Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    A high-quality genome sequence of Rosa chinensis to elucidate ornamental traits
    Hibrand Saint-Oyant, L. ; Ruttink, T. ; Hamama, L. ; Kirov, I. ; Lakhwani, D. ; Zhou, N.N. ; Bourke, P.M. ; Daccord, N. ; Leus, L. ; Schulz, D. ; Geest, H. van de; Hesselink, T. ; Laere, K. Van; Debray, K. ; Balzergue, S. ; Thouroude, T. ; Chastellier, A. ; Jeauffre, J. ; Voisine, L. ; Gaillard, S. ; Borm, T.J.A. ; Arens, P. ; Voorrips, R.E. ; Maliepaard, C. ; Neu, E. ; Linde, M. ; Paslier, M.C. Le; Bérard, A. ; Bounon, R. ; Clotault, J. ; Choisne, N. ; Quesneville, H. ; Kawamura, K. ; Aubourg, S. ; Sakr, S. ; Smulders, M.J.M. ; Schijlen, E. ; Bucher, E. ; Debener, T. ; Riek, J. De; Foucher, F. - \ 2018
    Nature Plants 4 (2018). - ISSN 2055-026X - p. 473 - 484.

    Rose is the world’s most important ornamental plant, with economic, cultural and symbolic value. Roses are cultivated worldwide and sold as garden roses, cut flowers and potted plants. Roses are outbred and can have various ploidy levels. Our objectives were to develop a high-quality reference genome sequence for the genus Rosa by sequencing a doubled haploid, combining long and short reads, and anchoring to a high-density genetic map, and to study the genome structure and genetic basis of major ornamental traits. We produced a doubled haploid rose line (‘HapOB’) from Rosa chinensis ‘Old Blush’ and generated a rose genome assembly anchored to seven pseudo-chromosomes (512 Mb with N50 of 3.4 Mb and 564 contigs). The length of 512 Mb represents 90.1–96.1% of the estimated haploid genome size of rose. Of the assembly, 95% is contained in only 196 contigs. The anchoring was validated using high-density diploid and tetraploid genetic maps. We delineated hallmark chromosomal features, including the pericentromeric regions, through annotation of transposable element families and positioned centromeric repeats using fluorescent in situ hybridization. The rose genome displays extensive synteny with the Fragaria vesca genome, and we delineated only two major rearrangements. Genetic diversity was analysed using resequencing data of seven diploid and one tetraploid Rosa species selected from various sections of the genus. Combining genetic and genomic approaches, we identified potential genetic regulators of key ornamental traits, including prickle density and the number of flower petals. A rose APETALA2/TOE homologue is proposed to be the major regulator of petal number in rose. This reference sequence is an important resource for studying polyploidization, meiosis and developmental processes, as we demonstrated for flower and prickle development. It will also accelerate breeding through the development of molecular markers linked to traits, the identification of the genes underlying them and the exploitation of synteny across Rosaceae.

