Evolutionary dynamics of insect symbiont associations

Bacterial symbionts of insects are pervasive and influence several ecologically relevant traits of their hosts. Although there has been rapid progress in understanding the extent and phenotypic outcome of these associations, there is a paucity of empirical data on fine-scale evolutionary processes that operate between the genomes of host and symbiont. Recently published papers examining symbiont relationships of aphids, fruit flies and butterflies are starting to reveal that these associations might be more dynamic than was appreciated previously.     

Incorporating the process of vertical transmission into understanding of host–symbiont dynamics

Variation exists in the frequency of obligate, vertically transmitted symbiotic organisms within and among host populations; however, these patterns have not been adequately explained by variable fitness effects of symbionts on their hosts. In this forum, we call attention to another equally important, but overlooked mechanism to maintain variation in the frequency of symbioses in nature: the rate of vertical transmission. On ecological time scales, vertical transmission can affect the equilibrium frequencies of symbionts in host populations, with potential consequences for population and community dynamics. In addition, vertical transmission has the potential to influence the evolution of symbiosis, by affecting the probability of fixation of symbiosis (and therefore the evolution of complexity) and by allowing hosts to sanction against costly symbionts. Here we use grass–epichloae symbioses as a model system to explore the causes and consequences of variation in vertical transmission rates. We identify critical points for symbiont transmission that emerge from considering the host growth cycle devoted to reproduction (asexual vs sexual) and the host capability to maintain homeostasis. We also use information on the process of transmission to predict the environmental factors that would most likely affect transmission rates. Altogether, we aim to highlight the vertical transmission rate as an important process for understanding the ecology and evolution of symbiosis, using grass–epichloae interactions as a case study.     

Consequences of grazing on the vertical transmission of a fungal Neotyphodium symbiont in an annual grass population

Symbiosis between cool‐season grasses and vertically transmitted fungal endophytes are common and significantly impact on ecosystem function. This makes the understanding of the underlying mechanisms to symbiotic individuals frequency in local populations much more interesting. Most studies have been focused on the differential fitness between symbiotic and non‐symbiotic counterparts (relative fitness), barely considering other mechanisms. We performed a microcosms experiment to evaluate whether grazing alters the dynamics of the endophyte Neotyphodium occultans in the annual grass Lolium multiflorum by simultaneously modifying the relative fitness and the endophyte efficiency to be transmitted from host plants to seeds. Grazing was simulated by means of clipping and trampling on symbiotic and non‐symbiotic plants growing separately, in soils obtained from paddocks, differing in their agronomic management history (natural grassland vs. ryegrass promotion). Seed production showed a complex pattern as it depended on the symbiotic status of the plants, the level of grazing and the agro‐ecological context. Grazed plants produced three times fewer seeds than ungrazed plants only in microcosms with endophyte‐symbiotic plants in soils from ryegrass promotion. Endophyte benefits on seed production were exclusively observed in ungrazed plants in the same soil. Symbiotic plants produced symbiotic and non‐symbiotic seeds in all the treatments. While the production of non‐symbiotic seeds by these plants was not affected by grazing and the soil, grazing reduced the production of symbiotic seeds in both contexts. Grazing negative effect on the density of fully infected spikes determined a significant increment in the transmission failures which were not modified by agro‐ecological contexts. Therefore, grazing can modulate symbiosis dynamics through reducing seed production and endophyte transmission efficiency. Transmission has been disregarded, but it is a context‐dependent process that could lead to a gradual reduction in the symbiotic plants frequency in a population if the mutualism effectiveness does not outweigh transmission failures.     

Horizontal transmission of the insect symbiont Rickettsia is plant-mediated

Bacteria in the genus Rickettsia, best known as vertebrate pathogens vectored by blood-feeding arthropods, can also be found in phytophagous insects. The presence of closely related bacterial symbionts in evolutionarily distant arthropod hosts presupposes a means of horizontal transmission, but no mechanism for this transmission has been described. Using a combination of experiments with live insects, molecular analyses and microscopy, we found that Rickettsia were transferred from an insect host (the whitefly Bemisia tabaci) to a plant, moved inside the phloem, and could be acquired by other whiteflies. In one experiment, Rickettsia was transferred from the whitefly host to leaves of cotton, basil and black nightshade, where the bacteria were restricted to the phloem cells of the plant. In another experiment, Rickettsia-free adult whiteflies, physically segregated but sharing a cotton leaf with Rickettsia-plus individuals, acquired the Rickettsia at a high rate. Plants can serve as a reservoir for horizontal transmission of Rickettsia, a mechanism which may explain the occurrence of phylogenetically similar symbionts among unrelated phytophagous insect species. This plant-mediated transmission route may also exist in other insect-symbiont systems and, since symbionts may play a critical role in the ecology and evolution of their hosts, serve as an immediate and powerful tool for accelerated evolution.     

