Multiple choice: hemiparasite performance in multi‐species mixtures

Hemiparasitic plants have green leaves, but extract water and solutes from neighbouring plants. It is still poorly understood how different host plants in communities contribute to parasite performance, as species that are good hosts in single‐host experiments may not necessarily be preferred hosts in mixtures. We grew the root hemiparasite Rhinanthus alectorolophus (Orobanchaceae) together with each of 13 host species (experiment 1) and with 15 different four‐species mixtures of these hosts (experiment 2) that differed in the number of legumes and of host functional groups. Parasites profited from mixtures including more legumes and from mixtures including different host functional groups. Some host species and mixtures were very tolerant of parasitism and supported large parasites without being strongly suppressed in their own growth, but the suppression of a species in the single‐host experiment did not explain the suppression of a species in a host mixture. We thus calculated for each host species an index of the difference in suppression between the two experiments which may be related to host use in a mixture. Host quality (mean parasite biomass with a host species) in the single‐host experiment could explain 64% of the variation in parasite biomass with a host mixture when it was weighted by the proportion of the host species in the mixture without the parasite and by the suppression difference index. Our results suggest that plant species which are the best hosts in single‐host experiments are not always those used most strongly by a parasite growing with a mixture. Together with the finding that hemiparasites benefit from a mixed diet based on hosts from different functional groups this suggests that parasites prefer certain host species to obtain a mixed diet.     

Interactions of the Hemiparasitic Species Rhinanthus minor with its Host Plant Community at Two Nutrient Levels

For root hemiparasites, host plants are both the source of water and nutrients below-ground, but competitors for light above-ground. Hemiparasites can reduce host biomass, and in this way considerably affect the whole plant community. To investigate these effects, we carried out two experiments in an oligotrophic meadow with a native population of Rhinanthus minor. In the first experiment, removal of R. minor was combined with fertilization in a factorial design, and in the second one, we manipulated R. minor density by thinning. The presence of R. minor decreased the biomass of its host community, mostly by suppressing grasses. In this way, the species was able to counterbalance the effect of fertilization, which increased community biomass and in particular that of grasses. Neither the presence of R. minor nor fertilization affected the total number of species or the Shannon-Wiener diversity index (H’) of the host community. However, H’ of grasses was higher and H’ of forbs (non-leguminous dicots) was lower in the presence of R. minor. Reduction of grasses by R. minor favored mainly the dominant forb Plantago lanceolata, which partly acquired the role of a competitive dominant. Effects of R. minor on community diversity seem to be highly dependent on the relative sensitivities of dominant and subordinate species. Fertilization increased the mortality of seedlings, resulting in a lower number of flowering plants. However, surviving individuals on average produced more flowers. Thinning resulted in lower mortality of R. minor plants. This indicates that intraspecific competition in R. minor populations results in negative density dependence.     

Effects of drought and N level on the interactions of the root hemiparasite Rhinanthus alectorolophus with a combination of three host species

• Increasing nitrogen deposition and more frequent drought events are likely to change plant interactions in natural grasslands. Both factors may also influence the interactions between hemiparasitic plants, regarded as keystone species in many grasslands, and their host species.
• We grew a combination of three suitable hosts, a grass, a forb and a legume, with and without the hemiparasite Rhinanthus alectorolophus at three levels of nitrogen (N) and two levels of water availability in a factorial design.
• Biomass of the hemiparasite and host community increased with N level and was reduced by drought to a similar degree. Larger plants in fertilised pots started to wilt earlier, and the presence of a hemiparasite further increased drought sensitivity. The hemiparasite strongly reduced biomass of the host community and overall productivity, and affected the competitive balance among host plants because it particularly reduced biomass of the dominant grass. These effects were the opposite of those of high N. The hemiparasite increased the root mass fraction of the hosts at all levels of N and water availability, indicating that the effect of the hemiparasite on the hosts was mainly due to loss of belowground resources.
• Our results indicate that hemiparasites will not always respond more strongly to increased N availability and drought than autotrophic plants, and that hemiparasites can have similarly strong effects on grassland communities as soil fertility and drought. By preferentially attacking dominant species the hemiparasites might alleviate the negative effects of nutrient enrichment on grassland diversity.