    Development of a molecular test to ensure good flowering of Viburnum Opulus Var. Roseum
    Kromwijk, J.A.M. ; Noort, F.R. van; Verhoef, N. ; Balk, P. ; Wordragen, M.F. van - \ 2013
    In: II International Symposium on Woody Ornamentals of the Temperate Zone. - Leuven : ISHS - p. 105 - 112.
    During wintertime Viburnum opulus var. roseum (snowball) shrubs are forced in warm greenhouses to harvest early cut flowers. Early forcing is occasionally unsuccessful. This is probably due to a lack of hours with low temperatures that is needed to break bud dormancy. To gain more insight about the effect of temperature on breaking dormancy and early forcing results, shrubs were transferred into cold storage in week 41, 43 and 45 (2009) for 4, 6 or 8 weeks at a temperature of 2, 5 or 8°C. After storage, the shrubs were forced at a day-/night temperature of 28/23°C. Shrubs stored for at least 8 weeks from week 41, 6 weeks from week 43 and 4 weeks from week 45 gave good forcing results, regardless of storage temperature. Since growing conditions outside vary every year and differ between nurseries, it is difficult to determine when forcing can be started. To prevent poor forcing results, we aimed to develop a molecular diagnostic assay to determine the moment at which winter dormancy of snowball flower buds is sufficiently broken. In order to identify genes involved in bud breaking dormancy, an experiment was performed on a commercial nursery from October 2010 to January 2011. Flower buds of snowball shrubs outside were sampled weekly and in December and January batches of shrubs were placed every week in a greenhouse to observe forcing results. Genes involved in bud breaking dormancy were identified by performing next generation Illumina RNA sequencing on bud samples with poor and good forcing results. A set of candidate genes was validated during a second trial at two different commercial nurseries in 2011-2012.
    Sink filling, inulin metabolizing enzymes and carbohydrate status in field grown chicory (Cichorium intybus L.).
    Arkel, J. van; Vergauwen, R. ; Sévenier, R. ; Hakkert, J.C. ; Laere, A. van; Bouwmeester, H.J. ; Meer, I.M. van der - \ 2012
    Journal of Plant Physiology 169 (2012)15. - ISSN 0176-1617 - p. 1520 - 1529.
    sucrose 1-fructosyl transferase - helianthus-tuberosus colombia - fructan metabolism - jerusalem-artichoke - fructosyl transferase - transgenic potato - mass fingerprint - sugar-beet - roots - storage
    Inulin is a fructose-based polymer that is isolated from chicory (Cichorium intybus L.) taproots. The degree of polymerization (DP) determines its application and hence the value of the crop. The DP is highly dependent on the field conditions and harvest time. Therefore, the present study was carried out with the objective to understand the regulation of inulin metabolism and the process that determines the chain length and inulin yield throughout the whole growing season. Metabolic aspects of inulin production and degradation in chicory were monitored in the field and under controlled conditions. The following characteristics were determined in taproots: concentrations of glucose, fructose and sucrose, the inulin mean polymer length (mDP), yield, gene expression and activity of enzymes involved in inulin metabolism. Inulin synthesis, catalyzed by sucrose:sucrose 1-fructosyltransferase (EC 2.4.1.99) (1-SST) and fructan:fructan 1-fructosyltransferase (EC 2.4.1.100) (1-FFT), started at the onset of taproot development. Inulin yield as a function of time followed a sigmoid curve reaching a maximum in November. Inulin reached a maximum mDP of about 15 in September, than gradually decreased. Based on the changes observed In the pattern of inulin accumulation, we defined three different phases in the growing season and analyzed product formation, enzyme activity and gene expression in these defined periods. The results were validated by performing experiments under controlled conditions in climate rooms. Our results show that the decrease in 1-SST that starts in June is not regulated by day length and temperature. From mid-September onwards, the mean degree of polymerization (mDP) decreased gradually although inulin yield still increased. The decrease in mDP combined with increased yield results from fructan exohydrolase activity, induced by low temperature, and the back transfer activity of 1-FFT. Overall, this study provides background information on how to improve inulin yield and quality in chicory. (C) 2012 Elsevier GmbH. All rights reserved.
    Interactie: over de ontbrekende schakel in de communicatiewetenschap
    Aarts, M.N.C. ; Renes, R.J. ; Laere, J. van; Molder, H.F.M. te - \ 2004
    In: Etmaal van de Communicatiewetenschap, Enschede 18-19 november 2004. - Enschede : Universiteit Twente - p. 119 - 119.
    Exploring new coordination scenarios for workflow in the Amsterdam Police Force - simulation games as organizational change instruments
    Laere, J. van - \ 2004
    The IGF2-intron3-G3072A substitution explains a major imprinted QTL effect on backfat thickness in a Meishan x European white pig intercross
    Jungerius, B.J. ; Laere, A.S. van; Pas, M.F.W. te; Oost, B.A. van; Andersson, L. ; Groenen, M.A.M. - \ 2004
    Genetical Research 84 (2004)2. - ISSN 0016-6723 - p. 95 - 101.
    quantitative trait loci - muscle mass - igf2 locus - growth - scan - maps
    A paternally expressed QTL for muscle growth and backfat thickness (BFT) has previously been identified near the IGF2 locus on the distal tip of pig chromosome 2 (SSC2p) in three experimental F-2 populations. Recently, a mutation in a regulatory element of the IGF2 gene was identified as the quantitative trait nucleotide (QTN) underlying the major QTL effect on muscle growth and BFT in crosses between Large White and Wild Boar or Pietrain. This study demonstrates that the IGF2 mutation also controls the paternally expressed QTL for backfat thickness in a cross between Meishan and European Whites. In addition, a comparison of QTL of backfat thickness measured by Hennessy grading probe (HGP) and by ultrasound measurement (USM) was made. In the USM analyses, the IFG2 mutation explains the entire QTL effect on SSC2p, whereas in the HGP analysis the presence of a second minor QTL can not be excluded. Finally, this study shows that this particular IGF2 mutation does not cause the paternally expressed QTL for teat number mapping to the same region of SSC2p as the BFT QTL.
