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CLONAL ORGANISMS.
Term Paper ID:24799
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Reproduction, genetic & phenotypic diversity, growth & life cycles of modular, sessile organisms (bryozoans, pathogens, snails).... More...
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Paper Abstract:
Reproduction, genetic & phenotypic diversity, growth & life cycles of modular, sessile organisms (bryozoans, pathogens, snails).
Paper Introduction:
Comparatively little attention has been paid to the life histories of many modular, sessile organisms such as sponges, bryozoans and cnidarians, as well as terrestrial and marine animals and plants (9). Vernon et al. (23) have noted that while clonal reproduction is commonly reported in literature describing metazoan life cycles, there is yet little understanding of how these organisms deal with natural enemies or habitat, and how such elements affect the expression of a modular lifestyle. Presented here are studies which describe selected clonal organisms, relating such aspects as reproduction, genetic and phenotypic diversity, and growth to the highly specialized mode of clonal life cycles.
Sackville Hamilton et al. (19) have observed that it has been difficult to assign a role--and even a consistent name--to
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Most DNA fingerprinting techniques examine the polymorphismsequences in only the chromosome or in the plasmids or only a single genefragment (22). Orive, M. Appl. A more recent work by Orive (17) further relatespopulation size to genetic relatedness, and provides a model which focuseson the time at which two alleles last shared a common ancestor to describetheir relationship in more informative, genetic terms. Proc. (19)present an overview of the life history and population biology of clonalorganisms, proposing a model to describe different strategies in relationto r and K-selection. This has proven especially useful inbacterial studies. (1 ) further investigated the role of environment in shaping clonalgrowth, in this instance, in stimulating the appearance of sexual versusasexual forms. (white clover), Hutchings et al. An examination ofa single wild population of these snails (7), showed that triploidparthenogenetic forms co-existed with the sexual forms across threedifferent habitat zones, with the clonal forms displaying high diversity(165 genotypes among 6 5 snails). Bifani,P.; Plikaytis, B.; Kapur, V.; Stockbauer, K.; Pan, X.;Lutfey, M.; Moghazeh, S.; Eisner, W. These authors predicted that in suchvertebrates, asexual reproductive phases will yield abundant hybridsinhabiting large ranges, frequently in harsh or disturbed environments.Here, reproduction in the absence of genetic recombination may serve toenhance adaptation, by producing large numbers of co-existing clonal sub-populations whose performance would vary in the face of differing selectivepressures. and Immun. Instead, success may be more a function of growth environmentoperating at the level of sexual or asexual phases within a givenpopulation. In the case of the clonal pathogenBorrelia burgdorferi, which causes Lyme borreliosis in man, an examinationof genetic trees of plasmid sequences taken from fifteen isolates, noevidence was seen for genetic exchange between chromosomal genes and clonesrarely exchanged plasmids between themselves. Random amplified polymorphic DNA markers have also revealed moresubtle aspects of the life histories of clonal organisms. The role of habitat partioning in influencing phenotypic expressionin clonal animals was further defined by Jokela et al. 63(6):22 6-12; 1995 (Jun.).17. 7. Irreversibility of cellular aging andneoplastic transformation: a clonal analysis. Dushyantha, W.; Hutchings, M. Proc. Additionally, habitat partioning seems especially to favor theappearance or maintenance of clonal forms in a population. 'Spontaneous' transformation as aberrant epigenesis.Differentiation. Gray, J.; Herwig, R. Semlitsch, R.; Hotz, H.; Guex, G.