In populations from crosses of resistant and susceptible plants of the bean (Phaseolus vulgaris) cultivar Black Turtle Soup, resistance to blackeye cowpea mosaic virus (BICMV) and to cowpea aphidborne mosaic virus (CAbMV) was conferred independently by single dominant factors that appear to be closely linked. The symbol Bcm was assigned to the gene for BICMV resistance and Cam to that for CAbMV. Linkage was determined by separately inoculating plants of the testeross with both viruses on different leaves. Since BICMV and CAbMV Incite identical symptoms, but do not cross protect against each other, their presence in each susceptible plant was determined by enzyme-linked immunosorbent assay (ELISA). This detection method was highly specific because the two viruses are distantly related serologically. The normally resistant plants responded with a lethal systemic hypersensitive reaction when approach-grafted to BICMV or CAbMV-infected plants, or when mechanically in oculated and held constantiy at 35°

Gonsalves, D.

Provvidenti, R.

Taiwo, M. A.

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is with the chinchilla (Cch) allele at the albinolocus of the house mouse3. This allele in the housemouse has a much more pronounced effect onphaeomelanin than eumelanin so that in agoutimice the subapical yellow band is much lighter.Since chinchilla has no influence on the length ofthe pigmented regions of agouti hairs in the housemouse while the gray mutation in spiny miceshows an extension of the black pigment furtherdown the spine, this homology seems less likely.A further alternate hypothesis, that the mutationis at the extension locus, also is less likely sincemutations at the extension locus that extendblack pigment are generally dominant alleles5'6.On the basis of the available data it is not possibleto be sure of the genetic homology. The mutationalso could be interfering with the normal processof coat maturation, thus maintaining the juvenilecoat color, even though it is not a mutation at acoat-color locus.This mutation seems to involve coat color only.There do not appear to be any other differencesbetween wild-type and gray animals. However,if metabolic or behavioral changes are associatedwith the mutation it will be difficult to studythem. The spiny mouse is not an ideal species forgenetic research because it has a small litter size(1 -6) and a relatively long gestation (38 days)2.Also, the best breeding pairs are established be-fore the animals are sexually mature. Matingsestablished after the animals are sexually matureare not very successful. In most cases the adultanimals are very aggressive towards strange an-imals. This makes it difficult to establish desiredmatings and to make test matings of one femaleto males of both phenotypes.References1. DICE, L. R. The inheritance of dichromatism in thedeer mouse, Peromyscus maniculatus blandus. Am.Nat. 67:571-574. 1933.2. DlETERLEN, F. Beitrage zur Biologie dcr Stachel-maus, Acomys cahirinus dimidiatus Cretzschmar.Z. Saugetierk 26:1 -13. 1961.3. FELDMAN, H. W. A fourth allelomorph in the al-bino series in mice. Am. Nat. 56:573-574. 1922.4. MARTIN-DENNIS, G. and B. PEITZ. Developmentof the adult pelage in normal and hormone-injectedspiny mice (Acomys cahirinus). J Exp. Zool.218:381-386. 1981.5. SEARLE, A. G. Comparative Genetics of CoatColour in Mammals. Academic Press, NY. 1968.6. SILVERS, W. K. The Coat Colors of Mice.Springer-Verlag, NY. 1979.The Journal of Heredity 74:60-61. 