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Discoloration may include (1) a white dorsal spot at the distal end of the opisthosoma order 500 mg naprosyn visa rheumatoid arthritis espanol, (2) two white stripes along the dorsal lateral sides of the body in the region of the Malpighian tubules generic 250 mg naprosyn free shipping neoplastic arthritis in dogs, or (3) a combination of both forms (Bjrnson et al order naprosyn 500mg overnight delivery osteoarthritis knee diet. Mites carrying discoloration(s) appeared lethargic and provided poor pest control (Steiner 1993b; Bjrnson et al order naprosyn paypal rheumatoid arthritis xeljanz. Rectal plugs, which were observed when symptoms were more pronounced, often disrupted normal excretion and might cause the affected individual to become stuck to the leaf surface (Bjrnson et al. However, in a follow-up study Bjrnson and Raworth (2003) found that the expression of white opisthosomal discolorations in P. Conclusions Several potential pathogens pathogens in the true sense and unidentied diseases have been reported for phytoseiid mites. However, the status and impact of many described entities on their host is unclear. Fourteen reports are descriptive with unknown host effects; three reports mention pathological manifestations without proving the nal cause of the symptoms and eleven reports describe endosymbiotic bacteria. From the latter group four reports refer to Microsporidia, one to a bacterium and one to a fungus. Microsporidian infections often appear not to be readily visible as no obvious external symptoms are present and female predators infected by A. Such infections may thus remain undetected for extended periods meanwhile spreading in the case of exchange of predator populations among producers and laboratories. Screening of these pathogens on a regular basis is therefore advisable for maintenance of healthy predator populations over long periods. However, as only few pathogens in the true sense are described up to now it is too early to plead for regular general pathogen screening in phytoseiid mite mass rearings. Yet, the reports on true pathogens, often made in response to unexpected phenomena in a mass rearing, show that pathogens of benecial mites can be an important factor hampering the quality of the mass-reared mites. The nal conclusion of this review therefore is that more research on diseases of benecial mites that are applied in biological pest control is needed. Acknowledgement We are grateful to Joop van Lenteren for his helpful comments on an earlier version of this manuscript. Can Entomol 135:129 138 Bjrnson S, Schutte C (2003) Pathogens of mass-produced natural enemies and pollinators. Biol Control 19:17 27 Blumel S, Hausdorf H (2002) Results of quality control tests with Phytoseiulus persimilis, Neoseiulus cucumeris and Orius laevigatus in Austria. Neth J Zool 38:148 165 Dicke M, Dijkman H, Wunderink R (1991) Response to synomones as a parameter in quality control of predatory mites. Exp Appl Acarol 22:311 333 Dicke M, Schutte C, Dijkman H (2000) Change in behavioral response to herbivore-induced plant volatiles in a predatory mite population. J Chem Ecol 26:1497 1514 Di Palma A (1996) Thyphlodromus rhenanoides Athias-Henriot e T. Exp Appl Acarol 42:75 85 Enigl M, Zchori-Fein E, Schausberger P (2005) Negative evidence of Wolbachia in the predaceous mite Phytoseiulus persimilis. Proc R Soc Lond B 270:2185 2190 Janssen A (1999) Plants with spider mite prey attract more predatory mites than clean plants under greenhouse conditions. Entomol Exp Appl 93:259 268 Keller S (1997) The genus Neozygites (Zygomycetes, Entomophthorales) with special reference to species found in tropical regions. J Invertebr Pathol 79:173 178 Poinar G, Poinar R (1998) Parasites and pathogens of mites. Annu Rev Entomol 43:449 469 Pukall R, Schumann P, Schutte C et al (2006) Acaricomes phytoseiuli gen. Exp Appl Acarol 24:709 725 Schutte C (2006) A novel bacterial disease of the predatory mite Phytoseiulus persimilis: disease syndrome, disease transmission and pathogen isolation. Exp Appl Acarol 38:275 297 Schutte C, Negash T, Poitevin O, Dicke M (2006b) A novel disease affecting the predatory mite Phyto- seiulus persimilis (Acari: Phytoseiidae): 2. Exp Appl Acarol 39:85 103 Schutte C, Poitevin O, Dicke M (2008a) A novel disease affecting the predatory mite Phytoseiulus persimilis (Acari: Phytoseiidae): evidence for the involvement of bacteria. Am Entomol 40:240 253 Skirvin D, Fenlon J (2003a) Of mites and movement: the effect of plant connectedness and temperature on movement of Phytoseiulus persimilis. Biol Control 27:242 250 Skirvin D, Fenlon J (2003b) The effect of temperature on the functional response of