Fungal symbionts (Harpellales) in Norwegian aquatic insect larvae

Collections of aquatic insect larvae in Norway, over a 40 d period in May and Aug 2002, resulted in finding more than 25 species and one new genus of Harpellales (Trichomycetes). Nine new fungal symbionts are described and named: Ephemerellomyces aquilonius (a new monotypic genus), Glotzia stenospora and Legeriosimilis europaeus in mayfly nymphs (Ephemeroptera); Genistelloides amplispora and G. communis in stonefly nymphs (Plecoptera); and Smittium biforme, Sm. precipitiorum, Stachylina acutibasilaris and St. lentica in midge larvae (Diptera: Chironomidae). Two possibly new species of Smittium in Chironomidae larvae are described but not formally named. New hosts and biogeographical distributions are recorded for 14 previously described species, including the rare occurrence of Smittium simulii in mosquito larvae.     

Episodes in insect evolution

This article derives from a society-wide symposium organized by Timothy Bradley and Adriana Briscoe and presented at the 2009 annual meeting of the Society for Integrative and Comparative Biology in Boston, Massachusetts. David Grimaldi provided the opening presentation in which he outlined the major evolutionary events in the formation and subsequent diversification of the insect clade. This presentation was followed by speakers who detailed the evolutionary history of specific physiological and/or behavioral traits that have caused insects to be both ecologically successful and fascinating as subjects for biological study. These include a review of the evolutionary history of the insects, the origins of flight, osmoregulation, the evolution of tracheal systems, the evolution of color vision, circadian clocks, and the evolution of eusociality. These topics, as covered by the speakers, provide an overview of the pattern and timing of evolutionary diversification and specialization in the group of animals we know as insects.     

Parallels in genome evolution in mitochondria and bacterial symbionts

Mitochondria, the energy-producing organelles of the eukaryotic cell, originate from an endosymbiotic alpha-proteobacterium. These organelles are believed to have arisen only once in evolutionary history, but despite their common ancestry, mitochondrial DNAs vary extensively throughout eukaryotes in genome architecture and gene content. New insights into early mitochondrial genome evolution come from the investigation of primitive mitochondriate eukaryotes, as well as the comparison between mitochondria and intracellular bacterial symbionts.     

Trichomycete insect symbionts in Great Smoky Mountains National Park and vicinity

Collections of trichomycete symbionts of larval aquatic insects in Great Smoky Mountains National Park and vicinity in the southern Appalachian region of the USA resulted in finding many taxa of Harpellales, including an unusual new monotypic genus, Barbatospora ambicaudata in Simuliidae, and five new species in Thaumaleidae or Chironomidae, Harpellomyces montanus, Smittium lentaquaticum, Sm. minutisporum, Stachylina gravicaudata and St. stenospora. In addition a new species of Amoebidium (Amoebidiales), A. appalachense, attached to the anal tubules of bloodworms (Chironomidae) is described. Axenic cultures of three of the new taxa were obtained, plus Sm. culisetae. Fourteen identified species representing 13 genera of previously known Harpellales are recorded from Plecoptera, Ephemeroptera and Diptera, as well as a new Dipteran host record for an unidentified harpellid that was found in a Blephariceridae. Also identified were Paramoebidium corpulentum and many undetermined species of Paramoebidium (Amoebidiales) from four orders of aquatic insect larvae. The occurrence of an Enterobryus species in Diplopoda and another Eccrinales from an aquatic beetle is noted. Amoebidiales,     