    Coordinating distributed work, exploring situated coordination with gaming-simulation
    Laere, J. van - \ 2003
    Delft University of Technology. Promotor(en): H.G. Sol. - Delft, Nijmegen : Delft University of Technology - ISBN 9789090175263 - 244
    coördinatie - werk - groepen - computersimulatie - organisaties - nederland - oefening - teamwork - werkploegen - politie - amsterdam - coordination - practice - teamwork - work teams - work - groups - computer simulation - organizations - netherlands - police - amsterdam
    Improving communication in distributed work groups, a framework for analysis and design
    Laere, J. van - \ 2003
    In: Etmaal van de communicatiewetenschap, 13-14 November 2003, Nijmegen. - Amsterdam : NESCOR Netherlands School of Communication Research - p. 89 - 89.
    Kom uit de kas: een kwalitatieve analyse van perceptieverschillen tussen management, middenkader en medewerkers in het cultuurveranderingsproces van PPO Glastuinbouw
    Laere, J. van; Goot, W. van der - \ 2003
    Wageningen : Wageningen Universiteit, LSG Communicatiemanagement - 32 p.
    Manage communities of practice in organisations : rule them, support them, or do not bother them?
    Laere, J. van - \ 2002
    In: Support for knowledge sharing in communities / Erik Andriessen, J.H., Soekijad, Maura, Keasberry, Helen J., - p. 71 - 89.
    Dietary fructooligosaccharides and transgalactooligosaccharides can affect fermentation characteristics in gut contents and portal plasma of growing pigs
    Houdijk, J.G.M. ; Verstegen, M.W.A. ; Bosch, M.W. ; Laere, K.J.M. van - \ 2002
    Livestock Production Science 73 (2002)2-3. - ISSN 0301-6226 - p. 175 - 184.
    We studied whether dietary non-digestible oligosaccharides (NDOs) affected pH and volatile fatty acids (VFAs) in gastrointestinal contents and in portal plasma of young pigs. Five groups of five 57-day-old pigs received for 44 days either a corn-based control diet or this diet with 7.5 or 15 g/kg fructooligosaccharides (FOSs, Raftilose P95?) or the control diet with 10 or 20 g/kg transgalactooligosaccharides (TOSs, Oligostroop?). The pigs weighed on average 45.5±1.3 kg during dissection, which took place 3 h after feeding. Dietary NDOs tended to lower the pH of the stomach content from 4.5 to 4.2 (P=0.06). Pigs fed the high TOS diet had more caecal VFAs than the control pigs (30.4 vs. 15.6 mmol, P<0.05). Compared to TOS-fed pigs, FOS-fed pigs had a higher proximal colon pH (6.5 vs. 6.2, P<0.01), lower proximal colon VFA concentration (131 vs. 166 mmol/l, P<0.01) and lower portal VFA concentration (0.9 vs. 1.6 mmol/l, P<0.05), with the control pigs being intermediate. However, the amount of colonic VFAs was similar across diets (~40 mmol). The results support the view that dietary FOSs and TOSs may have different effects on fermentation characteristics of gut contents of pigs.
    Managing communities of practice in organisations
    Laere, J. van - \ 2002
    In: Support for knowledge sharing in communities / Andriessen, J.H.E., Soekijad, M., Keasberyy, H.J., Delft : Delft University Press - p. 51 - 89.
    Isolation and structural analysis of new fructans produced by chicory
    Timmermans, J.W. ; Slaghek, T.M. ; Iizuka, M. ; Ende, W. van den; Roover, J. de; Laere, A. van - \ 2001
    Journal of carbohydrate chemistry 20 (2001)5. - ISSN 0732-8303 - p. 375 - 395.
    Process for the manufacture of a fermented health-promoting product
    Kiers, J.L. ; Nout, M.J.R. ; Rombouts, F.M. ; Koops, B.C. ; Laere, K.M.J. van; Wissing, E. ; Hagemann, R.J.J. ; Meulen, J. van der - \ 2001
    Octrooinummer: WO02086136, gepubliceerd: 2001-04-25.
    The present invention relates to a process that yields a product that can suitably be used in the treatment and prevention of diarrhea without giving rise to any undesirable side-effects. The invention is based on the discovery that aqueous fractions obtained from the fermentation of certain plant materials with fungi, may effectively be used to prevent, manage and/or treat diarrhea. Thus, in one aspect, the invention relates to a process for the manufacture of a health promoting product, said process comprising the subsequent steps of: a) preparing a substrate containing dry matter of which at least 50 wt.s derived from a plant material selected from legume, pulses, fruit, nuts, beans, seeds, grain, tubers and mixtures thereof, said substrate containing between 7.5 and 70 rotein, between 20 and 67 arbohydrates and less than 40 ipids by weight of dry matter, b) inoculating said substrate with a specified fungus, c) allowing the fungus to ferment the substrate for at least 6 hours at a temperature in excess of 25 DEG C, d) isolating an aqueous fraction from the fermentate so obtained, e) drying said aqueous fraction so as to obtain a powder which expresses at least 10 U/g of protease activity. Another aspect of the present invention relates to a composition for use in a method of treating diarrhea, wherein said method comprises administering an effective amount of a powder obtained by the process as described above
    Parasite-host interactions between the Varroa mite and the honey bee : a contribution to sustainable Varroa control
    Calis, J.N.M. - \ 2001
    Wageningen University. Promotor(en): J.C. van Lenteren; M.W. Sabelis. - S.l. : S.n. - ISBN 9789058084460 - 144
    honingbijen - bijenziekten - varroa jacobsoni - gastheer parasiet relaties - mijtenbestrijding - honey bees - bee diseases - varroa jacobsoni - host parasite relationships - mite control