Phytoseiulus persimilis (Acari: Phytoseiidae). Nature 361:66 68 Sut akova G (1988) Electron microscopic study of developmental stages of Rickettsiella phytoseiuli in Phytoseiulus persimilis Athias-Henriot (Gamasoidea: Phytoseiidae) mites. Acta Virol 32:50 54 Sut akova G (1991) Rickettsiella phytoseiuli and its relation to mites and ticks. Academia, Praque, pp 45 48 Sut akova G (1994) Phenomenon of Rickettsiella phytoseiuli in Phytoseiulus persimilis mite. Acta Entomol Bohemoslov 87:431 434 Sut akova G, Rehacek J (1989) Experimental infection with Rickettsiella phytoseiuli in adult female Der- macentor reticulatus (Ixodidae): an electron microscopy study. Exp Appl Acarol 7:299 311 Sut akova G, Ruttgen F (1978) Rickettsiella phytoseiuli and virus-like particles in Phytoseiulus persimilis (Gamasoidea: Phytoseiidae) mites. Int J Syst Evol Microbiol 54:961 968 Zemek R, Nachman G (1999) Interaction in tritrophic acarine predator-prey metapopulation system: prey locationanddistancemovedbyPhytoseiuluspersimilis(Acari:Phytoseiidae). ExpApplAcarol23:21 40 Symbionts, including pathogens, of the predatory mite Metaseiulus occidentalis: current and future analysis methods Marjorie A. Jeyaprakash Originally published in the journal Experimental and Applied Acarology, Volume 46, Nos 1 4, 329 347. Until molecular tools became available, analysis meth- ods were limited primarily to microscopic observations; some viruses and rickettsia-like organisms were observed infecting diseased M. A new phylogenetic analysis of the Bacteroidetes-Flavobacterium group suggests the unnamed Bacteroidetes in M. However, much of our current information about the role these microorganisms play in the biology of M. We also currently lack any knowledge of the importance of these microorganisms under Weld conditions. Keywords Phytoseiidae Metaseiulus (= Typhlodromus or Galendromus) occidentalis Microbial symbionts Pathogens Assessment methods Metagenomics Bacteroidetes Wolbachia Cardinium Enterobacter Oligosporidium Viruses Serratia M. It has been imported and established in Australia and New Zealand in classical biological control programs for the control of mites in apple and peach orchards (Readshaw 1975; Field 1978). Between 1970 and early 1981, at least 470 papers were published on the Phytoseiidae (Tanigoshi 1982) and between 1960 and 1994, more than 420 papers were published on M. The number of chromosomes (3 and 6 in males and females, respectively) and the genetic system of M. Whether this unusual genetic system is inXuenced by the microbial associates (Wolbachia or Cardinium) of M. Whether pesticide-resistant microbial symbionts are associated with these resistances also remains unknown, but many soil microorganisms have been documented to degrade pesticides (Felsot 1989) and microbial gut symbionts of a tephritid have been implicated in the pesticide resistance of its host (Boush and Matsumura 1967). Because mitochondrial organelles are derived from endosymbiotic bacteria, these unusual features will be discussed brieXy below. Diseases of Mites and Ticks 331 Despite this extensive research conducted over the past 60 years, until recently we knew little about the microbial associates of M. We adhere to the term symbiosis in its original deWnition of organisms living together, usu- ally in close association with one another, to the beneWt of at least one organism, with the partners referred to as symbionts.
In the past buy naprosyn online pills bacterial arthritis definition, it has been commonplace for newborn The ber level in milk replacers is a rough correlation calves to receive colostrum until 3 to 4 days of age and to plant origin sources of protein in some instances order naprosyn 250mg with amex rheumatoid arthritis qof. This is no the advent of acceptable soy protein sources discount naprosyn 500 mg arthritis medication starting with t, however order naprosyn toronto arthritis spine diet, longer so frequently practiced, and milk replacer may be ber levels cannot be the sole means of evaluation. Yet another milk replacers with soy our or other soy source added common feeding error for farmers using milk replacer is could be judged somewhat by crude ber because each not increasing the amount fed as the calf ages. Therefore step-by-step discussion with the owner and by careful crude ber is not of great value when evaluating current observation can the veterinarian detect and correct some milk replacer protein content. Yet another controversial aspect of milk replacer High-quality calf starter grains can mask the effect of feeding involves physiologic clotting in the calf ab- a poor-quality milk replacer; some authors believe that omasum. Milk fed by conventional means causes reex up to 