Low Host Abundance Research Articles

Low occurrence of ranavirus in the Prairie Pothole Region of Montana and North Dakota (USA) contrasts with prior surveys.

Ranaviruses are emerging pathogens that have caused mortality events in amphibians worldwide. Despite the negative effects of ranaviruses on amphibian populations, monitoring efforts are still lacking in many areas, including in the Prairie Pothole Region (PPR) of North America. Some PPR wetlands in Montana and North Dakota (USA) have been contaminated by energy-related saline wastewaters, and increased salinity has been linked to greater severity of ranavirus infections. In 2017, we tested tissues from larvae collected at 7 wetlands that ranged in salinity from 26 to 4103 mg Cl l-1. In 2019, we used environmental DNA (eDNA) to test for ranaviruses in 30 wetlands that ranged in salinity from 26 to 11754 mg Cl l-1. A previous study (2013-2014) found that ranavirus-infected amphibians were common across North Dakota, including in some wetlands near our study area. Overall, only 1 larva tested positive for ranavirus infection, and we did not detect ranavirus in any eDNA samples. There are several potential reasons why we found so little evidence of ranaviruses, including low larval sample sizes, mismatch between sampling and disease occurrence, larger pore size of our eDNA filters, temporal variation in outbreaks, low host abundance, or low occurrence or prevalence of ranaviruses in the wetlands we sampled. We suggest future monitoring efforts be conducted to better understand the occurrence and prevalence of ranaviruses within the PPR.

Low Host Abundance and High Temperature Determine Switching from Lytic to Lysogenic Cycles in Planktonic Microbial Communities in a Tropical Sea (Red Sea).

The lytic and lysogenic life cycles of marine phages are influenced by environmental conditions such as solar radiation, temperature, and host abundance. Temperature can regulate phage infection, but its role is difficult to discern in oligotrophic waters where there is typically low host abundance and high temperatures. Here, we study the temporal variability of viral dynamics and the occurrence of lysogeny using mitomycin C in a eutrophic coastal lagoon in the oligotrophic Red Sea, which showed strong seasonality in terms of temperature (22.1–33.3 °C) and large phytoplankton blooms. Viral abundances ranged from 2.2 × 106 to 1.5 × 107 viruses mL−1 and were closely related to chlorophyll a (chl a) concentration. Observed high virus-to-bacterium ratio (VBR) (4–79; 16 ± 4 (SE)) suggests that phages exerted a tight control of their hosts as indicated by the significant decrease in bacterial abundance with increasing virus concentration. Heterotrophic bacterial abundance also showed a significant decrease with increasing temperature. However, viral abundance was not related to temperature changes and the interaction of water temperature, suggesting an indirect effect of temperature on decreased host abundance, which was observed at the end of the summertime. From the estimated burst size (BS), we observed lysogeny (undetectable to 29.1%) at low percentages of 5.0% ± 1.2 (SE) in half of the incubations with mitomycin C, while it increased to 23.9% ± 2.8 (SE) when the host abundance decreased. The results suggest that lytic phages predominate, switching to a moderate proportion of temperate phages when the host abundance reduces.

Offspring production and self-superparasitism in the solitary ectoparasitoidSpalangia cameroni(Hymenoptera: Pteromalidae) in relation to host abundance

Parasitoid fitness strongly depends on the availability and quality of hosts, which provide all resources required for larval development. Several factors, such as host size and previous parasitation, may affect host quality. Because self-superparasitism induces competition among a female's offspring, it should only occur if there is an imperfect recognition of self-parasitized hosts or if there is a fitness advantage to self-superparasitism. Against this background, we investigated self-superparasitism and offspring production in Spalangia cameroni (Hymenoptera: Pteromalidae) in relation to the abundance of a novel host, Ceratitis capitata (Diptera: Tephritidae). Individual pairs of parasitoids were provided with either two (low host abundance) or ten (high host abundance) pupae per day. Under high host abundance, lifetime fecundity (number of eggs laid), offspring number, number of pupae parasitized and hosts killed were greater than under low host abundance, whereas the number of eggs per host was lower; and the proportion of hosts that did not produce offspring tended to be lower. The latter suggests the occurrence of ovicide, when hosts are scarce due to an at least imperfect recognition of previously self-parasitized hosts. Offspring production per parasitized pupa was higher when hosts were scarce and levels of self-superparasitism high, suggesting the existence of beneficial effects of self-superparasitism.