    Introduction

    Varroa mites as parasites of honey bees

    Varroa destructor (Anderson & Trueman, 2000), is the most important pest of European races of the Western honey bee, Apis mellifera L., weakening bees and vectoring bee diseases (Matheson, 1993). Over the past decades it has spread all over the world and control measures are required to maintain healthy honey bee colonies.

    Originally, this mite only occurred in colonies of the Eastern honey bee, Apis cerana Fabr., in Asia. Varroa destructor was formerly known as V. jacobsoni Oud. (Anderson & Trueman, 2000). The Varroa mite was described in 1904 by Oudemans as a parasite of Eastern honey bees in Indonesia. Although the actual damage inflicted by the mite to the Eastern honey bee has never been determined, the Varroa mite is not considered to be a problem in colonies of its original host. However, Varroa turned into a serious pest of Western honey bees when beekeepers moved the Western honey bee into the area of distribution of the Eastern honey bee. The mite appeared to be a harmful parasite on its new host, but before this was realised it had already spread over the world through shipments of colonies and queens (De Jong et al., 1982; Matheson, 1993).

    Varroa mites may ruin Western honey bee colonies because parasitised bees suffer from malformations and a shortened life span (Beetsma et al., 1989). The Varroa mite feeds on both adult bees and brood, but reproduction is restricted to brood cells, which mites invade during the final larval developmental stage of the honey bee. Offspring is produced during the period that the immature bee develops in the capped brood cell and the mother and her progeny emerge together with the young bee. In addition to direct damage to bees through feeding, mites act as vectors of honey bee pathogens and increase the incidence of honey bee diseases (Ball, 1994). This threat of Varroa mites to beekeeping resulted in the development of acaricides and nowadays several effective acaricides are available which are applied world-wide (Koeniger & Fuchs, 1988; Ritter, 1990). However, the use of acaricides has important disadvantages. Acaricides contaminate bee products like honey and wax (De Greef, 1994) and thus the use of these acaricides is in conflict with the status of honey and wax as natural products. Another disadvantage is that mites have become resistant to these acaricides and this resistance is spreading world-wide, which implies the need for alternative ways of control.

    Towards sustainable Varroa control

    In this thesis, I present studies on biotechnical methods of Varroa control and studies on how variation in the honey bee's susceptibility to Varroa affects the mite population growth. In theory, biotechnical control methods in which mites are trapped in brood cells and removed from the colony, so-called trap-comb methods, are simple. In practice, however, these methods may become complicated because timing of application needs to be integrated in other activities of the beekeeper, such as swarm prevention. In addition, application of these methods is usually labour intensive. Effective trap-comb methods are available, but reduction of labour intensity is still needed. Much research is therefore directed to breed honey bees that are less susceptible to Varroa mites (Woyke, 1989; Büchler, 1994; Moritz, 1994). In this field, I investigated whether reduced developmental time of bee brood and attractiveness of bee brood to mites are suitable traits for selection aiming at reduced susceptibility of honey bees to Varroa mites. If less susceptible honey bees are available, the high effectiveness of control methods needed for successful control may be relaxed. This in turn may allow simplification of biotechnical control methods. The aim of my thesis is to develop acaricide-free beekeeping by using alternative methods for effective control of Varroa .

    Objectives and research questions

    Applying knowledge on invasion behaviour in the development of biotechnical control methods and population modelling

    The parasite-host interactions between the mite and the honey bee have been intensively studied, because such knowledge may lead to new ways of control. In earlier work, I collaborated with Beetsma and Boot (1995) to study invasion behaviour of mites into brood cells. Varroa mites survive on adult bees, but reproduction is restricted to the capped brood cell (Ifantidis & Rosenkranz, 1988). The rate of brood cell invasion defines the distribution of mites over bees and brood and, therefore, the population dynamics of the mite. The rate of invasion appeared to depend mainly on the ratio of brood cells that are being capped per bee in the colony, as reviewed in Chapter 1. In this thesis I applied this knowledge to design control methods that are based on trapping mites in bee brood. I investigated if it is possible to predict the effectiveness of trap-comb methods using a model based on the calculated invasion rate of the mites in brood cells from the ratio of capped brood cells per bee (Chapters 2&3). Using this model, concepts of trap-comb-methods were evaluated (Chapter 4). I also applied knowledge on invasion behaviour to gain more insight in the mite's population dynamics in general (Chapter 5).

