower elevations due to a limitation of suitable habitats. The eastern honeybee (Apis cerana) is such a species and this study explored the effects of long and narrow geographic distributions on honeybee populations. Genetic differentiation and diversity were assessed across populations of the southeastern Qinghai-Tibet Plateau. A total of 492 honeybee samples from eight sampling sites in four valleys were analyzed for the genetic differentiation and diversity of 31 microsatellite loci and mitochondrial tRNA^leu-COII fragments. The following results were obtained: (1) Microsatellite genetic differentiation coefficients (F _ST) ranged from 0.06 to 0.16, and mitochondrial F _ST estimates ranged from 0.18 to 0.70 for different sampling sites in the same valley, indicating genetic differentiation. (2) Honeybees in adjacent valleys were also genetically differentiated. The F _ST of microsatellites and mitochondria were 0.04-0.29 and 0.06-0.76, respectively. (3) Likely a result of small population sizes, the observed genetic diversity was low. The observed impedance of honeybee gene flow among valleys increased both genetic differentiation and population numbers in the Qinghai-Tibet Plateau. This study contributes significantly to the current understanding of the mechanism underlying population genetic differentiation and highlights the potential effects of utilizing genetic resources that are subject to the ecological conditions of the long and narrow geographic distributions of plateau-valley landforms.

Genetic diversity and population structure analyses of Plectranthus edulis (Vatke) Agnew collections from diverse agro-ecologies in Ethiopia using newly developed EST-SSRs marker system

BACKGROUND

Plectranthus edulis (Vatke) Agnew (locally known as Ethiopian dinich or Ethiopian potato) is one of the most economically important edible tuber crops indigenous to Ethiopia. Evaluating the extent of genetic diversity within and among populations is one of the first and most important steps in breeding and conservation measures. Hence, this study was aimed at evaluating the genetic diversity and population structure of this crop using collections from diverse agro-ecologies in Ethiopia.

RESULTS

Twenty polymorphic expressed sequence tag based simple sequence repeat (EST-SSRs) markers were developed for P. edulis based on EST sequences of P. barbatus deposited in the GenBank. These markers were used for genetic diversity analyses of 287 individual plants representing 12 populations, and a total of 128 alleles were identified across the entire loci and populations. Different parameters were used to estimate the genetic diversity within populations; and gene diversity index (GD) ranged from 0.31 to 0.39 with overall mean of 0.35. Hierarchical analysis of molecular variance (AMOVA) showed significant but low population differentiation with only 3% of the total variation accounted for variation among populations. Likewise, cluster and STRUCTURE analyses did not group the populations into sharply distinct clusters, which could be attributed to historical and contemporary gene flow and the reproductive biology of the crop.

CONCLUSIONS

These newly developed EST-SSR markers are highly polymorphic within P. edulis and hence are valuable genetic tools that can be used to evaluate the extent of genetic diversity and population structure of not only P. edulis but also various other species within the Lamiaceae family. Among the 12 populations studied, populations collected from Wenbera, Awi and Wolaita showed a higher genetic diversity as compared to other populations, and hence these areas can be considered as hot spots for in-situ conservation as well as for identification of genotypes that can be used in breeding programs.

Genome-wide single nucleotide polymorphism scan suggests adaptation to urbanization in an important pollinator, the red-tailed bumblebee (Bombus lapidarius L.)

Urbanization is considered a global threat to biodiversity; the growth of cities results in an increase in impervious surfaces, soil and air pollution, fragmentation of natural vegetation and invasion of non-native species, along with numerous environmental changes, including the heat island phenomenon. The combination of these effects constitutes a challenge for both the survival and persistence of many native species, while also imposing altered selective regimes. Here, using 110 314 single nucleotide polymorphisms generated by restriction-site-associated DNA sequencing, we investigated the genome-wide effects of urbanization on putative neutral and adaptive genomic diversity in a major insect pollinator, Bombus lapidarius, collected from nine German cities and nine paired rural sites. Overall, genetic differentiation among sites was low and there was no obvious genome-wide genetic structuring, suggesting the absence of strong effects of urbanization on gene flow. We nevertheless identified several loci under directional selection, a subset of which was associated with urban land use, including the percentage of impervious surface surrounding each sampling site. Overall, our results provide evidence of local adaptation to urbanization in the face of gene flow in a highly mobile insect pollinator.