50% to 75% of calf weight gain before weaning esophageal groove closure and diversion into the ab- may result from high-quality calf starter intake. In the abomasum, replacers containing antibiotics or decoquinate are ad- milk quickly is separated into a casein and milk fat vertised widely, but their value is difcult to assess be- coagulum and a liquid component, whey. The true etiology of milk owner will be adamant that an infectious disease is re- replacer-related calf mortality varies but would include: sponsible because so many calves appear to be affected Poor-quality milk replacer (i. Careful reading of the Feeding at the wrong dilution instructions on milk replacers and/or consultation with Feeding the wrong amount (usually not enough) a nutritionist afliated with the manufacturer may reveal that mixing at hot temperatures (104 to 106o F) is re- Clinical Signs quired for complete solubilization of fat in the replacer; Calves suffering malnutrition from poor-quality milk subsequent cooling to body temperature is necessary for replacer appear thin, have dull hair coats with patchy acceptance by the calf. The sight of a whole complain about calf mortality that usually occurs at 3 to group of malnourished but hungry and bright nursing 6 weeks of age and attribute death to diarrhea. Calves calves in a barn that usually has well-conditioned calves may die suddenly but often remain hungry and willing to almost guarantees that the owner has switched to a new nurse even if recumbent 1 to 2 days before death. Calves that survive to wean- and allows other diseases to be ruled out following sub- ing often do well on high-quality solid feeds and regain mission of appropriate samples. Prevention Ancillary Aids Correction and prevention merely require the feeding of If calves are dying as early as 3 weeks of age, enteric a high-quality milk replacer at proper dilution and in pathogens and parasites must be ruled out by submis- proper quantities. The owner must be convinced that sion of either fecal samples from live animals or feces milk replacer is not the place to save pennies. It may be nec- given the increased costs associated with calf losses essary to assess blood selenium and vitamin E values in such cases, it can be stated that the most expensive from calves that become recumbent. Total protein val- milk replacer a producer can buy is often the cheapest ues may be low because of persistent low protein intake one. Milk replacer is never as good as whole milk for or fecal losses associated with enteritis, a result of poor- calves; therefore whenever possible, owners should be quality protein sources. Blood work is normal unless a encouraged to feed calves whole milk that is at least stress leukogram exists. Assessment of adequacy of pas- 22% crude protein and 20% crude fat (dry matter basis) sive transfer is also prudent. Milk dis- rophy of fat in the epicardial grooves, omentum, and carded because of antibiotic residues is not ideal and perirenal areas. It carries an either poor digestion of nutrients in the cachectic state increased risk for transmission of several contagious, or opportunistic, secondary enteric infections of the infectious diseases unless pasteurization is performed. Pneumonia may be present as a Many owners need to reassess the costs of feeding milk concurrent condition. Use of a Diagnosis usually can be made by inspection of the pasteurizer for feeding waste milk to calves has been calves coupled with a careful history and evaluation of shown to be of economic benet on larger dairies. Differential teurization of waste milk is worthy of consideration for diagnoses include infectious causes of diarrhea, coccid- those operations that routinely produce calves for sale iosis, and selenium deciency. Calves that are less than as replacement stock because milk-borne transmission 3 weeks of age require careful consideration of infectious of infectious agents of concern (e. Regularly monitor the preparation and feeding tem- quently discourage farmers from using these feeds. If the veterinarian has made a diagnosis of diarrhea An excellent calf starter, adequate feed intake, and good and emaciation due to milk replacer issues and feeding management may mask the effects of a poor-quality milk of pasteurized whole milk cannot be done, the follow- replacer. This is why some farms seem to have starving ing instructions can be followed: calves on a specic milk replacer, whereas others seem to 1. Ensure adequate colostral feeding to ensure passive achieve acceptable growth with the same product. Feed colostrum for the rst 3 days of the calf s life of Age) Diagnostic Protocol at 10% to 12% body weight, but only if sure the Table 6-4 gives a diagnostic plan