Intraspecific Variation In Resistance Research Articles

Effects of the floral phytochemical eugenol on parasite evolution and bumble bee infection and preference

Ecological and evolutionary pressures on hosts and parasites jointly determine infection success. In pollinators, parasite exposure to floral phytochemicals may influence between-host transmission and within-host replication. In the bumble bee parasite Crithidia bombi, strains vary in phytochemical resistance, and resistance increases under in vitro selection, implying that resistance/infectivity trade-offs could maintain intraspecific variation in resistance. We assessed costs and benefits of in vitro selection for resistance to the floral phytochemical eugenol on C. bombi infection in Bombus impatiens fed eugenol-rich and eugenol-free diets. We also assessed infection-induced changes in host preferences for eugenol. In vitro, eugenol-exposed cells initially increased in size, but normalized during adaptation. Selection for eugenol resistance resulted in considerable (55%) but non-significant reductions in infection intensity; bee colony and body size were the strongest predictors of infection. Dietary eugenol did not alter infection, and infected bees preferred eugenol-free over eugenol-containing solutions. Although direct effects of eugenol exposure could influence between-host transmission at flowers, dietary eugenol did not ameliorate infection in bees. Limited within-host benefits of resistance, and possible trade-offs between resistance and infectivity, may relax selection for eugenol resistance and promote inter-strain variation in resistance. However, infection-induced dietary shifts could influence pollinator-mediated selection on floral traits.

A rare major plant QTL determines non-responsiveness to a gall-forming insect in willow

Growth and defence are plant traits that determine attack success by sessile herbivorous insects. Models of plant resistance suggest induced resistance to be an especially important mechanism in growth-dominated plant species. Little is known, however, about the effects of absence of induced responses as an alternative resistance mechanism. Large genetically based intraspecific variation in resistance (neonate larval mortality) occurs in Salix viminalis against the gall midge Dasineura marginemtorquens. In certain resistant genotypes, larval mortality is associated with formation of necrotic lesions and biochemical changes commonly found in hypersensitive response (HR). More detailed studies, however, have shown that resistance can occur without HR symptoms. By means of a quantitative trait locus (QTL) analysis, this study tested whether (1) resistance is caused by the HR or (2) by the failure of the midge to initiate a gall (non-responsiveness). One QTL explained 68 and 18 % of the variation in resistance in the experimental population during 2 years of investigation. Co-location of QTLs for resistance and HR was found in only one of the years, i.e. when HR was suppressed. The data suggest that the mechanism of resistance involves non-responsiveness, implying that the insect starves to death due to lack of resources. Such symptomless defence may be more important than previously thought because it is difficult to observe in nature.

Variation Within and Among Vitis spp. for Foliar Resistance to the Powdery Mildew Pathogen Erysiphe necator.

To complement existing control strategies, grape growers desire cultivars with resistance to powdery mildew caused by Erysiphe necator. Numerous disease resistance screens of diverse Vitis germplasm have been conducted previously to identify powdery mildew resistance but ratings of named cultivars were inconsistent and identities of resistant individuals in wild species were not typically provided. In the current study, controlled inoculations of a single isolate were made onto detached leaves from 1,025 Vitis accessions. The results were compared with natural epidemics in two vineyards: the cold-hardy Vitis spp. repository in Geneva, NY, in 2007-08, and a replicated vineyard of 89 Vitis accessions in Fredonia, NY in 2006-08. Of the genotypes screened using both natural infection and single-isolate inoculation, 33% were resistant to a single isolate but susceptible to diverse isolates in either or both vineyards, possibly due to race-specific resistance. This was exemplified by selection of E. necator genotypes virulent to Vitis labrusca in the Fredonia, NY vineyard, which is surrounded by production of the interspecific labrusca hybrids Concord and Niagara. Otherwise, there was good correlation of ratings between the vineyard and single-isolate ratings (r = 0.55 to 0.56) and between Geneva and Fredonia vineyard ratings (r = 0.75). No accession rated in all three screens was immune from infection. Although individual accessions of V. aestivalis, V. palmata, Vitis × doaniana, and Ampelopsis brevipedunculata were resistant in Geneva and Fredonia, each well-represented species had notable intraspecific variation in resistance. For 129 interspecific hybrids in this and previous studies, ratings infrequently corresponded among previous studies (39%) and between the current and previous studies (17 to 46%). However, three cultivars (Cayuga White, Diana, and Mars) were consistently rated as resistant across four independent studies. The results underscore the importance of uniform testing in multiple environments and the need for strategies for the development of cultivars with durable resistance.

