Graham N. Stone

The adaptive significance of insect gall morphology

Insect galls are dramatic examples of extended phenotypes: although composed of host plant tissues, their development is largely controlled by insect genes. Adaptive explanations for gall traits should thus be expressed in terms of impacts on insect fitness, but the extent to which interspecific variation in gall structure is adaptive, and the possible selective pressures driving diversification in gall form remain controversial. In colonial aphids and thrips, gall structures probably diversified in response to selection for enhancement of the surface area available for feeding. In other taxa, such as gall wasps and gall midges, diversity is expressed predominantly in non-nutritive tissues, particularly those on the gall surface. All natural enemies attack the occupants of closed galls by penetrating gall tissue, and modifications that reduce enemy attack rates should thus be favoured. Recent studies of intraspecific variation in gall form strongly support a defensive role for several traits, but, to date, there is little empirical support for enemies as a cause of interspecific variation in gall form. Selection imposed by enemies nevertheless remains the most probable adaptive explanation for the evolution of diversity. We suggest that this hypothesis has yet to be tested explicitly, and discuss the requirements for an appropriate cross-species analysis.

Evolution and diversity of Rickettsia bacteria

BackgroundRickettsia are intracellular symbionts of eukaryotes that are best known for infecting and causing serious diseases in humans and other mammals. All known vertebrate-associated Rickettsia are vectored by arthropods as part of their life-cycle, and many other Rickettsia are found exclusively in arthropods with no known secondary host. However, little is known about the biology of these latter strains. Here, we have identified 20 new strains of Rickettsia from arthropods, and constructed a multi-gene phylogeny of the entire genus which includes these new strains.ResultsWe show that Rickettsia are primarily arthropod-associated bacteria, and identify several novel groups within the genus. Rickettsia do not co-speciate with their hosts but host shifts most often occur between related arthropods. Rickettsia have evolved adaptations including transmission through vertebrates and killing males in some arthropod hosts. We uncovered one case of horizontal gene transfer among Rickettsia, where a strain is a chimera from two distantly related groups, but multi-gene analysis indicates that different parts of the genome tend to share the same phylogeny.ConclusionApproximately 150 million years ago, Rickettsia split into two main clades, one of which primarily infects arthropods, and the other infects a diverse range of protists, other eukaryotes and arthropods. There was then a rapid radiation about 50 million years ago, which coincided with the evolution of life history adaptations in a few branches of the phylogeny. Even though Rickettsia are thought to be primarily transmitted vertically, host associations are short lived with frequent switching to new host lineages. Recombination throughout the genus is generally uncommon, although there is evidence of horizontal gene transfer. A better understanding of the evolution of Rickettsia will help in the future to elucidate the mechanisms of pathogenicity, transmission and virulence.

Differential var gene transcription in Plasmodium falciparum isolates from patients with cerebral malaria compared to hyperparasitaemia

The Plasmodium falciparum variant erythrocyte surface antigens known as PfEMP1, encoded by the var gene family, are thought to play a crucial role in malaria pathogenesis because they mediate adhesion to host cells and immuno-modulation. Var genes have been divided into three major groups (A, B and C) and two intermediate groups (B/A and B/C) on the basis of their genomic location and upstream sequence. We analysed expressed sequence tags of the var gene DBLα domain to investigate var gene transcription in relation to disease severity in Malian children. We found that P. falciparum isolates from children with cerebral malaria (unrousable coma) predominantly transcribe var genes with DBLα1-like domains that are characteristic of Group A or B/A var genes. In contrast, isolates from children with equally high parasite burdens but no symptoms or signs of severe malaria (hyperparasitaemia patients) predominantly transcribe var genes with DBLα0-like domains that are characteristic of the B and C-related var gene groups. These results suggest that var genes with DBLα1-like domains (Group A or B/A) may be implicated in the pathogenesis of cerebral malaria, while var genes with DBLα0-like domains promote less virulent malaria infections.