     

Impact of hemiparasitic Rhinanthus angustifolius and R. minor on nitrogen availability in grasslands

Root hemiparasites like Rhinanthus angustifolius C.C. Gmel and R. minor L. have a potential to accelerate the restoration of semi-natural grasslands because they may decrease above-ground biomass of the vegetation. This, in turn, may be beneficial for species diversity. It is known that hemiparasites often accumulate high nutrient concentrations in their above-ground parts, resulting in high quality litter. Because of the short life cycle of many parasitic plants, litter is released early in the season and the main part is not removed from the grassland by hay-making. This has been shown to yield an increased nutrient availability locally. We performed an introduction experiment with R. angustifolius and R. minor in three semi-natural grasslands in Flanders (Belgium). In the second year after sowing, the above-ground nitrogen (N) content of the grasses and of the potential host vegetation (excluding the hemiparasite), was increased in the parasitized plots. The reduction of grass (and legume) above-ground biomass in parasitized plots resulted in a decrease in the total above-ground N uptake of grasses, host and total vegetation (ex- and including the parasite, respectively) of the parasitized plots compared to the control. Furthermore, with a tracer experiment (¹⁵N), we demonstrated that the N from the added tracer was relatively less available in parasitized plots, suggesting larger soil N pools in these treatments. This is probably the consequence of increased mineralization, resulting from the high-quality, parasitic litter. Further experiments should be conducted to investigate the impact of hemiparasitic Rhinanthus spp., e.g. on the availability of other nutrients such as phosphorus.     

Heterotrophic carbon gain by the root hemiparasites, Rhinanthus minor and Euphrasia rostkoviana (Orobanchaceae)

Hemiparasitic plants gain virtually all mineral nutrients and water from their host plant whilst organic carbon is provided, at least in part, by their own photosynthetic activity, although their rates of assimilation are substantially lower than that found in non-parasitic plants. Hence, hemiparasites must gain at least some of their organic carbon heterotrophically from the host plant. Despite this, heterotrophic carbon gain by root hemiparasites has been investigated only for a few genera. We investigated heterotrophic carbon gain by two root hemiparasites, Rhinanthus minor L. and Euphrasia rostkoviana Hayne (Orobanchaceae), using natural abundance stable isotope (δ¹³C) profiles of both parasites attached to C₃ (wheat) and C₄ (maize) hosts coupled to a linear two-source isotope-mixing model to estimate the percentage of carbon in the parasite that was derived from the host. Both R. minor and E. rostkoviana attached to maize hosts were significantly more enriched in ¹³C than those attached to wheat hosts with R. minor becoming more enriched in ¹³C than E. rostkoviana. The natural abundance ¹³C profiles of both parasites were not significantly different from their wheat hosts, but were less enriched in ¹³C than maize hosts. Using a linear two-source isotope-mixing model, we estimated that R. minor and E. rostkoviana adult plants derive c. 50 and 25% of their carbon from their hosts, respectively. In light of these results, we hypothesise that repeatedly observed negative effect of competition for light on hemiparasites acts predominantly in early ontogenetic stages when parasites grow unattached or the abstraction of host nutrients is less effective.     

Response of a root hemiparasite to elevated CO2 depends on host type and soil nutrients

