Search

Eigenlijk is een bol gewoon een plant met een wat eigenaardige vorm. De meeste processen verlopen hetzelfde. Maar waarom gaat die vermeerdering zo langzaam? En hoe kan het, dat een prachtige bol toch een slechte bloei geeft? Tijd voor meer inzicht in de fysiologie van de bloembol.

Diseases that are transmitted by arthropod vectors from animal hosts to humans – so called zoonotic vector-borne diseases – have increased in incidence in the last decades. In North America and Europe, tick-borne pathogens cause the majority of vector-borne diseases, including Lyme borreliosis and tick-borne encephalitis. The pathogens causing these diseases are transmitted by tick species within the Ixodes ricinus complex. These are generalist ticks that have a multi-year lifecycle with three active stages, larva, nymph and adult. Each stage passively waits for a vertebrate host by questing in the vegetation. Once a host is encountered these ticks feed on the host for several days sucking blood, after which they detach and moult to the next stage or lay eggs. Although these ticks spend the majority of their life in the vegetation, the availability of hosts is an important determinant of tick densities. In Europe, the Sheep tick (Ixodes ricinus) is the most important vector for tick-borne pathogens. These pathogens include Borrelia burgdorferi sensu lato (s.l.), the causative agent of Lyme borreliosis, Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis, Borrelia miyamotoi, the causative agent of acute febrile illness and Candidatus Neoehrlichia mikurensis, the causative agent of neoehrlichiosis. There are several genospecies within the B. burgdorferi s.l. complex, among which B. afzelii, B. bavariensis, B. garinii, B. lusitaniae, B. spielmanii, and B. valaisiana are found in questing ticks and patients in the Netherlands. All of these pathogens are maintained and amplified by vertebrate hosts. Host species differ in their ability to transmit the different pathogens (reservoir competence), as well as in their competence for ticks. Therefore, it has been hypothesized that changes in vertebrate assemblage composition can change tick-borne pathogen dynamics and thereby tick-borne disease risk, where a decrease in host species diversity might lead to an increased disease risk, the so-called dilution effect of host species richness hypothesis. In his thesis, Tim Hofmeester describes his research on the role of different vertebrate host species in maintaining I. ricinus populations and in infecting I. ricinus larvae with different tick-borne pathogens. By performing a systematic review, Hofmeester found that for both mammals and birds, there was a positive correlation between host body mass and tick burden for the different stages. Nymphal burden was positively correlated with infection prevalence with B. burgdorferi s.l. in hosts, which was again positively correlated with the average number of larvae that got infected with B. burgdorferi s.l. while feeding on a host. He also showed that the majority of I. ricinus individuals of the three stages (larva, nymph and adult) feed on only a few vertebrate host species (rodents, thrushes and deer, respectively). Using camera traps, Hofmeester showed that the presence of deer, such as Roe deer and Red deer, was a more important determinant of I. ricinus density than the number of deer available to ticks in twenty forested areas in the Netherlands. Ixodes ricinus densities were significantly reduced after two years of excluding deer by fencing four 0.75 ha forest plots in a forest near Apeldoorn, the Netherlands. Therefore, tick-borne disease risk can be reduced by placing fences around small forested areas with a high recreational pressure. Hofmeester showed that tick burdens on rodents were higher in areas with large numbers of deer, while they were lower in areas with large numbers of carnivores. These differences in tick burden on rodents between areas were strongly correlated to the number of questing nymphs in the vegetation that were infected with tick-borne pathogens that are transmitted by rodents. This implies that changes in vertebrate assemblage can lead to cascading effects on rodent-transmitted tick-borne disease risk, via larval burden on rodents. Furthermore, Hofmeester found that the percentage of ticks infected with a specific pathogen was correlated to the number of animals in an area that could transmit this pathogen, while this percentage decreased with the number of animals that could not transmit this pathogen. These parameters were, however, not correlated to species richness, something that was expected based on the dilution effect of species richness hypothesis. Therefore, there is no support for a dilution effect of species richness on tick-borne pathogens in the Netherlands. In his synthesis, Hofmeester presents a mathematical model in which the importance of spatial behaviour of hosts for tick-borne pathogens is shown and he proclaims the need for the integration of the field of behavioural ecology into disease ecology to better understand the effect of changes in vertebrate assemblages on pathogen prevalence and ultimately, disease risk. The data presented in this thesis show that it is not host diversity but the presence, abundance and behaviour of specific host species that drives tick-borne pathogen dynamics (identity effect). Vertebrate species change their behaviour in the presence or absence of predators and competitors. Hofmeester shows that this, theoretically, can have a major influence on the density of infected nymphs in the vegetation. Therefore, behavioural changes of reservoir-competent hosts should be taken into account when modelling the effect of changes in vertebrate assemblage composition on tick-borne disease risk. The behaviour of vertebrate species in Europe is changing, as multiple species have adapted to human-dominated and fragmented landscapes. The adaptation of small mammals, thrushes and deer to fragmented landscapes might be one of the driving factors behind the increase in tick-borne disease incidence in Europe. A further adaptation of important host species to urbanized landscapes might be expected as these are the safest areas for vertebrate species trying to avoid predation. This might result in an increase in population density of reservoir-competent host species in urban areas with a corresponding increase in tick-borne pathogen prevalence and therefore, tick-borne disease risk. Concluding, our world is changing and as a consequence vertebrate assemblages are also changing. This may lead to changes in I. ricinus density and infection prevalence with tick-borne pathogens. From the studies presented in his thesis Hofmeester concludes that the abundance and behaviour of several host species (e.g., Bank vole, Blackbird, Red deer, Red fox, Roe deer, and Wood mouse) determines tick-borne disease risk. Therefore, studying the drivers of animal abundance and behaviour related to ticks and pathogens will be the next step in better understanding and describing tick-borne disease risk. The ecology of tick-borne pathogens is very complex and targeting vertebrate hosts for intervention strategies will be both inefficient and costly due to the intricate interplay between multiple vertebrate host species. Therefore, Hofmeester concludes that prevention of tick bites is the best way to reduce tick-borne disease incidence.

