Gillian Lye

Population structure and inbreeding in a rare and declining bumblebee, Bombus muscorum (Hymenoptera: Apidae)

Owing to habitat loss populations of many organisms have declined and become fragmented. Vertebrate conservation strategies routinely consider genetic factors, but their importance in invertebrate populations is poorly understood. Bumblebees are important pollinators, and many species have undergone dramatic declines. As monoandrous social hymenopterans they may be particularly susceptible to inbreeding due to low effective population sizes. We study fragmented populations of a bumblebee species, on a model island system, and on mainland Great Britain where it is rare and declining. We use microsatellites to study: population genetic structuring and gene flow; the relationships between genetic diversity, population size and isolation; and frequencies of (sterile) diploid males — an indicator of inbreeding. We find significant genetic structuring (θ = 0.12) and isolation by distance. Populations > 10 km apart are all significantly differentiated, both on oceanic islands and on the mainland. Genetic diversity is reduced relative to closely related common species, and isolated populations exhibit further reductions. Of 16 populations, 10 show recent bottlenecking, and 3 show diploid male production. These results suggest that surviving populations of this rare insect suffer from inbreeding as a result of geographical isolation. Implications for the conservation of social hymenopterans are discussed.

Cryptic differences in dispersal lead to differential sensitivity to habitat fragmentation in two bumblebee species

Habitat loss has led to fragmentation of populations of many invertebrates, but social hymenopterans may be particularly sensitive to habitat fragmentation due to their low effective population sizes. The impacts of fragmentation depend strongly on dispersal abilities, but these are difficult to quantify. Here, we quantify and compare dispersal abilities of two bumblebee species, Bombus muscorum and Bombus jonellus, in a model island system. We use microsatellites to investigate population genetic structuring, dispersal and spatial patterns in genetic diversity. Populations of both species showed significant structuring, and isolation by distance, but this was markedly greater in B. muscorum (θ = 0.13) than in B. jonellus (θ = 0.034). This difference could reflect a higher effective population size in B. jonellus compared to B. muscorum, but this is not consistent with the observed abundance of the two species. We argue that it is more likely that B. jonellus has a higher propensity to disperse than B. muscorum. This will influence their relative susceptibility to habitat fragmentation and may in part explain differential declines of mainland populations of these and other bumblebee species.

Reconstructing demographic events from population genetic data: the introduction of bumblebees to New Zealand

Four British bumblebee species (Bombus terrestris, Bombus hortorum, Bombus ruderatus and Bombus subterraneus) became established in New Zealand following their introduction at the turn of the last century. Of these, two remain common in the United Kingdom (B. terrestris and B. hortorum), whilst two (B. ruderatus and B. subterraneus) have undergone marked declines, the latter being declared extinct in 2000. The presence of these bumblebees in New Zealand provides an unique system in which four related species have been isolated from their source population for over 100 years, providing a rare opportunity to examine the impacts of an initial bottleneck and introduction to a novel environment on their population genetics. We used microsatellite markers to compare modern populations of B. terrestris, B. hortorum and B. ruderatus in the United Kingdom and New Zealand and to compare museum specimens of British B. subterraneus with the current New Zealand population. We used approximate Bayesian computation to estimate demographic parameters of the introduction history, notably to estimate the number of founders involved in the initial introduction. Species‐specific patterns derived from genetic analysis were consistent with the predictions based on the presumed history of these populations; demographic events have left a marked genetic signature on all four species. Approximate Bayesian analyses suggest that the New Zealand population of B. subterraneus may have been founded by as few as two individuals, giving rise to low genetic diversity and marked genetic divergence from the (now extinct) UK population.

Impacts of the use of Nonnative Commercial Bumble Bees for Pollinator Supplementation in Raspberry