Matching loci surveyed to questions asked in phylogeography

Although mitochondrial DNA (mtDNA) has long been used for assessing genetic variation within and between populations, its workhorse role in phylogeography has been criticized owing to its single-locus nature. The only choice for testing mtDNA results is to survey nuclear loci, which brings into contrast the difference in locus effective size and coalescence times. Thus, it remains unclear how erroneous mtDNA-based estimates of species history might be, especially for evolutionary events in the recent past. To test the robustness of mtDNA and nuclear sequences in phylogeography, we provide one of the largest paired comparisons of summary statistics and demographic parameters estimated from mitochondrial, five Z-linked and 10 autosomal genes of 30 avian species co-distributed in the Caucasus and Europe. The results suggest that mtDNA is robust in estimating inter-population divergence but not in intra-population diversity, which is sensitive to population size change. Here, we provide empirical evidence showing that mtDNA was more likely to detect population divergence than any other single locus owing to its smaller Ne and thus faster coalescent time. Therefore, at least in birds, numerous studies that have based their inferences of phylogeographic patterns solely on mtDNA should not be readily dismissed.

Diploid male production correlates with genetic diversity in the parasitoid wasp Venturia canescens: a genetic approach with new microsatellite markers

Sex determination is ruled by haplodiploidy in Hymenoptera, with haploid males arising from unfertilized eggs and diploid females from fertilized eggs. However, diploid males with null fitness are produced under complementary sex determination (CSD), when individuals are homozygous for this locus. Diploid males are expected to be more frequent in genetically eroded populations (such as islands and captive populations), as genetic diversity at the csd locus should be low. However, only a few studies have focused on the relation between population size, genetic diversity, and the proportion of diploid males in the field. Here, we developed new microsatellite markers in order to assess and compare genetic diversity and diploid male proportion (DMP) in populations from three distinct habitat types - mainland, island, or captive -, in the parasitoid wasp Venturia canescens. Eroded genetic diversity and higher DMP were found in island and captive populations, and habitat type had large effect on genetic diversity. Therefore, DMP reflects the decreasing genetic diversity in small and isolated populations. Thus, Hymenopteran populations can be at high extinction risk due to habitat destruction or fragmentation.

Microsatellite Analysis of Museum Specimens Reveals Historical Differences in Genetic Diversity between Declining and More Stable Bombus Species

Worldwide most pollinators, e.g. bumblebees, are undergoing global declines. Loss of genetic diversity can play an essential role in these observed declines. In this paper, we investigated the level of genetic diversity of seven declining Bombus species and four more stable species with the use of microsatellite loci. Hereto we genotyped a unique collection of museum specimens. Specimens were collected between 1918 and 1926, in 6 provinces of the Netherlands which allowed us to make interspecific comparisons of genetic diversity. For the stable species B. pascuorum, we also selected populations from two additional time periods: 1949-1955 and 1975-1990. The genetic diversity and population structure in B. pascuorum remained constant over the three time periods. However, populations of declining bumblebee species showed a significantly lower genetic diversity than co-occurring stable species before their major declines. This historical difference indicates that the repeatedly observed reduced genetic diversity in recent populations of declining bumblebee species is not caused solely by the decline itself. The historically low genetic diversity in the declined species may be due to the fact that these species were already rare, making them more vulnerable to the major drivers of bumblebee decline.

Molecular tools and bumble bees: revealing hidden details of ecology and evolution in a model system

Bumble bees are a longstanding model system for studies on behaviour, ecology and evolution, due to their well-studied social lifestyle, invaluable role as wild and managed pollinators, and ubiquity and diversity across temperate ecosystems. Yet despite their importance, many aspects of bumble bee biology have remained enigmatic until the rise of the genetic and, more recently, genomic eras. Here, we review and synthesize new insights into the ecology, evolution and behaviour of bumble bees that have been gained using modern genetic and genomic techniques. Special emphasis is placed on four areas of bumble bee biology: the evolution of eusociality in this group, population-level processes, large-scale evolutionary relationships and patterns, and immunity and resistance to pesticides. We close with a prospective on the future of bumble bee genomics research, as this rapidly advancing field has the potential to further revolutionize our understanding of bumble bees, particularly in regard to adaptation and resilience. Worldwide, many bumble bee populations are in decline. As such, throughout the review, connections are drawn between new molecular insights into bumble bees and our understanding of the causal factors involved in their decline. Ongoing and potential applications to bumble bee management and conservation are also included to demonstrate how genetics- and genomics-enabled research aids in the preservation of this threatened group.

