AN OVERVTEWOF CHEMOTAXONOMY AND tTS ROLE rN CREATINGA PHYLOGENETICCLASSIFICATION SYSTEM COLEENA MANNHEIMER NationalBotanical Research lnstitute Ministryof Agriculture,Water and RuralDevelopment, P/Bag13184,Windhoek. INTRODUCTION chemotaxonomicinvestigations have been employed at all levelsof thetaxonomic heirarchy, from subvariety to division (Smithin Street, 1978; Stace, 1980). lt isthought that when Chemotaxonomyis the systematicstudy of chemical the groupsin questiondifferentiated, the abilityto form a variationbetween olant taxa. Evidenceof chemicalvariation chemicalsubstance was retainedby virtueof metabolic has essentiallybeen usedfor classificationpurposes ever processesretained by the group or its ancestors. By since 'folktaxonomies', based on certainobvious plant implicationwe seethat if the pathwayof chemicalevolution characteristicswere instinctivelyemployed by mankind were establishedthen this mightoff er insightto the centuriesago. Thesecategories, such as edibility,taste, evolutionaryhistory of the group,as well as to the colour,smell and medicinalvalue were founded, however understandingof the present-dayrelationships within and unknowingly,on chemicalproperties. As longago as the betweengroups (Erdtmann, cited in Davis& Heywood, first centuryafter Christthe aromaticmints had been 1973). recognisedand groupedtogether by Dioscorides(Jones & Luchsinger,l986). Researchhas shown that there is generallyan inverse relationshipbetween the taxonomicdistribution of a plantsgrew from Awarenessof the chemicalcomplexity of compoundor class of compoundsand its biogenetic the desiresof Europeansfor exoticspices and condiments complexitysuch that if it is biosyntheticallysimple and properties. as wellas investigationsinto their medicinal Early widespreadit may be assumedto be primitive,while those knowledgeabout the subjectwas summarisedin herbals, morelimited in theirdistribution and morecomolex to bio- physiologically andconcentrated on informationabout active synthesisemay be assumedmore advanced (Gershenzon secondarymetabolites such as alkaloidsand saponins & Mabry,1983). Howeverthese assumptionsprove (Jones in Street,1978). simplisticwhen phylogenetic inferences are to be madefrom chemicaldata. Davisand Heywood(1973) indicate the Duringthe eighteenth and nineteenth centuries knowledge followinggeneral complications:- in the field increased,and sometaxonomists made use of severalchemical characteristics inattempts to classifyplants 1. Chemicalcompounds and/or biosynthetic pathways andto demonstratetheir phylogeny. However, although the are not alwaysenvironmentally stable, and so their chemicalcharacters they usedwere recognised, they were presencemay be affectedquantitatively or manifestationsof processesor compoundsnot yet qualitativelyby factors such as temperatureor mineral completelyidentified (Jones & Luchsinger,1986) and so their deficiencies. use was basedon inadequateknowledge and evidence. 2. Biosyntheticknowledge is inadequateabout a great Graduallythe numberof recognisednatural plant products manyof the chemicalcharacters used in taxonomy, increased,extending to includeproteins, nucleic acids and so that parallelismor convergencemight confuse the majorpolysaccharide categories. At the same time perceptionof phylogeny.A compoundmight arise researchinto plantmetabolism revealed similarities and viatwoor moreentirely distinct biosynthetic pathways uniformitiesin the chemicalfunctioning of plants,while (convergence),and its presencewould thus not simultaneouslyhighlighting biochemical peculiarities which necessarilyimply a relationshipbetween taxa in which mightbe taxonomicallyor phylogeneticallysignificant. it occurs.(Gershenzon and Mabry (1983) quote the Successfulattemots were made to correlatethis variation caseof napthoquinones,which may arise in higher with knownclassifications, and manyclaims were made as plants vrafour different pathways). Parallelism may to thetaxonomic merit of variouschemical characters (Smith causesimilar end products to arisein relatedgroups, in Street,1978). Howeverit is only in recentdecades that particularlywhen they are derivedfrom ubiquitous reasonablyrapid surveys of plant extractshave become metabolicintermediates. feasible,due to improvedtechniques of chemicalanalysis and the elucidationof the structuresof many organic 3. ln manycases it is not knownwhether metabolic compounds(Radford et al,1974).Technological advances, pathwaysare reversibleor not, and reversalof a particularlyelectrophoresis and chromatography,have pathwayconfuses the issueof whethera character simplifiedand speededup analyses,and also oftenmade is primitiveor derivedin a specifictaxon. analysisof smalleramounts of materialviable. This is particularlyvaluable when rare herbarium material must