Responses of Brassica oleracea cultivars to infestation by the aphid Brevicoryne brassicae: an ecological and molecular approach

Intraspecific variation in resistance or susceptibility to herbivorous insects has been widely studied through bioassays. However, few studies have combined this with a full transcriptomic analysis. Here, we take such an approach to study the interaction between the aphid Brevicoryne brassicae and four white cabbage (Brassica oleracea var. capitata) cultivars. Both under glasshouse and field conditions, two of the cultivars clearly supported a faster aphid population development than the other two, indicating that aphid population development was largely independent of the environmental conditions. Genome-wide transcriptomic analysis using 70-mer oligonucleotide microarrays based on the Arabidopsis thaliana genome showed that only a small number of genes were differentially regulated, and that this regulation was highly cultivar specific. The temporal pattern in the expression behaviour of two B. brassicae-responsive genes in all four cultivars together with targeted studies employing A. thaliana knockout mutants revealed a possible role for a trypsin-and-protease inhibitor in defence against B. brassicae. Conversely, a xyloglucan endotransglucosylase seemed to have no effect on aphid performance. Overall, this study shows clear intraspecific variation in B. brassicae susceptibility among B. oleracea cultivars under glasshouse and field conditions that can be partly explained by certain differences in induced transcriptional changes.

SEX AND LIFE‐HISTORY STAGE ALTER HERBIVORE RESPONSES TO A CHEMICALLY DEFENDED RED ALGA

Intraspecific variation in resistance to herbivory among genders and life-history phases of primary producers can significantly alter the ecological and evolutionary consequences of plant-herbivore interactions. Seaweeds (macroalgae) with complex life histories have multiple distinct phases with associated variation in traits that can potentially lead to differences in resistance to consumers and provide a unique system in which to simultaneously test the effects of sex and life-history stage on herbivory. We tested the susceptibility to grazing of the three life-history stages and separate sexes of the chemically defended red alga Asparagopsis armata against the sea hare Aplysia parvula, and we related this to the plant quality traits of different stages and genders. Differences in nutrient content and halogenated secondary metabolites between life-history phases were highly sex dependent. Male gametophytes had a low concentration of secondary metabolites and the highest nutrient content. The highest secondary metabolite content was found within the female gametophyte, in the wall of the reproductive structures (cystocarps) that contain the microscopic carposporophyte phase. Feeding choices by A. parvula were consistent with differences in algal quality and defense and resulted in the haploid male gametophytes being the most preferred food type. The diploid carposporophyte found inside the chemically rich cystocarps was the least consumed life-history stage. Selective herbivory of male gametophytes by A. parvula is consistent with an observed shift in gametophyte sex ratio in the field from unity at the beginning of the reproductive season to female bias at the end. The variation in susceptibility to herbivory found between sex and life-history stages of A. armata represents the first example of sex-biased consumption in seaweeds and may contribute to the maintenance of complex life histories such as those found in red algae.

Intraspecific variation in resistance of frog eggs to fungal infection

Documenting sources of variation in host viability at pathogen exposure within and among populations is an important task in order to predict host-pathogen evolutionary dynamics. In the present study, we investigated family and population variation in the degree of embryonic infection of the pathogenic fungus Saprolegnia spp., by infecting moor frog (Rana arvalis) eggs from six populations and exposing them to two different temperatures. We found a significant family effect on the degree of Saprolegnia-infection of eggs and embryos, suggesting that there is genetic variation in resistance among embryos, or variation among females in some aspect of maternally induced resistance. Furthermore, infection level differed significantly between temperatures, with most families having more infected eggs in the relatively colder temperature. However, eggs and embryos from the different populations showed different degrees of Saprolegnia-infection in the two temperatures, i.e., there was a significant population x temperature interaction on the proportion of infected eggs. Thus, the degree of Saprolegnia-infection is sensitive to variation at the level of the family, population and environmental conditions, suggesting that responses to fungal outbreaks will vary geographically and will be difficult to predict. (Less)

Lack of intraspecific variation in resistance to defoliation in a grass that evolved under light grazing pressure

Grass species frequently show marked intraspecific variation inmorphology and tolerance to defoliation. Remarkably, most of this evidencecomesfrom grasslands with long evolutionary history of grazing. Here, we explore theintraspecific variation in grazing tolerance and morphometric traits associatedwith grazing avoidance of Paspalum dilatatum (Poir.), agrass from the Flooding Pampa (Argentina), where grazing is a novel disturbancein evolutionary time. We performed a clipping experiment in a greenhouse withtwo populations from sites with contrasting short term grazing regime:continuous grazing and 20 year-old grazing exclosure. The populations did notdiffer in their tolerance to clipping, and showed minor differences in the wayclipping affected plant height, a trait associated with grazing avoidance. Ourresults indicate that there are exceptions to the generalized findings of highlevels of intraspecific variation in grazing resistance among populations ofgrasses. These exceptions may be associated to evolutionary history of grazing.