    Towards less susceptible honey bees

    Differential reproduction of mites in both host-species, A. cerana and A. mellifera , seems to be a key factor in susceptibility of honey bees to Varroa (Büchler, 1994; Rosenkranz & Engels, 1994). In European A. mellifera colonies mites reproduce in both worker and drone brood and mite numbers increase rapidly. In colonies of its original host, A. cerana , mites invade both types of brood cells but refrain from reproducing in worker cells (Boot et al., 1997). Thus, in A. cerana mite numbers can only increase when drones are being reared. In African and africanised A. mellifera races a high percentage of mites that invade worker brood also refrain from reproducing (Camazine, 1986; Ritter, 1993). Therefore, like A. cerana, African and africanised honey bees are less susceptible to Varroa . I studied whether refraining from reproduction in worker brood is due to a trait of the honey bee or due to a trait of the mite (Chapter 6). By transferring Varroa mites originating from A. mellifera colonies to A. cerana worker brood and vice versa there appeared to be two distinct mite populations with a different reproductive strategy. Mites originating from A. mellifera reproduced in worker brood in both species of honey bee, whereas mites originating from A. cerana reproduced in drone brood only. Later, genetic studies of Varroa mites (Anderson & Trueman, 2000) made clear that the two populations in fact belong to different species. The mites that parasitise Western honey bees originate from Korea and Japan and were erroneously called V. jacobsoni and have been recently named V. destructor (Anderson & Trueman, 2000).

    Selection for honey bee traits that reduce reproductive success in worker brood is reminiscent of the situation we in the original host-parasite relationship where mites reproduce exclusively in drone brood. I studied honey bee traits that may play a role in the reproductive success of Varroa mites in worker brood: the duration of the capped brood stage and attractiveness of the brood cells. A short duration of the capped brood stage will limit the development of nymphs (Chapter 7). Reduced attractiveness will decrease the rate of invasion and hence the rate of reproduction (Chapter 8).

    Summary

    Structure of the thesis

    The chapters in this thesis are articles in which a separate part of the work is introduced and results are presented and discussed. The first six chapters have been published in periodicals and the final two chapters are submitted for publication.

    Invasion behaviour of Varroa mites: from bees into brood cells (Chapter 1)

    Varroa mites may invade worker or drone brood cells when worker bees bring them into close contact with these cells. The attractive period of drone brood cells is two to three times longer than that of worker brood cells. The attractiveness of brood cells is related to the distance between the larva and the cell rim and the age of the larva. The moment of invasion of the mite into a brood cell is not related to the duration of its stay on adult bees. The fraction of the phoretic mites that invade brood cells is determined by the ratio of the number of suitable brood cells and the size of the colony. The distribution of mites over drone and worker brood in a colony is determined by the specific rates of invasion and the number of both brood types. Knowledge of mite invasion behaviour has led to effective biotechnical control methods and increased insight in the mite's population dynamics.

    Control of Varroa mites by combining trapping mites in honey bee worker brood with formic acid treatment of the capped brood outside the colony: Putting knowledge on brood cell invasion into practice (Chapter 2)

    Biotechnical Varroa control methods are based on the principle that mites inside brood cells are trapped and then removed from the bee colony. Initially, methods were studied in which worker brood was used for trapping. Trapped mites were killed with a formic acid treatment that left the worker brood unharmed. The observed percentage of mites trapped and killed by formic acid treatment was 87% and 89% in two experiments which matched predictions based on knowledge on brood cell invasion. Hence, knowledge on the mites' behaviour with respect to brood cell invasion proved to be a useful tool for designing and improving trap-comb methods for Varroa control.

    Effective biotechnical control of Varroa mites: Applying knowledge on brood cell invasion to trap mites in drone brood (Chapter 3)

    Trapping mites in brood cells is most efficient when drone brood is used while the colonies are otherwise broodless. In theory, one trap-comb using drone brood is enough to achieve effective control. I designed and tested two methods using trap-combs with drone brood. To reduce labour intensity, application of trap-combs was integrated in swarm prevention techniques. In the first method, effectiveness of the control method varied considerably, from 67% to 96%. Effectiveness depended on the number of drone cells that had been available for mite trapping. The observed effectiveness in each separate colony could be predicted from the numbers of bees and brood cells, thereby showing the validity of our approach. In the second method, we adjusted the method to improve production of drone brood on the trap-combs, because this appeared to be crucial for trapping efficiency. The observed effectiveness of 93.4 % demonstrates that trap-combs with drone brood can effectively trap mites, thereby offering a non-chemical method of Varroa control.