Lyme borreliosis is the most prevalent vector-borne disease in the temperate regions of the northern hemisphere. The bacteria that cause it are members of the Borrelia burgdorferi sensu lato complex, a group of spirochaetes which are transmitted by hard ticks of the Ixodes ricinus complex. In several European countries, including The Netherlands, the incidence of Lyme borreliosis has been on the rise for the last decades. The acarological risk of human infection with Borrelia burgdorferi s.l. has been defined as the density of infected questing nymphs. This definition assumes that the distribution of the various genospecies of Borrelia in Lyme borreliosis is reflected in their distribution in questing ticks; furthermore, it assumes that all Borrelia genospecies are considered equally hazardous for humans. In order to define effective intervention strategies for controlling the disease, more insight in the transmission dynamics of tick-borne pathogens, both between animals and ticks, but also from ticks to humans is needed. As part of a Dutch research programme – “Shooting the messenger” – this PhD thesis focussed on linking the transmission cycles of Lyme spirochaetes to the different clinical manifestations of Lyme borreliosis. To that end, I explored aspects of the ecology and molecular adaptations of B. burgdorferi s.l. at various scales, from complex to genospecies level. The ecological adaptations of B. burgdorferi s.l. are underpinned by a complex genomic structure and gene expression, with large genetic variation among and within the genospecies. In Chapter 3, we prove that the 5S-23S rDNA intergenic spacer (IGS) is a suitable molecular marker for identification of B. burgdorferi s.l. to genospecies level, but also to characterize the genetic diversity at intragenospecies level and to detect genetic differentiation among the subpopulations of Borrelia. Consequently, we used this marker in combination with other genetic markers, in the studies addressing the genetic diversity of Borrelia in small mammals and humans. The main transmission route of these bacteria is the interstadial one, from larvae to nymphs and from nymphs to adult ticks. Larvae of I. ricinus can become infected during a blood meal from an infected host and during a blood meal in the vicinity of an infected nymph feeding on a host, a process known as co-feeding. The infected engorged larvae then moult into infected nymphs, which can transmit the spirochaetes to new hosts. The same process is repeated in the next developmental stage – nymph to adult. Thus, the maintenance of the bacteria in enzootic cycles is dependent on various species of vertebrates and the ticks that feed on them. In order to identify the main vertebrate hosts responsible for the maintenance of B. burgdorferi in enzootic cycles, but also for feeding I. ricinus ticks, we conducted a meta-analysis on literature data (Chapter 2). Our quantification method suggests that only a few host species, which are amongst the most widespread species in the environment (rodents, thrushes and deer), feed the majority of I. ricinus individuals and that rodents infect the majority of I. ricinus larvae with B. burgdorferi s.l.. The increase in distribution and abundance of these species, could be one of the main causes for the increase in Lyme borreliosis incidence in Europe in recent decades. While at genospecies level, there is host specificity, with B. afzelii associated with rodents and B. garinii with birds, we wanted to see if the same holds true at intragenospecies level, for the various genotypes of Borrelia. Chapter 4 focuses on the rodents, which were identified in the literature meta-analysis as being the main hosts for I. ricinus larvae as well as for Borrelia afzelii. We tested the multiple niche polymorphisms hypothesis, using IGS, dbpA and ospC as molecular markers for typing B. afzelii genotypes in fed larvae collected from rodents in various areas in The Netherlands. Despite the high genetic diversity within B. afzelii, there was no difference between wood mice and bank voles in the number and types of B. afzelii haplotypes they transmit. Additionally, we compared the quantitative role of bank voles and wood mice in B. afzelii and Neoehrlichia mikurensis maintenance, another emerging tick-borne pathogen in Europe. Neoehrlichia mikurensis prevalence was positively associated with B. afzelii. Mathematical models including tick burden and infection prevalence indicated that bank voles are better amplifiers of these two bacteria than wood mice. Our study suggests that wood mice and bank voles differ in their contribution to the dynamics of B. afzelii, and possibly other TBP, in questing ticks but not in their contribution to the genetic diversity of these microorganisms. The density of the vertebrate hosts and the feeding preferences of the ticks should determine the prevalence of B. burgdorferi s.l. genospecies in questing ticks. We address this topic in Chapter 5, by testing 5,570 questing I. ricinus nymphs from 22 different areas in The Netherlands. We