ABSTRACT Evidence for pollinator declines has led to concern that inadequate pollination services may limit crop yields. The global trade in commercial bumble bee (Bombus spp.) colonies provides pollination services for both glasshouse and open-field crops. For example, in the United Kingdom, commercial colonies of nonnative subspecies of the bumble bee Bombus terrestris L. imported from mainland Europe are widely used for the pollination of raspberries, Rubus idaeus L. The extent to which these commercial colonies supplement the services provided by wild pollinators has not been formally quantified and the impact of commercial bumble bees on native bees visiting the crop is unknown. Here, the impacts of allowing commercially available bumble bee colonies to forage on raspberry canes are assessed in terms of the yield of marketable fruit produced and the pollinator communities found foraging on raspberry flowers. No differences were found in the abundance, diversity, or composition of social bee species observed visiting raspberry flowers when commercial bumble bees were deployed compared with when they were absent. However, weight of marketable raspberries produced increased when commercial bees were present, indicating that wild pollinator services alone are inadequate for attaining maximum yields. The findings of the study suggests that proportional yield increases associated with deployment of commercial colonies may be small, but that nevertheless, investment in commercial colonies for raspberry pollination could produce very significant increases in net profit for the grower. Given potential environmental risks associated with the importation of nonnative bumble bees, the development of alternative solutions to the pollination deficit in raspberry crops in the United Kingdom may be beneficial.

Niche differentiation of a cryptic bumblebee complex in the Western Isles of Scotland

Abstract.  1. It has recently become apparent that there is cryptic diversity in bumblebees, notably in the Bombus lucorum complex which appears to contain three distinct taxa (B. lucorum, B. magnus, and B. cryptarum). We know almost nothing about how these species differ in their ecology or distribution.

Aggregations of male Bombus muscorum (Hymenoptera: Apidae) at mature nests. Incestuous brothers or amorous suitors?

Aggregations of male bumblebees are occasionally observed at the nest entrances of conspecifics. It has always been assumed that they originate from nearby nests and are hoping to mate with emerging gynes. However, it is possible that they are males from within the nest that have not yet dispersed, or are hoping to mate with their sisters. Inbreeding in Hymenoptera with single locus complimentary sex determination (sl-CSD) is costly and should be avoided. Nevertheless, other taxa with sl-CSD have been found to inbreed in this way. We use microsatellites to compare aggregating males with workers from within a Bombus muscorum nest. All males have genotypes inconsistent with origination from within the nest. Estimates of FIS are near zero, indicating low rates of non-random mating. Clearly male bumblebees can detect mature nests, and congregate in the hope of mating with virgin queens. These data suggest that mechanisms may exist to avoid brother-sister matings at the nest, and perhaps beyond.ZusammenfassungGelegentlich findet man Ansammlungen von männlichen Hummeln an den Ausgängen von voll entwickelten Hummelnestern. Obwohl dies keine bevorzugte Paarungsstrategie der Hummeln darstellt, wurde trotzdem angenommen, dass diese Männchen von anderen Nestern der Umgebung stammen in der Hoffnung, sich mit einer unbegatteten Königin zu paaren, wenn diese das Nest verlässt. Es gibt aber auch die Möglichkeit, dass diese Männchen aus eben diesem Nest stammen und vor dem Ausfliegen hier verweilen, möglicherweise in der Hoffnung sich mit einer ihrer Schwestern zu paaren. Es wird allerdings angenommen, dass sich Inzucht gerade bei Hymenopteren negativ auswirkt und daher vermieden werden sollte. Trotzdem ist bei verschiedenen Hymenopteren-Arten die Paarung zwischen Geschwistern weit verbreitet.In dieser Arbeit verwendeten wir genetische Marker um festzustellen, ob Männchen, die sich um ein reifes Nest herum sammeln, mit den Individuen innerhalb des Nestes verwandt sind. Die Untersuchungen wurden an zufällig gefundenen Nestern auf einer isolierten schottischen Insel durchgeführt. Zusätzlich überprüften wir anhand des Inzuchtgrades innerhalb der gesamten Population, ob Geschwister-Paarungen üblich sind. Unsere Ergebnisse lassen darauf schließen, dass keines der vor den Nesteingängen versammelten Männchen aus dem jeweiligen Nest stammt. Die Männchen müssen daher aus der Umgebung durch bisher unbekannte Reize angelockt werden. Der Inzuchtgrad innerhalb der Population lag nahe Null, was mit den sehr seltenen Bruder-Schwester-Paarungen übereinstimmt. Unsere Ergebnisse lassen vermuten, dass die Mehrzahl der Männchen sich vom Nest entfernt, um Paarungen mit den eigenen Schwestern zu vermeiden. Wir diskutieren frühere Arbeiten bei verwandten Taxa, die eine bevorzugte Paarung mit nicht verwandten Partnern nachgewiesen haben. Wir vermuten, dass Hummeln in der Lage sind, den Verwandtschaftsgrad anhand von kutikulären Duftstoffmustern festzustellen und diese Fähigkeit zur Vermeidung von Inzucht verwenden.