Matthew C. Tinsley

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.

The harlequin ladybird, Harmonia axyridis: global perspectives on invasion history and ecology

The harlequin ladybird, Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae), is native to Asia but has been intentionally introduced to many countries as a biological control agent of pest insects. In numerous countries, however, it has been introduced unintentionally. The dramatic spread of H. axyridis within many countries has been met with considerable trepidation. It is a generalist top predator, able to thrive in many habitats and across wide climatic conditions. It poses a threat to biodiversity, particularly aphidophagous insects, through competition and predation, and in many countries adverse effects have been reported on other species, particularly coccinellids. However, the patterns are not consistent around the world and seem to be affected by many factors including landscape and climate. Research on H. axyridis has provided detailed insights into invasion biology from broad patterns and processes to approaches in surveillance and monitoring. An impressive number of studies on this alien species have provided mechanistic evidence alongside models explaining large-scale patterns and processes. The involvement of citizens in monitoring this species in a number of countries around the world is inspiring and has provided data on scales that would be otherwise unachievable. Harmonia axyridis has successfully been used as a model invasive alien species and has been the inspiration for global collaborations at various scales. There is considerable scope to expand the research and associated collaborations, particularly to increase the breadth of parallel studies conducted in the native and invaded regions. Indeed a qualitative comparison of biological traits across the native and invaded range suggests that there are differences which ultimately could influence the population dynamics of this invader. Here we provide an overview of the invasion history and ecology of H. axyridis globally with consideration of future research perspectives. We reflect broadly on the contributions of such research to our understanding of invasion biology while also informing policy and people.

Are we underestimating the diversity and incidence of insect bacterial symbionts? A case study in ladybird beetles

Vertically transmitted bacterial symbionts are common in arthropods. However, estimates of their incidence and diversity are based on studies that test for a single bacterial genus and often only include small samples of each host species. Focussing on ladybird beetles, we collected large samples from 21 species and tested them for four different bacterial symbionts. Over half the species were infected, and there were often multiple symbionts in the same population. In most cases, more females than males were infected, suggesting that the symbionts may be sex ratio distorters. Many of these infections would have been missed in previous studies as they only infect a small proportion of the population. Furthermore, 11 out of the 17 symbionts discovered by us were either in the genus Rickettsia or Spiroplasma, which are rarely sampled. Our results suggest that the true incidence and diversity of bacterial symbionts in insects may be far greater than previously thought.

Impacts of inbreeding on bumblebee colony fitness under field conditions.

BackgroundInbreeding and the loss of genetic diversity are known to be significant threats to small, isolated populations. Hymenoptera represent a special case regarding the impact of inbreeding. Haplodiploidy may permit purging of deleterious recessive alleles in haploid males, meaning inbreeding depression is reduced relative to diploid species. In contrast, the impact of inbreeding may be exacerbated in Hymenopteran species that have a single-locus complementary sex determination system, due to the production of sterile or inviable diploid males. We investigated the costs of brother-sister mating in the bumblebee Bombus terrestris. We compared inbred colonies that produced diploid males and inbred colonies that did not produce diploid males with outbred colonies. Mating, hibernation and colony founding took place in the laboratory. Once colonies had produced 15 offspring they were placed in the field and left to forage under natural conditions.ResultsThe diploid male colonies had a significantly reduced fitness compared to regular inbred and outbred colonies; they had slower growth rates in the laboratory, survived for a shorter time period under field conditions and produced significantly fewer offspring overall. No differences in success were found between non-diploid male inbred colonies and outbred colonies.ConclusionOur data illustrate that inbreeding exacts a considerable cost in Bombus terrestris through the production of diploid males. We suggest that diploid males may act as indicators of the genetic health of populations, and that their detection could be used as an informative tool in hymenopteran conservation. We conclude that whilst haplodiploids may suffer less inbreeding depression than diploid species, they are still highly vulnerable to population fragmentation and reduced genetic diversity due to the extreme costs imposed by the production of diploid males.

Senescence of the cellular immune response in Drosophila melanogaster.