found an overall prevalence of 11.8% for B. burgdorferi s.l., with large and consistent variations among the various locations. As expected based on the results of Chapter 2, Borrelia afzelii was predominant (6.7 % of the questing ticks) among the B. burgdorferi s.l. genospecies. It was followed by B. garinii/B. bavariensis (1.5 %), B. valaisiana (1.2 %), and B. burgdorferi sensu stricto (0.2 %). We noticed that, over the usual range of questing ticks’ densities, the density of infected ticks is increasing with the overall density of questing ticks, and a downward trend might be observed only for questing tick densities of over 200/100 m2. This indicates that the density of questing nymphs is the main driver of the acarological risk of human exposure to B. burgdorferi s.l. We also screened for the presence of other tick-borne pathogens that have previously been detected in questing ticks in The Netherlands: Rickettsia helvetica, Anaplasma phagocytophilum, Neoehrlichia mikurensis and several Babesia spp. (Chapter 5). To test whether these pathogens might share similar enzootic cycles we looked for patterns of coinfection and seasonal dynamics of infection in questing I. ricinus nymphs. One-third of the Borrelia-positive ticks were infected with at least one other pathogen. Coinfection of B. afzelii with N. mikurensis and with Babesia spp. occurred significantly more often than single infections, indicating the existence of mutual reservoir hosts. The diversity of tick-borne pathogens detected in I. ricinus in this study and the frequency of their coinfections with B. burgdorferi s.l. underline the need to consider them when evaluating the risks of infection and subsequently the risk of disease following a tick bite. Chapter 6 addresses the pathogenicity of B. burgdorferi s.l. genospecies and genotypes for humans, using the eight multilocus sequence typing scheme housekeeping genes (MLST) and IGS as molecular markers. The frequency of the Borrelia spp. in humans is compared to the frequency in questing ticks to assess the infectivity of the various genospecies and genotypes. The fraction of STs that were isolated from human samples was significantly higher for the genospecies that are known to be maintained in enzootic cycles by mammals (B. afzelii, B. bavariensis, and spielmanii) than for genospecies that are maintained by birds (B. garinii and B. valaisiana) or lizards (B. lusitaniae). Just as in questing ticks, B. afzelii was the most prevalent Borrelia in in human Lyme borreliosis. Borrelia afzelii was associated with acrodermatitis chronica atrophicans, while B. garinii and B. bavariensis were associated with neuroborreliosis. Despite its high incidence in ticks and erythema migrans, in terms of disease burden (as measured by disability-adjusted life year), B. afzelii is of least concern for public health. Other Borrelia spirochaetes that are rarely found in questing I. ricinus ticks, such as B. bavariensis, seem to be responsible for most of the neuroborreliosis cases – a more severe clinical symptom of Lyme borreliosis. This implies that the prevalence of B. burgdorferi s.l. in questing ticks does not necessarily reflect the incidence of human Lyme borreliosis. We found six multilocus sequence types that were significantly associated with clinical manifestations in humans and five IGS haplotypes that were associated with the human Lyme borreliosis cases. While IGS could perform just as well as the housekeeping genes in the MLST scheme for predicting the infectivity of B. burgdorferi s.l., the advantage of MLST is that it can also capture the differential invasiveness of the various STs. In this thesis, I have identified the most important vertebrate hosts for maintenance of B. burgdorferi s.l. in enzootic cycles. I have also shown that their density is reflected in the prevalence of B. burgdorferi s.l. in questing ticks. The comparative study of questing ticks and Lyme borreliosis indicated that some of the Borrelia genospecies have similar prevalences in the two sources. The findings in my thesis indicate, thus, that there is a link between the density of suitable hosts for ticks and Borrelia spp., the density of infected ticks and the distribution of the B. burgdorferi s.l. genospecies in Lyme borreliosis. There are exceptions, however, that cannot be explained by this simple thread line. Such a situation is the perceived association of B. bavariensis with rodents that is not reflected by its extremely low prevalence in questing ticks. Furthermore, this low prevalence cannot explain the overrepresentation of B. bavariensis in Lyme borreliosis. As result of the study of pathogenicity of the various Borrelia genospecies and genotypes, I suggest the separate hazard assignment for the Borrelia genospecies; this, in combination with the exposure (prevalence in questing ticks) would allow for individual genospecies/genotypes risk assessment. The findings in this thesis stress the importance of both ecological and clinical studies for addressing the public health issue of Lyme borreliosis.