Evolutionary dynamics and population biology of a polymorphic insect

Conspicuous heritable polymorphisms are useful to address the question if morph frequencies are stable or whether they fluctuate between generations. Ecological geneticists have studied colour polymorphisms in the past, but there are few long-term studies of genetic dynamics across multiple generations. We studied morph-frequency dynamics and female fecundity in the trimorphic blue-tailed damselfly (Ischnura elegans). The morphs include a male-coloured (androchrome) type of female, which is thought to be maintained by frequency-dependent sexual conflict. Morph frequencies changed significantly between years across all populations. There was evidence for directional frequency change since androchrome females increased in 9 of 10 populations across a 4-year period. There was heterogeneity between populations in their evolutionary trajectories, partly caused by population age: androchrome frequencies were initially high in young populations but gradually decreased and approached the level of old populations. We discuss the possible causes of morph-frequency fluctuations, and the role of morph-specific fecundity, dispersal and other forces influencing evolutionary dynamics in this system.     

Symbiont physiology and population dynamics before and during symbiont shifts in a flexible algal‐cnidarian symbiosis

For cnidarians that can undergo shifts in algal symbiont relative abundance, the underlying algal physiological changes that accompany these shifts are not well known. The sea anemone Anthopleura elegantissima associates with the dinoflagellate Symbiodinium muscatinei and the chlorophyte Elliptochloris marina, symbionts with very different tolerances to light and temperature. We compared the performance of these symbionts in anemones maintained in an 8–11.5 month outdoor common garden experiment with simulated intertidal conditions and three levels of shading (2, 43, and 85% ambient irradiance). Symbiont densities, mitotic indices, photophysiology and pigments were assessed at three time points during the summer, a period of high irradiance and solar heating during aerial exposure. Whereas S. muscatinei was either neutrally or positively affected by higher irradiance treatments, E. marina responded mostly negatively to high irradiance. E. marina in the 85% irradiance treatment exhibited significantly reduced P_max and chlorophyll early in the summer, but it was not until nearly 3 months later that a shift in symbiont relative abundance toward S. muscatinei occurred, coincident with bleaching. Symbiont densities and proportions remained largely stable in all other treatments over time, and displacement of S. muscatinei by E. marina was not observed in the 2% irradiance treatment despite the potentially better performance of E. marina. While our results support the view that rapid changes in symbiont relative abundance are typically associated with symbiont physiological dysfunction and bleaching, they also show that significant temporal lags may occur between the onset of symbiont stress and shifts in symbiont relative abundances.     

The early life of a leaf‐cutter ant colony constrains symbiont vertical transmission and favors horizontal transmission

Colonial organisms host a large diversity of symbionts (collectively, parasites, mutualists, and commensals) that use vertical transmission (from parent colony to offspring colony) and/or horizontal transmission to disperse between host colonies. The early life of some colonies, characterized by the dispersal and establishment of solitary individuals, may constrain vertical transmission and favor horizontal transmission between large established colonies. We explore this possibility with the miniature cockroach Attaphila fungicola, a symbiont of leaf‐cutter ants and the mutualist fungal gardens they cultivate. The early life of a leaf‐cutter colony is characterized by the dispersal of a female alate (winged “queen”) carrying a fungal pellet, and the subsequent establishment of a foundress (workerless “queen”) raising her incipient fungal garden and colony. Roaches hitchhike on female alates during leaf‐cutter nuptial flights, which strongly suggests that roaches are vertically transmitted to foundresses and their incipient colonies; however, weak compatibility between roaches and incipient gardens may constrain roach vertical transmission. Reciprocally, opportunities for horizontal transmission between large established colonies with abundant fungal gardens may weaken selection against roach‐induced harm (virulence) of incipient gardens. We use a laboratory experiment, behavioral observations, field surveys, and a transmission model to estimate the effect roaches have on the survivorship of incipient gardens and the frequency of roach vertical transmission. Contrary to traditional assumptions, our results indicate that roaches harm incipient gardens and predominantly use horizontal transmission between established leaf‐cutter colonies. Ultimately, “costs of generalism” associated with infecting disparate stages of a host''s lifecycle (e.g., incipient vs. established colonies) may constrain the vertical transmission of roaches and a broad range of symbionts.     

