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Cycad Toxins and Neurological Diseases In HORTSCIENCE 40(6):1598–1606. 2005. for ALS–PDC must be put to rest before the horticulture industry can expect increased use of this magnifi cent specimen plant. Cycad Toxins and Neurological One troublesome facet of past research has been the highly variable concentration Diseases in Guam: Defi ning of various toxins from batch to batch within a study, and from study to study within and Theoretical and Experimental among research teams (e.g., Palm and Cycad Societies of Australia, 1993, and citations therein). Indeed, this variability has been a Standards for Correlating Human reason for failures to embrace the cycad hy- pothesis. However, this variability need not be Disease with Environmental Toxins viewed through the eyes of a skeptic using an 1 enigmatic fi lter. We believe the documented Thomas E. Marler variability would have been explicable if College of Natural and Applied Sciences, University of Guam, UOG Station, background biotic and abiotic habitat factors Mangilao, Guam 96923 had been recorded, and appropriate sampling procedures had been used. Vivian Lee and Christopher A. Shaw We focus on several steryl glucoside sub- Department of Ophthalmology, University of British Columbia, Vancouver, types in our research (see Marler et al., 2005) BC, Canada and the present paper for two reasons. First, other known cycad toxins such as methyl azoxyl- Additional index words. ALS–PDC, BMAA, cycad, Cycas micronesica, Guam, neurotoxins, methanol (MAM) and β-methyl-amino-alanine secondary compounds, steryl glucosides (BMAA) have been studied for decades in spite of the fact that most do not generate behav- Abstract. Consumption of Cycas micronesica seed tissue has been associated with the amyo- ioral or pathological outcomes that resemble trophic lateral sclerosis–parkinsonism dementia complex (ALS–PDC) of the Western Pacifi c. ALS–PDC (reviewed by Wilson et al., 2002). However, failures to document vital plant and neighborhood descriptors and pronounced Our current work strongly implicates steryl variability in toxin concentrations noted within and among studies obfuscate decades of glucoside variants as contributing to ALS–PDC- research on this subject. We discuss the theoretical and experimental constraints of plant like outcomes in cycad fed mice (Khabazian et tissue sampling in relation to human disease research. Comparisons are made between this al., 2002; Shaw and Wilson, 2003; Wilson et approach and methods used throughout the history of ALS–PDC research, most notably al., 2002), mirroring in vivo data obtained by very recent reports concerning -methyl-amino-alanine. Methods for studying possible plant β earlier researchers (Ambike and Rao, 1967). neurotoxins need to be standardized and must follow rigorous criteria to be valid in principle. Second, the well-known cycad toxins such as Our discussions reveal why these criteria are essential and highlight the impact that natural MAM and BMAA are removed from cycad variations have on environmental toxin quantifi cation and interpretation. Past research tissue during the water-leaching used in tradi- on cycad toxins is defi cient on experimental and theoretical grounds, and interpretation of tional fl our preparation methods, but the steryl published data is dominated by ambiguities. This area of study as conventionally conceived glucosides are insoluble in water. Our ongoing and carried out needs transforming. We argue that future empirical studies should honor research includes all known neurotoxins in cycad appropriate plant science standards concomitantly with medical science standards. This dual tissue, since the role of these metabolites in focus will ensure appropriate sampling scheme, sample size, and reporting of background cycad plants has never been addressed in past plant and community factors known to infl uence phenotypic plasticity. research (Brenner et al., 2003). Theoretical and The island of Guam claims an endemic toxins have been published throughout recent experimental standards described in the pres- cycad species and an unwelcome high historical decades, and many of these were in pursuit of a ent study will be used to defi ne procedural and incidence of neurological disease among the reasonable link between exposure to cycad com- conceptual concerns as other candidate toxins Chamorro residents. The fi rst written record pounds and the prevalence of ALS–PDC (e.g., are given full scrutiny. suggesting that consumption of the cycad seed Kurland, 1993 and citations therein). However, Our aim in the present paper is to describe tissue was a probable cause of the neurologi- no article within this sizeable body of literature the multitude of omissions and shortcomings cal diseases was made by Spanish Lieutenant adequately described fi