-D. 5. J. Rubin (18) examined the response of such cells when the levelof nutrient in the growth environment was altered. Ecol. 8. Ramets or individuals that share thesame genotype for all polymorphic loci can be described as a clone (11).This nomenclature impacts studies of clonal organisms in several ways.Reproductive success can be measured in terms of successful growth ofgenets, modules or both. The creation of models to determine genetic relatedness andinbreeding in organisms has shed considerable light on the degree ofgenetic diversity seen in clonal organisms. Theor. In Uvularia,habitats marked by good physiological conditions (i.e., sunny patches)favored the appearance of sexual, or seed-producing forms. Reports of segregation of asexual and sexual forms in some animalswere used by Schenk and Vrijenhoek (2 ) to argue against an "ecologicallyneutral" relationship between such forms. Comparatively little attention has been paid to the life histories ofmany modular, sessile organisms such as sponges, bryozoans and cnidarians,as well as terrestrial and marine animals and plants (9). All clone colonies in the study were verysimilar genetically and all suffered parasitism, to the possible extent ofan observed population collapse. 62(11):4 49-59; 1996 (Nov.). Shady areas wereseen to promote the existence of vegetatively-reproducing ramets, often forperiods extending to years. Probability of clonalidentity: inferring the relative success of sexual versus clonalreproduction from spatial genetic patterns. Individuals within clonal populationsmay apparently retain the ability to alter their phenotype in response toenvironmental cues. Lond.(B)1375:1313-8; 1996. DNA fingerprinting techniques for micro-organisms: a proposal for classification and nomenclature. In a study of a single clone lineage of Daphnia pulex, Lynch (15)concluded that clonal divergence yielded between 1 percent and 21 percentof the total phenotypic variation expected for certain life-history traitsseen in this organism. Clonal life cycles permitpopulations to amplify the phenotypic diversity already inherent in theirsexual genomes. Genotypic variation and clonal structure in coralpopulations with different disturbance histories. J. Researchers understand that much of the variation seen in the lifehistories of clonal organisms revolves around sex (9). Harada, Y.; Kawano, S.; Iwasa, Y. Flat reaction normsand "frozen" phenotypic variation in clonal snails (Potamopyrgusantipodarum). Support for the "frozen-niche theory" is also found in a study bySemlitsch et al. 5 (4):1541-1548; 1996. The freshwater snail (Potampyrgus antipodarum) occurs naturally as co-existing populations of sexual and clonal subpopulations. Theobserved pattern of clonal expansion was likely due, in part, to antibioticselection operating on randomly occurring mutants in the diseasepopulation, in addition to the more disturbing likelihood of inadequatelytreated infection. Gray and Herwig (8) successfully extracted microbial DNAfrom marine sediments, yielding high molecular weight DNA for amplificationand analysis. While markers were able to delineatefour distinct clones of bryozoan, there was no evidence that the success ofthe most common clone was directly related to the relative level ofparasitism it experienced. Envir. Jokela, J.; Lively, C.; Fox, J.; Dybdahl, M. Using a computer simulation program in combination withgenetic marker data from multi-locus allozymes, randomly amplifiedpolymorphic DNA (RAPD), and mitochondrial DNA, these authors generated aseries of spatial genetic grids, and proposed a recurrence formulaindicating the probability of clonal identity in their plants. Chow, M.; Rubin, H. 275(6):452- 7; 1996 (Feb.). For invertebrates displaying both clonal and sexually reproducingforms, several studies have examined the impact of habitat in regulating orinfluencing these forms. In general, success and growth in clonal organisms appear to be aresult of many, interacting variables. Sci.USA. Can. Microbiol. 44(1):25-32; 1997 (Jan.-Feb.). Growth sensitivity to outside stimulus has also been described interrestrial clonal plants. (19) have observed that it has beendifficult to assign a role--and even a consistent name--to describeacceptably the individual when this concept is examined from the viewpointsof animal and plant biologists. Intraspecific variation in Cryptocaryouirritans. Evol. Both have contributedgreatly to a clearer understanding of how variation occurs in clonalpopulations. The effects of environmental heterogeneity on the growth of clonalorganisms has been studied in a multitude of species. Comparative studies of the role of habitat in producing a variety ofecologically different forms have further delineated the highly complexnature of the life histories of clonal organisms. 