Relationship between chemical functional groups on Eucalyptus secondary metabolites and their effectiveness as marsupial antifeedants

Eucalyptus displays strong intraspecific variation in resistance to browsing by marsupial folivores that can be attributed to variation in the concentration and type of diformylphloroglucinol compounds (DFPCs) in the foliage. In this study, we ask which functional groups of diformylphloroglucinol compounds determine their effectiveness in deterring feeding. We used a simple and highly deterrent compound, jensenone, as a model DFPC and compared its activity to structural variants that differ in the types of functional groups on the phloroglucinol molecule. Torquatone, a naturally occurring compound in the steam volatile fraction of Eucalyptus torquata foliage, has neither the aldehyde nor phenol groups that are believed to contribute to the antifeedant actions of jensenone. From the naturally occurring compounds we have synthesized two intermediates, a capped phenol/free aldehyde compound (acetyl-jensenone) and a free phenol/no aldehyde compound (demethyl-torquatone). Addition of jensenone and acetyl-jensenone to diets of common ringtail possums (Pseudocheirus peregrinus) substantially reduced their food intakes. Torquatone showed less activity, and there was little reduction in food intake when demethyl-torquatone was added to the diet. We conclude that at least the aldehyde groups attached to the aromatic nucleus are important in determining whether these compounds deter feeding by common ringtail possums, whereas the phenol groups may play only a minor role.

Inter- and intraspecific variation in resistance to desiccation by adult Aedes (Stegomyia) spp. (Diptera: Culicidae) from Indonesia.

Desiccation survival times of adult mosquitoes were studied for yellow fever mosquito, Aedes aegypti (L.) (6 strains), Asian tiger mosquito, Ae. albopictus (Skuse) (5 strains), and Ae. paullusi Stone & Farmer (1 strain) colonized from South Sulawesi, Indonesia. At both 90 and 70% RH, Ae. aegypti males and females outlived the other species. The forest species Ae. paullusi was least resistant to desiccation. Strains of Ae. aegypti and Ae. albopictus from urban areas were more resistant to desiccation than conspecific strains from rural villages. When water was accessible, Ae. albopictus females outlived Ae. aegypti females, indicating that death from deprivation of water was not a result of energy depletion. Ae. aegypti was largest in body size, followed by Ae. albopictus and Ae. paullusi. Among strains within species, the relationship between body size and survival time was significant only for Ae. aegypti females at 70% RH. Adult desiccation resistance may be a factor that influences distribution and abundance of Aedes (Stegomyia) species.

Within-Population Variation in Response of Red Oak Seedlings to Herbivory by Gypsy Moth Larvae

The potential for an evolutionary response to gypsy moth (Lymantna dispar L.) herbivory was investigated in red oak (Quercus rubra L.), a preferred host. Seedlings of nine open-pollinated families were grown in a greenhouse and experimentally defoliated by fourth instar larvae in the summer of 1991 to assay for intraspecific variation in resistance to and recovery from herbivory. Defoliation increased mortality and reduced growth differentially among the nine families. Family differences in plant size before defoliation explained much of the variation in mortality and growth following defoliation. Different allocation patterns among families explained some of the variation in seedling growth. Absolute growth rate of larvae was significantly different among families and higher on individuals with larger leaf area, but resistance to defoliation and larval growth efficiency were not significantly different among families. Since we have not measured the additive genetic variation in seedling traits, the magnitude of the potential evolutionary response cannot be projected from this study, but intraspecific variability in red oak does exist for tolerance and response to defoliation and possibly for resource allocation. Phenotypic selection on traits related to response to defoliation may result in evolutionary change in natural populations of red oak.

Intraspecific variation in resistance to desiccation in the land snailTheba pisana

Theba pisana is a bush-dwelling snail with a circum-Mediterranean distribution. In Israel it is limited to a narrow coastal strip, along which there is a north-to-south gradient in mean annual rainfall (700–300 mm per year). In this study we found significant intraspecific differences among populations ofT. pisana in resistance to desiccation along this gradient, and in a population from Greece, which may be grossly correlated with the climatic gradient. The Greek population was the least resistant to desiccation with an extremely high rate of water loss. The differences in total mass loss were mainly determined by the rate of water loss during the first 4 days of desiccation. A general phenomenon during desiccation was the close regulation of the soft body water content at the expense of the extra-pallial fluid compartment.