During the last decade azole resistance has increasingly been reported in Aspergillus fumigatus, which is a fungal pathogen involved in the vast majority of invasive aspergillosis infections in humans, and is now a global public health concern. Antifungal azoles, especially triazoles, are the drugs of choice for medical treatment. However, this treatment is hampered by the emergence of multi-azole resistant A. fumigatus isolates, especially the highly resistant variants TR34/L98H and TR46 /Y121F/T289A. Therefore, to control this disease, it is essential to elucidate by what mechanisms resistance emerges, how resistance spreads and how resistant genotypes persist in environments without azoles. The presented thesis shows the relevance of the life cycle of A. fumigatus to the development of azole resistance and possible evolutionary routes that lead to it. The work highlights the importance of fungal biology and evolution towards understanding the development of azole resistance in fungi. We conclude that azole resistance in A. fumigatus is a consequence of selection pressure by azole in the environment on the genetic variation generated via various aspects in the A. fumigatus life cycle. This thesis also introduces an experimental evolution approach to study the dynamics and mechanisms of the evolution of azole resistance. In addition, we investigate what condition can lead an environment to be a possible hotspot for the development of resistance. Finally, we link this to the potential conditions under which resistance can emerge and spread in the lungs of humans and how this depends on the specific azole used.

Soil is one of the most important natural resource for life on Earth and provides important ecosystem services, such as food production, carbon sequestration, water regulation and contaminant attenuation. Soil quality, defined as the soil’s ability to provide these services, is drastically reduced in many locations and regions worldwide due to human activities. This loss in soil quality ultimately leads to soil degradation, erosion and desertification, imposing a severe and increasing risk for the growing human population. It is therefore essential that we are aware of the importance of protecting soil, and at the same time understand processes that build up and regenerate soil. The key objective of the present study was to obtain a better understanding of soil food web structure and functioning, and how these develop in stages along the soil life cycle. Using field surveys, I investigated the soil food web structure and functioning in different sites along the soil life cycle, including soils developing in glacial chronosequences, productive soils under different land use and management, and soils under risk of degradation. The soil food web was expected to build up in biomass and structure, be highest in the intermediate soils, and decrease in soils at or nearby degradation. This was indeed the case when comparing developing soils in the chronosequences, and comparing productive soils with degrading soils. But also land use type turned out to be very important for the structure of the soil food web. Biological measures such as biomass, activity and diversity of soil organisms, especially that of soil microarthropods, were found to be indicative for soil quality in all sites. I also investigated the possible role of soil organisms in the soil ecosystem functioning, in terms of soil structure formation and C and N mineralisation. Although soil organisms are known to have an important role on soil structure formation, no clear indications of such a role were found for that function in the studied sites. However, soil microbial biomass and activity, and the biomass of other trophic groups, did play a crucial role in soil ecosystem process rates, especially the C and N mineralisation rates. In conclusion, I have found that soil food webs assemble in a directive manner: organism biomass and activity increase with soil productivity. In productive soils, land use type and land management are the main drivers affecting soil food web structure and functioning, although this effect is limited to the topsoil. Under harsh conditions, soil organisms reach a relatively low biomass and are sensitive to aspects of intensive agricultural land use.

De Bryum knowltonii (Roodmondknikmos) en B. intermedium (Middelst knikmos) zijn twee knikmossen die op de Rode Lijst voorkomen en in Zeeuws-Vlaanderen te vinden zijn. Beide worden zowel aan de zuid- als aan de noordrand van de streek gevonden. Rienk-Jan Bijlsma tekende voor de identificatie van de twee knikmossen, die bij nadere kennismaking vrij karakteristieke kapsels blijken te hebben, althans als ze in het goede stadium zijn. Die van B. knowltonii, kort en gewoonlijk maar half voorover buigend, lijken op een pijpenkop. Bryum intermedium valt op door zijn scheef gebekte kapsels. De tweede editie van de Prodromus (Abeleven 1893) geeft voor tal van mossen een uitvoerige opsomming van standplaatsen, zo ook voor Bryum intermedium.

The effect of the dioxin-like PCB 126 (3,3¿,4,4¿,5-pentachlorobiphenyl) on the early development of the marine flatfish sole (Solea solea) was tested in a newly developed early life stage (ELS) test that includes the metamorphosis of the symmetric larvae into an asymmetrical flatfish. Early life stages of sole were exposed to a concentration series of PCB 126 in seawater until 4, 8, 10 and 15 days post fertilisation (dpf). Subsequently the development of the larvae was registered under further unexposed conditions. The LC50s at the start of the free-feeding stage (12 dpf) ranged between 39 and 83 ng PCB 126/l depending on exposure duration. After the fish had completed the metamorphosis, the LC50 values ranged between 1.7 and 3.7 ng PCB 126/l for the groups exposed for 4, 8 and 10 dpf, respectively. Thus exposure for only 4 days, covering only the egg stage, was sufficient to cause adverse effects during a critical developmental phase two weeks later. The internal dosages of these larvae, determined by means of an in vitro gene reporter assay as dioxin-equivalent values (TEQ), revealed a LD50 of 1 ng TEQ/g lipid, which is within the same order of magnitude as TEQ levels found in fish from highly polluted areas. This study indicates that ELS fish tests that are terminated shortly after the fish becomes free-feeding, underestimate the toxic potential of compounds with low acute toxicity such as PCBs. Our prolonged ELS with this native marine flatfish suggests that reproductive success of fish populations at contaminated sites can be affected De standaardprocedure die moet vaststellen hoe giftig bepaalde stoffen voor vislarfjes zijn, heeft een blinde vlek. Dat ontdekten Wageningse biologen door proeven met pasgeboren tongetjes. Hun onderzoek geeft waarschijnlijk een aanvullende verklaring voor de achteruitgang van de visstand

Eigenlijk is een bol gewoon een plant met een wat eigenaardige vorm. De meeste processen verlopen hetzelfde. Maar waarom gaat die vermeerdering zo langzaam? En hoe kan het, dat een prachtige bol toch een slechte bloei geeft? Tijd voor meer inzicht in de fysiologie van de bloembol.