1983.Inheritance of resistance toblackeye cowpea mosaic andcowpea aphid-borne mosaicviruses in Phaseolus vufgarisR. Prowldentl, D. Gonsalves, andM. A. TalwoABSTRACT: hi populations from crosses of resistant andsusceptible plants of the bean (Phaseolus wlgaris)cultivar Black Turtle Soup, resistance to blackeyecowpea mosaic virus (BICMV) and to cowpea aphid-borne mosaic virus (CAbMV) was conferred Indepen-dently by single dominant factors that appear to beclosely linked. The symbol Bern was assigned to thegene for BICMV resistance and Cam to that for CAbMV.Linkage was determined by separately inoculating plantsof the testcross with both viruses on different leaves.Since BICMV and CAbMV Incite identical symptoms,but do not cross protect against each other, their pres-ence in each susceptible plant was determined by en-zyme-Jinked Immunosorbent assay (ELISA). This de-tection method was hfghly specific because the two vi-ruses are distantly related serotoglcalry. The normallyresistant plants responded with a lethal systemic hy-persensitive reaction when approach-grafted to BICMV-or CAbMV-infected plants, or when mechanically In-oculated and held constantly at 35°C.RECENTLY we reported that blackeye cowpeamosaic virus (BICMV) and cowpea aphid-bornemosaic virus (CAbMV) are distantly relatedThe first two authors are affiliated with the De-partment of Plant Pathology, New York State Agri-cultural Experiment Station, Cornell University, Ge-neva, New York; the third author is affiliated with theDepartment of Biological Sciences, University of Lagos,Lagos, Nigeria.© 1983, American Genetic Association.serologically, do not cross protect against eachother, and resistance to them in cowpea (Vignaunguiculala (L.) Walp.) is conferred by distinctgenetic factors7"9. Thus, it was concluded thatthese viruses are two distinct members of thepotyvirus group4-8'9. However, BICMV andCAbMV share some common features, such asthe length of virus particle, size of capsid protein,and sedimentation rate of nucleic acid9. In ad-dition, bean cultivars resistant or susceptible toone of these viruses also are resistant or suscep-tible to the other9. In susceptible bean genotypesthe symptoms incited by BICMV or CAbMVare strikingly similar.The aim of this study was to determine themode of inheritance and the nature of resistanceto BICMV and CAbMV in identical geneticpopulations of a cultivar of Phaseolus vulgarisL., and to define the relationship between theresistance factors.Materials and MethodsGenetic populations were derived from crossesbetween two selections of the same bean cultivar,Black Turtle Soup. Black Turtle I (BTl) is re-sistant to BICMV and CAbMV, whereas BlackTurtle 2 (BT2) is susceptible to both viruses.These two lines have been used in genetic studiesinvolving other viruses5-6.To determine the mode of inheritance of re-sistance, plants of BTl, BT2, and their Fi, F2,and reciprocal backcross generations were in-oculated with the Florida isolate of BICMV(BICMV-Fla)9 and the Moroccan isolate ofCAbMV (CAbMV-Mor)4-9. Inocula for me-chanical transmissions were prepared from leavesof cowpea plants infected with either of these twoviruses. Foliar tissue was homogenized withphosphate buffer (K+) (pH 7.4) and extractswere rubbed onto primary leaves of bean plantsthat had been dusted with 400 mesh carborun-dum. To minimize escapes among susceptiblegenotypes, plants were reinoculated a week lateron the first trifoliolates. All plants, regardless oftheir reaction to the viruses, were assayed forboth BICMV and CAbMV by the enzyme-linkedimmunosorbent assay (ELISA) method, asoriginally described by Clark and Adams2 andmodified by Taiwo and Gonsalves8. Antiserumto BICMV-Fla and to CAbMV-Mor had beenprepared by Taiwo and Gonsalves8.To determine the nature of resistance in re-sistant BTl plants, they were approach-graftedto BT2 plants, which were then inoculated withBICMV or CAbMV.For linkage determination between factors forresistance to BICMV and CAbMV, each plantof the testcross was inoculated separately withthese viruses, one virus on each primary leaf.Since preliminary studies indicated that the in-cubation period in susceptible BT2 plants at25°C is 5-6 days for BICMV and 7-8 days forCAbMV, inoculations with BICMV were de-layed about 60 hours after leaves were inoculatedwith CAbMV. The local and systemic presenceof these two viruses was ascertained by ELISAtests. Each plant was assayed for both virusesusing tissue from inoculated (local