Landscape alteration and habitat modification: impacts on plant-pollinator systems

Insect pollinators provide an important ecosystem service to many crop species and underpin the reproductive assurance of many wild plant species. Multiple, anthropogenic pressures threaten insect pollinators. Land-use change and intensification alters the habitats and landscapes that provide food and nesting resources for pollinators. These impacts vary according to species traits, producing winners and losers, while the intrinsic robustness of plant-pollinator networks may provide stability in pollination function. However, this functional stability might be eroded by multiple, interacting stressors. Anthropogenic changes in pollinator-mediated connectivity will alter plant mating systems (e.g. inbreeding level), with implications for plant fitness and phenotypes governing trophic interactions. The degree to which plant populations can persist despite, or adapt to, pollination deficits remains unclear.

The relationship between managed bees and the prevalence of parasites in bumblebees

Honey bees and, more recently, bumblebees have been domesticated and are now managed commercially primarily for crop pollination, mixing with wild pollinators during foraging on shared flower resources. There is mounting evidence that managed honey bees or commercially produced bumblebees may affect the health of wild pollinators such as bumblebees by increasing competition for resources and the prevalence of parasites in wild bees. Here we screened 764 bumblebees from around five greenhouses that either used commercially produced bumblebees or did not, as well as bumblebees from 10 colonies placed at two sites either close to or far from a honey bee apiary, for the parasites Apicystis bombi, Crithidia bombi, Nosema bombi, N. ceranae, N. apis and deformed wing virus. We found that A. bombi and C. bombi were more prevalent around greenhouses using commercially produced bumblebees, while C. bombi was 18% more prevalent in bumblebees at the site near to the honey bee apiary than those at the site far from the apiary. Whilst these results are from only a limited number of sites, they support previous reports of parasite spillover from commercially produced bumblebees to wild bumblebees, and suggest that the impact of stress from competing with managed bees or the vectoring of parasites by them on parasite prevalence in wild bees needs further investigation. It appears increasingly likely that the use of managed bees comes at a cost of increased parasites in wild bumblebees, which is not only a concern for bumblebee conservation, but which may impact other pollinators as well.

Fine-scale spatial genetic structure of common and declining bumble bees across an agricultural landscape

Land-use changes have threatened populations of many insect pollinators, including bumble bees. Patterns of dispersal and gene flow are key determinants of species' ability to respond to land-use change, but have been little investigated at a fine scale (<10 km) in bumble bees. Using microsatellite markers, we determined the fine-scale spatial genetic structure of populations of four common Bombus species (B. terrestris, B. lapidarius, B. pascuorum and B. hortorum) and one declining species (B. ruderatus) in an agricultural landscape in Southern England, UK. The study landscape contained sown flower patches representing agri-environment options for pollinators. We found that, as expected, the B. ruderatus population was characterized by relatively low heterozygosity, number of alleles and colony density. Across all species, inbreeding was absent or present but weak (FIS  = 0.01-0.02). Using queen genotypes reconstructed from worker sibships and colony locations estimated from the positions of workers within these sibships, we found that significant isolation by distance was absent in B. lapidarius, B. hortorum and B. ruderatus. In B. terrestris and B. pascuorum, it was present but weak; for example, in these two species, expected relatedness of queens founding colonies 1 m apart was 0.02. These results show that bumble bee populations exhibit low levels of spatial genetic structure at fine spatial scales, most likely because of ongoing gene flow via widespread queen dispersal. In addition, the results demonstrate the potential for agri-environment scheme conservation measures to facilitate fine-scale gene flow by creating a more even distribution of suitable habitats across landscapes.