51(4):1249-1261;1997.22. In studies of clonal pathogens where clonal identity may becritical to the creation of successful vaccine, fingerprinting techniqueshave made significant contributions. Effects of spatial scale ofenvironmental heterogeneity on the growth of a clonal plant: anexperimental study with Glechoma hederacea. When forced to grow into habitats of extremepatchiness, plants behaved as though the niche were uniformly orhomogeneously poor. Hutchings, M.; Turkington, R.; Carey, P; Klein, E. Differential infection by parasites withinthe host population may be based on genotype in ways that may not always beclearly understood. In addition to producing large numbers of potentiallyrapidly dispersing forms of a population, clonal reproduction may serve toreduce or change the critical impact of many selective pressures, amongthem parasitism, on a population. In this way,natural selection operates to promote the appearance of clonal forms withina population, allowing for enhanced distribution and diversity ofphenotypic forms into harsher or ecologically undesirable habitats. Habitat also appears to influence theappearance of sexually reproductive forms in corals, as seen in a multi-species comparative analysis which indicated that colony growth,irrespective of species type, is favored at the expense of sexualreproduction until some size-specific point in relation to habitat isobtained (9). Using a comparison of amplifiedrDNA derived from sixteen isolates, and checking for spacer regions,Diggles and Adlard (5) showed that the corals taken from estuarine sourcesvaried signficantly in sequence from those found in reef habitats. Norris, S.; Howell, J.; Garza, S.; Ferdows, M.; Barbour, A. Some of these alterations may be heritable (3, 18),specifically in the instance of growth constraints. Changes did not spread across the colony as a whole, arguing forhigh plasticity and complexity in the 'neighbor response' and raisingquestions about the ability of clones to respond locally to the intrusiveeffects of physical stimuli, including that provided by neighbors. By sheer numbers aswell as their unusually marked abilities to colonize, these organisms havecome to represent a highly adaptive alternative strategy as far as habitatexploitation is concerned. The frequency of triploid clonalsincreased with increasing lake depth, with a proportionately greater numberof clone individuals showing marked habitat specificity. Damman, H.; Cain, M. In many studies, actual variation can be revealed byamplified fragment length polymorphism (AFLP) or restriction fragmentlength polymorphism (RFLP) or by electrophoresis, with the use of variouscombinations of techniques revealing the greatest levels of taxonomicdifferences. Effective population size in organisms with complex life-histories. 75:1382-1393; 1997.14. 53(2):123-37; 1993 (Jun.).19. 6. Harshman andFutayama (11) described high diversity in the rare asexual forms which weregathered from a natural populationl. Evol.39(2):315-324; 1985(a).12. Sackville Hamilton, N.; Schmidt, B.; Harper, J. Additional evidence that clonal isolates can be highlydistinct comes from studies of reef corals. Harshman, L.; Futayama, D. 47(4):1213-1228; 1993.13. Yet the capabilityto reverse environmentally-induced transformation was not uniformlyexpressed across the two cell cultures examined. 85(5):591-6 ;1997.11. He found that the growthbehavior of NIH 3T3 cells changed in successive passages and that changeswere dependent on the individual passage regime. In some terrestrial clonal plants, not only the physical environmentbut also the presence and type of neighbor organism has been shown toimpact growth. Pop. These authors assessed the changes in growth response tovariations in nutrient supply in three genets of white clover selected froma field population. Reproductive strategies of modularorganisms: comparative studies of reef-building corals. Spatial and temporal factorsaffecting co-existence among sexual and clonal forms of Poeciliopsis.Evol. Fitness can be examined similarly, with geneticdescendants arising from varying proportions of these sources. In the geometrid moth (Alsophila pomateria) newasexual genotypes typically arise from the sexual population, and singlefemale moths can produce both sexual and asexual offspring. Schenk, R.; Vrijenhoek, R. Literature Cited 1. Infect. Ecol.77(3):95 -963; 1996.1 . Many of these techniques have revealed significant levels ofvariation within clonal populations. Evol. 9. (14). Fox, J.; Dybdahl, M.; Jokela, J.; Lively, C.; Genetic structure