Agapetus fuscipes is a caddisfly that only seems to occur in unimpacted streams and therefore can be a suitable indicator species for natural conditions. The species has decreased in the Netherlands because of human activities which caused organic pollution and hydromorphological degradation. Literature was reviewed to study the autecology and life cycle of A. fuscipes in order to reveai the ecological requirements of this species. By taking Agapetus fuscipes as an example, it is shown that the autecology and life cycle of an indicator species can give important clues for its presence in unimpacted and absence in impacted streams Agapetus fuscipes is a caddisfly that only seems to occur in unimpacted streams and therefore can be a suitable indicator species for natural conditions. The species has decreased in the Netherlands because of human activities which caused organic pollution and hydromorphological degradation. Literature was reviewed to study the autecology and life cycle of A. fuscipes in order to reveal the ecological requirements of this species. By taking Agapetus fuscipes as an example, it is shown that the autecology and life cycle of an indicator species can give important clues for its presence in unimpacted and absence in impacted streams. A. fuscipes is very susceptible to organic pollution and to a lesser degree to discharge dynamics (dropping water level and discharge peaks). The species copes with dynamic discharge events by maintaining a high population density and recolonisation of disturbed habitats from refuges. However, the vulnerability of the species strongly depends on the life stage of the animals (e.g., the ability to migrate, the oxygen demand and the habitat requirements differ between instars). Although several adaptations to dynamic conditions, a high frequency of discharge peaks or a long period of drought can cause the population to decline. Once a population has totally disappeared from a stream it will take the species a long time to recolonise the stream because of its low dispersion capacity. To protect this species stream restoration should focus on water quality (avoid organic pollution and agricultural run off) and on stabilising the discharge by taking care of natural infiltration in the catchment area instead of fast removal of rain water by drainage systems.

Beschrijving van de ziektecyclus, symptomen en bestrijding van verticillium-verwelking van bomen. Verwelkingsziekte van bomen, veroorzaakt door de schimmel Verticillium dahliae, komt vaker voor dan gedacht wordt, en veroorzaakt schade in de boomteelt, aan laanbomen en aan oudere bomen in stad en landschap. Vooral de esdoorn is gevoelig. De schimmel infecteert zijn waardplanten via de wortel, wat leidt tot verstopte vaten, belemmering van het watertransport en droogteverschijnselen. Vaak wordt de ziekte bij bomen niet herkend; karakteristiek is dat de ziekte vaak eenzijdig aan een boom optreedt. De kansen op herstel variëren per boomsoort. Behandeling van zieke bomen is niet goed mogelijk. Onderzoekers van PRI en PPO-Boskoop werken aan resistente rassen en milieuvrindelijke methoden ter bestrijding van de schimmel in de bodem. Voorlopig blijft het voorkomen van een aantasting de enige effectieve bestrijding (schoon plantmateriaal op een schone bodem)

Opmerkelijke waarnemingen van insectenplagen op bomen en struiken in 2001: de stormachtige opmars van de paardenkastanjemineermot; een nieuwe exoot, de robiniamineermot; knopsterfte bij rhododendrons door de schimmel Pycnostysanus azaleae die overgebracht wordt door de rhododendroncicade; vraat aan lariksnaalden door rupsen van de lariksspinselbladwesp; aantasting van Weymouthdennen door rupsen van de sparappelboorder; naaldval door de groene sparrenluis; spinselmotten op verschillende soorten waardplanten (o.a. de wilgenspinselmot in de Biesbosch); een iets oplopend aantal meldingen van de eikenprocessierups. De levenscyclus van de plaaginsecten, de schadebeelden en de vindplaatsen en verspreiding van de plagen worden beschreven. In een tabel de graad van aantasting per provincie in 2001 voor bladvretende, naaldvretende, zuigende, bladminerende en galvormende insecten, en houtboorders, bast- en zaadvreters. Jaarlijkse monitoring in het kader van het DWK programma functievervulling natuur, bos en landschap. Correctie op tabel 'De graad van insectenaantasting per provincie in 2001' in Vakblad Natuurbeheer 41(2002)8:139

Bij een besmetting met stengelaaltjes (Ditylenchus dipsaci) in bolgewassen is de economische schade groot, omdat het een quarantaine-organisme betreft. Binnen deze aaltjessoort zijn verschillende rassen te onderscheiden, met elk een andere waardplantenreeks. Het tulpenstengelaaltje heeft de breedste waardplantenreeks en is daarmee ook het meest gevreesde ras. Om onderzoek te doen naar de overleving, verspreiding en bestrijding van stengelaaltjes, zijn technieken nodig voor rassenidentificatie, in stand houding en vermeerdering van de aaltjes. Het ontbreken van geschikte technieken vormt een grote belemmering voor het onderzoek. Dit project heeft nieuwe methoden opgeleverd voor de bewaring en detectie van (tulpen-)stengelaaltjes.

Bij een besmetting met stengelaaltjes (Ditylenchus dipsaci) in bolgewassen is de economische schade groot, omdat het een quarantaine-organisme betreft. Binnen deze aaltjessoort zijn verschillende rassen te onderscheiden, met elk een andere waardplantenreeks. Het tulpenstengelaaltje heeft de breedste waardplantenreeks en is daarmee ook het meest gevreesde ras. Om onderzoek te doen naar de overleving, verspreiding en bestrijding van stengelaaltjes, zijn technieken nodig voor rassenidentificatie, in stand houding en vermeerdering van de aaltjes. Het ontbreken van geschikte technieken vormt een grote belemmering voor het onderzoek. Dit project heeft nieuwe methoden opgeleverd voor de bewaring en detectie van (tulpen-)stengelaaltjes.