Although elevated CO₂ may affect various forms of ecological interactions, the effect of elevated CO₂ on interactions between parasitic plants and their hosts has received little attention. We examined the effect of elevated CO₂ (590 μl l⁻¹) at two nutrient (NPK) levels on the interactions of the facultative root hemiparasite Rhinanthus alectorolophus with two of its hosts, the grass Lolium perenne and the legume Medicago sativa. To study possible effects on parasite mediation of competition between hosts, the parasite was grown with each host separately and with both hosts simultaneously. In addition, all combinations of hosts were grown without the parasite. Both the parasite and the host plants responded to elevated CO₂ with increased growth, but only at high nutrient levels. The CO₂ response of the hemiparasite was stronger than that of the hosts, but depended on the host species available. With L. perenne and M. sativa simultaneously available as hosts, the biomass of the parasite grown at elevated CO₂ was 5.7 times that of parasites grown at ambient CO₂. Nitrogen concentration in the parasites was not influenced by the treatments and was not related to parasite biomass. The presence of the parasite strongly reduced both the biomass of the hosts and total productivity of the system. This effect was much stronger at low than at high nutrient levels, but was not influenced by CO₂ level. Elevated CO₂ did not influence the competitive balance between the two different hosts grown in mixture. The results of this study support the hypothesis that hemiparasites may influence community structure and suggest that these effects are robust to changes in CO₂ concentration. Received: 17 August 1998 / Accepted: 3 March 1999     

Molecular approaches reveal weak sibship aggregation and a high dispersal propensity in a non‐native fish parasite

Inferring parameters related to the aggregation pattern of parasites and to their dispersal propensity are important for predicting their ecological consequences and evolutionary potential. Nonetheless, it is notoriously difficult to infer these parameters from wildlife parasites given the difficulty in tracking these organisms. Molecular‐based inferences constitute a promising approach that has yet rarely been applied in the wild. Here, we combined several population genetic analyses including sibship reconstruction to document the genetic structure, patterns of sibship aggregation, and the dispersal dynamics of a non‐native parasite of fish, the freshwater copepod ectoparasite Tracheliastes polycolpus. We collected parasites according to a hierarchical sampling design, with the sampling of all parasites from all host individuals captured in eight sites spread along an upstream–downstream river gradient. Individual multilocus genotypes were obtained from 14 microsatellite markers, and used to assign parasites to full‐sib families and to investigate the genetic structure of T. polycolpus among both hosts and sampling sites. The distribution of full‐sibs obtained among the sampling sites was used to estimate individual dispersal distances within families. Our results showed that T. polycolpus sibs tend to be aggregated within sites but not within host individuals. We detected important upstream‐to‐downstream dispersal events of T. polycolpus between sites (modal distance: 25.4 km; 95% CI [22.9, 27.7]), becoming scarcer as the geographic distance from their family core location increases. Such a dispersal pattern likely contributes to the strong isolation‐by‐distance observed at the river scale. We also detected some downstream‐to‐upstream dispersal events (modal distance: 2.6 km; 95% CI [2.2–23.3]) that likely result from movements of infected hosts. Within each site, the dispersal of free‐living infective larvae among hosts likely contributes to increasing genetic diversity on hosts, possibly fostering the evolutionary potential of T. polycolpus.     

Habitat fragmentation and vegetation structure impact gastrointestinal parasites of small mammalian hosts in Madagascar

Deleterious effects of habitat loss and fragmentation on biodiversity have been demonstrated in numerous taxa. Although parasites represent a large part of worldwide biodiversity, they are mostly neglected in this context. We investigated the effects of various anthropogenic environmental changes on gastrointestinal parasite infections in four small mammal hosts inhabiting two landscapes of fragmented dry forest in northwestern Madagascar. Coproscopical examinations were performed on 1,418 fecal samples from 903 individuals of two mouse lemur species, Microcebus murinus (n = 199) and M. ravelobensis (n = 421), and two rodent species, the native Eliurus myoxinus (n = 102) and the invasive Rattus rattus (n = 181). Overall, sixteen parasite morphotypes were detected and significant prevalence differences between host species regarding the most common five parasites may be explained by parasite–host specificity or host behavior, diet, and socioecology. Ten host‐ and habitat‐related ecological variables were evaluated by generalized linear mixed modeling for significant impacts on the prevalence of the most abundant gastrointestinal parasites and on gastrointestinal parasite species richness (GPSR). Forest maturation affected homoxenous parasites (direct life cycle) by increasing Lemuricola, but decreasing Enterobiinae gen. sp. prevalence, while habitat fragmentation and vegetation clearance negatively affected the prevalence of parasites with heterogenic environment (i.e., Strongyloides spp.) or heteroxenous (indirect cycle with intermediate host) cycles, and consequently reduced GPSR. Forest edges and forest degradation likely change abiotic conditions which may reduce habitat suitability for soil‐transmitted helminths or required intermediate hosts. The fragility of complex parasite life cycles suggests understudied and potentially severe effects of decreasing habitat quality by fragmentation and degradation on hidden ecological networks that involve parasites. Since parasites can provide indispensable ecological services and ensure stability of ecosystems by modulating animal population dynamics and nutrient pathways, our study underlines the importance of habitat quality and integrity as key aspects of conservation.     

