Cell surface components of Leishmania: identification of a novel parasite lectin?

Protozoan parasites of the genus Leishmania have a glycoconjugatesurface coat (the glycocalyx) that acts as the interface betweenthe parasite and its external environment. The prinicipal componentsof the glycocalyx, the lipophosphoglycans and the glycoinositolphospholipids,have a variety of functions that facilitate parasite survivalin both the extracellular and the intracellular stages of thelife cycle. Recently, a novel hydrophilic Leishmania protein,the Gene B protein, has been identified on the surface of infectiveparasite stages. Attachment to the surface appears to be byassociation between a region of repeated amino acids in thismolecule and components of the glycocalyx. As a consequence,the Gene B protein is exposed on the parasite surface whileother peptides are buried beneath the glycocalyx. The putativefunctions of this unusual molecule are discussed. differentially regulated genes glycoconjugates infective parasites Leishmania surface protein     

Xylem Transport and Storage of Amino Acids by S.W. Australian Mistletoes and their Hosts

Amino acid composition of xylem (tracheal) sap and ethanolicextracts of shoots of mistletoes (Amyema spp. and Lysiana casuarinae)and their hosts were compared, using material collected in theirnative habitats. Data indicated that certain host xylem soluteswere transferred directly to the parasite xylem, while otherswere either not absorbed or were metabolized prior to transfer.Certain solutes were major constituents of parasite xylem, butundetected or only in trace amount in the host. Shoot aminoacid pools of parasites differed markedly from those of hosts.The mistletoe, Amyema preissii, exhibited differential storageand transport of arginine when parasitizing three differentspecies, but accumulated proline on only two of these hosts.Host- specific amino acids (djenkolic acid in Acacia saligna,and tyramine in Acacia acuminata) were transported and accumulatedin relatively large amounts by the parasite, but were not detectedin other associations. Proline was the major solute of Amyemalinophyllum parasitizing Casuarina obesa, but arginine predominatedin Lysiana casuarinae on the same host. However, when L. csuarinaeparasitized A. linophyllum, in turn parasitic on C. obesa, theLysiana accumulated equal amounts of proline and arginine andmore asparagine than when directly on the Casuarina. Xylem feedingof ¹⁵N-labelled aspartic acid or ¹³N-(amide labelled) asparagineto cut shoots or whole haustoria-bearing plants of the mistletoeA. preissii resulted in 68–73% of the ¹⁵N of aspartateand 24–30% of that of asparagine appearing in ethanol-solubleshoot amino compounds other than the fed solute. ¹⁵N labellingpatterns of detached shoots were not noticeably different fromthat of whole plants suggesting that the haustorium had relativelylittle effect on processing incoming solutes. Alanine, glutamine,and arginine were principal recipients of ¹⁵N from aspartate,alanine and glutamine in the case of fed asparagine. It is estimatedthat 24% of the carbon requirements for dry matter accumulationin Amyema linophyllm were met by intake of xylem sap solutesfrom its host Casuarina obesa. Key words: Amino acids, xylem transport, mistletoes, host: parasite relations, N metabolism     

Changes in the Levels of Free Amino Acids in Various Regions of Cuscuta during Growth

The angiospermous plant parasite Cuscuta derives reduced carbonand nitrogen compounds primarily from its host. Free amino acidsalong Cuscuta vines in three zones, viz., 0 to 5 cm, 5 to 15cm, and 15 to 30 cm, which in a broad sense represent the regionof cell division, cell elongation and differentiation and vasculartissue differentiation respectively, were quantitatively estimated.The free amino acid content was the highest in the 0 to 5 cmregion and progressively decreased along the posterior regionsof the vine. The haustorial region showed the lowest contentof free amino acids. In general, the free amino acid contentin samples collected at 7 p.m. was found to be higher than thatin the samples collected at 7 a.m. Three basic amino acids,histidine, the uncommon amino acid -hydroxyarginine, and arginineconstituted more than 50% of the total free amino acids in allthe zones studied except the haustorial region. Aspartic acidand glutamic acid constituted the major portion in the acidicand neutral fraction of amino acids. Glutamine, asparagine,threonine, and serine were eluted together and occurred in substantialamounts. -Hydroxyarginine constituted the largest fraction inthe cut end exudate of Cuscuta and presumably appeared to bethe major form of transport amino acid. -Hydroxyarginine wasalso a major constituent of the basic amino acids in Cuscutavines parasitizing host plants from widely separated families,suggesting that this amino acid is a biosynthetic product ofthe parasite rather than that of the hosts. Also, U-¹⁴C argininewas converted to -hydroxyarginine by cut Cuscuta vines, suggestingthat -hydroxyarginine is synthesized de novo from arginine byCuscuta. (Received March 30, 1987; Accepted June 7, 1988)     