Mature bream (>30 cm LF) dominated the fish communities in floodplain lakes along the lower River Rhine in The Netherlands, in terms of biomass, in all lakes at all times. In some lakes bream made up 99 f the total fish biomass. While immature, medium-sized bream (10–30 cm) and white bream and roach were abundant in the main channel of the river, these species were almost absent from the floodplain lakes. Inundation of the floodplains did not lead to spatial homogenization of species and length distributions. Small bream (

Over schimmels en insecten, als belagers van grassen en golfvelden, is relatief veel bekend. Minder bekend zijn nematoden ofwel aaltjes. Vanuit met name Ierland en het Verenigd Koninkrijk, golflanden bij uitstek, worden de laatste jaren regelmatig problemen gemeld met slecht groeiend gras op golfbanen veroorzaakt door nematoden. Vooral de greens laten veel schade zien.

Een overzicht van de stand van zaken m.b.t. de Aal (Paling; Anguilla anguilla) en de aalvisserij in Europa: levenscyclus en biologie; verspreiding van de aal en de aalvisserij; langjarige trends in de populatiedichtheid en het bestand, de intrek van glasaal en de vangsten. Criteria en mogelijke maatregelen voor een duurzame aalvisserij worden besproken

LCA (levenscyclus-analyse) is een methode om de milieu-effecten van productie en gebruik van een object in kaart te brengen. In het artikel wordt getracht het landgebruik van een activiteit en het daarmee gepaard gaande verlies aan biodiversiteit te kwantificeren en mee te nemen in LCA-studies.Een en ander op mondiaal niveau en uitgewerkt voor verschillende soorten menselijke activiteiten (o.a. zandwinning in Europees landbouwgebied; bauxietwinning in Zuid-amerikaans tropisch bos)

Op 15 juni 2004 promoveerde Jessica Vereijssen aan Wageningen Universiteit op haar proefschrift getiteld: 'Cercospora leaf spot in sugar beet: Epidemiology, life cycle components and disease management'. De promotor was prof. dr. MJ Jeger en co-promotoren waren dr. ir. JHM Schneider (IRS (Instituut voor Rationele Suikerproduktie)) en dr. ir. AJ Termorshuizen (Biologische Bedrijfssystemen, WU)

Veel volken van de honingbij overleven tegenwoordig de winter niet. Allerlei mogelijke oorzaken passeren de revue, waaronder de straling van het mobiele telefonie netwerk. Daar is weinig onder- zoek aan gedaan, maar het is bekend dat bijen magnetische en elektromagnetische velden kunnen detecteren en er op kunnen reageren. Het kan dus niet op voorhand worden uitgesloten dat bijen erdoor beïnvloed worden. Doelstelling van het onderzoek is om een relatie te kunnen leggen tussen aan de ene kant de dosis EMV die bijen in hun larvale en pupale stadium ondergaan en aan de andere kant hun ont- wikkeling en enkele kenmerken in hun volwassen leven (levensduur, vliegprestaties en morfolo- gische en fysiologische kenmerken). Ook wordt de ontwikkeling van het bijenvolk door het jaar heen (die een resultante is van de larvale, de pupale en de volwassen ontwikkeling) gemonitord.

Veel volken van de honingbij overleven tegenwoordig de winter niet. Allerlei mogelijke oorzaken passeren de revue, waaronder de straling van het mobiele telefonie netwerk. Daar is weinig onder- zoek aan gedaan, maar het is bekend dat bijen magnetische en elektromagnetische velden kunnen detecteren en er op kunnen reageren. Het kan dus niet op voorhand worden uitgesloten dat bijen erdoor beïnvloed worden. Doelstelling van het onderzoek is om een relatie te kunnen leggen tussen aan de ene kant de dosis EMV die bijen in hun larvale en pupale stadium ondergaan en aan de andere kant hun ont- wikkeling en enkele kenmerken in hun volwassen leven (levensduur, vliegprestaties en morfolo- gische en fysiologische kenmerken). Ook wordt de ontwikkeling van het bijenvolk door het jaar heen (die een resultante is van de larvale, de pupale en de volwassen ontwikkeling) gemonitord.

Wortelknobbelaaltjes (Meloidogyne spp.) behoren tot de meest geavanceerde parasieten onder de ziekteverwekkers van planten. Ze onderscheiden zich van veel obligaat biotrofe pathogenen door hun brede waardplantenreeks. Eenmaal gevestigd in het wortelstelsel onttrekken ze vervolgens gedurende enkele weken voedingsstoffen aan de plant via speciaal daarvoor aangelegde voedingsstructuren - de reuzecellen. Ze kunnen zich al naar gelang de omstandigheden dat vereisen, op allerlei manieren voortplanten.De laatste jaren zijn door de toepassing van moleculaire technieken een aantal bijzondere ontdekkingen gedaan die betrekking hebben op deze aanpassingen. Ingegaan wordt op de ontwikkelingen met nadruk op de genen, die betrokken zijn bij de parasitaire levensstijl van de nematode