infection) andnoninoculated trifoliates (systemic infection). Allthe experiments were conducted in an insect-freegreenhouse maintained at 25°C.ResultsReaction of parents. Plants of the resistantparent BTl inoculated with BICMV or CAbMVremained free of local and systemic infection.Plants of the susceptible parent, BT2, reactedsimilarly to both viruses. Symptoms included leafepinasty, local chlorotic spots, and a severe sys-temic mottle and distortion. Plant growth wasconsiderably reduced, older leaves becamechlorotic and abscised prematurely and no podswere produced. The reaction of BTl plants thathad been approach-grafted to BT2 plants inoc-ulated with BICMV or C^bMV, was lethal.Within 15 days of inoculation, all BTl plants haddeveloped apical and stem necrosis followed 6ydeath. Identical reactions were noted in plants ofBTl when they were mechanically inoculatedwith BICMV or CAbMV and incubated con-stantly at 35°C. :60 The Journal of Heredity NotesReaction of breeding lines. Plants of (BT1 XBT2)F, inoculated with BICMV or CAbMVremained free of symptoms and assays revealedthe absence of local and systemic infections.However, when these plants were approach-grafted to infected BT2 plants, they collapsedwith apical and stem necrosis. Plants of the F2j generation segregated closely to the ratio of 1susceptible to 3 resistant to BICMV (Table I)and to CAbMV (Table II). A few resistant plantsoccasionally developed some local veinal necrosis,which in two plants progressed to cause apicalnecrosis and later death. Plants of the backcross» (BT1 X BT2) X BT1 remained free of local andsystemic infection with both viruses. Plants of the* testcross (BT1 X BT2) X BT2 segregated ap-proximately to the ratio 1 resistant to 1 suscep-tible. From the data presented in Tables I and II,it is concluded that resistance to BICMV orCAbMV is conditioned by single dominant fac-tors.Linkage between resistance factors. Followinginoculation with BICMV and CAbMV, allplants of the testcross population segregated intotwo classes, resistant or susceptible. In eachsusceptible plant, the local and systemic presenceof these viruses was demonstrated by ELISA^ tests. The data presented in Table III clearly in-dicate that the gene for resistance to BICMVand that for CAbMV are closely linked.DiscussionSimilarities in mode of inheritance, nature ofresistance, and an apparent close linkage betweenresistant factors, suggest a common gene for re-sistance to BICMV and CAbMV in the bean lineBT1. However, until an analysis of nucleotidesequence homologies of these two viruses and theresistance genes of BT1 becomes available, it isprudent to consider the two factors for resistanceas independent entities. Thus, the symbol Bern(Blackeye cowpea mosaic) is assigned to thesingle dominant factor for resistance to BICMV,and Cam (Cowpea aphid-borne mosaic) to thatconferring resistance to CAbMV.The use of the same plants of the testcross todetermine linkage between Bern and Cam waspossible because of the lack of cross protectionand the absence of a close serological relationshipbetween BICMV and CaBMV4-8'9. Distant orintermediate serological relationships amongsome viruses are usually not detected withELISA3. The lack of cross reactivity with theELISA system between BICMV-Fla andCAbMV-Mor was noted by Taiwo and Gon-salves8, and exploited in this work. Using ELISA,it was established that each plant of the testcrosswas either resistant or susceptible to both viruses.The difference in incubation periods was easilyovercome by delaying inoculations with BICMV,the virus having the shorter incubation period.The combined effect of both viruses in susceptiblegenotypes was no more severe than the symptomsincited by each virus separately.The systemic hypersensitive reaction in plantsof BT1 when graft-inoculated or mechanicallyinoculated with BICMV or CAbMV and incu-bated at high temperature is known to occur withother viruses5.For BICMV as well as CAbMV, seed trans-mission in cowpea represents