    Model evaluation of methods for Varroa mite control based on trapping in honey bee brood (Chapter 4)

    The trap-comb model that was used to predict mite-trapping effectiveness in our experiments was used to estimate and compare effectiveness of different trap-comb methods described by several authors. Predictions of the model showed that for effective control by trapping with worker brood is labour intensive because a large amount of brood is needed to trap a sufficient number of mites. An extra input of labour is the demand for treatment of the capped worker brood to selectively kill the mites, because beekeepers want to save the brood. The model predicted that trapping with drone brood demands much less brood cells for effective mite control. Labour intensity is less compared to trap-combs with worker brood. This is because drone brood with trapped mites is usually destroyed instead of saved and preparation of trap-combs with drone brood can be integrated into swarm-prevention-techniques.

    Population modelling of Varroa mites (Chapter 5)

    To understand population dynamics of the mite, Fries et al. (1994) incorporated knowledge on Varroa mite-honey bee interactions into a mite population model. I updated and extended this model by incorporating more recent data, in particular on mite invasion from bees into brood cells. This allowed predictions of invasion into and emergence from brood cells, and hence the distribution of mites over bees and brood. As mite control treatments usually only affect mites either in brood cells or on adult bees, the model can be used to evaluate their effectiveness and timing. Mite population growth proved to be especially sensitive to the length of the brood period, the number of drone cells and reproductive success in the brood cells.

    Natural selection of Varroa explains the different reproductive strategies in colonies of Apis cerana and Apis mellifera (Chapter 6)

    In colonies of European A. mellifera, Varroa reproduces both in drone and in worker brood. In colonies of its original Asian host, A. cerana, the mites invade both drone and worker brood cells, but reproduce only in drone cells. Absence of reproduction in worker cells is probably crucial for the tolerance of A. cerana towards Varroa because it means that the mite population can only grow during periods of drone rearing. To test whether the absence of mite reproduction in worker brood of A. cerana is due to a trait of the mites or of the honey bee species, mites from bees in A. mellifera colonies were introduced into A. cerana worker brood cells and vice versa. Approximately 80% of the mites originating from A. mellifera reproduced in worker cells of both A. mellifera and A. cerana. Conversely, only 10% of the mites originating from A. cerana colonies reproduced in worker cells of A. cerana and A. mellifera. Hence, absence of reproduction in worker cells is due to a trait of the mites. Additional experiments showed that A. cerana removed 84% of the worker brood that was artificially infested with mites from A. mellifera colonies. Brood removal started 2 days after artificial infestation, which suggests that the bees responded to behaviour of the mites. Because removal behaviour of the bees will have a large impact on the mite's fitness, it probably plays an important role in selection for differential reproductive strategies. These findings have large implications for selection programmes to breed less-susceptible bee strains. If differences in mites (i.e. whether they reproduce in worker brood or not) are mite-specific, we should not only look for mites not reproducing as such, but for colonies in which mites are selected for not reproducing in worker cells. Hence, in selection programmes reproductive success of mites that reproduce in both drone and worker cells should be compared to the reproductive success of mites that reproduce exclusively in drone cells.

    Reproductive success of Varroa mites in honey bee brood with differential development times (Chapter 7)

    Reproduction of Varroa mites has been extensively studied and many aspects of its life history such as number of eggs laid, timing of egg laying, and mortality of immature mites, are well known. However, estimates of the actual reproductive success after one brood cycle, i.e. how many mites can be found alive on the bees after emergence of an infested cell, are still fairly theoretical. Because this parameter is crucial for understanding population growth of the mites, several methods were used to measure the actual reproductive success. To evaluate how development time of the capped brood stage may affect population growth of the mites, measurements were done in bee strains with different development times of worker brood. In brood with a relatively short developmental time, reproductive success of mites was lower. Increased developmental time resulted in higher egg production and lower mortality of offspring before or shortly after emergence of the mites from the brood cell. The results show that the number of mites emerging alive from worker cells with relatively short development times, may become lower than the initial number that invaded the cells. This results in a decline of the mite population if only worker cells are available. In addition, the low reproductive success in worker brood with a short development time, explains that the phenomenon of mites not reproducing in worker cells, as found in A. cerana and in several A. mellifera races, evolves if these mites survive to reproduce in drone brood the next brood cycle.