THE STRUCTURE OF CYNIPID OAK GALLS : PATTERNS IN THE EVOLUTION OF AN EXTENDED PHENOTYPE

Galls are highly specialized plant tissues whose development is induced by another organism. The most complex and diverse galls are those induced on oak trees by gallwasps (Hymenoptera: Cynipidae: Cynipini), each species inducing a characteristic gall structure. Debate continues over the possible adaptive significance of gall structural traits; some protect the gall inducer from attack by natural enemies, although the adaptive significance of others remains undemonstrated. Several gall traits are shared by groups of oak gallwasp species. It remains unknown whether shared traits represent (i) limited divergence from a shared ancestral gall form, or (ii) multiple cases of independent evolution. Here we map gall character states onto a molecular phylogeny of the oak cynipid genus Andricus, and demonstrate three features of the evolution of gall structure: (i) closely related species generally induce galls of similar structure; (ii) despite this general pattern, closely related species can induce markedly different galls; and (iii) several gall traits (the presence of many larval chambers in a single gall structure, surface resins, surface spines and internal air spaces) of demonstrated or suggested adaptive value to the gallwasp have evolved repeatedly. We discuss these results in the light of existing hypotheses on the adaptive significance of gall structure.

PARTITIONING OF POLLINATORS DURING FLOWERING IN AN AFRICAN ACACIA COMMUNITY

Competition for pollination is an important factor structuring flowering in many plant communities. We examined mechanisms reducing interspecific pollen flow in a community of 10 Acacia species in a highly seasonal savannah habitat in Tanzania. Partitioning is achieved, in part, through separation of flowering in space and seasonal time, and through interspecific differences in pollinator guilds. Nevertheless, coflowering Acacia species shared several pollinators; this means that interspecific pollen transfer is possible. We analyzed daily patterns of pollinator activity and pollen release in 10 Acacia assemblages containing a total of 10 Acacia species. Pollinator activity was scored using counts at flowers over constant time intervals throughout the day. Pollen availability was assessed using a simple method which allows quantification of pollen exposed on the surface of the Acacia inflorescence. Sympatric co-flowering Acacia species each show high intra- specific synchrony but release their pollen at different times of day. Pollinators rapidly harvest available pollen and move from one Acacia species to the next, following the daily sequence of pollen release. The activity of shared pollinators is structured throughout the day as a result of temporal patterns of pollen release across Acacia species. The observed temporal structuring of pollen release is compatible with patterns predicted to result from competitive displacement. Additional support for a competition-based explanation for this patterning comes from the observation that an Acacia species flowering without competitors

Out of Anatolia: longitudinal gradients in genetic diversity support an eastern origin for a circum‐Mediterranean oak gallwasp Andricus quercustozae

Many studies have addressed the latitudinal gradients in intraspecific genetic diversity of European taxa generated during postglacial range expansion from southern refugia. Although Asia Minor is known to be a centre of diversity for many taxa, relatively few studies have considered its potential role as a Pleistocene refugium or a potential source for more ancient westward range expansion into Europe. Here we address these issues for an oak gallwasp, Andricus quercustozae (Hymenoptera: Cynipidae), whose distribution extends from Morocco along the northern coast of the Mediterranean through Turkey to Iran. We use sequence data for a fragment of the mitochondrial gene cytochrome b and allele frequency data for 12 polymorphic allozyme loci to answer the following questions: (1) which regions represent current centres of genetic diversity for A. quercustozae? Do eastern populations represent one refuge or several discrete glacial refugia? (2) Can we infer the timescale and sequence of the colonization processes linking current centres of diversity? Our results suggest that A. quercustozae was present in five distinct refugia (Iberia, Italy, the Balkans, southwestern Turkey and northeastern Turkey) with recent genetic exchange between Italy and Hungary. Genetic diversity is greatest in the Turkish refugia, suggesting that European populations are either (a) derived from Asia Minor, or (b) subject to more frequent population bottlenecks. Although Iberian populations show the lowest diversity for putatively selectively neutral markers, they have colonized a new oak host and represent a genetically and biologically discrete entity within the species.