Estimating population size and transmission bottlenecks in maternally transmitted endosymbiotic bacteria

Many species of bacterial endosymbionts are acquired by animal hosts before birth, through direct transmission from mothers to eggs or embryos. This vertical transmission imposes a reduction in numbers or "bottleneck," and the size of this bottleneck affects the population structure and evolution of symbiotic lineages. We have estimated the size of the transmission bottleneck in Buchnera, the bacterial symbiont of aphids, using basic light and electron microscopy techniques. By serial-sectioning whole aphid abdomens, their eggs, and embryos, we determined the following parameters: (i) The average size of a Buchnera cell is 2.9 mm in diameter. (ii) The total number of Buchnera in an Acyrthosiphon pisum embryo was around 36,700 whereas a first instar nymph contained more than 119,000. (iii) The number of symbionts per bacteriocyte was around 800 in an embryo and 3200 in a first instar nymph. (iv) The total number of Buchnera transmitted to each sexual egg ranged from 850 in Nasonovia to 1800 in A. pisum to more than 8000 in Uroleucon ambrosiae. (v) The total number of secondary endosymbionts in A. pisum was 12,170 for an embryo and 18,360 for a first instar nymph. Secondary symbionts were arranged both extracellularly and in clusters of 2000–8000 bacteria inside bacteriocytes. These numbers are consistent with the few previous estimates of symbiont population sizes based on counts of gene copies.     

Follicular population dynamics during the estrous cycle of the mare

Ovarian follicles ≥2 mm were studied in 40 mares by daily ultrasound examinations. There were significant differences among days of the estrous cycle in the populations of 2–5 mm, 16–20 mm, and > 20 mm follicles. Significant orthogonal contrasts of the mean follicular profiles were cubic (P < 0.0001) for each of these categories. The cubic profile in the 2–5 mm follicle population was characterized by an apparent increase in numbers of follicles which began before ovulation, reached a maximum on approximately Day 5 (ovulation=Day 0), then decreased until approximately Day 14. In the 16–20 mm and > 20 mm categories, the cubic profile appeared to be due to increased mean numbers of follicles which began between Days 6 to 10. A reciprocal temporal association existed between the decrease in mean number of 2–5 mm follicles and the increase in mean numbers of 16–20 mm and > 20 mm follicles. A decrease (P < 0.05) was observed in the mean number of > 20 mm follicles and in the mean diameter of the second largest follicle between Day-7 to Day-1. There was a striking reciprocal relationship between the mean numbers of 2–10 mm and > 10 mm follicles. An effect (P < 0.01) of season was observed between groups studied from May to July vs. August to October in the 2–5 mm and the > 20 mm categories although there was not a day by season interaction. The mean number of follicles was greater for the May–July group than for the August–October group for both endpoints. Results supported the hypotheses that the number of entries into the 2–5 mm pool was not constant during the interovulatory interval, and the total number of follicles ≥2 mm did not change throughout the estrous cycle. Results did not support a two-wave theory of follicular growth. There was a continuous increase in number of follicles greater than 10 mm until Day-6 or Day-7 when the follicle destined to ovulate was selected.     

Life table responses and population dynamics of four cladoceran zooplankton during a reservoir flood

The life table responses and population dynamics of four cladocerans(Ceriodaphnia lacustris, Daphnia parvula, Diaphonosoma leuchtenbergianumand Moina micrura) were measured during a flood of a large turbidreservoir (Lake Texoma, Oklahoma-Texas). Mean recruitment andlongevity of Ceriodaphnia and Daphnia in life table experimentswere reduced during the flood when abundant silt particles andpicoplankton (<2 µm) replaced algal particles in thewater column. Individual growth rates measured in the life tableexperiments were slower for Ceriodaphnia during the flood. Fieldcollections revealed that Ceriodaphnia and Daphnia populationsdeclined immediately after the arrival of turbid water, andremaining females carried fewer eggs. Populations of Moina andDiaphanosoma increased during the flood, and life table experimentsshowed that these species were able to grow well in silt-ladenwater.     