for herd neonatal diar- cow is not shedding M. Begin feeding a high-quality milk replacer on day 4 at 10% body weight: Coccidiosis Minimum 22% protein most or all of milk origin if possible Etiology Minimum 20% fat Coccidiosis has become one of the most serious prob- Minimal crude ber lems encountered in raising dairy calves when the 4. After the rst week, quantity can gradually be in- calves are grouped and housed in mini free stall barns creased to maintain 10% to 12% body weight intake. Be sure dilution factors for milk replacer are correct ment currently is very popular because it decreases la- to mimic the total solids of milk. All calves should have fresh water and a high-quality Although up to 20 species of Eimeria may infect cat- calf starter available at all times. Sporulated oocysts that are infective enough that the starter does not spoil in the feeder. Feces collected immediately after onset of diarrhea to diagnostic two or three consecutive affected calves lab on at least two or three consecutive affected calves A. Whenever possible, more than one calf should be sampled before blaming the whole herd problem on a single isolate. If patient older than 2 weeks, consider poor replacer rather than infectious diseases. Therefore calves remain viable for more than 1 year in favorable condi- exposed to coccidia oocysts and simultaneously sub- tions that include moisture and absence of temperature jected to stress, poor nutrition, exogenous corticoste- extremes. Rectal pro- calves licking themselves and ingesting feces or fecal- lapse may occur secondary to proctitis and prolonged stained hair, or from browsing on a contaminated bed- tenesmus. Affected calves appear dehydrated, thin, ded pack under the roofed area of the housing unit. Milder cases merely show Coccidia are quite host-specic intracellular para- mild diarrhea without systemic signs and many cases sites that complete both the asexual and sexual phases are subclinical. Feces stain the (meronts) that release merozoites that then infect epi- perineum, tail, and hocks of typical cases. Second-generation calves in the group are infected, only those with severe schizonts then form in these cells and subsequently infection show dramatic signs. Coccidiosis is a perfect release merozoites that begin the sexual phase of the example of the weak sister law in parasitology this law reproductive cycle by invading yet another host epithe- states that when a group of animals are parasitized, the lial cell and become microgamonts (male) or macro- most seriously affected bring attention to the problem gamonts (female). Microgamonts release microgametes and act as a signal that the entire group needs treatment. Invasion of cells and subse- the patient from concentrating on eating or drinking. The magnitude of enteric pathology appears to be related to the dose of oocysts ingested. Small doses of ingested oocysts may result in inapparent infection and eventual induction of immu- nity. Oocysts are observed in feces (patent infection) ap- proximately 17 to 20 days following infection with E.
Manure storage systems to control environmental toring of the latter are attractive tools for herd monitor- degradation return more potassium to the soil buy 500 mg naprosyn visa arthritis in knee and cycling. Therefore the urine pH of dry cows has been chased grains and concentrates are brought to the farm 250 mg naprosyn for sale arthritis treatment raisins and gin, suggested as a parameter to monitor or judge the effec- there is a net accumulation of potassium purchase discount naprosyn line dogs with arthritis in back legs. Cows on unsupple- vertently feed more and more potassium to our dry mented diets typically have urine pH of 8 to 8 purchase 250mg naprosyn with visa lower back arthritis relief. Un- followed by hyperexcitability will be seen in cattle whose doubtedly many veterinarians include phosphorus sup- magnesium values decrease rapidly to 1. Unfortunately convincing scientic evidence for hypophosphatemia as a blood levels, particularly in advanced cases in which con- contributor or absolute cause of recumbency is lacking. Mea- uids, urine and cerebrospinal uid will more reliably surement of phosphorous levels in blood taken from the determine the antemortem magnesium status of an ani- jugular vein routinely underestimates phosphorus ob- mal found moribund, or if sampled within 12 hours of tained from the coccygeal vein by up to 0. Mild hy- centration of ammonia or ammonium is high in rumen pophosphatemia (between 2 and 4 mg/dl) is not associ- uid. The combined effect of a low magnesium and high ated with discernible clinical signs in the absence of other nitrogen content in rapidly growing grass causes clinical