Generation of constitutive and inducible trans-sialylation dominant-negative phenotypes in Trypanosoma brucei and Trypanosoma cruzi

Trans-sialylation is a unique enzymatic process that is restrictedto some trypanosome species. By expressing developmentally regulatedtrans-sialidases, these protozoan parasites cleave sialic acidsfrom host glycoconjugates and transfer them to acceptors ontheir own cell surfaces. The biological function of this processis not understood, but trans-sialylation is expected to be importantin the invasion of mammalian cells by Trypanosoma cruzi andthe survival of Trypanosoma brucei within its insect vector.Since a conventional gene knockout approach was precluded, wedeveloped a dominant-negative strategy, in which fusion proteinsconsisting of a bacterial sialidase and trypanosome proteinswere expressed in T.brucei and T.cruzi. The strong recombinantsialidase activity shifted the reaction equilibrium from sialicacid transfer to hydrolysis, in this way creating a sialic-acid-negativephenotype. Taking advantage of a recently introduced inducibleexpression system, we were able to control the expression ofsialidase fusion proteins in T.brucez. Reversion of the sialic-acid-negativestate to wild-type sialylation was accomplished by selectiveinhibition of the foreign sialidase, leaving the parasite trans-sialidaseunaffected. Both desialylation and resialylation of trypanosomeswas rapidly achieved. Our results show that neither T.bruceinor T.cruzi require sialic acids for survival in vitro, rulingout the involvement of sialylation in cell surface integrity.The versatile system introduced here will allow a detailed invivo study of the role of trans-sialylation during the trypanosomeinfection cycle. Furthermore, cell-surface sialic acids areimplicated in a multitude of (patho-) biochemical processesin other organisms. The quantitative and qualitative manipulationof cell surface sialic acids, by expressing of counteractingenzymes, constitutes a novel approach with potentially broadapplications in glycobiology. sialidase trans-sialidase sialic acids PARP procyclin dominant-negative phenotype     

Chemical Composition of Viscin Mucilage from Three Mistletoe Species--A comparison

The viscin of mistletoes is a unique tissue which serves toprovide strong adherence of the seed of the parasite to thehost branch. The viscin mucilage of three species, Phoradendroncalifornicum and Arceuthobium americanum (Viscaceae) and Phthirusapyrifolia (Loranthaceae), was analysed. The major componentof the mucilage of Pho. californicum and A. americanum was polysaccharidic;of this, neutral sugars comprised the largest portion, but substantialamounts of uronic acids and proteins were also present. Xyloseand arabinose were the most abundant of the neutral sugars,and analysis showed glycine to be the most abundant amino acidas well as the presence of higher-than-average amounts of histidine.In contrast, in the mucilage of Pht. pyrifolia glucose was themost abundant neutral sugar and the protein content was substantiallyhigher. Based on the differences in composition observed amongthe different species the future use of viscin components astaxonomic markers is suggested. Viscin mucilage, mistletoes, Arceuthobium americanum, Phthirusa pyrifolia, Phoradendron californicum, g.l.c., amino acid analysis     