ofco-existing sexual and clonal subpopulations in a freshwater snailPotampyrgus antipodarum. Sackville Hamilton et al. (13) found that the observedresponse of any given clone varied in relation to the genotype of theneighbor in its growth path. More important, by varying nutrient suppliesand proximity to a neighboring clone in three genets of field populationsof clover, these workers were able to establish that the observed changeswere localized to those areas of the clone in closest proximity to theneighbor. Eukar. (B)232:35-57; 1987.2 . This indicated that of theclones examined, all were so highly divergent that vaccines developed tocombat one would be highly unlikely to effectively immunize against any ofthe others, with serious implications for control (16). In this study, evidence from electrophoretic assay of enzymesobtained from coral branches suggested that genotypic diversity in thisclonal organism was directly related to habitat disturbance (whose criteriaincluded consideration of the degree of damaged or broken branches, and thespace available for growth). Ecol. Studies of cell linesin culture, specifically those of NIH 3T3, have permitted many workers toclosely examine elements of cellular aging and neoplastic transformation.Populations of these cells can be followed in successive generationalpasses, and their response correlated with exposure to differentenvironments. 93(18):9793-8; 1996 (Sept.). Browne, R.; Hoopes, C. Mol. Population growth and viability analyses ofthe clonal woodland herb, Asarum canadense. Life- historyconcepts and the population biology of clonal organisms. Ecol. Hunter (12)reported variations in genotype and clone structure after colonies of reefcoral (Porites compressa) were exposed to different histories ofdisturbance. Vernon et al.(23) have noted that while clonal reproduction is commonly reported inliterature describing metazoan life cycles, there is yet littleunderstanding of how these organisms deal with natural enemies or habitat,and how such elements affect the expression of a modular lifestyle.Presented here are studies which describe selected clonal organisms,relating such aspects as reproduction, genetic and phenotypic diversity,and growth to the highly specialized mode of clonal life cycles. Measurements ofroot:shoot ratios and overall changes in biomass revealed that in thisherb, growth is modified, with coarse-grained foraging displayed whenhabitat patches were large. In sessile marine animals such as corals, the growth response ofmodular organism has also been equated with foraging behavior. Analyzing the relationshipsbetween naturally occurring sexual and asexual forms of topminnow(Poeciliopsis), these authors found evidence that spatial segregation washabitat-controlled, specifically at the level of patchiness or graininess.In this study, broad seasonal shifts affected clones in different habitats,resulting in the differential appearance of sexual and clonal fish. Spontaneous mutations for life history characters in anobligate parthenogen. Hunter, C. Indeed, studies of the response of clonal organisms to the effects oftheir immediate habitat has provided much insight into more complicatedelements of the modular lifestyle. Evol. J. Nat. Further, these workers showed that thedistribution of clonal forms was heavily dependent on host plants. J. Diggles, B.; Adlard, R. In a study whichexamined over time the relationship between a fresh-water bryozoanpopulation and its associated parasite, Vernon et al. The related question ofthe reversibility of such transformations in NIH 3T3 cells was studied byChow and Rubin (3), who found that experimental cells appeared to modifytheir growth based upon the density of the population. In support of suchmodels, the last decade has seen the emergence of genetic markers andreproducible techniques of DNA fingerprinting. Sampling technique, whether by observation, genetic markers orallozyme procedures, can be a critical aspect of clonal population studies.Scale is especially important; insufficient or inadequate sampling mayentirely miss the spatial genetic structure of a population (1 ) andconfound the identification of clonal replicants versus the appearance ofgenuinely unique genotypes. Biotech.6(2):115-42; 1996 (Oct.).23. 