Nutrient requirements differ in two Pedicularis species in the absence of a host plant: implication for driving forces in the evolution of host preference of root hemiparasitic plants

Background and Aims

Facultative root hemiparasitic plants generally have a wide host range, but in most cases show an obvious host preference. The reasons for the marked difference in growth performance of hemiparasites when attached to different hosts are not fully understood. In this study, the hypothesis was tested that hemiparasites showing a preference for different hosts have different nutrient requirements.

Methods

Two facultative root hemiparasitic Pedicularis species (P. rex and P. tricolor) with a different host dependency and preference were used to test their responses to inorganic solutes. The effects of nitrogen, phosphorus and potassium on growth of the hemiparasitic plants not attached to a host were determined, using an orthogonal design in pot cultivation under greenhouse conditions. Variables including biomass, shoot nutrient concentration, root:shoot (R:S) ratios and the number of haustoria were measured.

Key Results

As in autotrophic plants, nutrient deficiency reduced dry weight (DW) and nutrient concentrations in the root hemiparasites. Nitrogen and phosphorus significantly influenced growth of both Pedicularis species, while potassium availability influenced only shoot DW of P. rex. Nitrogen had far more effect on growth of P. rex than on P. tricolor, while phosphorus deficiency caused more marked growth depression in P. tricolor than in P. rex. Pedicularis rex grew faster than P. tricolor in a range of nutrient supplies. Different patterns of biomass allocation between the two Pedicularis species were observed. While P. rex invested more into roots (particularly fine rootlets) than P. tricolor, the number of haustoria produced by P. rex was relatively much lower than that produced by P. tricolor, which had a much smaller root system.

Conclusions

The two Pedicularis species differ in nutrient requirements and biomass allocation. Distinct interspecific traits in growth and nutrient requirements can be driving forces for the differential interactions between hemiparasites and their hosts.     

Is palatability of a root-hemiparasitic plant influenced by its host species?

Palatability of parasitic plants may be influenced by their host species, because the parasites take up nutrients and secondary compounds from the hosts. If parasitic plants acquired the full spectrum of secondary compounds from their host, one would expect a correlation between host and parasite palatability. We examined the palatability of leaves of the root-hemiparasite Melampyrum arvense grown with different host plants and the palatability of these host plants for two generalist herbivores, the caterpillar of Spodoptera littoralis and the slug Arion lusitanicus. We used 19 species of host plants from 11 families that are known to contain a wide spectrum of anti-herbivore compounds. Growth of M. arvense was strongly influenced by the host species. The palatability of the individual host species for the two herbivores differed strongly. Both A. lusitanicus and S. littoralis discriminated also between hemiparasites grown with different host plants. There was no correlation between the palatability of a host species and that of the parasites grown on that host, i.e., hemiparasites grown on palatable host species were not more palatable than those grown on unpalatable hosts. We suggest an interacting pattern of specific effects of chemical anti-herbivore defences and indirect effects of the hosts on herbivores through effects on growth and tissue quality of the parasites.     