Crocodilians and Their Helminth Parasites: Macroevolutionary Considerations

Crocodilian relationships supported by the phylogenetic relationshipsof digenean and nematode parasites are compared with currentestimates of crocodilian phylogeny. The parasite data support(1) the placement of Gavialis as the sister-group of the alligatoridsand the crocodylids, (2) the monophyly of alligators and caimans,(3) the placement of Caiman (as a monophyletic group) as thesister-group of Melanosuchus plus Paleosuchus, and (4) ancientorigins of Crocodylus consistent with patterns of continentaldrift. The parasite data do not support the monophyly of Crocodylus,but the "misplaced" species (C. palustris and Osteolaemus) havehad few parasites reported from them. There is evidence of widespreadhost-switching, but most of the ambiguity appears to resultfrom uneven representation of parasite groups in host species.This is probably due both to uneven sampling by parasitologistsand to parasite extinctions associated with crocodilian extinctions.     

The Initiation of the Secondary Haustorium in Alectra vogelii Benth

Anatomical studies were carried out on initiation of the secondaryhaustorium in Alectra vogelii, a root parasite of leguminouscrops in Nigeria. In both the normal and self-haustorium, theformation of the haustorial initial on the parasite root soonafter initial contact between the host and parasite roots isfollowed by the penetration of the host root by the haustorium.Specialized penetrating cells (intrusive cells) at the haustorialfront prise apart and loosen the host root cortical cells, whichlater become digested. Through the same processes, a few ofthese columnar intrusive cells at the haustorial front piercethe endodermis to make contact with the xylem of the host root.Thereafter, a true conductive bridge consisting of short, isodiametric,reticulate vessel elements is established between the parasiteand host roots through the secondary haustorium. No pholem tissuewas observed in the connection. There is a close similaritybetween the mode of initiation of the secondary haustorium ofAlectra vogelii and that previously described for its primaryhaustorium. Alectra vogelii Benth, haustorium, self-haustorium, root parasite, hemiparasitism, Vigna unguiculata, Arachis hypogaea     

Glycans as targets for monoclonal antibodies that protect rats against Trichinella spiralis

We have investigated the role of glycans on Trichinella spiralisantigens in recognition by rat monoclonal antibodies (mAbs)which protect rat pups against challenge with the parasite.In pups born to infected dams or pups passively immunized withmAbs, antibodies eliminate a challenge dose from the intestinewithin hours (‘rapid expulsion’). Because such dramaticprotection can be afforded by mAbs, we have sought to characterizethe parasite antigens they target In this report we show thatprotective antibodies were unable to bind excretory/secretory(ES) antigens de-glycosylated with trifluoromethanesulphonicacid (TFMS). In addition, oligosaccharides isolated from glycoproteinsby alkaline hydrolysis or peptlde: Nglycosidase F (PNGase F)digestion were bound by protective, but not non-protective,mAbs. Glycans affinity purified with protective mAb 9D boundto all but one protective mAb. These antibodies have been shownpreviously to bind to the surfaces of intact larvae, indicatingthat the glycan is exposed on the parasite surface. Candidateglycans that may be involved in binding protective mAbs haveunusual tri- and tetra-antennary structures with terminal tyvelosemoieties (Reason et al., Glycobiology, 4, 000-000, 1994). Coatingof the larval surface with such glycans may serve to protectthe parasite and its secreted products from enzymatic attackas the parasite travels to and resides in its epithelial niche. glycans protective antibodies Trichinella spiralis     

Water Relations of the Mistletoe Amyema fitzgeraldii and its Host Acacia acuminata