Immune system effectiveness generally declines as animals age, compromising disease resistance. In Drosophila, expression of a variety of immune-related genes elevates during ageing; however how this is linked to increasing pathogen susceptibility in older flies has remained unclear. We investigated whether changes in the Drosophila cellular immune response might contribute to immunosenescence. Experiments studied fly cohorts of different ages and compared the numbers and activity of the circulating haemocytes involved in pathogen defence. In female wildtype Samarkand and Oregon R flies the haemocyte population fell by 31.8% and 10.2% respectively during the first four weeks of adulthood. Interestingly we detected no such decline in male flies. The impact of ageing on the phagocytic activity of haemocytes was investigated by injecting flies with fluorescently labelled microbes or latex beads and assessing the ability of haemocytes to engulf them. For all immune challenges the proportion of actively phagocytosing haemocytes decreased as flies aged. Whilst 24.3%±1.15% of haemocytes in one-week-old flies phagocytosed Escherichia coli bacteria or Beauveria bassiana fungal spores, this decreased to 16.7%±0.99% in four-week-old flies. This clear senescence of the Drosophila cellular immune response may underpin increased disease susceptibility in older flies.

Small steps or giant leaps for male-killers? Phylogenetic constraints to male-killer host shifts.

BackgroundArthropods are infected by a wide diversity of maternally transmitted microbes. Some of these manipulate host reproduction to facilitate population invasion and persistence. Such parasites transmit vertically on an ecological timescale, but rare horizontal transmission events have permitted colonisation of new species. Here we report the first systematic investigation into the influence of the phylogenetic distance between arthropod species on the potential for reproductive parasite interspecific transfer.ResultsWe employed a well characterised reproductive parasite, a coccinellid beetle male-killer, and artificially injected the bacterium into a series of novel species. Genetic distances between native and novel hosts were ascertained by sequencing sections of the 16S and 12S mitochondrial rDNA genes. The bacterium colonised host tissues and transmitted vertically in all cases tested. However, whilst transmission efficiency was perfect within the native genus, this was reduced following some transfers of greater phylogenetic distance. The bacteriums ability to distort offspring sex ratios in novel hosts was negatively correlated with the genetic distance of transfers. Male-killing occurred with full penetrance following within-genus transfers; but whilst sex ratio distortion generally occurred, it was incomplete in more distantly related species.ConclusionThis study indicates that the natural interspecific transmission of reproductive parasites might be constrained by their ability to tolerate the physiology or genetics of novel hosts. Our data suggest that horizontal transfers are more likely between closely related species. Successful bacterial transfer across large phylogenetic distances may require rapid adaptive evolution in the new species. This finding has applied relevance regarding selection of suitable bacteria to manipulate insect pest and vector populations by symbiont gene-drive systems.

Kin recognition and inbreeding reluctance in bumblebees

Inbreeding frequently has a costly impact on fitness, thus selection has favoured the evolution of kin recognition and inbreeding avoidance behaviour in many species. As haplodiploid Hymenoptera, bumblebees are susceptible to additional costs of inbreeding due to their single-locus complementary sex determination (sl-CSD) system, which means that incest can result in the production of costly diploid males. Here we test whether Bombus terrestris reproductives are able to discriminate between kin and non-kin and whether their willingness to mate is adjusted accordingly. We found that B. terrestris reproductives took significantly longer to mate with siblings compared to non-relatives. This indicates that this species exhibits kin recognition and uses this information to determine mating behaviour.ZusammenfassungBei vielen Tierarten führt die Paarung mit nahen Verwandten bei den Nachkommen zu einer geringerer Fitness, ein Phänomen, das als Inzuchtdepression bekannt ist. Solche Arten sollten daher in der Lage sein, ihre Verwandten zu erkennen und eine Paarung mit ihnen zu vermeiden. Das Ziel dieses Experimentes war es zu prüfen, ob Hummeln aus der Art Bombus terrestris ihre Verwandten erkennen können, da Hummeln allgemein als besonders anfällig gegenüber Inzuchteffekte gelten. Dies vor allem deshalb, da aufgrund der genetischen Strukturen im Hummelvolk die Paarung zwischen verwandten Individuen zu diploiden Männchen führen kann. Diploide Männchen sind steril und werden auf Kosten der fleißigen Arbeiterinnen produziert, wodurch das Hummelvolk geschwächt wird. Daher sollte die natürliche Selektion zu Verwandtschaftserkennung und Inzuchtvermeidung führen, um die Kosten für die Produktion diploider Männchen zu umgehen. Das Paarungsexperiment wurde in einem großen Flugkäfig (70 cm × 70 cm × 70 cm) aus Gaze durchgeführt und den jungen Königinnen wurden entweder ihre Brüder oder unverwandte Männchen als Paarungspartner angeboten. Die Bereitschaft der Königinnen sich mit ihren Brüdern bzw. den unverwandten Männchen zu paaren wurde ermittelt, indem die Zeitspanne zwischen dem Freilassen der Paarungspartner (Königin und Männchen) und der erfolgreichen Kopulation gemessen wurde. Durchschnittlich 10,8 Minuten (± 0,94) vergingen, bis eine Verwandtenpaarung stattfand, während im Durchschnitt lediglich 4,5 Minuten (± 1,15) für eine Paarung zwischen nicht verwandten Partnern benötigt wurden. Diese Ergebnisse lassen vermuten, dass B. terrestris die Fähigkeit zur Verwandtschaftserkennung besitzt und entsprechend dem Verwandtschaftsgrad das Paarungsverhalten ändert. Weitere Untersuchungen sollten die Mechanismen der Verwandtschaftserkennung aufklären.