Experimental evolution of nodule intracellular infection in legume symbionts

Soil bacteria known as rhizobia are able to establish an endosymbiosis with legumes that takes place in neoformed nodules in which intracellularly hosted bacteria fix nitrogen. Intracellular accommodation that facilitates nutrient exchange between the two partners and protects bacteria from plant defense reactions has been a major evolutionary step towards mutualism. Yet the forces that drove the selection of the late event of intracellular infection during rhizobium evolution are unknown. To address this question, we took advantage of the previous conversion of the plant pathogen Ralstonia solanacearum into a legume-nodulating bacterium that infected nodules only extracellularly. We experimentally evolved this draft rhizobium into intracellular endosymbionts using serial cycles of legume-bacterium cocultures. The three derived lineages rapidly gained intracellular infection capacity, revealing that the legume is a highly selective environment for the evolution of this trait. From genome resequencing, we identified in each lineage a mutation responsible for the extracellular–intracellular transition. All three mutations target virulence regulators, strongly suggesting that several virulence-associated functions interfere with intracellular infection. We provide evidence that the adaptive mutations were selected for their positive effect on nodulation. Moreover, we showed that inactivation of the type three secretion system of R. solanacearum that initially allowed the ancestral draft rhizobium to nodulate, was also required to permit intracellular infection, suggesting a similar checkpoint for bacterial invasion at the early nodulation/root infection and late nodule cell entry levels. We discuss our findings with respect to the spread and maintenance of intracellular infection in rhizobial lineages during evolutionary times.     

Dynamics of genome evolution in facultative symbionts of aphids

Aphids are sap-feeding insects that host a range of bacterial endosymbionts including the obligate, nutritional mutualist Buchnera plus several bacteria that are not required for host survival. Among the latter, 'Candidatus Regiella insecticola' and 'Candidatus Hamiltonella defensa' are found in pea aphids and other hosts and have been shown to protect aphids from natural enemies. We have sequenced almost the entire genome of R. insecticola (2.07 Mbp) and compared it with the recently published genome of H.?defensa (2.11 Mbp). Despite being sister species the two genomes are highly rearranged and the genomes only have ～55% of genes in common. The functions encoded by the shared genes imply that the bacteria have similar metabolic capabilities, including only two essential amino acid biosynthetic pathways and active uptake mechanisms for the remaining eight, and similar capacities for host cell toxicity and invasion (type 3 secretion systems and RTX toxins). These observations, combined with high sequence divergence of orthologues, strongly suggest an ancient divergence after establishment of a symbiotic lifestyle. The divergence in gene sets and in genome architecture implies a history of rampant recombination and gene inactivation and the ongoing integration of mobile DNA (insertion sequence elements, prophage and plasmids).     

Bacterial symbionts as mediators of ecologically important traits of insect hosts

1. Bacterial symbionts play a prominent role in insect nutritional ecology by aiding in digestion of food or providing nutrients that are limited or lacking in the diet. Thereby, endosymbionts open niches to their insect host that would otherwise be unavailable. 2. Currently, several other ecologically relevant traits mediated by endosymbionts are being investigated, including enhanced parasite resistance, enhanced heat tolerance, and influences on insect–plant interactions such as manipulation of plant physiology to the benefit of the insect. 3. Traits mediated by endosymbionts are often identified by correlative studies where traits are found to be altered in the presence of a particular symbiont. Recent developments in genomic tools offer the opportunity for studying the impact of bacteria–insect symbioses under natural conditions in a population and community ecology context. In vivo experiments specifically testing putative functions of endosymbionts in parallel to population‐level studies on the prevalence of endosymbionts allow disentangling host versus symbiont contribution to phenotypic variability observed in individuals. Effects of symbionts on host phenotype are often large and relevant to host fitness, e.g. by significantly enhancing survival or fecundity in a context‐dependent manner. 4. Predominantly vertically transmitted endosymbionts contribute to the heritable genetic variation present in a host species. Phenotypic variation on which selection can act may be due to differences either among host genomes, symbiont genomes, or genotype × genotype interactions. Therefore the holobiont, i.e. the host including all symbionts, should be regarded as the unit of selection as the association between host and symbionts may affect the fitness of the holobiont depending on the environment.     

Intracellular symbionts and the evolution of uniparental cytoplasmic inheritance.

Uniparental inheritance of cytoplasmic elements is widespread among eukaryotic organisms and is achieved by a diverse range of mechanisms. This paper shows that the cytoplasmic genetic system would be expected to evolve towards uniparental inheritance, given the existence of deleterious symbionts capable of invading the host cytoplasm together with nuclear genes that lead to the elimination of cytoplasmic elements from one of the gamete types. The reason for this is that, under biparental inheritance, foreign symbionts with strong deleterious effects are able to spread through host populations. A nuclear modifier gene which leads to the loss of cytoplasmic elements from one gamete type gains a net advantage as a symbiont spreads, because the modifier sometimes gives rise to a symbiont-free zygote. Insofar as small gametes reduce the rate of symbiont transmission to the zygote, modifier genes causing small gamete size would tend to accumulate, so that cytoplasmic inheritance would become associated with maternal rather than paternal gametes. Once uniparental inheritance predominates in the host population, the population is protected from invasions by a large class of harmful symbionts, but at the same time those symbionts that benefit their hosts are still able to increase in frequency.     