Water relations of the mistletoe Amyema fitzgeraldii and itshost Acacia acuminata were studied near Geraldton, Western Australia.Transpiration rates of host and parasite under unstressed winterconditions varied more than 300–fold between day and nightwhile leaf water potential gradients between the partners remainedwithin the range 0·4–0·6 MPa. Plots of transpirationagainst leaf water potential indicated closely similar fluidphase resistances in host and parasite during daytime but divergentbehaviour at night due to an apparently large increase in resistanceof the haustorial junction between the partners. Data for summerand winter studies across a full range of light intensitiesshowed the parasite to transpire, on average, 1·4 timesfaster and to exhibit noticeably lower water use efficienciesthan its host. Experiments following restorative changes atnight in leaf water potentials of host and parasite on detachedhost branches supplied through their cut ends with water indicatedthat the haustorium offered a major resistance to water uptakeby the parasite. Restoration of leaf water potentials by theparasite lagged markedly behind that of the host, especiallyduring winter, leading to a rapid build up in potential gradientbetween partners. A phase of rapid flow into the parasite thenfollowed, presumably motivated by lowering of the haustorialresistance. Reversal of the potential gradient between hostand parasite was recorded in a night-time study involving abagged (non-transpiring) mistletoe attached to a host branchfrozen at the base to prevent further water uptake. Mechanismsare proposed to account for the apparently highly variable natureof the resistance of the haustorium. Key words: Mistletoe, transpiration, haustorial resistance, Amyema fitzgeraldii, Acacia acuminata     

The Regulation of the Water Potential Gradient in the Host and Parasite Relationship betweenSorghum bicolorandStriga hermonthica

A glasshouse experiment was carried out to investigate the factorscontrolling the abstraction of xylem fluid from its host bythe parasiteStriga hermonthica(Scrophulariaceae).Strigahad amean daily transpiration rate far exceeding that of its hostsorghum (Sorghum bicolor), with infestation byStrigaalso shownto lower the transpiration rate of the host. Stopping the host'stranspiration was shown to decrease the transpiration rate ofthe parasite. Stopping the parasite's transpiration only gavean initial increase in the host's transpiration rate which wasnot sustained. The parasite had a lower water potential thanits host, values being -0.42 MPa and -0.23 MPa, respectively,and an accompanying higher osmotic pressure of 0.68 MPa against0.51 MPa for sorghum. Modifying the water potential gradientby bagging both partners together showed that the differentialin osmotic pressure and water potential was largely maintainedby the parasite's higher rate of transpiration. A favourablewater potential gradient towards the parasite still existedfollowing the cessation of transpiration, this being generatedby the haustorial resistance to hydraulic conductivity whichwas found to be some 1.5–4.5 times greater than that offeredby the parasite shoot. Both the high rate of transpiration andthe increased resistance across the haustoria would appear tobe necessary means to facilitate the diversion of host resourcesto the parasite.Copyright 1997 Annals of Botany Company Striga hermonthica; sorghum; water relations; haustorium; root parasite     

Pectolytic Activity by the Haustorium of the Parasitic PlantOrobancheL. (Orobanchaceae) in Host Roots

The possible involvement of enzymes in the penetration of intrusivecells of the parasitic angiospermOrobancheinto host root tissueswas studied using cytochemical and immunocytochemical methods.Pectin methyl esterase (PME) was detected, with specific antibodies,in the cytoplasm and cell walls ofOrobancheintrusive cells andin adjacent host apoplast. Depletion and chemical changes ofpectins in host cell walls were shown by histochemical stainingwith PATAg, which detects carbohydrates that are sensitive toperiodic acid, especially pectins, and with the monoclonal antibodiesJIM 5 and JIM 7 that label pectins with low and high rates ofesterification, respectively. Galacturonic sequences with lowrates of esterification were more abundant in host cell wallsadjacent to the parasite, which is consistent with pectin de-methylationby PME release from the parasite. Pectins were absent in middlelamellae and in host cell walls neighbouring mature intrusivecells of the parasite, consistent with further degradation ofpectins by other enzymes. These results provide the first directevidence for the presence and activity of a pectolytic enzymein the infection zone of the haustorium of a parasitic angiosperminsitu.Copyright 1998 Annals of Botany Company Broomrape;Orobanche; parasitic weed; haustorium; pectin methyl esterase; pectin; cell wall.     