The life cycle of most plants starts, and ends, at the seed stage. In most species mature seeds are shed and dispersed on the ground. At this stage of its life cycle the seed may be dormant and will, by definition, not germinate under favourable conditions (Bewley, 1997). Seasonal dormancy cycling is a characteristic found in plant seeds. Being able to cycle in and out of dormancy allows the seed to survive decades or even centuries, allowing germination to be spread over time, but only when optimal conditions are available, not only for germination but especially for seedling establishment. In this thesis we have attempted to further elucidate the mechanisms behind dormancy, germination and dormancy cycling. Sisymbrium officinale seeds need nitrate and light to start germination (Chapter 2, 3, 4, 6). Nitrate acts in part by reducing the abscisic acid (ABA) levels (a plant hormone that elevates dormancy levels). The action of light and nitrate can also be reached by applying gibberellins (GAs) to the seeds (Chapter 2, 3, 4, 6). GAs are capable of inducing enzymes that hydrolyze the ensdosperm walls (Debeaujon and Koornneef, 2000; Chen and Bradford, 2000; Nonogaki et al., 2000; Manz et al., 2005) In this way GAs could be involved in lowering the physical restrictions imposed by the resistance of the seed coat and the endosperm. On the other hand, GAs may also increase the embryo growth potential. For successful survival of the dormant seed, metabolic activity is reduced to avoid rapid depletion of reserves. The metabolic activity of the seed was measured using electron paramagnetic resonance (EPR), with TEMPONE as a spin probe, and the respiratory activity was measured with the Q2-test (Chapter 2).We showed that primary dormancy was accompanied by hardly any metabolic or respiratory activity, and this increased considerably when dormancy was broken by nitrate. However, when the light pulse was not given and the seeds had become secondary dormant the metabolic activity slowed down. Regulation of dormancy is tightly linked with abiotic stress factors from the environment. The regulation and survival of the seed under stress conditions is largely dependent on the composition of the cytoplasm. We tested this by EPR, using carboxyl-proxyl (CP) spin probe (Chapter 4). The primary dormant and sub-dormant seeds possessed a higher viscosity than the germinating seeds. The viscosity of secondary dormant seeds appeared intermediate; however, the ease at which the vitrified water melted was similar to that of primary dormant seeds. As a result of the differences in viscosity, the temperature of vitrified water melting differed between the different dormancy states. The changes in cytoplasmic viscosity and vitrified water melting may be linked to changes in metabolism and the content of high molecular weight compounds. As membranes are the primary target for temperature perception, they are often implicated in regulating dormancy. Therefore, Hilhorst (1998) put forward a hypothesis in which changes in responsiveness to dormancy breaking factors like nitrate and light was a function of cellular membrane fluidity. In Chapter 3 we indeed showed that dormancy is a function of membrane fluidity. Primary dormant seeds of Sisymbrium officinale appeared to have very rigid membranes, whereas breaking dormancy increased membrane fluidity considerably. However, when sub-dormant seeds became secondary dormant membrane fluidity decreased again, but not to the rigidity seen in primary dormant seeds. One of the most common ways in which cells control membrane fluidity is by homeoviscous adaptation with the help of desaturases. Desaturase involvement in changes in membrane fluidity due to changes in dormancy was tested in Chapter 3 (using Sisymbrium officinale) and Chapter 5 (using Arabidopsis thaliana). Here we found that although desaturase activity may change the membrane fluidity or influence the germination/dormancy phenotype, the two are not linked, unless the effects of these enzymes are very local within the seed. Finally, in Chapter 7, we presented a new model in which a membrane anchored dormancy related protein/transcription factor is activated by changes in membrane fluidity. The activated form is transported to the nucleus, where it starts the germination process, which includes changes in metabolism and mobilization of storage reserves.

The life cycle of most plants starts, and ends, at the seed stage. In most species mature seeds are shed and dispersed on the ground. At this stage of its life cycle the seed may be dormant and will, by definition, not germinate under favourable conditions (Bewley, 1997). Seasonal dormancy cycling is a characteristic found in plant seeds. Being able to cycle in and out of dormancy allows the seed to survive decades or even centuries, allowing germination to be spread over time, but only when optimal conditions are available, not only for germination but especially for seedling establishment. In this thesis we have attempted to further elucidate the mechanisms behind dormancy, germination and dormancy cycling. Sisymbrium officinale seeds need nitrate and light to start germination (Chapter 2, 3, 4, 6). Nitrate acts in part by reducing the abscisic acid (ABA) levels (a plant hormone that elevates dormancy levels). The action of light and nitrate can also be reached by applying gibberellins (GAs) to the seeds (Chapter 2, 3, 4, 6). GAs are capable of inducing enzymes that hydrolyze the ensdosperm walls (Debeaujon and Koornneef, 2000; Chen and Bradford, 2000; Nonogaki et al., 2000; Manz et al., 2005) In this way GAs could be involved in lowering the physical restrictions imposed by the resistance of the seed coat and the endosperm. On the other hand, GAs may also increase the embryo growth potential. For successful survival of the dormant seed, metabolic activity is reduced to avoid rapid depletion of reserves. The metabolic activity of the seed was measured using electron paramagnetic resonance (EPR), with TEMPONE as a spin probe, and the respiratory activity was measured with the Q2-test (Chapter 2).We showed that primary dormancy was accompanied by hardly any metabolic or respiratory activity, and this increased considerably when dormancy was broken by nitrate. However, when the light pulse was not given and the seeds had become secondary dormant the metabolic activity slowed down. Regulation of dormancy is tightly linked with abiotic stress factors from the environment. The regulation and survival of the seed under stress conditions is largely dependent on the composition of the cytoplasm. We tested this by EPR, using carboxyl-proxyl (CP) spin probe (Chapter 4). The primary dormant and sub-dormant seeds possessed a higher viscosity than the germinating seeds. The viscosity of secondary dormant seeds appeared intermediate; however, the ease at which the vitrified water melted was similar to that of primary dormant seeds. As a result of the differences in viscosity, the temperature of vitrified water melting differed between the different dormancy states. The changes in cytoplasmic viscosity and vitrified water melting may be linked to changes in metabolism and the content of high molecular weight compounds. As membranes are the primary target for temperature perception, they are often implicated in regulating dormancy. Therefore, Hilhorst (1998) put forward a hypothesis in which changes in responsiveness to dormancy breaking factors like nitrate and light was a function of cellular membrane fluidity. In Chapter 3 we indeed showed that dormancy is a function of membrane fluidity. Primary dormant seeds of Sisymbrium officinale appeared to have very rigid membranes, whereas breaking dormancy increased membrane fluidity considerably. However, when sub-dormant seeds became secondary dormant membrane fluidity decreased again, but not to the rigidity seen in primary dormant seeds. One of the most common ways in which cells control membrane fluidity is by homeoviscous adaptation with the help of desaturases. Desaturase involvement in changes in membrane fluidity due to changes in dormancy was tested in Chapter 3 (using Sisymbrium officinale) and Chapter 5 (using Arabidopsis thaliana). Here we found that although desaturase activity may change the membrane fluidity or influence the germination/dormancy phenotype, the two are not linked, unless the effects of these enzymes are very local within the seed. Finally, in Chapter 7, we presented a new model in which a membrane anchored dormancy related protein/transcription factor is activated by changes in membrane fluidity. The activated form is transported to the nucleus, where it starts the germination process, which includes changes in metabolism and mobilization of storage reserves.