4 (5):1 6 -1 7 ; 1986.21. Ecol. Phylogenetic analysis of the bacterialcommunities in marine sediments. Competition amongtadpoles of co-existing hemiclones of hybridogenetic Rana esculenta:support for the Frozen Niche Variation Model. Evol. Based upon these relatively high levels ofvariation, he proposed that the clonal lifestyle was entirely sufficient tomaintain significant levels of phenotypic evolution, due solely tomutation. Morphologicalplasticity in Trifolium repens L.: the effects of clone genotype, soilnutrient level, and the genotype of conspecific neighbors. In a study using polymorphic GC-rich repetitive sequences (PGRS),hybridization patterns, and automated DNA gene sequencing, Bifani et al.(1) tracked the origin and spread of a multi-drug resistant clone family ofMycobacterium tuberculosis across state lines. Hall, V.; Hughes, T. 51(4):112 -1129; 1997.15. 44(3):316-4 ; 1993 (Dec.).18. 3. Nat.Soc. Using allozymic and mitochondrial DNA analysis, Browne and Hoope(2) demonstrated similar evidence of high phenotypic diversity in theasexually reproducing brine shrimp (Artemia), suggesting that meioticprocesses could in themselves generate allozymic diversity in naturallyoccurring populations. By comparing distributionpatterns of disease isolates taken from actual patients, these authorscould show that the pathogen in question was in fact a closely relatedfamily of resistant organisms that had shared a recent common ancestor. (23) found highlevels of clonality and parasitism. For animalsexhibiting more than one sexual phenotype, questions arise concerning thenature of population structure and genotypic dynamics and the advantagesand disadvantages of sexual versus asexual reproduction in modularorganisms. Highand low infecting phenotypes of clonal populations of in vitro-culturedBorrelia burgdorferi. In the process of natural selection, they then"freeze" certain genotypes ("frozen-niche theory") which are alreadypresent in the ancestral sexual population. Royal Soc. In these organisms, the clonal growth habitessentially permits lateral expansion by placing daughter ramets at somedistance from the parent plant, and is thus commensurate with foragingbehavior (13). Biol. Evol. In an examination of morphological plasticity of Trifoliumrepens L. Genetic diversity and selection in asexualbrine shrimp (Artemia). Vaneechoutte, M. Temporalpatterns of clonality and parasitism in a population of freshwaterbryozoans. 39(4):8 4-818; 1985.16. 44(4):1 35-1 51; 199 . 4. Clones may effectively actto randomly sample habitats. J.Bot. The origin and distribution of clonaldiversity in Alsophila pomateria (Lepidoptera, Geometridae). Strict density dependence does notappear to be a directly limiting cause maintaining constant populationsizes (4). 86(1):13-26; 1998. Sackville Hamilton et al. 2. Acad. The advantages and disadvantages of sexual versus asexualreproduction will continue to be debated, but the ecological success ofclonal organisms is no longer a matter of question. In a very elegant study of the perennial Uvularia (Liliaceae), Haradaet al. Observationsof naturally occurring Potampyrgus led these authors to propose that clonescan display signficant differences in phenotype. Dushyantcha andHutchings (6) exposed the clonal herb Glechoma to a variety of testenvironments, including two different soil types and concluded that growthresponse was related to the scale of heterogeneity encountered by theplant, i.e., the patchiness of the foraging area. Vernon, J.; Okamura, B.; Jones, C.; Noble, L. Lynch, M. Individual clonal units are generallyreviewed in terms of genets (the result of mitotic growth derived from asingle zygote), and modules (the multicellular 'iterative unit', seenduring the process of genet growth). 85(1):17-28; 1997. Microbiol. Rubin, H. This response to the frequency of habitat heterogeneityled the authors to propose a time-factor of response, where plants might beunable to adequately to detect such changes and adjust their morphologyrapidly enough to confront swiftly changing environmental pressures. Origin and interstate spread of aNY City multi-drug resistant Mycobacterium tuberculosis clone family.JAMA. Proc. Lond. (21) which examined competition among hemi-clone tadpolesof frogs (Rana esculenta). Foraging behavior, as measured by stolon growth wasaffected by nutrient availability, with longer stolons produced in responseto nutrient-poor conditions and fewer, more compact ones in favorable ones.In this way, clones tended to consolidate their growth within richhabitats, and to increase the probability of escaping the poorer niches bygrowing past them.
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