Cell Wall Degrading Enzymes in Cuscuta reflexa and Its Hosts

Pectin degrading enzymes, hemicellulose degrading enzyme andcellulose degrading enzymes were studied in Cuscuta reflexaRoxb., its susceptible hosts, Brassica campestris L., Cocciniaindica W. & A. Datura innoxia Mill, Helianthus annuus L.,Holoptelea indica Planch, Lantana camara L., Medicago sativaL., Manihot utilissima Pohl, Petunia hybrida X Hort exvilm,Pisum sativum L., Phaseolus vulgaris L. and Solanum nigrum L.and non-susceptible plants Ipomoea batata Lam. and Solanum tuberosumL. Pectin esterase and polygalacturonase were present in higheramounts in Cuscuta parasitic on P. vulgaris and S. nigrum, whichneeded more time for haustorial establishment. Exo-l, 4-ß-D-glucosidaseactivity was found in Cuscuta but could not be detected in itshosts. Xylanase and cellulase activity of host plants increasedwhile cellobiase activity decreased as a result of infectionby the parasite. Higher pectin esterase, polygalacturonase,xylanase and exo-l, 4-ß-D-glucosidase activities inthe haustorial region of the parasite is likely to bring aboutthe lysis of the cell wall of the host plant and thus facilitatethe penetration of the parasite haustoria into the host sieveelement, which is necessary for the transport of nutrients betweenthe host and the parasite. Key words: Cell wall degrading enzymes, Cuscuta reflexa     

Does legume nitrogen fixation underpin host quality for the hemiparasitic plant Rhinanthus minor?

The high quality of leguminous hosts for the parasitic plantRhinanthus minor (in terms of growth and fecundity), comparedwith forbs (non-leguminous dicots) has long been assumed tobe a function of the legume's ability to fix atmospheric nitrogen(N) from the air and the potential for direct transfer of compatibleamino compounds to the parasite. Using associations betweenRhinanthus minor and Vicia faba (Fabaceae) that receive N eitherexclusively via symbiotic associations with rhizobia supplyingorganic N fixed from N₂ or exclusively through the supply ofinorganic nitrate to the substrate, the underlying reasons forthe quality of legumes as hosts for this parasite are unravelled.It is shown that sole dependence of the host, V. faba, on Nfixation results in lower growth of the attached parasite thanwhen the host is grown in a substrate supplied exclusively withinorganic N. In contrast, the host plants themselves achieveda similar biomass irrespective of their N source. The physiologicalbasis for this is investigated in terms of N and abscisic acid(ABA) partitioning, haustorial penetration, and xylem sap aminoacid profiles. It is concluded that legume N fixation does notunderpin the quality of legumes as hosts for Rhinanthus butrather the well-developed haustorium formed by the parasite,coupled with the lack of defensive response of the host tissuesto the invading haustorium and the presence of sufficient nitrogenouscompounds in the xylem sap accessible to the parasite haustoria,would appear to be the primary factors influencing host qualityof the legumes. Key words: ABA, haustorium, legume, nitrogen fixation, nodules, parasitic plant Received 14 November 2007; Revised 7 January 2008 Accepted 8 January 2008     

Elevated spring testosterone increases parasite intensity in male red grouse

The expression of testosterone-dependent sexual traits mightsignal the ability of their bearers to cope with parasite infections.According to the immunocompetence handicap hypothesis (IHH),such signals would be honest because physiological costs oftestosterone, such as a reduced ability to control parasiteinfections, would prevent cheating. We tested whether testosteronewould affect the outcome of a standardized parasite challengein red grouse, using a main parasite of the species, the nematodeTrichostrongylus tenuis. We caught males in spring, removedtheir nematode parasites, and implanted them with testosteroneor empty implants, as controls. After 1 month, they were reinfectedwith a standard dose of infective T. tenuis parasites. Whenchallenged, testosterone males had relatively less globulinrelative to albumin plasma proteins than control males, an indicationthat they had experienced increased physiological stress. Testosterone-treatedmales had significantly more T. tenuis parasites than controlsin the next autumn and also had more coccidia and lost moreweight than controls. Testosterone-treated males neverthelessbenefited from their elevated spring testosterone: they hadbigger sexual ornaments than controls both in spring and autumn,and they tended to have a higher pairing and breeding successthan controls. Our results supported the IHH in showing thatelevated testosterone impaired the ability of males to copewith a standardized challenge by a dominant parasite. Testosteronethus plays a key role in mediating trade-offs between reproductiveactivities and parasite defense, and testosterone-dependentcomb size might honestly signal the ability of red grouse tocontrol T. tenuis infection.     