signicant macroelement or electrolyte disturbances. The mech- tle with severe hypophosphatemia (plasma phosphorus anism of high ammonia concentration leading to inhibi- 1 mg/dl) may be recumbent, but the absolute relevance tion of magnesium absorption is not known. Dry cow of their hypophosphatemia is clouded by the fact that diets based on ammoniated corn silage or the use of urea such individuals are usually hypocalcemic, hypoglycemic, as the primary protein supplement may inadvertently and hypomagnesemic. The clinical that most cows with milk fever will also be hypophospha- signs of affected cows are similar to milk fever rather than temic (cows with plasma calcium 5 mg/dl will typically the classic grass tetany of hypomagnesemia. The hypomagnesemic cattle are also typically mildly to mod- biologically active form of phosphorus is in the form of erately hypocalcemic. The clinical relevance of low grade inorganic phosphate, and any attempts to therapeutically hypomagnesemia in lactating dairy cattle is hard to char- address real or perceived hypophosphatemia should re- acterize; however, chronic, low magnesium levels are ect this. Sampling of individual, anorectic cows for a herd Sterile Fleet solutions are a good source of phosphorus issue with hypomagnesemia is of dubious value, but the and can be given subcutaneously or intravenously when demonstration of plasma magnesium levels of less than diluted. Oral phosphorous sup- 24 hours of freshening on a farm should be taken as a plementation can be given in the form of 200 to 250 g of problem with magnesium availability or absorption in sodium monophosphate (providing approximately 50 g the transition diet. Similar testing can be performed on of phosphate), usually combined with other drench comp- groups of cows in early lactation. This ratio corrects for the degree of water conservation by the Hypomagnesemia in dairy cattle very rarely assumes the kidney and better reects magnesium status than does severe clinical presentation with which veterinarians who magnesium concentration alone. Guidelines for target work with pastured, spring-calving beef herds will be all values of this ratio have not been developed by North too familiar. Normal plasma magnesium concentration American laboratories as they have been in New Zealand. Nor- is it usually suggests either compromised renal function mal plasma potassium is between 3. Severely hypokalemic cows in which the plasma potassium has decreased to less than 2. Recumbency may be anticipated Treatment of cattle with hypomagnesemic grass tetany when the potassium level decreases to less than 2. Typical premonitory signs of obvious muscle fascicula- or convulsing cattle will occasionally rst need to be se- tions and increased time lying down will have been no- dated before parenteral administration of magnesium. It should be emphasized, however, that severe appropriate in such cases, but caution needs to be taken hypokalemia is a rare cause of recumbency in dairy cattle with regard to the speed of infusion because of its poten- compared with hypocalcemia or musculoskeletal and tial cardiotoxicity. Retro- tions containing multiple macroelements such as calcium spective clinical observations have been validated by ex- and phosphorus are used, the magnesium content of these perimental reproduction of severe hypokalemia and should be checked before infusion to verify that there is weakness following multiple administrations of the drug. The infusion should be However, it has become evident in recent years that the performed over at least 5 to 10 minutes. To prevent re- condition can be seen in the absence of isoupredone lapses over the next 12 to 24 hours, a further 250 ml of acetate administration. It is appropriate to select infusion lem are uncertain; however, many affected cattle have a solutions that also contain calcium because many indi- history of chronic refractory ketosis, or at least repeated viduals will be concurrently hypocalcemic, and the relapse treatments for presumed ketosis with a variety of agents rate appears to be lower and the initial response rate that may induce hyperglycemia. Theoretically the repeated greater in cattle that receive parenteral calcium also. This intracellular shifting may be exacerbated by and also runs the risk of overstimulation and a return to the inevitable metabolic alkalosis that accompanies pro- tetany if used prematurely in severe cases. Cattle with prolonged an- however, oral magnesium supplementation is a safe and orexia may also have whole body potassium depletion effective way to address less severe hypomagnesemia in caused by inadequate intake in feed, coupled with contin- cattle. Many drenches, commercial or home made, that