Caste load and the evolution of reproductive skew

Reproductive skew theory seeks to explain how reproduction is divided among group members in animal societies. Existing theory is framed almost entirely in terms of selection, though nonadaptive processes must also play some role in the evolution of reproductive skew. Here I propose that a genetic correlation between helper fecundity and breeder fecundity may frequently constrain the evolution of reproductive skew. This constraint is part of a wider phenomenon that I term "caste load," which is defined as the decline in mean fitness caused by caste-specific selection pressures, that is, differential selection on breeding and nonbreeding individuals. I elaborate the caste load hypothesis using quantitative and population genetic arguments and individual-based simulations. Although selection can sometimes erode genetic correlations and resolve caste load, this may be constrained when mutations have similar pleiotropic effects on breeder and helper traits. I document evidence for caste load, identify putative genomic adaptations to it, and suggest future research directions. The models highlight the value of considering adaptation within the boundaries imposed by genetic architecture and incidentally reaffirm that monogamy promotes the evolutionary transition to eusociality.

Migration and dispersal may drive to high genetic variation and significant genetic mixing: the case of two agriculturally important, continental hoverflies (Episyrphus balteatus and Sphaerophoria scripta)

Population structure of pests and beneficial species is an important issue when designing management strategies to optimize ecosystem services. In this study, we investigated for the first time the population structure at a continental scale of two migratory species of hoverflies providing both pest regulation and pollination services [Episyrphus balteatus and Sphaerophoria scripta (Diptera: Syrphidae)]. To achieve this objective, we used two sets of 12 species-specific microsatellite markers on a large-scale sampling from all over Europe. Our findings showed a high level of genetic mixing resulting in a lack of genetic differentiation at a continental scale and a great genetic diversity in the two species. All the pairwise FST values between European localities were less 0.05 in the two species. These low values reflect a large-scale genetic mixing probably caused by the existence of frequent migratory movements in the two species. Mantel tests revealed isolation-by-distance pattern on the East-West axis, but not on the North-South axis. This isolation-by-distance pattern confirms the existence of North-South migratory movements in both directions and suggests an important step by step dispersal. Population features shown by this study are common in invasive species and pests, but are not often observed in beneficial species. They reflect great colonization abilities and a high adaptive potential when dealing with a changing environment. Our results highlight the two studied species as particularly interesting beneficial insects for pollination and pest predation in the current context of global change.

Lack of variation at phosphoglucose isomerase (pgi) in bumblebees: implications for conservation genetics studies

Assessing genetic variation underlying ecologically important traits is increasingly of interest and importance in population and conservation genetics. For some groups generally useful markers exist for examining the relative role of selection and drift in shaping genetic diversity e.g. the major histocompatibility complex in vertebrates and self-incompatibility loci in plants. For invertebrates there is no such generally useful locus. However, phosphoglucose isomerase (Pgi) has been proposed as a useful functional marker in the conservation genetics of invertebrates. Where thermal microclimate varies, balanced polymorphisms may be maintained due to trade-offs between thermally stable and kinetically advantageous allelic forms. We here report very low levels of Pgi variation in bumblebees rendering this locus to be of little use as an adaptive marker in a conservation genetics context in this group. Potential explanations for this lack of variation are considered.

Genetic differentiation of the Euglossini (Hymenoptera, Apidae) populations on a mainland coastal plain and an island in southeastern Brazil

Euglossini bees are among the main pollinators of plant species in tropical and subtropical forests in Central and South America. These bees are known as long-distance pollinators due to their exceptional flight performance. Here we assessed through microsatellite loci the gene variation and genetic differentiation between populations of four abundant Euglossini species populations sampled in two areas, Picinguaba (mainland) and Anchieta Island, Ubatuba, São Paulo State, southeastern Brazil. There was no significant genetic differentiation between the island and mainland samples of Euglossa cordata (Fst = 0.008, P = 0.60), Eulaema cingulata (Fst = 0.029, P = 0.29) and Eulaema nigrita (Fst = 0.062, P = 0.38), but a significant gene differentiation between mainland and island samples of Euglossa stellfeldi (Fst = 0.028, P = 0.016) was detected. As expected, our results showed that the water body that separates the island from the mainland does not constitute a geographic barrier for these Euglossini bees. The absence of populational structuring of three out the four species studied corroborates previous reports on those bees, characterized by large populations, with high gene diversity and gene flow and very low levels of diploid males. But the Eg. stellfeldi results clearly point that dispersal ability is not similar to all euglossine bees, what requires the development of different conservationist strategies to the Euglossini species.

