Thispaper waspresented atacoloquium entitled "Self-Defense byPlants: Induction andSignalling Pathways," organized byClarence A.Ryan, Christopher J.Lamb, AndreT.Jagendorf, andPappachan E.Kolattukudy, held September 15-17, 1994, bytheNational Academy ofSciences, inIrvine, CA.

DNAprobes fromtheL6rust resistance gene offlax(Linum usitatissimum) hybridize toresistance genes at theunlinked M locus, indicating sequence similarities between genes atthetwoloci. Genetic andmolecular dataindicate that theLlocus issimple andcontains asingle gene with13alleles andthat theM locus iscomplex andcontains atandemarray ofgenes ofsimilar sequence. Thustheevolution ofthese two related loci hasbeendifferent. Theconsequence ofthecon- trasting structures oftheL andM loci ontheevolution of different rustresistance specificities cannowbeinvestigated atthemolecular level. There arenumerous examples ofplant pathogen interactions wherethehostplant contains multiple resistance gene spec- ificities whicheachconfer resistance tosomebutnotall isolates ofaparticular pathogen species. Frequently, the multiple resistance specificities maptoonly afewgenetic loci wherethey'either arealleles ofasingle gene orareclosely linked genes. Well-studied examples include theRplrust resistance locus atthetip oftheshort armofchromosome 10 ofmaize, thedowny mildew resistance loci oflettuce, theMla powdery mildew resistance locus inbarley, andtheL andM rustresistance gene lociofflax(1). Insomeexamples, clustering ofgenes controlling resistance todiverse pathogen species hasbeenobserved. Forexample, resistance tothe fungus Cladosporium fulvum andthenematode Meloidogyne incognita areclosely linked intomato(2), andinwheat resistance genes forrustandpowdery mildewareclosely linked (3). Oneexplanation forclustering isthat thegenes have arisen byduplication ofanancestral gene followed bydiver- gence toproduce different specificities toward strains ofa single pathogen species andevenresistance against pathogens fromdifferent species andkingdoms. Whilethegenerality of this viewremains tobedemonstrated, thecloning andse- quencing ofthreedifferent resistance genes-the L6rust resistance gene offlax, theRPS2bacterial resistance gene of Arabidopsis, andthetobacco mosaic virus resistance gene N of tobacco-have demonstrated that inthese three cases, there is somestructural andorganizational conservation between the gene products that involves theATP/GTP binding motif, the Ploop, intheamino-terminal half ofthegene products anda leucine-rich region comprising almost halfofthecarboxyl- terminal portion oftheprotein (4-6). Onequestion concerning resistance gene evolution iswhy havesomegroups ofresistance genes evolved astandemarrays andothers asmultiple allelic series? Genetic evidence forthese twosituations comesfromtheanalysis oftheunlinked L and M rustresistance loci offlax(Linum usitatissimum). Thirteen different rustresistance specificities mapattheL locus, and extensive attempts torecombine different pairs ofLspecific-