Spatio-temporal dynamics of bumblebee nest parasites (BombussubgenusPsythirusssp.) and their hosts (Bombusspp.)

1. A 39-year bumblebee data base was used to study the codistribution of six cuckoo bumblebees in the subgenus Psythirus of Bombus (hereafter called Psythirus) and their free-living bumblebee hosts in the British Isles. 2. A model of nest parasitism predicted host threshold densities and stable deterministic dynamics, with fluctuations only emerging as a result of environmental or demographic stochasticity. 3. Standardized transects indicated that variation in total number of records could be largely attributed to variation in observer effort; analyses were therefore carried out using relative abundance. 4. Spatially, parasite-free zones were evident in areas of low host abundance, but the host threshold for parasite presence differed among species and locations. 5. Temporally, the relative numbers of the parasite and host species remained relatively constant, except that the nest parasite P. campestris declined significantly since 1990. 6. There were consistent negative effects of the parasitic species on the numbers of hosts in the following year, and this pattern was seen over large geographic areas. 7. The spatio-temporal patterns confirmed a high degree of host specificity, except that P. campestris may be parasitizing not only B. pascuorum but also other species in the subgenus Thoracobombus.

Emergence, spread, persistence and fade-out of sylvatic plague in Kazakhstan

Predicting the dynamics of zoonoses in wildlife is important not only for prevention of transmission to humans, but also for improving the general understanding of epidemiological processes. A large dataset on sylvatic plague in the Pre-Balkhash area of Kazakhstan (collected for surveillance purposes) provides a rare opportunity for detailed statistical modelling of an infectious disease. Previous work using these data has revealed a host abundance threshold for epizootics, and climatic influences on plague prevalence. Here, we present a model describing the local space-time dynamics of the disease at a spatial scale of 20 × 20 km(2) and a biannual temporal scale, distinguishing between invasion and persistence events. We used a Bayesian imputation method to account for uncertainties resulting from poor data in explanatory variables and response variables. Spatial autocorrelation in the data was accounted for in imputations and analyses through random effects. The results show (i) a clear effect of spatial transmission, (ii) a high probability of persistence compared with invasion, and (iii) a stronger influence of rodent abundance on invasion than on persistence. In particular, there was a substantial probability of persistence also at low host abundance.

Host plant use by apple maggot, western cherry fruit fly, and other Rhagoletis species (Diptera: Tephritidae) in central Washington state

Host plant use by apple maggot fly, Rhagoletis pomonella (Walsh), 1867, western cherry fruit fly, Rhagoletis indifferens Curran, 1932, and other Rhagoletis species (Diptera: Tephritidae) in five cities or towns and several habitats of different commercial importance within south central Washington state was determined in 2004 to 2006. Rhagoletis pomonella larvae infested 0 to 1.6% of non-commercial apple trees, Malus domestica (Borkh.) Borkh., and 0 to 22.5% of wild black hawthorn, Crataegus douglasii Lindl. (Rosaceae), the first data showing R. pomonella has established within this part of the state. Rhagoletis indifferens infested 81.8 to 100% of non-commercial sweet and sour cherry trees (Prunus avium (L.) L. and Prunus cerasus L.) (0% in one area) and 21.7 to 94.7% of wild bitter cherry trees, Prunus emarginata (Dougl. ex Hook.) D. Dietr. (Rosaceae). Rhagoletis zephyria Snow, 1894, Rhagoletis basiola (Osten Sacken), 1877, Rhagoletis tabellaria (Fitch), 1855, Rhagoletis berberis Curran, 1932, and Rhagoletis completa Cresson, 1929 infested 0 or 15.4 to 100% of their normal host plants. In south central Washington, the host breadth of R. pomonella is narrow perhaps because of the hot and dry conditions, low fly densities, and low host abundance outside of orchards. Here, R. pomonella likely can be managed to meet the strict quarantine requirements of apple export. Rhagoletis indifferens management can be confined to sweet and sour cherry trees. Responses of various Rhagoletis species to plant cues and survival of flies in dry climates need to be determined to understand patterns of host use and population densities in Washington.