A molecular and ecological study of Grillotia (Cestoda: Trypanorhyncha) larval infection in small to mid‐sized benthonic sharks in the Gulf of Naples,Mediterranean Sea

AimTrypanorhyncha cestodes comprise a wide range of heteroxenous parasites infecting elasmobranchs as definitive hosts. Limited data exist on the larval infection of these cestodes and the role of intermediate and paratenic hosts in the life cycle of these parasites. We investigated the factors that determine the occurrence and the level of infection of Grillotia plerocerci in the skeletal muscles of various benthonic sharks and analyzed the parasites through an integrative taxonomic approach.LocationMediterranean Sea.MethodsSharks obtained as bycatch of commercial trawling activities (i.e., Etmopterus spinax, Galeus melastomus, and Scyliorhinus canicula) were used in this study. Data from a limited number of Dalatias licha and Scyliorhinus stellaris were also included. Grillotia plerocerci were molecularly characterized using the partial 28S large subunit rDNA. Boosted regression trees were used to model the relationship between the abundance of infection with both morphological and physiological predictors in each host.ResultsPlerocerci of Grillotia were detected in all shark species except S. stellaris. Host species significantly differed in terms of parasite abundance, with the highest and lowest prevalence and abundance of infection detected in G. melastomus and E. spinax, respectively. The relative influence of the traits involved in explaining the parasite abundance was related to the host size in G. melastomus, while both morphology‐ and physiology‐related traits explained the patterns observed in E. spinax and S. canicula. The 28S rDNA sequences shared an identity of ∼99.40% with a Grillotia species previously found in the Mediterranean Sea. At intraspecific level, two different genotypes were found. A first type was retrieved only from D. licha, whereas a second type was found in G. melastomus, E. spinax, and S. canicula.Main conclusionsPresent results suggest that the two genotypes could be involved in different consumer‐resource systems and confirm most of the examined shark species as transport hosts of Grillotia species for unknown larger top predators.     

Interactions between hemiparasitic plants and their hosts: The importance of organic carbon transfer

Hemiparasitic plants display a unique strategy of resource acquisition combining parasitism of other species and own photosynthetic activity. Despite the active photoassimilation and green habit, they acquire substantial amount of carbon from their hosts. The organic carbon transfer has a crucial influence on the nature of the interaction between hemiparasites and their hosts which can oscillate between parasitism and competition for light. In this minireview, we summarize methodical approaches and results of various studies dealing with carbon budget of hemiparasites and the ecological implications of carbon heterotrophy in hemiparasites.Key words: haustorium, heterotrophy, parasitic plant, mistletoe, Rhinanthus, Striga, δ¹³CHemiparasitic plants withdraw resources from the vascular system of their hosts through a specialized transfer organ called haustorium.¹ Hemiparasites attack the host''s xylem, in contrast to the holoparasites that infect both phloem and xylem, and as a consequence, hemiparasitic plants have access to water and mineral nutrients but little carbon.¹ Due to their reduced or non-existing root networks, hemiparasitic plants acquire virtually all mineral nutrients and water from the host while organic carbon is provided, at least in part, by their own photosynthetic activity.^2,3 This is in contrast to holoparasitic plants which rely on the host for the supply of both organic and inorganic nutrients. The location of the attachment to the host and the degree of host dependency represent the most important characters defining the three basic functional types within hemiparasitic plants. Root hemiparasites attack host roots but their above-ground appearance is usually not substantially different from that of a non-parasitic plant. This group can be further divided in two—facultative and obligate hemiparasites consisting of plants that are able (at least sometimes) or unable to complete their life cycle without an attachment to the host respectively. Stem hemiparasites are attached to the host stem (usually trunk or branches) and are all obligate parasites, unable to survive without a host.Hemiparasitic plants have an ambiguous relationship with their hosts which, on the one hand, represent exclusive sources of inorganic nutrients but on the other hand, the co-occurrence of these host plants in the hemiparasite vicinity imposes competition for light. The nature and intensity of this competitive relationship varies across different groups and species of hemiparasites. The ability of hemiparasites to acquire organic carbon (largely in the form of xylem-mobile organic and amino acids) is certainly the key factor affecting this interaction since hemiparasites that are capable of efficient organic carbon abstraction should be minimally affected by shading from their host. The fact that hemiparasites can exhibit substantial carbon heterotrophy is now supported by a large number of studies, although a traditional point of view on hemiparasites that highlights the importance of inorganic resources (mainly nitrogen) acquisition is still prevailing. Therefore, we decided to summarize available information on hemiparasite heterotrophy, outline techniques for assessing the proportion of heterotrophy and estimating the overall carbon budget, and discuss possible implications of this phenomenon on hemiparasite ecology.     