Structural diversity of symbionts and related cellular mechanisms underlying vertical symbiont transmission in cicadas

Many insects depend on symbiont(s) for survival. This is particularly the case for sap-feeding hemipteran insects. In this study, we revealed that symbionts harboured in cicadas are diverse and complex, and the yeast-like fungal symbionts (YLS) are present in most cicada species, but Hodgkinia is absent. During vertical transmission, Sulcia became swollen with the outer membrane drastically changed, while Hodgkinia became shrunken and changed from irregular to roughly spherical. Sulcia and/or Hodgkinia were exocytosed from the bacteriocytes to the intercellular space of bacteriomes, where they gathered together and were extruded to haemolymph. YLS and associated facultative symbiont(s) in the fat bodies were released to the haemolymph based on bacteriocyte disintegration. The obligate symbiont(s) were endocytosed and exocytosed successively by the epithelial plug cells of the terminal oocyte, while associated facultative symbiont(s), and possibly also YLS, may take a ‘free ride’ on the transmission of obligate symbiont(s) to gain entry into the oocyte. Then, the intermixed symbionts formed a characteristic ‘symbiont ball’ in the oocyte. Our results suggest that YLS in cicadas represent a new example of a relatively early stage of symbiogenesis in insects and contribute to a better understanding of the diversity and transmission mechanisms of symbionts in insects.     

Analysis of genetic bottlenecks during horizontal transmission of Cucumber mosaic virus

Genetic bottlenecks may occur in virus populations when only a few individuals are transferred horizontally from one host to another, or when a viral population moves systemically from the infection site. Genetic bottlenecks during the systemic movement of an RNA plant virus population were reported previously (H. Li and M. J. Roossinck, J. Virol. 78:10582-10587, 2004). In this study we mechanically inoculated an artificial population consisting of 12 restriction enzyme marker mutants of Cucumber mosaic virus (CMV) onto young leaves of squash plants and used two aphid species, Aphis gossypii and Myzus persicae, to transmit the virus populations from infected source plants to healthy squash plants. Horizontal transmission by aphids constituted a significant bottleneck, as the population in the aphid-inoculated plants contained far fewer mutants than the original inoculum source. Additional experiments demonstrated that genetic variation in the artificial population of CMV is not reduced during the acquisition of the virus but is significantly reduced during the inoculation period.     

Insect-microbe mutualism without vertical transmission: a stinkbug acquires a beneficial gut symbiont from the environment every generation

The broad-headed bug Riptortus clavatus (Heteroptera: Alydidae) possesses a number of crypts at a posterior midgut region, which house a dense population of a bacterial symbiont belonging to the genus Burkholderia. Although the symbiont is highly prevalent (95 to 100%) in the host populations, the symbiont phylogeny did not reflect the host systematics at all. In order to understand the mechanisms underlying the promiscuous host-symbiont relationship despite the specific and prevalent association, we investigated the transmission mode and the fitness effects of the Burkholderia symbiont in R. clavatus. Inspection of eggs and a series of rearing experiments revealed that the symbiont is not vertically transmitted but is environmentally acquired by nymphal insects. The Burkholderia symbiont was present in the soil of the insect habitat, and a culture strain of the symbiont was successfully isolated from the insect midgut. Rearing experiments by using sterilized soybean bottles demonstrated that the cultured symbiont is able to establish a normal and efficient infection in the host insect, and the symbiont infection significantly improves the host fitness. These results indicated that R. clavatus postnatally acquires symbiont of a beneficial nature from the environment every generation, uncovering a previously unknown pathway through which a highly specific insect-microbe association is maintained. We suggest that the stinkbug-Burkholderia relationship may be regarded as an insect analogue of the well-known symbioses between plants and soil-associated microbes, such as legume-Rhizobium and alder-Frankia relationships, and we discuss the evolutionary relevance of the mutualistic but promiscuous insect-microbe association.     

Characterization of a culturable alphaproteobacterial symbiont common to many marine sponges and evidence for vertical transmission via sponge larvae

A closely related group of alphaproteobacteria were found to be present in seven genera of marine sponges from several locations and were shown to be transferred between sponge generations through the larvae in one of these sponges. Isolates of the alphaproteobacterium were cultured from the sponges Axinella corrugata, Mycale laxissima, Monanchora unguifera, and Niphates digitalis from Key Largo, Florida; Didiscus oxeata and Monanchora unguifera from Discovery Bay, Jamaica; an Acanthostronglyophora sp. from Manado, Indonesia; and Microciona prolifera from the Cheasapeake Bay in Maryland. Isolates were very similar to each other on the basis of 16S rRNA gene sequence (>99% identity) and are closely related to Pseudovibrio denitrificans. The bacterium was never isolated from surrounding water samples and was cultured from larvae of M. laxissima, indicating that it is a vertically transmitted symbiont in this sponge. Denaturing gradient gel electrophoresis, 16S rRNA gene clone library analysis, and fluorescent in situ hybridization with probes specific to the alphaproteobacterium confirmed the presence of this bacterium in the M. laxissima larvae. The alphaproteobacterium was densely associated with the larvae rather than being evenly distributed throughout the mesohyl. This is the first report of the successful culture of a bacterial symbiont of a sponge that is transferred through the gametes.     