Where is the UK's pollinator biodiversity? The importance of urban areas for flower-visiting insects

Insect pollinators provide a crucial ecosystem service, but are under threat. Urban areas could be important for pollinators, though their value relative to other habitats is poorly known. We compared pollinator communities using quantified flower-visitation networks in 36 sites (each 1 km2) in three landscapes: urban, farmland and nature reserves. Overall, flower-visitor abundance and species richness did not differ significantly between the three landscape types. Bee abundance did not differ between landscapes, but bee species richness was higher in urban areas than farmland. Hoverfly abundance was higher in farmland and nature reserves than urban sites, but species richness did not differ significantly. While urban pollinator assemblages were more homogeneous across space than those in farmland or nature reserves, there was no significant difference in the numbers of rarer species between the three landscapes. Network-level specialization was higher in farmland than urban sites. Relative to other habitats, urban visitors foraged from a greater number of plant species (higher generality) but also visited a lower proportion of available plant species (higher specialization), both possibly driven by higher urban plant richness. Urban areas are growing, and improving their value for pollinators should be part of any national strategy to conserve and restore pollinators.

Behavioral, ecological, and physiological determinants of the activity patterns of bees

Publisher Summary This chapter discusses the behavioral, ecological, and physiological determinants of the activity patterns of bees. An activity pattern is the change in levels of a particular activity through time. Bees are an excellent taxon within which to investigate both the issues of constraint and currency. Their physiological constraints are better understood than those of almost any other invertebrate, allowing an unparalleled opportunity to integrate physiology into behavioral ecology. Many bees require elevated body temperatures ( T b ) to fly, and hence thermal properties of their environment significantly constrain their activity. However, many species are heterothermic, being able to elevate their T b endothermically when necessary, which gives some degree of escape from the usual thermal constraints on other entirely ectothermic insects. The chapter discusses the factors structuring activity patterns in a wide range of bees. Observed activity patterns are generated by interactions between properties of the bees themselves (intrinsic factors) and properties of their environment (extrinsic factors). Intrinsic factors include physiological differences between species and between the sexes. Extrinsic factors are the fluctuating properties of the environment, and are either abiotic or biotic.

Controlling for non-independence in comparative analysis of patterns across populations within species

How do we quantify patterns (such as responses to local selection) sampled across multiple populations within a single species? Key to this question is the extent to which populations within species represent statistically independent data points in our analysis. Comparative analyses across species and higher taxa have long recognized the need to control for the non-independence of species data that arises through patterns of shared common ancestry among them (phylogenetic non-independence), as have quantitative genetic studies of individuals linked by a pedigree. Analyses across populations lacking pedigree information fall in the middle, and not only have to deal with shared common ancestry, but also the impact of exchange of migrants between populations (gene flow). As a result, phenotypes measured in one population are influenced by processes acting on others, and may not be a good guide to either the strength or direction of local selection. Although many studies examine patterns across populations within species, few consider such non-independence. Here, we discuss the sources of non-independence in comparative analysis, and show why the phylogeny-based approaches widely used in cross-species analyses are unlikely to be useful in analyses across populations within species. We outline the approaches (intraspecific contrasts, generalized least squares, generalized linear mixed models and autoregression) that have been used in this context, and explain their specific assumptions. We highlight the power of ‘mixed models’ in many contexts where problems of non-independence arise, and show that these allow incorporation of both shared common ancestry and gene flow. We suggest what can be done when ideal solutions are inaccessible, highlight the need for incorporation of a wider range of population models in intraspecific comparative methods and call for simulation studies of the error rates associated with alternative approaches.

Pollination ecology of acacias (Fabaceae, Mimosoideae)

We review the pollination ecology of acacias worldwide, discussing (1) the rewards provided to flower visitors, (2) the temporal patterns of flowering and reward provision and (3) the taxonomic composition of flower visitor assemblages. The flowers of most acacias (including all members of the subgenus Phyllodineae) offer only pollen to flower visitors and floral nectar is limited to a minority of species in the subgenera Acacia and Aculeiferum. The most important pollinators of acacias are social and solitary bees, although other insects and nectar-feeding birds are important in specific cases. Acacias that secrete nectar attract far more species-rich assemblages of flower visitors, although many of these are probably not important as pollinators. Most acacias in the subgenus Phyllodineae have long-lived protogynous flowers, without clear daily patterns in reward provision and visitation. In contrast, most members of the other two subgenera have flowers that last for a single day, appear to be protandrous and have clear daily patterning in reward provision and visitation. The generality of these patterns should not be assumed until the pollination ecology of many more phyllodinous acacias has been studied, particularly in arid environments. The accessibility of the floral rewards in acacia flowers makes them important examples of two general issues in plant communities—the partitioning of shared pollinators and the evolution of floral ant repellents. SBogy as G. N.