Effective population size in eusocial Hymenoptera with worker-produced males

In many eusocial Hymenoptera, a proportion of males are produced by workers. To assess the effect of male production by workers on the effective population size N(e), a general expression of N(e) in Hymenoptera with worker-produced males is derived on the basis of the genetic drift in the frequency of a neutral allele. Stochastic simulation verifies that the obtained expression gives a good prediction of N(e) under a wide range of conditions. Numerical computation with the expression indicates that worker reproduction generally reduces N(e). The reduction can be serious in populations with a unity or female-biased breeding sex ratio. Worker reproduction may increase N(e) in populations with a male-biased breeding sex ratio, only if each laying worker produce a small number of males and the difference of male progeny number among workers is not large. Worker reproduction could be an important cause of the generally lower genetic variation found in Hymenoptera, through its effect on N(e).

Triploid bumblebees indicate a direct cost of inbreeding in fragmented populations

Hymenopteran species with single-locus complimentary sex-determination (sl-CSD) face an additional cost of inbreeding because of a loss of diversity at the sex-determining locus. Laboratory studies of a range of Hymenoptera have found that a small percentage of diploid males produce viable diploid sperm, and that if these males mate, then the resultant females produce triploid offspring that are sterile. Here, we use microsatellite markers to determine the frequency of triploid individuals of Bombus muscorum and B. jonellus in a model island system. Triploids were found in populations of both species. Observed triploid frequencies of up to 8% were detected, and estimated total frequencies peaked at 20% with respect to normal diploid workers. For both species, triploid frequency was negatively correlated with surrogates of population size, providing direct evidence for inbreeding in small populations. Populations limited to <∼15 km(2) of suitable habitat were particularly likely to harbour triploids. Estimated total triploid frequencies were higher in B. muscorum than in B. jonellus, perhaps due to the greater dispersal range of the latter species. Implications for the conservation of rare social hymenopterans are discussed.

Patterns of range-wide genetic variation in six North American bumble bee (Apidae: Bombus) species

The increasing evidence for population declines in bumble bee (Bombus) species worldwide has accelerated research efforts to explain losses in these important pollinators. In North America, a number of once widespread Bombus species have suffered serious reductions in range and abundance, although other species remain healthy. To examine whether declining and stable species exhibit different levels of genetic diversity or population fragmentation, we used microsatellite markers to genotype populations sampled across the geographic distributions of two declining (Bombus occidentalis and Bombus pensylvanicus) and four stable (Bombus bifarius; Bombus vosnesenskii; Bombus impatiens and Bombus bimaculatus) Bombus species. Populations of declining species generally have reduced levels of genetic diversity throughout their range compared to codistributed stable species. Genetic diversity can be affected by overall range size and degree of isolation of local populations, potentially confounding comparisons among species in some cases. We find no evidence for consistent differences in gene flow among stable and declining species, with all species exhibiting weak genetic differentiation over large distances (e.g. >1000 km). Populations on islands and at high elevations experience relatively strong genetic drift, suggesting that some conditions lead to genetic isolation in otherwise weakly differentiated species. B. occidentalis and B. bifarius exhibit stronger genetic differentiation than the other species, indicating greater phylogeographic structure consistent with their broader geographic distributions across topographically complex regions of western North America. Screening genetic diversity in North American Bombus should prove useful for identifying species that warrant monitoring, and developing management strategies that promote high levels of gene flow will be a key component in efforts to maintain healthy populations.

Genetic diversity, parasite prevalence and immunity in wild bumblebees

Inbreeding and a consequent loss of genetic diversity threaten small, isolated populations. One mechanism by which genetically impoverished populations may become extinct is through decreased immunocompetence and higher susceptibility to parasites. Here, we investigate the relationship between immunity and inbreeding in bumblebees, using Hebridean island populations of Bombus muscorum. We sampled nine populations and recorded parasite prevalence and measured two aspects of immunity: the encapsulation response and levels of phenoloxidase (PO). We found that prevalence of the gut parasite Crithidia bombi was higher in populations with lower genetic diversity. Neither measure of immune activity was correlated with genetic diversity. However, levels of PO declined with age and were also negatively correlated with parasite abundance. Our results suggest that as insect populations lose heterozygosity, the impact of parasitism will increase, pushing threatened populations closer to extinction.