    Attractiveness of brood cells of different honey bee races to Varroa mites (Chapter 8)

    Reproduction of the Varroa mite only occurs inside capped brood cells of honey bees. Therefore, invasion into brood cells is crucial for the mite's reproduction and the rate of invasion will affect the growth of the mite population. I investigated the invasion response of the mites to drone or worker larvae of different honey bee races, because selection for less attractive brood may help Varroa control. The observed differences in invasion response of Varroa mites to worker brood of the tested colonies were not statistically significant. The results suggest that not the racial origin of the worker brood, but the distance between the larva and the cell rim affects the invasion response of the Varroa mites to worker brood cells. Because measuring the distance between the larva and the cell rim in drone brood cells is inaccurate due to curved cell caps of neighbouring cells, the results for drone brood cells are difficult to interpret. Possibilities to obtain less attractive brood via selection or comb manipulation are discussed.

    Epilogue

    Towards a future in which beekeeping does not depend on the use of acaricides for effective control of Varroa

    Considering the conflict between the use of synthetic acaricides and the status of honey bee products as natural products and the spreading resistance of Varroa to these acaricides, there is a clear need for alternative ways of Varroa control. Our research on biotechnical control methods and susceptibility of honey bees to Varroa contributes to sustainable Varroa control. Knowledge on invasion behaviour of mites into brood cells proved to be useful to understand the possibilities and limitations for improvement of biotechnical control methods. Using drone brood on trap-combs, an effective biotechnical control method has become available providing a non-chemical way of controlling the mite population. Integration of knowledge on invasion behaviour into a population model of the Varroa mite allows us to gain more insight in the mite's population dynamics and evaluate traits of honey bees that via selection may decrease susceptibility of honey bee colonies. Selection for honey bee traits that reduce reproductive success in worker brood in A. mellifera may lead to selection of mites towards the situation we know from the original host-parasite relationship were mites only reproduce in drone brood. The duration of the capped brood stage seems a good candidate because selection for a short development time will reduce reproductive success of the mites. Attractiveness of brood cells is a less suitable trait because differences in attractiveness of brood of different race were not detected. Although less susceptible honey bees are not available yet, selectable traits have been identified that may reduce the effect of Varroa infestation on honey bee colonies. Nowadays, beekeeping is not dependent on the use of synthetic acaricides to control the Varroa mite. Next to trap-comb methods, much research has been successfully directed towards Varroa control using organic acids and essential oils (Imdorf, 1999). Reducing susceptibility of honey bees together with effective control by means of biotechnical and other 'organic' control methods provides a perspective for beekeeping that does not rely on synthetic acaricides to kill Varroa mites.

    Acknowledgements

    I thank M. Beekman, WJ Boot, JC van Lenteren and M.W. Sabelis for their valuable comments on the manuscript.

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    Non-digestible oligosaccharides (NDOs) are oligosaccharides, which resist digestion in the upper gastrointestinal tract, and which are fermented in the colon by intestinal bacteria. Some NDOs are considered bifidogenic, meaning that they selectively stimulate the growth of bifidobacteria in the colon microbiota. The degradative fermentation of structurally different oligosaccharides by intestinal bacteria was studied in this thesis, in order to establish the potentially bifidogenic effects of various types of NDOs. Structurally different oligosaccharides were produced using different routes. Arabino-, (arabino-)galacto-, (arabino-)xylo-, galacturono-, and rhamnogalacturono- oligosaccharides were derived by enzymatic hydrolysis of plant polysaccharides. Through transglycosylation reactions using glycosidases, transgalactooligosaccharides of theα- andβ-glycosyl linkage type were obtained.

    The chemical structure of the oligosaccharides clearly influenced their fermentation behaviour. It was concluded that species belonging to different groups, so not only bifidobacteria, have the capability of hydrolysing these oligosaccharides. Bi. adolescentis, being a major bifidobacterial species of the adult intestinal microflora, was able to utilise a wide range of oligosaccharides showing its wide range of glycosidases. Two novel arabinoxylan degrading enzymes were purified from Bi. adolescentis and these enzymes in combination with aβ-xylosidase are involved in the complete degradation of arabinoxylooligosaccharides. For the utilisation ofα-galactooligosaccharides Bi. adolescentis produced anα-galactosidase. Thisα-galactosidase was characterised as a retaining glycosidase and was used for the production of new types ofα-galactooligosaccharides. Theseα-1→6 linked-galactooligosaccharides could be utilised by various strains belonging to bifidobacteria and lactobacilli. Upon growth of Bi. adolescentis on transgalactooligosaccharides (TOS) a novelβ-galactosidase was produced, involved in the degradation of TOS. It was speculated that this enzyme was membrane or cell wall associated. After growth of Bi. adolescentis on TOS or on hydrolysed arabinogalactan theβ-galactosidase production of Bi. adolescentis increased compared to growth on galactose and this increasedβ-galactosidase activity could be linked to increased activity towards both polymeric and oligomeric galactan. In vivo dietary intervention with TOS also resulted in increased levels of v-galactosidase activity in feces. Although the nature and specificity of theβ-galactosidase is not yet known it can be concluded that glycosidase activity of the intestinal bacteria might be a useful biomarker of the colonic metabolic activity.