Growth and partitioning of C and fixed N in the shrub legume Acacia littorea in the presence or absence of the root hemiparasite Olax phyllanthi

Nodulated plants of Acacia littorea were pot cultured singlyin minus nitrogen sand culture in the presence or absence ofa transplanted seedling of the root hemiparasite Olax phyllanthiand harvests of cultures made 4 and 8 months after introducingthe parasite. Parasitism decreased host shoot growth while increasingroot growth to a similar extent. Final shoot:root dry weightratio was 2.2 for parasitized versus 4.3 for unparasitized Acacia.Partitioning of fixed N showed 4-fold larger N increments inshoots than roots of unparasitized plants, whereas parasitizedplants lost a small amount of shoot N, made a root gain of Ndouble that of unparasitized plants and lost over half of theirN to Olax. The increment of fixed N in the host:parasite associationwas similar to that of unparasitized Acacia. Data on dry mattergain per unit foliage area and mean CO₂ assimilation rates pershoot of Olax and Acacia (parasitized or unparasitized) werediscussed in relation to an estimated heterotrophic gain ofxylem C from the host equivalent to 40% of the increment ofdry matter C made by the parasite. Growth of Olax was accompaniedby large increases in numbers of haustoria, 9% of which wereattached to root nodules as opposed to roots. Structural andnutritional features of direct parasitism of nodules are described.Models of flow and utilization of C and N in the Acacia:Olaxassociation and unparasitized Acacia are discussed in relationto published data for other host:parasite associations. Key words: Olax phyllanthi, host-parasite relationships, C and N partitioning, Acacia, N₂ fixation     

Infection of the diatom Asterionella by a chytrid. II. Effects of light on survival and epidemic development of the parasite

A model is formulated to investigate the ability of chytridparasites to survive or become epidemic within populations oftheir algal hosts The model is used for an analysis of the effectsof light on the occurrence of Rhizophydium planktonicum Canteremend., a chytrid parasite of the freshwater diatom Asterionellaformosa Hass., using the information on the growth parametersof host and parasite presented in the first part of this article(J. Plankton Res ., 13, 103–117). According to the model,conditions for survival of the parasite are optimal when thehost grows at saturating light conditions. Under limiting lightconditions, Rhizophydium needs higher host densities in orderto maintain itself. The parasite is not able to survive prolongedperiods of severe light limitation of the host Epidemic development,however, turned out to be facilitated by a moderate light limitationof the host. Both light saturation and severe light limitationhamper epidemic development, but in the first case, epidemicdevelopment is still possible at sufficiently high host densities.     

Parasites and Bright Male Plumage in the Satin Bowerbird (Ptilonorhynchus violaceus)

Results from our field studies of the satin bowerbird (Ptilonorhynchusviolaceus) suggest that females choose males as mates basedon their level of infection with the ectoparasite (Myrsideaptilonorhynchi: Menoponidae). We evaluated predictions fromthree hypotheses for why this pattern of choice might evolve.The bright male and correlated infection models both suggestthat females choose parasite free males because these malesare more likely to sire parasite resistant offspring. The brightmale hypothesis suggests that females are able to gauge infectionbased on plumage brightness. The correlated infection hypothesisclaims that females assess resistance to endoparasites througha correlated effect on ectoparasites. In the parasite avoidancemodel female choice is shaped by the proximate benefits of avoidinginfection. Six predictions from these models were tested usinginformation on patterns of infection in satin bowerbirds. Ofthese models the parasite avoidance model was best supportedby the available data.     