Responses of insect herbivores to sharing a host plant with a hemiparasite: impacts on preference and performance differ with feeding guild

1. Root hemiparasites are common components of many ecosystems and can affect both the biomass and the nutritional quality of the plants they infect. The consequences of these modifications for the preference and performance of three herbivore feeding guilds sharing a host with the hemi‐parasite were examined. 2. It was predicted that as the hemiparasite increased in biomass its impact on the host would increase, as would the indirect impacts on the herbivores. It was also predicted that herbivores from different feeding guilds would respond differently to the presence of the hemiparasite, reflecting the extent to which they utilise resources disrupted by the parasite and hence are in competition with it. 3. The preference and performance of phloem‐feeding aphids, xylem‐feeding spittle bugs, and leaf‐feeding grasshoppers were measured on the host grass species, Holcus lanatus L. (Poaceae), with and without attachment from the hemi‐parasite, Rhinanthus minor L. (Orobanchaceae). 4. The effects of R. minor on the host were dependent on the hemiparasite's stage of growth, being most pronounced when it was at peak biomass. At this stage it caused a significant reduction in the biomass, water content, and total nitrogen content of the host plants. 5. Overall, herbivores benefited from, or preferred, shared host plants more than uninfected plants. The aphid benefited from sharing a host with R. minor, showing increased population growth on, and preference for, parasitised plants. The spittle bug also showed a preference for parasitised plants. The grasshopper, Chorthippus brunneus Thunberg (Orthoptera: Acrididae), did not show a preference for, or a performance response to, parasitised hosts, but it consumed significantly more plant material when caged on parasitised plants. 6. These data support the prediction that invertebrate herbivores responded to changes in host plant traits driven by the hemiparasite, and strongly suggest that these indirect interactions could impact on population and community processes within natural communities.     

Do Parasitic Trematode Cercariae Demonstrate a Preference for Susceptible Host Species?

Many parasites are motile and exhibit behavioural preferences for certain host species. Because hosts can vary in their susceptibility to infections, parasites might benefit from preferentially detecting and infecting the most susceptible host, but this mechanistic hypothesis for host-choice has rarely been tested. We evaluated whether cercariae (larval trematode parasites) prefer the most susceptible host species by simultaneously presenting cercariae with four species of tadpole hosts. Cercariae consistently preferred hosts in the following order: Anaxyrus ( = Bufo) terrestris (southern toad), Hyla squirella (squirrel tree frog), Lithobates ( = Rana) sphenocephala (southern leopard frog), and Osteopilus septentrionalis (Cuban tree frog). These host species varied in susceptibility to cercariae in an order similar to their attractiveness with a correlation that approached significance. Host attractiveness to parasites also varied consistently and significantly among individuals within a host species. If heritable, this individual-level host variation would represent the raw material upon which selection could act, which could promote a Red Queen “arms race” between host cues and parasite detection of those cues. If, in general, motile parasites prefer to infect the most susceptible host species, this phenomenon could explain aggregated distributions of parasites among hosts and contribute to parasite transmission rates and the evolution of virulence. Parasite preferences for hosts belie the common assumption of disease models that parasites seek and infect hosts at random.     