the major source ofinoculum110. However, no information is avail-able regarding the seedborne nature of BICMVor CAbMV in seed of susceptible bean genotypes,because the severity of symptoms usually causespremature death of infected plants. Data are notavailable concerning the natural occurrence ofthese two viruses in bean crops. But, if a suscep-tible cultivar is grown in the vicinity of a BICMVor CAbMV infected cowpea field, conceivablythe viruses could spread to the bean plants withdevastating consequences. Both of these virusesare readily spread by a number of aphid speciesin a stylet-borne manner110.Some of the bean lines currently used assources of resistance to root-rot pathogens (PI109859, PI 165435, PI 203598, PI 203958, andothers) were determined to be susceptible toBICMV and CAbMV, consequently adequatetesting of breeding lines deriving from these plantintroductions is recommended. However, it isreassuring to know that the majority of theleading commercial bean cultivars possess re-sistance to BICMV and CAbMV9.Table I. Segregation ratios of cross andbackcross populations of Phaseolus vulgarisBlack Turtle 1 (BT1) with Black Turtle 2 (BT2)for resistance to blackeye cowpea mosaicNo. plants* Exp. Goodness-ratio of-fit (/>)PopulationsBT1BT2(BTI X BT2)F,BT1 X BT2)F2(BTI X BT2)F,X BTI(BTI X BT2)F,X BT2R1100461847854S01290540493:11:10.430.62* R = resistant; S = susceptibleTable II. Segregation ratios of cross andbackcross populations of Phaseolus vulgarisBlack Turtle 1 (BTI) with Black Turtle 2 (BT2)for resistance to cowpea aphid-borne mosaicvirusNo. plants Exp. Goodness-Populations R S ratio of-fit (P)BTIBT2(BTI(BTI(BTIX(BTIXX BT2)F,X BT2)F2X BT2)F,BTIX BT2)F,BT2106049214955401170680493:11:10.740.62Table III. Reaction of testcross plants of Phaseolus vulgaris Black Turtle 1 (BTI) with BlackTurtle 2 (BT2), when inoculated with blackeye cowpea mosaic virus (BICMV) and cowpea aphid-borne mosaic virus (CAbMV) on different leaves. Both viruses were detected locally andsystemically by enzyme-linked immunosorbent assay (ELISA) in each susceptible plantPopulation(BTI X BT2)X BT2Virusi BICMVI CAbMVNo.R*5454plantsSt4949Exp.ratio1:1Goodness-of-fit (P)0.62* Plants resistant to BICMV also were resistant to CaBMV• Plants susceptible to BICMV also were susceptible to CaBMVReferences1. BOCK, K. R. and M. CoNTl. Cowpea aphid-bornemosaic virus, C M.I./A.A.B. Description of plantviruses. No. 134. 1974.2. CLARK, M F. and A. N ADAMS Characteriza-tion of microplate method of enzyme-linked im-munosorbent assay for the detection of plant vi-ruses. J Gen. Virol. 34.475-483. 1977.3. KOENlG, R. ELISA in study of homologous andhcterologous reactions of plant viruses. /. Gen.Virol. 40:309-318. 1978.4. LIMA, J. A., D. E. PURCIFULL, and E. HlEBERT.Purification, partial characterization and serologyof blackeye cowpea mosaic virus. Phytopathology69:1252-1258. 1979.5. PROVVIDENTI, R. Inheritance of resistance towatermelon mosaic virus 2 in Phaseolus vulgaris.Phytopathology 64:1448-1450 1974.6. , D. GONSALVES, and P. RANALLI. Inher-itance of resistance to soybean mosaic virus inPhaseolus vulgaris. J Hercd 73:302-303.19827. TAIWO, M. A., R. PROVVIDENTI, and D. GON-SALVES. Inheritance of resistance to blackeyecowpea mosaic virus in Vigna unguiculata. J.Hered. 72:433-434. 1981.8. and D. GONSALVES Serological groupingof blackeye cowpea mosaic and cowpea aphid-borne mosaic viruses. Phytopathology 72583-589.1982.9. , , R. PROVVIDENTI, and H. D.THURSTON. Partial characterization and groupingof isolates of blackeye cowpea mosaic and cowpeaaphid-borne mosaic viruses. Phytopathology 72:590-596. 1982.10. ZETTLER, F. W. and I. R. EVANS. Blackeyecowpea mosaic virus in Florida- host range andincidence in certified cowpea seed. Proc. Fla. StateHort.Soc. 85:99-101. 1972.Notes January / February 1983 61

Inheritance of resistance to blackeye cowpea mosaic and cowpea aphid-borne mosaic viruses in Phaseolus vulgaris

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