Photosynthesis in the Tapinanthus-Diplorhynchus Mistletoe-Host Relationship

Hill reaction activity and gas exchange were studied in thesouthern African mistletoe Tapinanthus vittatus, its host treeDiplorhynchus condylocarpon and uninfected trees of the samespecies. Photosynthetic potential was assessed by measuringHill reaction activities in isolated thylakoids: reaction ratesin the parasite were less than half those of the host or uninfectedtrees. In addition, parasite thylakoids were more sensitiveto heat treatment than those of the trees. Mean maximum CO₂assimilation rates (±s.e.) were significantly lower (4·2±0·24µmol m^-1 s^-1) in the parasite, compared to both uninfectedtrees (7·6±0·22), and those of the infectedhost (5·2±0·27). Chlorophyll contents (±s.e.)were 800±40 mg m^-2 in the parasite, lower in uninfectedtrees (490±80) and lowest in infected trees (310±70).CO₂ assimilation rates calculated on a per unit chlorophyllbasis were similar in parasitized and non-parasitized trees,58±4·3 and 56±2·2 µmol mgchlorophyll^-1 h^-1 respectively, whereas the parasite's maximumCO₂ assimilation rate was considerably lower (20±1·2).Parasite transpiration rates were approximately double thoseof infected trees throughout the daylight period, and somewhathigher than uninfected trees from mid-morning onwards. Overall,parasite were generally less photosynthetically active thanhost plants; additionally, parasite infection was seen to reducehost carbon assimilation rates compared to those of uninfectedtrees.Copyright 1993, 1999 Academic Press Tapinanthus vittatus, Diplorhynchus condylocarpon, mistletoe, photosynthesis, Hill reaction, Zimbabwe     

Stage-specific manipulation of a mosquito's host-seeking behavior by the malaria parasite Plasmodium gallinaceum

We present experimental evidence that different stages of themalaria parasite Plasmodium gallinaceum differentially affectthe host-seeking behavior of its mosquito vector Aedes aegypti.In uninfected mosquitoes, host-seeking behavior is continuedif mosquitoes have ingested less than a threshold volume ofblood, whereas a larger blood meal inhibits host seeking. Weinvestigated the parasite's effect on this behavior by feedinginfected and uninfected mosquitoes for variable amounts of timeand assaying 30-45 min later whether they continued their attemptsat blood-feeding. Mosquitoes infected with oocysts (which cannotbe transmitted) had a smaller threshold volume and were lesslikely to return for further probing, whereas individuals infectedwith transmissible sporozoites increased the threshold volumerequired to inhibit host-seeking behavior. We conclude thatthe stage-specific effect of the parasite on host-seeking behavioris likely to be an active manipulation by the parasite to increaseits transmission success.     

The parasitic angiosperm Striga hermonthica can reduce photosynthesis of its sorghum and maize hosts in the field

Two cultivars of sorghum (CK60 and Ochuti) and one cultivarof maize (H511) were grown in field plots in western Kenya inthe presence or absence of the parasitic angiosperm Striga hermonthica,with or without a single addition of nitrogen fertilizer (150kg N ha^–1) using a factorial design. A progressive declinein rates of photosynthesis of Striga-infected plants were observedfor the sorghum cultivar CK60 from 30 d after planting (DAP)and for maize from 40 DAP, until measurements ended 63 DAP.At this time photosynthetic rates were 46% and 31% lower inthe Striga-infected sorghum and maize cultivars, respectively,compared to uninfected control plants. No decline in photosynthesiswas observed in the second sorghum cultivar studied, Ochuti,a local land race reported to show some tolerance to the parasite.The trends in photosynthesis reflected stunting of the cereals,as determined by the height of the youngest emerged ligule,however, only the grain yield of the sorghum cultivar CK60 wassignificantly reduced by the presence of the parasite. The nitrogenapplication influenced neither the growth nor the photosyntheticparameters measured, and possible explanations for the absenceof responses are discussed. It is concluded that S. hermonthicacan reduce photosynthetic rates of field-grown sorghum and maize,and suggest that an ability to maintain high rates of photosynthesiswhilst infected may be an important correlate of tolerance tothe parasite. Key words: Parasitic angiosperm, photosynthesis, nitrogen, tropical weeds, tropical agriculture