Comparison of lysogeny (prophage induction) in heterotrophic bacterial and Synechococcus populations in the Gulf of Mexico and Mississippi river plume

Lysogeny has been documented as a fundamental process occurring in natural marine communities of heterotrophic and autotrophic bacteria. Prophage induction has been observed to be prevalent during conditions of low host abundance, but factors controlling the process are poorly understood. A research cruise was undertaken to the Gulf of Mexico during July 2005 to explore environmental factors associated with lysogeny. Ambient physical and microbial parameters were measured and prophage induction experiments were performed in contrasting oligotrophic Gulf and eutrophic Mississippi plume areas. Three of 11 prophage induction experiments in heterotrophic bacteria (27%) demonstrated significant induction in response to Mitomycin C. In contrast, there was significant Synechococcus cyanophage induction in seven of nine experiments (77.8%). A strong negative correlation was observed between lysogeny and log-transformed activity measurements for both heterotrophic and autotrophic populations (r=-0.876, P=0.002 and r=-0.815, P=0.025, respectively), indicating that bacterioplankton with low host growth favor lysogeny. Multivariate statistical analyses indicated that ambient level of viral abundance and productivity were inversely related to heterotrophic prophage induction and both factors combined were most predictive of lysogeny (rho=0.899, P=0.001). For Synechococcus, low ambient cyanophage abundance was most predictive of lysogeny (rho=0.862, P=0.005). Abundance and productivity of heterotrophic bacteria was strongly inversely correlated with salinity, while Synechococcus was not. This indicated that heterotrophic bacterial populations were well adapted to the river plume environments, thus providing a possible explanation for differences in prevalence of lysogeny observed between the two populations.

Effect of Nutrient Addition and Environmental Factors on Prophage Induction in Natural Populations of Marine Synechococcus Species

A series of experiments were conducted with samples collected in both Tampa Bay and the Gulf of Mexico to assess the impact of nutrient addition on cyanophage induction in natural populations of Synechococcus sp. The samples were virus reduced to decrease the background level of cyanophage and then either left untreated or amended with nitrate, ammonium, urea, or phosphate. Replicate samples were treated with mitomycin C to stimulate cyanophage induction. In five of the nine total experiments performed, cyanophage induction was present in the non-nutrient-amended control samples. Stimulation of cyanophage induction in response to nutrient addition (phosphate) occurred in only one Tampa Bay sample. Nutrient additions caused a decrease in lytic (or control) phage production in three of three offshore stations, in one of three estuarine experiments, and in a lysogenic marine Synechococcus in culture. These results suggest that the process of cyanophage induction as an assay of Synechococcus lysogeny was not inorganically nutrient limited, at least in the samples examined. More importantly, it was observed that the level of cyanophage induction (cyanophage milliliter(-1)) was inversely correlated to Synechococcus and cyanophage abundance. Thus, the intensity of the prophage induction response is defined by ambient population size and cyanophage abundance. This corroborates prior observations that lysogeny in Synechococcus is favored during times of low host abundance.