Life cycle and population dynamics of Trochosa terricola Thorell (Araneae: Lycosidae) in a Norfolk grass heath

Abstract. 1. The population of the lycosid Trochosa terricola Thorell was sampled from April 1973 to August 1975 at Weeting Heath NNR, a Breckland grass heath.
2. Four sampling methods were compared for efficiency. Hand searching gave density estimates between 38.3 and 70.1% of heat extraction.
3. The temperature range in the sward at +1 cm was –5°C to 39°C with January and July means of 3.2°C and 17.4°C.
4. Eight male and nine female instars were determined and the life cycle extended over 2 or 3 years.
5. Adults were nocturnal but the juveniles diurnal. An annual diplochrone activity pattern was observed for adult males.
6. The horizontal distribution within the sward was aggregated, the structure and microhabitat being important determining factors. The population density was greater in moist, young Festuca spp. tussocks. An equation relating population density to habitat characteristics was derived.
7. The overall population density ranged from 14.0 m^-2 to 76.0 m^-2 and was maximal in autumn after breeding. The population biomass was greatest during autumn (291.2 mg d.wt m^-2).
8. Mean number of juveniles emerging from an egg sac was 77.3 (first sac) and 38.0 (second sac). The natality in 1973 was 66.8 individuals m^-2.
9. The survivorship curve until maturity varied between types I and III in different years.
10. The population dynamics were compared and are discussed in the light of other data. The variable population characteristics suggested that Den Boer's "spreading of risk" theory applied to the T.terricola population.     

Life history and population dynamics of Atriplex triangularis

The dynamics of a population of the stoloniferous Potentilla anserina was studied during four years. The mortality of established ramets followed a log-linear pattern superimposed on a seasonal fluctuation with an increased mortality during the period of vegetative spread. The major external mortality factors were unfavourable weather conditions such as drought and freezing. The turnover rate of the population was slow (31.6 yr). Vegetative spread dominated the recruitment to the population. Seedlings occurred but were exposed to a much higher mortality than vegetative daughter ramets. The juvenile period of seedlings is at least 4 yr in the field. This contrast to vegetative daughter ramets which are potentially flowering and stoloniferous the year after they are established, even if age affects both reproduction and vegetative spread. The density of most of the population is below the level where intraspecific regulative effects operate. However, in very dense stands both stolon and flower production were negatively density dependent.Nomenclature follows Lid (1974), Norsk og Svensk Flora, 2nd ed. Oslo.I am indebted to Lenn Jerling, Peter Torstensson, Anders Telenius and Gudrún Jónsdóttir for their help, and inspiring criticism throughout this study.     

Climate-mediated population dynamics enhance distribution range expansion in a rice pest insect

Environmental fluctuations can influence invertebrate population dynamics over large spatial scales, and effects of climate change are of particular importance in understanding phenology. In this study, we tested whether changing climate patterns could increase voltinism and emergence synchrony in Stenotus rubrovittatus and drive the mirid bug’s expansion into currently uninhabited areas of Japan. This expansion could have potentially serious economic consequences for the rice industry. We modelled development of S. rubrovittatus in the field applying the effective accumulated temperature model to calculate the theoretical number of generations and the egg hatching dates from 2003 to 2012 based on a high-resolution, daily weather database. We then performed a regional analysis to assess the relationship between population dynamics and range expansion across the study region and also included a local analysis to evaluate how population parameters affect the presence of S. rubrovittatus at local sites in each year. Results showed that distribution expanded with a relative increase in voltinism and with synchrony of egg hatching date. Moreover, we showed that increased voltinism in the previous year positively influenced local population occurrence. This positive effect suggests that the species’ distribution range expands through increased reproduction at both the regional and local scale. Climate-mediated population dynamics play a significant role in range expansion of the mirid bug.