are ued obligate losses in urine and feces. Administration of used as nonspecic supportive enteral uid therapy in any drugs with mineralocorticoid action will further exac- lactating cows now contain 200 to 250 g of magnesium erbate urinary losses. Many affected animals are unable to even istration represents a potentially acute cardiotoxic risk. Recommendations include oral ad- factors including dry matter intake, concurrent metabolic ministration of up to 0. Indeed, the inevitably of mild hypoka- lemia in association with anorexia in the postpartum cow has led to the inclusion of potassium supplementa- A tion by many practitioners to cows that receive oral uids for whatever reason. Low level supplementation in the order of 60 to 125 g is well tolerated and safe when large volume orogastric uids are administered. A and B, Cow with severe hypokalemia and recum- Elcher R: Evaluation of the metabolic and nutritional situation in bency. The cow exhibited accid paralysis manifested as dairy herds: diagnostic use of milk components. Bones Shallow cavity around tailhead with No cavity around tailhead and fatty of pelvis and short ribs sharp and some fatty tissue lining it and cover- tissue easily felt over whole area. Thick layer of tissue covering surfaces can be felt with slight pres- top of short ribs, which can still be sure. For clostridial myositis to develop, both the organism and a suitable anaerobic environment for its vegetative growth must be present. Clostridium perfringens myositis in the right hind limb of Malignant edema implying any clostridial myositis a Holstein cow subsequent to an intramuscular injec- rather than specic C. Young cattle ap- organism survives in soil, but it is not known whether it pear to be at greatest risk for C. However, we investigated a herd ently allows the vegetative form to proliferate in the gut epidemic of C. The cows in this outbreak had grazed pastures the muscle and liver, the organism remains innocuously in previous summer, but the epidemic occurred during the the spore form unless the surrounding tissue is injured in winter months and was triggered by muscle bruising and some way that creates an anaerobic environment suitable trauma as a result of crowding through a narrow passage for vegetative growth of C. Note the extensive surgical that is leukocidal and increases capillary permeability. However, it may not be so much the drug itself but produce toxins other than the alpha-toxin, depending on the fact that some people injecting a drug do not clean, the serotype involved. Regardless of the causative species, most clostridial The signs of clostridial myositis include fever, depres- myositis in dairy cattle occurs by exogenous routes.
The variable part of the pilin gene has alternate cassettes stored in adjacent locations order cheap naprosyn on line arthritis after back fusion. Inverted repeats ank the pair of alternate cassettes purchase naprosyn american express dog arthritis medication over the counter, causing the whole complex occasionally to ip orientation buy line naprosyn rheumatoid arthritis mouth sores. Several bacteriophage use a similar inversion system to switch genes encoding their tail bers cheap naprosyn master card arthritis neck food, which determine host range (Kamp et al. These low-diversity switches provideonlyalimitedadvantage against immunity because, even if the switch rates were low, an infection would soon contain all variants at appreciable abundance. Thus, these switch mechanisms may serve mainly to generate alternative attachment vari- ants. Antigenic vari- ation appears to be common and to be caused by diverse mechanisms. Infection and reproduction in host erythrocytes determine the build- up of parasite numbers within the host (Mims et al. Each parasite exports only one var type to the erythrocyte surface, but a clone of par- asites switches between var types(Smith et al. Switching between var loci does not depend on the mechanism of gene conversion found in Borrelia hermsii and Trypanosoma brucei. There are at least eleven and perhaps as many as fty discrete genes that encode variants of p235 (Borre et al. Within an erythrocyte, the parasite develops a multinucleate stage and then divides into new merozoites that burst the host cell. They suggest that upon division into separate merozoites, each merozoite presents a dierent p235 protein on its surface. The various p235 mole- cules may facilitate invasion of dierent classes of erythrocytes. Other Plasmodium species express surface proteins that are distantly relatedtop235, but in those cases the surface molecules do not arise from an antigenically diverse, multicopy gene family (Barnwell 1999). Some of the Plasmodium species have