An ancient mitochondrial polymorphism in Adalis bipunctata linked to a sex-ratio-distorting bacterium.

Sex-ratio-distorting microbes are common parasites of arthropods. Although the reasons they have invaded and spread though populations are well understood, their subsequent dynamics within those populations are virtually unknown. We have found that different strains of a male-killing Rickettsia bacterium infecting the beetle Adalia bipunctata are associated with distinct mitochondrial haplotypes, which is expected as both the mitochondria and the bacteria are maternally transmitted. These mitochondrial haplotypes shared a common ancestor >2 million years ago, and their overall diversity is significantly greater than expected under neutrality from comparisons with a nuclear gene. Furthermore, a variety of statistical tests show strong deviations from neutrality in mitochondrial but not in nuclear genes. We therefore conclude that natural selection is probably maintaining a polymorphism of different Rickettsia strains in this species. Despite the age of the different mitochondrial haplotypes, there is very little genetic diversity within them. Furthermore, there is considerable variation in mitochondrial haplotype and bacterial strain frequency between populations, despite it being thought that this species has fairly low levels of population structure. We conclude that the fitness of these male killers may be negatively frequency dependent or different strains may be favored in different populations. These hypotheses await experimental confirmation.

Social learning drives handedness in nectar-robbing bumblebees

Bumblebees have been found to observe and copy the behaviour of others with regard to floral choices, particularly when investigating novel flower types. They can also learn to make nectar-robbing holes in flowers as a result of encountering them. Here, we investigate handedness in nectar-robbing bumblebees feeding on Rhinanthus minor, a flower that can be robbed from either the right-hand side or the left-hand side. We studied numerous patches of R. minor spread across an alpine landscape; each patch tended to be robbed on either the right or the left. The intensity of side bias increased through the season and was strongest in the most heavily robbed patches. We suggest that bees within patches learn robbing strategies (including handedness) from one another, either by direct observation or from experience with the location of holes, leading to rapid frequency-dependent selection for a common strategy. Primary robbing was predominantly carried out not only by a specialist robbing species, Bombus wurflenii, but also by Bombus lucorum, a widespread generalist. Both species adopted the same handedness within particular flower patches, providing the first evidence for social learning crossing the species boundary in wild insects.

Immune response costs are associated with changes in resource acquisition and not resource reallocation

Summary 1. Evolutionary ecologists frequently argue that parasite defence is costly because resources must be reallocated from other life-history traits to fuel the immune response. However, this hypothesis is rarely explicitly tested. An alternative possibility is that immune responses impair an organism’s ability to acquire the resources it needs to support metabolism. Here, we disentangle these opposing hypotheses for why the activation costs of parasite resistance arise. 2. We studied fecundity costs associated with immune stimulation in Drosophila melanogaster. Then, by measuring correlated changes in metabolic rate, food consumption and body weight, we assessed whether responses were consistent with immunity costs originating from altered resource allocation or from impaired resource acquisition. 3. Microbial injection resulted in a 45% fecundity decrease. It also triggered a mean decline in metabolic rate of 6% and a mean reduction in food intake of 31%; body weight was unaffected. Metabolic rate downregulation was greater in males than in females, whereas declines in food ingestion were of similar magnitude in both sexes. These physiological shifts did not depend on whether microbial challenges were alive or dead, thus they resulted from immune system activation not pathogenesis. 4. These costs of immune activation are significant for individuals that successfully resist infection and might also occur in other situations when immune responses are upregulated without infection. 5. Whilst we found significant activation costs of resistance, our data provide no compelling evidence for the popularly argued hypothesis that immune deployment is costly because of reallocation of energetic resources to the immune system. Instead, reduction in resource acquisition due to ‘infection-induced anorexia’ may be the principal driver of metabolic changes and fecundity costs resulting from immune response activation.