Social evolution: community policing in insects

A recent study shows that, in social insects where workers suppress or 'police' the reproduction of nestmate workers, only a subset of workers act as police. This confirms that policing can serve a collective rather than a selfish interest.     

Social evolution: cooperation by conflict

A recent study suggests that aggression between wasps depends upon the costs and benefits of fighting, as determined by the position of individuals in a dominance hierarchy.     

Social evolution: evolving sex ratios

A recent study comparing sex ratios produced by experimental evolution in spider mites with those predicted by Hamilton's Local Mate Competition Theory clearly demonstrates Evolutionary Theory's success as a quantitatively predictive science.     

Extreme rates and heterogeneity in insect DNA evolution

Summary DNA-DNA hybridization studies of insects, more specificallyDrosophila and cave crickets, have revealed interesting patterns of genome evolution that contrast markedly with what has been seen in other taxa, especially mammals and birds. Insect genomes are composed of sections of single-copy DNA with extreme variation in rates of evolutionary change. This variation is more extreme than between introns and exons; introns fall into the relatively conserved fraction of the genome. Attempts to calculate absolute rates of change inDrosophila DNA have all led to estimates some 5–10 times faster than those found in most vertebrates; this is true even for the more conservative part of the nuclear genome. Finally we point out that morphological similarity, chromosomal similarity, and/or ability to form interspecific hybrids is often associated with quite high levels of single-copy DNA divergence in insects as compared to mammals and birds.     

Coexistence of novel gammaproteobacterial and Arsenophonus symbionts in the scale insect Greenisca brachypodii (Hemiptera,Coccomorpha: Eriococcidae)

Scale insects are commonly associated with obligate, intracellular microorganisms which play important roles in complementing their hosts with essential nutrients. Here we characterized the symbiotic system of Greenisca brachypodii, a member of the family Eriococcidae. Histological and ultrastructural analyses have indicated that G. brachypodii is stably associated with coccoid and rod‐shaped bacteria. Phylogenetic analyses have revealed that the coccoid bacteria represent a sister group to the secondary symbiont of the mealybug Melanococcus albizziae, whereas the rod‐shaped symbionts are close relatives of Arsenophonus symbionts in insects – to our knowledge, this is the first report of the presence of Arsenophonus bacterium in scale insects. As a comparison of 16S and 23S rRNA genes sequences of the G. brachypodii coccoid symbiont with other gammaprotebacterial sequences showed only low similarity (～90%), we propose the name ‘Candidatus Kotejella greeniscae’ for its tentative classification. Both symbionts are transovarially transmitted from one generation to the next. The infection takes place in the neck region of the ovariole. The bacteria migrate between follicular cells, as well as through the cytoplasm of those cells to the perivitelline space, where they form a characteristic ‘symbiont ball’. Our findings provide evidence for a polyphyletic origin of symbionts of Eriococcidae.     

Social evolution in toothed whales

Two contrasting results emerge from comparisons of the social systems of several odontocetes with terrestrial mammals. Researchers have identified remarkable convergence in prominent features of the social systems of odontocetes such as the sperm whale and bottlenose dolphin with a few well-known terrestrial mammals such as the elephant and chimpanzee. In contrast, studies on killer whales and Baird's beaked whale reveal novel social solutions to aquatic living. The combination of convergent and novel features in odontocete social systems promise a more general understanding of the ecological determinants of social systems in both terrestrial and aquatic habitats, as well as the relationship between relative brain size and social evolution.     

Expansion and evolution of insect GMC oxidoreductases

Background  

The GMC oxidoreductases comprise a large family of diverse FAD enzymes that share a homologous backbone. The relationship and origin of the GMC oxidoreductase genes, however, was unknown. Recent sequencing of entire genomes has allowed for the evolutionary analysis of the GMC oxidoreductase family.