De larve van de wortelvlieg, Psila rosae, kan in een aantal schermbloemige gewassen grote schade veroorzaken. Overal in Nederland waar peen wordt geteeld, komt aantasting voor. In Nederland lijken de biologische telers goed met het probleem wortelvlieg om te gaan; het aantal meldingen van schade en afkeuring valt mee. Met slimme teeltmaatregelen zijn de problemen beheersbaar en dankzij inzicht in de biologie en levenscyclus van de wortelvlieg kunnen passende maatregelen de schade beperken. In dit BioKennisbericht een overzicht en tips.

De larve van de wortelvlieg, Psila rosae, kan in een aantal schermbloemige gewassen grote schade veroorzaken. Overal in Nederland waar peen wordt geteeld, komt aantasting voor. In Nederland lijken de biologische telers goed met het probleem wortelvlieg om te gaan; het aantal meldingen van schade en afkeuring valt mee. Met slimme teeltmaatregelen zijn de problemen beheersbaar en dankzij inzicht in de biologie en levenscyclus van de wortelvlieg kunnen passende maatregelen de schade beperken. In dit BioKennisbericht een overzicht en tips.

Phytophthora infestans is de veroorzaker van de beruchte aardappelziekte. In glasteelten wordt alleen tomaat incidenteel door deze ziekteverwekker aangetast. Maar een groot aantal gewassen in de glastuinbouw is wel gevoelig voor andere Phytophthora-soorten, zoals paprika, tomaat, aardbei, cymbidium, bloeiende potplanten, groene en bonte planten, snijhortensia, rozen en lelie.

Tripsschade komt in veel gewassen voor. In de biologische teelt zijn maar weinig middelen tegen trips opgewassen. Schade is op verschillende manieren in de hand te houden, al blijkt het in de praktijk nauwelijks mogelijk om een gewas volledig te vrijwaren. In dit BioKennisbericht leest u meer over de leefwijze van trips en de mogelijke maatregelen om schade te beperken.

Phytophthora infestans is de veroorzaker van de beruchte aardappelziekte. In glasteelten wordt alleen tomaat incidenteel door deze ziekteverwekker aangetast. Maar een groot aantal gewassen in de glastuinbouw is wel gevoelig voor andere Phytophthora-soorten, zoals paprika, tomaat, aardbei, cymbidium, bloeiende potplanten, groene en bonte planten, snijhortensia, rozen en lelie.

Tripsschade komt in veel gewassen voor. In de biologische teelt zijn maar weinig middelen tegen trips opgewassen. Schade is op verschillende manieren in de hand te houden, al blijkt het in de praktijk nauwelijks mogelijk om een gewas volledig te vrijwaren. In dit BioKennisbericht leest u meer over de leefwijze van trips en de mogelijke maatregelen om schade te beperken.

Myrothecium roridum komt veel voor bij de teelt van pot- en perkplanten, bij sierteelt in de grond zoals lisianthus en antirrhinum en soms ook bij teelten op substraat (bouvardia). De schimmel manifesteert zich op de bladeren, stengel en overgang tussen lucht en grond of substraat en verspreidt zich snel onder warme en vochtige omstandigheden via enorme aantallen sporen die gemakkelijk kiemen.

Myrothecium roridum komt veel voor bij de teelt van pot- en perkplanten, bij sierteelt in de grond zoals lisianthus en antirrhinum en soms ook bij teelten op substraat (bouvardia). De schimmel manifesteert zich op de bladeren, stengel en overgang tussen lucht en grond of substraat en verspreidt zich snel onder warme en vochtige omstandigheden via enorme aantallen sporen die gemakkelijk kiemen.