Meta-analysis of standing crop reduction byRhinanthus spp. and its effect on vegetation structure

We performed a quantitative literature review on the effect of the root hemiparasiteRhinanthus on vegetation standing crop. (1) Across all available experimental studies in mixed vegetation and in pots, above-ground biomass of co-occurring species is generally reduced, with on average 40% and 60% of the value in the controls respectively. Total above-ground biomass, as the sum of parasite biomass and biomass of co-occurring species, decreases in most cases. For field experiments this reduction amounts, on average, to 26% of the control value. This implies that there is no compensation by the parasites’ biomass for the loss of biomass of co-occurring species due to parasite infection. This can be attributed to the low resource-use efficiency of hemiparasites. Meta-analysis confirmed these trends. (2) In pot experiments, the negative effect of the parasite on the above-ground biomass of the host increases with the number ofRhinanthus plants. In field experiments, we found no relationship between biomass reduction andRhinanthus density. (3) Total above-ground biomass reduction in field experiments increases with standing crop of the vegetation. However, reduction in above-ground biomass of co-occurring species seems to decrease with standing crop. Functional and species diversity buffer the community against negative effects ofRhinanthus. (4) In field experiments, functional groups are affected differently byRhinanthus spp. Grasses and legumes are mostly strongly reduced by the hemiparasites. Non-leguminous dicots mostly benefit from the presence ofRhinanthus. (5) In one out of four weeding experiments,Rhinanthus spp. has a significant (positive) effect on species number. However, the response of plant diversity to invasion of parasitic plants requires further research.     

Dormancy and germination of sixRhinanthus species in relation to climate

The genusRhinanthus (Orobanchaceae) consists of annual hemiparasites that occur in a wide range of climates. Patterns of dormancy and germination were studied for six species sampled in areas ranging from the Pyrenees to Northern Scandinavia, and from sea level up to about 2500 m altitude in the Alpine region. Dormancy was broken by a 2 to 6 months period of cold stratification. Optimal temperature and length of the stratification period appeared to vary between and within species. Two patterns of dormancy and germination were distinguished. Seeds of the first group, including the widely distributedR. minor andR. angustifolius, further referred to as the LW group, require a relatively long period of cold stratification. Moreover, their germination is accelerated if they are subjected to a widening range of higher temperatures in the last weeks of the stratification period. In the other species (R. alectorolophus, R. glacialis, R. mediterraneus and perhapsR. antiquus) the release of dormancy is completed in a rather short period. Higher temperatures in the last weeks of the stratification period hardly affect the germination process of this SN-group, with short dormancy and no accelerated germination at higher temperatures. In both groups, temperatures above the low values during stratification reduce germination percentages and induce secondary dormancy in non-germinated seeds. The effects strongly vary with timing and temperature. The patterns have a genetic basis and seem to be species-specific. They do not vary with climate conditions, since samplesR. minor, whether collected in sub-arctic or sub-alpine areas or at sea level, generally react according to the LW pattern, and samples ofR. alectorolophus from areas at sea level up to montane regions according to the SN one. The ecological significance of the various stratification requirements and effects of higher temperatures on germination is discussed in relation to the local climate of the species and the evolutionary history of the genus.     

In Vivo Microbial Coevolution Favors Host Protection and Plastic Downregulation of Immunity

Microbiota can protect their hosts from infection. The short timescales in which microbes can evolve presents the possibility that “protective microbes” can take-over from the immune system of longer-lived hosts in the coevolutionary race against pathogens. Here, we found that coevolution between a protective bacterium (Enterococcus faecalis) and a virulent pathogen (Staphylococcus aureus) within an animal population (Caenorhabditis elegans) resulted in more disease suppression than when the protective bacterium adapted to uninfected hosts. At the same time, more protective E. faecalis populations became costlier to harbor and altered the expression of 134 host genes. Many of these genes appear to be related to the mechanism of protection, reactive oxygen species production. Crucially, more protective E. faecalis populations downregulated a key immune gene, , known to be effective against S. aureus infection. These results suggest that a microbial line of defense is favored by microbial coevolution and may cause hosts to plastically divest of their own immunity.     