Intraspecific variation in resistance to desiccation and climatic gradients in the distribution of the bush‐dwelling land snail Trochoidea simulata

Resistance to desiccation was examined in six populations of the Israeli bush‐dwelling snail Trochoidea simulata, a desert species distributed mainly between the 100–200 mm isohyets.The present study revealed significant intraspecific differences in resistance to desiccation which are correlated with habitat and climatic gradients within the distribution range of the species. Populations from more arid sites were more resistant to desiccation and heat exposure than those from more mesic areas. However, the population from the Rift Valley (an extremely arid region) was surprisingly poorly resistant. Rates of population water loss under the controlled experimental conditions in the laboratory generally matched the calculated water losses during natural summer aestivation.

Intraspecific variation in resistance to desiccation and climatic gradients in the distribution of the land snail Xeropicta vestalis

Land snails are subject to desiccating conditions in their terrestrial habitat. Our previous studies have revealed significant differences in resistance to desiccation among closely related species and among genera that share a similar life style, suggesting that the distribution pattern is correlated with the abiotic regime in its specific habitat. Our present study extends the scope of comparison to the intraspecific level, by examining the resistance to desiccation in five populations of Xeropicta vestalis, a Mediterranean‐dwelling species that ranges from the 1000 mm to the 200 mm isohyet.The resistance to desiccation varied among populations in correlation with the specific habitat of each population and with the north‐to‐south and the west‐to‐easl climatic gradients. Furthermore, in one case it exceeded the resistance of other, desert‐dwelling species (Trochoidea simulata, Sphincterochila zonata). We suggest that, in spite of its physiological capacity to invade deserts, X. vestalis is prevented from doing so because it is an annual, semelparous species. The Negev Desert is an unpredictable environment, susceptible to year‐to‐year fluctuations to such an extent that one rainless year would wipe out all its populations.

Interspecific and intraspecific variation in resistance to ichthyophthiriasis among poeciliid and goodeid fishes

A study was undertaken on genetic variation in resistance to infection by Ichthyophthirius multifiliis in two goodeid (Ameca splendens and Ilyodon xantusi) and two poeciliid species (Xiphophorus maculatus and Xiphophorus variatus), and among four strains of X. maculatus (red, red wagtail, yellow comet tail, blue). Analysis of infection data was performed with a generalized linear interactive modelling package (GLIM). Partitioning of the variation was such that the minimal variation attributable to each of several factors was computed. Size of fish, parasite isolate, temperature/tank effects, and various interactions all affect infection levels. Removal of these effects allowed the identification of highly significant differences in both inter‐ and intraspecific variation in resistance. Three distinct groups of fish were identified in terms of their resistance. Species recently derived from wild populations (the two goodeids) were more susceptible than the two poeciliid species which have a much longer history of domestication. The blue strain of X. maculatus had significantly higher resistance than the other three strains.

Fire Blight Resistance in the Geneva Apple Rootstock Breeding Program1

Abstract Reactions of 122 apple clones, representing 26 pure and hybrid species of Malus, to Erwinia amylovora (Burrill) Winslow et al. were evaluated in the greenhouse, orchard, and field nursery. Vigorously growing shoots, 7 to 9 weeks old, were tip inoculated by injecting a 24-hour broth culture of a highly pathogenic isolate of E. amylovora. Lesion development, measured 5 to 7 weeks after inoculation, was expressed as a percentage of the current season’s shoot length killed by fire blight. Inter- and intraspecific variation in resistance, similar to that reported for Pyrus, was observed. The highest level of resistance, typified by a small necrotic lesion 2-3 mm in diameter around the inoculation point, occurred in small-fruited selections derived from Asiatic species. Few selections of the cultivated apple, Malus pumila Mill., were resistant. Fire blight ratings were assigned the clones based on their potentials as parents for breeding apple rootstocks.

Some aspects of the toxicity of p,p′-DDT, p,p′-DDE and p,p′-DDD to the freshwater planarian Polycelis felina (tricladida)

The effects of p,p′-DDT, p,p′-DDE and p,p′-DDD on the freshwater Triclad, Polycelis felina, were studied. The 96 h LC50 of the three chemicals were 1·05, 1·23 and 0·74 ppm, respectively. The chemicals appeared to exert their effects through the nervous system as revealed by the loss of coordinated movement and the retardation of the flip-over times of treated animals. DDT also tended to inhibit asexual fission at concentrations much lower than the observed 96 h LC50. The carrier solvent acetone had no permanent effect on the gross behaviour of the animals. P. felina responded consistently to experimental treatment and the observed intraspecific variation in resistance to organochlorine chemicals, by animals of approximately the same size, was low. The use of this and other species of flatworms as bioassays of organochlorine chemicals is discussed.