eld sampling methods, that have defi ned historical and ongoing research Colonel Felipe de la Corte in 1865 (as cited by habitat, conspecifi c and interspecifi c neighbors methods in this fi eld and illustrate the need Rogers, 1995). His opinions were ignored until of sampled plants, scaling and allometric rela- for appropriate theoretical and experimental the mid-20th century when the disease now tions, plant life history characteristics and events, directions in future research to remove the am- known as ALS–PDC and the cycad now known and other factors that are known to infl uence biguities that are defi ning of this large literature as Cycas micronesica K.D. Hill fi rst received secondary metabolism of plants. Unfortunately, base. Modifying the patterns of past research joint attention from the medical research com- this assertion holds true for the most recent addressing the link between the Guam cycad munity (reviewed in Kurland, 1988; Kurland et publications in this fi eld (e.g., Banack and Cox, and Guam’s neurological diseases to include al., 1994; Mabry, 2001). Consumption of cycad 2003; Yagi, 2004), despite the exposure of this appropriate plant science standards is critical megagametophyte tissue as processed fl our was manifest shortcoming of inadequate sampling for gaining a complete understanding of the commonly practiced, and the resulting inadver- methods by Zhang et al. (1996). chemical ecology in these stunning plants. tent ingestion of cycad toxins was considered the The endemic C. micronesica has not been We begin with two sections discussing the vehicle for human exposure (Whiting, 1963). popular in Guam’s horticulture industry. Al- various plant and habitat characteristics that Numerous articles and review chapters on cycad ternatively, C. revoluta Thunb. is one of the likely infl uence phenotypic plasticity in cycad most common landscape specimens in the biochemistry, factors that have been ignored Received for publication 29 Dec. 2004. Accepted commercial and residential landscape. Our in past research. for publication 27 Apr. 2005. We thank J. Chung, C. recent surveys revealed that the purported Melder and N. Dongol for assistance. Support pro- vided by USDA CSREES Special Grant in Tropical/ link between the native cycad and the dreaded Ignoring what Cannot be Ignored: Subtropical Agricultural Research to TEM and U.S. human disease is partly to blame for the lack Aerial Factors Army Medical and Materiel Command (DAMD17- of interest in using this handsome plant in 02-1-0678), NSERC Canada, and Scottish Rite the local horticulture industry. As a result, Control over known and probable biases Charitable Foundation of Canada to CAS. unanswered questions surrounding the hy- must be included in the sampling protocols in 1Corresponding author; e-mail tmarler @ uog.edu. pothesis that cycad toxin exposure is causal environmental research to ensure accuracy and 1598 HORTSCIENCE VOL. 40(6) OCTOBER 2005 OOctober.indbctober.indb 11598598 88/11/05/11/05 88:55:17:55:17 AAMM COVER STORIES repeatability. Following is a short list of factors research to clarify the infl uence of seed age on level is controlling of many plastic responses, that defi ne the aerial environment of cycad plants neurotoxin concentration is warranted. Seed age including synthesis and allocation of second- and that elicit phenotypic plasticity of plant has never been reported in past publications in ary compounds (e.g., Callaway et al., 2003; secondary chemistry. These are the major fac- this area of research. Herms and Mattson, 1992). Moreover, the tors that we have documented and standardized The timing and magnitude of resource alloca- level of incident light at microsites within a for our own research (Marler et al., 2005). We tions within a plant are determined by modular plant canopy is known to infl uence reproduc- suggest documenting each of these factors as organization, with each growth module behav- tive growth characteristics of those microsites a minimum for reliable interpretation of future ing semi-autonomously (Herms and Mattson, (Childers, 1995). For example, seeds on the research on cycad secondary metabolism. 1992). The synchrony of these growth modules north and south orientations of a cycad plant The chemical constituency of a plant organ among C. micronesica individuals within a land- may experience less incident light than on the observed at one point in time is the result of scape is increased by typhoon damage (Hirsh east and west orientations. Moreover, infl uence many processes in plant life history. For concen- and Marler, 2002). With each successive growth of shade from neighboring plants on cycad tration, pool size, and allocation of secondary module thereafter, the population becomes less secondary metabolism is unknown. We suggest
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