Characteristics of the mitochondrial genome of Rana omeimontis and related species in Ranidae: Gene rearrangements and phylogenetic relationships

The Omei wood frog (Rana omeimontis), endemic to central China, belongs to the family Ranidae. In this study, we achieved detail knowledge about the mitogenome of the species. The length of the genome is 20,120 bp, including 13 protein‐coding genes (PCGs), 22 tRNA genes, two rRNA genes, and a noncoding control region. Similar to other amphibians, we found that only nine genes (ND6 and eight tRNA genes) are encoded on the light strand (L) and other genes on the heavy strand (H). Totally, The base composition of the mitochondrial genome included 27.29% A, 28.85% T, 28.87% C, and 15.00% G, respectively. The control regions among the Rana species were found to exhibit rich genetic variability and A + T content. R. omeimontis was clustered together with R. chaochiaoensis in phylogenetic tree. Compared to R. amurensis and R. kunyuensi, it was more closely related to R. chaochiaoensis, and a new way of gene rearrangement (ND6‐trnE‐Cytb‐D‐loop‐trnL2 (CUN)‐ND5‐D‐loop) was also found in the mitogenome of R. amurensis and R. kunyuensi. Our results about the mitochondrial genome of R. omeimontis will contribute to the future studies on phylogenetic relationship and the taxonomic status of Rana and related Ranidae species.     

Local adaptation, resistance, and virulence in a hemiparasitic plant-host plant interaction

Abstract.— Coevolution may lead to local adaptation of parasites to their sympatric hosts. Locally adapted parasites are, on average, more infectious to sympatric hosts than to allopatric hosts of the same species or their fitness on the sympatric hosts is superior to that on allopatric hosts. We tested local adaptation of a hemiparasitic plant, Rhinanthus serotinus (Scrophulariaceae), to its host plant, the grass Agrostis capillaris . Using a reciprocal cross-infection experiment, we exposed host plants from four sites to hemiparasites originating from the same four sites in a common environment. The parasites were equally able to establish haustorial connections to sympatric and allopatric hosts, and their performance was similar on both host types. Therefore, these results do not indicate local adaptation of the parasites to their sympatric hosts. However, the parasite populations differed in average biomass and number of flowers per plant and in their effect on host biomass. These results indicate that the virulence of the parasite varied among populations, suggesting genetic variation. Theoretical models suggest that local adaptation is likely to be detected if the host and the parasite have different evolutionary potentials, different migration rates, and the parasite is highly virulent. In the interaction between R. serotinus and A. capillaris all the theoretical prerequisites for local adaptation may not be fulfilled.     

Host-specific races in the holoparasitic angiosperm Orobanche minor: implications for speciation in parasitic plants

Background and Aims

Orobanche minor is a root-holoparasitic angiosperm that attacks a wide range of host species, including a number of commonly cultivated crops. The extent to which genetic divergence among natural populations of O. minor is influenced by host specificity has not been determined previously. Here, the host specificity of natural populations of O. minor is quantified for the first time, and evidence that this species may comprise distinct physiological races is provided.

Methods

A tripartite approach was used to examine the physiological basis for the divergence of populations occurring on different hosts: (1) host–parasite interactions were cultivated in rhizotron bioassays in order to quantify the early stages of the infection and establishment processes; (2) using reciprocal-infection experiments, parasite races were cultivated on their natural and alien hosts, and their fitness determined in terms of biomass; and (3) the anatomy of the host–parasite interface was investigated using histochemical techniques, with a view to comparing the infection process on different hosts.

Key Results

Races occurring naturally on red clover (Trifolium pratense) and sea carrot (Daucus carota ssp. gummifer) showed distinct patterns of host specificity: parasites cultivated in cross-infection studies showed a higher fitness on their natural hosts, suggesting that races show local adaptation to specific hosts. In addition, histological evidence suggests that clover and carrot roots vary in their responses to infection. Different root anatomy and responses to infection may underpin a physiological basis for host specificity.

Conclusions

It is speculated that host specificity may isolate races of Orobanche on different hosts, accelerating divergence and ultimately speciation in this genus. The rapid life cycle and broad host range of O. minor make this species an ideal model with which to study the interactions of parasitic plants with their host associates.Key words: Parasitic plant, Orobanche, speciation, divergence, host-specificity, host-specific races