diverged tens of millions of years ago, so it is not surprising that they have dierent strategies for attach- ment, immune evasion, and antigenic variation. The parasite expresses only a small subset of these genes in an infected erythrocyte. Sera from twenty-ve previously infected hosts provided a panel of antibodies to test for prior exposure to the vir gene products. One of the expressed proteins reacted with the serum from only one host, the other proteinreacted with sera from two hosts. Thus, vir gene products are immunogenic, but each variant appears to be expressed rarely the hallmarks of antigenic variation from a large archival library. Thediversityofgene families in Plasmodium that play a role in antigenic variation provides an excellent opportunity for comparative, evolutionary studies. These vsp loci are silent, archival copies that can, by gene conversion, be copied into the single expression site. The genes dier by 30 40% in amino acid sequence, providing sucient diversity to reduce or elimi- nate antigenic cross-reactivity within the host. Those analysesfocuson attributes such as runs of similar nucleotides between loci that occur more often than would be likely if alleles di- verged only by accumulating mutations withineachlocus. The archival antigenic repertoire of Trypanosoma brucei evolves rap- idly (Pays and Nolan 1998). This species has a large archival library and multiple expression sites, but only one expression site is active at any time. New genes can be created within an active expression site when several donor sequences convert the site in a mosaic pattern (Pays 1989; Barbet and Kamper 1993). When an active expression site becomes inac- tivated, the gene within that site probably becomes protected from fur- ther gene conversion events (Pays et al. Perhaps new genes in silent expression sites can move into more per- manent archival locations by recombination, but this has not yet been observed. These examplesillustrate the scattered reports of recombination and the evolution of archived repertoires. These preliminary studiesshow the promise for understanding the interaction between mechanisms that create diversity and the strong forces of natural selectionimposed by immune recognition. Thecombination of generative mechanisms and selection shapes the archival antigenic repertoire. Reassortment of inuenza A s neuraminidase and hemagglutinin sur- face antigens provides the most famous example (Lamb and Krug 2001). When two or more viruses infect a single cell, the parental segments all replicate separately and then are packaged together into new viral particles. New neuraminidase-hemagglutinin combinations present novel anti- genic properties tothehost. Rare segregation events have introduced hemagglutinin from bird inuenza into the genome of human inuenza (Webster et al. The novel hemagglutinins cross-reacted very lit- tle with those circulating in humans, allowing the new combinations to sweep through human populations and cause pandemics. Intergenomic recombination occurs when chromosomes from dier- ent lineages exchange pieces of their nucleotide sequence. In protozoan parasites such as Plasmodium and Trypanosoma,recombination hap- pens as part of a typical Mendelian cycle of outcrossing sex (Jenni et al. Recombination can occur in viruses when two or more particles infect a singlecell. In all cases, even rare recombination can provide an important source for new antigenic variants. To what extent have molecular attributes of antigenic genes been shaped by the costs and benets of generat- ing variants? Do microbes under intense immune pressure have higher genome-wide mutation rates compared with similar organisms that do not face immune attack? To what extent have nucleotide sequences of antigens been shaped by the tendency of particular motifs to generate replication errors a form of local hypermutation? Idescribed various mechanisms bywhichgene expression shifts between archived variants. The rate at which switches occur prob- ably aects the parasite s ability to extend infection. If switches happen too quickly, then novel variants will not be expressed after the immune response develops against the many variants expressed early in infec- tion. If switches happen too slowly, then the parasite may be cleared before the variants are expressed. Thus, natural selection can strongly inuence the molecular details of the switch process in order to adjust the rate of change between variants. One could also test the evolution of the switch rate in vivo, comparing situations that imposed dierent immune pressures on rates of change and on particular orders in which variants are expressed.
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