Lysogeny and virus‐induced mortality of bacterioplankton in surface, deep, and anoxic marine waters

Lysogeny (bacteria containing inducible prophages) and lytic viral infection (bacteria in a lytic stage of infection) were investigated at the community level in contrasting marine environments such as estuarine versus offshore waters, surface versus deep waters, and oxic versus anoxic waters in the Mediterranean and Baltic Seas. The frequency of lysogenic cells (FLC) in bacterioplankton communities ranged from not detectable to 84% as estimated by prophage induction due to mitomycin C, and highest values were typically found in deep waters (800–2,000 m). Transmission electron microscopy based estimates of virus‐induced mortality of bacterioplankton (VMB) ranged from a few percent to 71%, and highest values were found in anoxic waters of the Baltic Sea. FLC and the frequency of infected cells (FIC) were related in form of a negative power function indicating that environments exist where one of the two viral life strategies prevails. Across all investigated environments, FLC was negatively related to bacterial abundance and production, whereas FIC showed a positive relationship with viral and bacterial parameters. FIC was higher and FLC was lower in moderately productive estuarine and offshore surface waters than in less productive mesopelagic and deep waters. Thus, lysogeny seems to be a survival strategy at low host abundance and activity, whereas high host abundance and activity seems to favor the lytic life cycle. The key process for the prevalence of lytic infection compared to prophage replication at high host abundance could be competition due to outnumbering. Between 11% and 88% (average, 35%) of the bacteria contained a functional (lytic or lysogenic) viral genome.

Agonistic Behaviour and Development of Territoriality During Ontogeny of the Sea Anemone Dwelling Crab Allopetrolisthes Spinifrons (H. Milne Edwards, 1837) (Decapoda: Anomura: Porcellanidae)

Allopetrolisthes spinifrons is an ectosymbiotic crab of the sea anemone Phymactis clematis. As a consequence of low host abundance, these represent a scarce and limited resource for the crab. Additionally, the relatively small size of the sea anemone host suggests that few symbiotic crabs can cohabit on one host individual, forcing crabs to adopt a territorial behaviour. In order to examine the potential presence and ontogenetic development of territoriality, the agonistic behaviour between crabs of various ontogenetic stages (adults, juveniles, and recruits) was studied in the laboratory. Laboratory experiments demonstrated that adult or juvenile crabs aggressively defended their sea anemone hosts against adult or juvenile intruders, respectively, but both adult and juvenile crabs tolerated recruits. Adult crabs behaved indifferently towards juvenile crabs, sometimes tolerating them, sometimes expelling them. Recruits never showed agonistic behaviour among themselves. The agonistic interactions observed in the laboratory and the uniform population distribution pattern on sea anemones recently described for A. spinifrons indicate that this species exhibits territorial behaviour, which develops during ontogeny. Territoriality in this species and other symbiotic decapods may function as a density-dependent mechanism of population regulation, being mediated by the availability of hosts. Resource monopolisation behaviours may be common among other symbiotic and free-living marine invertebrates inhabiting discrete habitats that represent a limiting resource.

Seasonal transmission ofRaphidascaris acus (Nematoda), a parasite of freshwater fishes, in definitive and intermediate hosts

The seasonal transmission ofRaphidascaris acus was studied in two small lakes on Manitoulin Island, Ontario. Dragonfly nymphs and caddisfly larvae, acting as paratenic hosts, contained second-stage larvae. Several fishes, including percids and cyprinids, were intermediate hosts with second, third, and fourth-stage larvae in the liver. Yellow perch,Perca flavescens, was the most important of these. Intensities were up to 928 and increased with length and age of the perch; prevalence was 100%. Abundance ofR. acus tended to be higher in females but was not related to condition of the perch. Second-stage larvae were acquired from invertebrates in summer and developed to the fourth stage by November. They became surrounded by fibrous capsules during the next summer but remained alive for at least another year. The longevity of larvae in the intermediate host may ensure survival of the parasite through periods of low host abundance after winterkill. Northern pike,Esox lucius, was the definitive host. Abundance ofR. acus tended to be greater in larger pike but was not related to sex or condition of the fish. The parasite was acquired in late fall. Prevalence was 100% and mean intensities were over 200 in winter and spring, declining to 64–100% and less than 15, respectively, in summer. Mature worms were present from early spring through summer. Seasonality of infection in the definitive host is not attributable to seasonal availability of larvae in perch. Instead it may be controlled by timing of predation on perch and rate of development and longevity of the parasite. Transmission to pike apparently continues in summer. Low intensity may result from low recruitment rate and rapid turnover of the parasite population.