    Characterization of a novel ß-galactosidase from Bifidobacterium adolescentis DSM 20083 active towards transgalactooligosaccharides
    Laere, K.M.J. van; Abee, T. ; Schols, H.A. ; Beldman, G. ; Voragen, A.G.J. - \ 2000
    Applied and Environmental Microbiology 66 (2000). - ISSN 0099-2240 - p. 1379 - 1384.
    This paper reports on the effects of both reducing and nonreducing transgalactooligosaccharides (TOS) comprising 2 to 8 residues on the growth of Bifidobacterium adolescentis DSM 20083 and on the production of a novel -galactosidase (-Gal II). In cells grown on TOS, in addition to the lactose-degrading -Gal (-Gal I), another -Gal (-Gal II) was detected and it showed activity towards TOS but not towards lactose. -Gal II activity was at least 20-fold higher when cells were grown on TOS than when cells were grown on galactose, glucose, and lactose. Subsequently, the enzyme was purified from the cell extract of TOS-grown B. adolescentis by anion-exchange chromatography, adsorption chromatography, and size-exclusion chromatography. -Gal II has apparent molecular masses of 350 and 89 kDa as judged by size-exclusion chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, respectively, indicating that the enzyme is active in vivo as a tetramer. -Gal II had an optimal activity at pH 6 and was not active below pH 5. Its optimum temperature was 35°C. The enzyme showed highest Vmax values towards galactooligosaccharides with a low degree of polymerization. This result is in agreement with the observation that during fermentation of TOS, the di- and trisaccharides were fermented first. -Gal II was active towards -galactosyl residues that were 14, 16, 13, and 11 linked, signifying its role in the metabolism of galactooligosaccharides by B. adolescentis
    Fermentation of plant cell wall derived polysaccharides and their corresponding oligosaccharides by intestinal bacteria
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    New types of nondigestible oligosaccharides were produced from plant cell wall polysaccharides, and the fermentation of these oligosaccharides and their parental polysaccharides by relevant individual intestinal species of bacteria was studied. Oligosaccharides were produced from soy arabinogalactan, sugar beet arabinan, wheat flour arabinoxylan, polygalacturonan, and rhamnogalacturonan fraction from apple. All of the tested substrates were fermented to some extent by one or more of the individual species of bacteria tested. Bacteroides spp. are able to utilize plant cell wall derived oligosaccharides besides their reported activity toward plant polysaccharides. Bifidobacterium spp. are also able to utilize the rather complex plant cell wall derived oligosaccharides in addition to the bifidogenic fructooligosaccharides. Clostridium spp., Klebsiella spp., and Escherichia coli fermented some of the selected substrates in vitro. These studies do not allow prediction of the fermentation in vivo but give valuable information on the fermentative capability of the tested intestinal strains.
    Purification and properties of a second fructan exohydrolase from the roots of Cichorium intybus
    Roover, J. de; Laere, A. van; Winter, M. de; Timmermans, J.W. ; Ende, W. van den - \ 1999
    Physiologia Plantarum 106 (1999)1. - ISSN 0031-9317 - p. 28 - 34.
    A 1-FEH II (1-fructan exohydrolase, EC 3.2.1.80) was purified from forced chicory roots (Cichorium intybus L. var. foliosum cv. Flash) by a combination of ammonium sulfate precipitation, concanavalin A (Con A) affinity chromatography and anion and cation exchange chromatography. This protocol produced a 70-fold purification and a specific activity of 52 nkat mg-1 protein. The apparent size of the enzyme was 60 kDa as estimated by gel filtration and 64 kDa on SDS-PAGE. Optimal activity was found between pH 5.0 and 5.5. The temperature optimum was around 35 °C. No product other than fructose could be detected with inulin as the substrate. The purified enzyme exhibited hyperbolic saturation kinetics with an apparent K(m) of 58 mM for 1-kestose (Kes) and 64 mM for 1,1-nystose (Nys). The purified 1-FEH II hydrolyzed the ̄(2 → 1) linkages in inulin, Kes and Nys at rates at least 5 times faster than the ̄(2 → 6) linkages in levan oligosaccharides and levanbiose. Fructose did not affect the 1-FEH II activity but sucrose (Suc) was a strong inhibitor of this 1-FEH II (K(i) = 5.9 mM). The enzyme was partially inhibited by Na-EDTA and CaCl2 (1 mM).
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