be 4. Largeand/or complex molecules are not necessarily used. the mostadvanced. Chemical reduction mav occur in plantsdue to metabolicchanges. It is now generallyaccepted that certaincompounds and relatedsubstances may be characteristicof certain 5. Chemicaland morphologicalfeatures may have taxonomicgroups (Davis & Heywood,1973), and certainly evolvedat differentspeeds due to differingselection AGRICOLA1998/1999 87 'primitive' pressures,and thus one might find chemical - they are chemicallystable, so that analysisof 'advanced' characterstogether with morphologyin a materialcan be doneVears after the materialis taxon. collected. - they are easilyisolated and identifiedeven from 6. Chemicalvariability occurs - for example,over a smallamounts of plantmaterial. Large numbers geographicrange, or vr,achemical mutants within of plantscan easilyand rapidlybe surveyedfor species. Chemistrymay also vary between flavonoidsusing paper, thin-layer, or oneor two- ontogeneticstages of organsanalysed. djmensionalchromatography, and in recentyears manyuseful results have been achieved (Vickery All theseproblems make it difficultto assessthe relative & Vickery,1981). degreeof advancementof differentcompounds or classes - theyoccur variously but ubiquitously in almostall of compoundsin any giventaxon. According to Davisand plants. Heywood(1973) chemosystematics is of far greateruse in - theycan be usedat alltaxonomic levels in most establishingconsanguinity (kinship) than in elucidationof groupsof plants. phylogenyproper, which involves tracing past evolutionary history. Althoughflavonoids are presentin all higherplants (Vickery& Vickery,1981), they are absentfrom Despitedrawbacks to potentialphylogenetic value, there are bacteriaand the majority of algae.The mostprimitive many instancestoday in whichphytochemical data have groupthat exhibitsthem is the Charophyceae contributedto substantialtaxonomic improvements (eg. (stoneworts),a groupof greenalgae considered to documentationof hybridisation),although they are still be advancedfor a numberof reasons. Simple underutilisedto a largeextent (Stace, 1980). In general, flavonoidshave been found in primitiveBryophyta, restrictedor uniqueoccurrence of compoundsis of greater whilefar morecomplex ones have been isolated from taxonomicvalue than widespreadoccurrence (Davis & the mostadvanced angiosperms (eg.Orchidaceae). Heywood,1973). Broadlyspeaking, four maincategories of charactersare usedin chemosvstematics:- Flavonoidshave been usedto revisethe families includedin the groupCentrospermae. The red anthocyaninand betacyaninpigments are mutually DIRECTLYVISIBLE PARTICLES exclusive,and so tooare the yellow betaxanthins and the carotenoids.A surveyof the Centrospermae Theseare particlessuch as starchgrains and raphides, showedthat while ten of the twelvefamilies included whichare often also regarded as morphologicalcharacters. in the groupcontained betacyanins and/or beta- An exampleof theiruse is in the utilisationof starchgrains xanthins,the Caryophyllaceaeand Molluginaceae to classifythe Poaceae(Polhill and Raven,eds, 1981). containedanthocyanins, in commonwith non- Centrospermaeplants. This argued for theirremoval to a separateorder from a chemosystematicviewpoint PRIMARY METABOLITES (Vickery& Vickery,1981). However,this removalis stilldebated, as their exclusion is indirect contradiction Althoughsome primarymetabolites are utilisedchemo- to the anatomicalevidence, Nevertheless,their systematically,on the wholetheir usefulness is limited,as anomalywas highlightedby theirflavonoid pattern. theyare intermediatesin or productsof essentialmetabolic 'fingerprint' pathways,and as suchare of ubiquitousdistribution (eg. Flavonoidscan be usedas a in some sugarsthat participatein the Calvincycle). Occasionally generawhere it has been found that each species theyaccumulate in unusuallyhigh concentrations or occur has a distinctflavonoid pattern. The Eaptisragenus as unusualstorage products, and it is largelyin such (Fabaceae),for example, contains 62 flavonoids,and instancesthat they are usedtaxonomically (Stace, 1980). eachof its 17 specieshas a characteristicflavonoid pattern. However,closely related genera exhibit severalspecies with almost identical patterns (Vickery SECONDARYMETABOLITES & Vickery,1981). Theseare very numerous and of morerestricted occurrence Oftenthe geographiccentre of originof a plantcan thanprimary metabolites, which makes them more valuable be deducedfrom its chemicalcharacteristics. as a sourceof taxonomicevidence (Stace, 1980), and a Geraniumspeciesr (Geraniaceae) exhibit a. wide large amountof informationon their distributionin the rangeof flavonoids,with primitivecompounds angiospermsis available.However, the data demonstrate predominatingin CentralEurasian species,