phaseolin diversity in the tepary bean, phaseolus

Plant Breeding 101, 292—30! (1988)© 1988 Paul Parey Scientific Publishers, Berlin and HamburgISSN 0179-9541

Department of Agronomy and Range Science. University of California, Davis (USA)

Phaseolin Diversity in the Tepary Bean,Phaseolus acutifolius A. Gray

CHRISTIANE SCHINKEE and P. GEPTS

Witb 4 figures and 2 tables

Received Mav !>, 1988 i Accepted August 6, 19S8

AbstractThe tepar),' bean {Phaseolus acutifolius A. Gra}') isgrown mainly in arid regions of Mexico and thesouth'ft'esrern U.S. as a subsistence crop by smallfarmers. It is also a store of genetic variability fortraits such as disease and pest resistance and stresstolerance to improve the common bean {P. viiigamL.). To determine geographic patterns of variationand the influence of domestication on genetic varia-bility, the genetic diversity of phaseolin, the majorseed storage protein, 'ft'as characterized among 55'R'ild and 8 cultivated teparies using polyacrylamidegel electropboresis. Fifteen electrophoretic phaseoiinpatterns '»'ere identified among wild forms, whereasonly one pattern was exhibited b)' cultivars. Thisresult suggests a single domestication m this speciesleading to a strong reduction in diversity. An addi-tional finding IS the divergence m phaseolin typesbefPk'een populations east and west of the SierraMadre Occidental mountains.

Key words: Phaseolus acutifolius — tepary bean —seed proteins — phaseolin — genetic diversity —domestication

The tepary bean (Phaseolus acutifolius A.Gray) is one of the five cultivated Phaseolustaxa. It IS grown mostly m Mexico and thesouthwestern United States by farmers m asubsistence agriculture setting (DUKE et al.1981, N.ABH.AN and FEEGEK 1978, NABHAN andTEIW'ES 1983, BousGAREN et al. 1983, PRATT andNABHAN 1988). In addition to cultivars, theprimary gene pool of P. acutifolius also C'On-tains wild forms: P. acutifolius var. acutifoliusand P. acutifolius var. tenuifolms (PRATT andNABHAN 1988), These two botanical varieties

can be distinguished b}' their leaflet shape andtheir ecological distribution, Var. tenuifohus ischaracterized b)' narrow leaflets and grows inmore open, nutrient deficient sites on slopesabove floodpiains; var, acutifolius, on the otherhand, exhibits subovate to ovate le,aflets andgro'ws in more tnesic, shaded and disturbedsites on streambanks and floodpiains. Becauseof the similarities in leaflet shape and adapta-tion of var. acutifolius types to di.sturbed sites,tepary cultivars are believed to have beendomesticated from var. acutifolius wild forms(PRATT and NABHAN 1988).

In recent years, the tepary bean h,as been thesubject of rene'W'ed interest as a crop adapted toand conditions and as a source of genetic varia-bility for the common bean (P. vulgaris). Ger-tain tepary cultivars show high levels of toler-ance to heat and drought stress and to salineconditio'ns (FREEM.AN 1912, MARGARIAN 1981,MoNTOYA-GoRONADO 1985, Cited by PRATT andNABHAN 1988, GOERTZ and KOBRIGER 19B5).Resistance to diseases and pests have also beenobserved in tepary beans. Several reports de-scribe the high levels of resistance against com-mon bacterial blight (Xanthomonas campestris)found in teparies in contrast with the moderatelevels of resistance found in cotnmon bean(e.g., GoYNE et al. 1963). This resistanee hasnow been transferred through interspecific hy-bridizations into common bean (THOMAS et al.1983, DRIJEHOUT and BEOK 1987). Additionalresistances include resistance to rast{Uromycesphaseoli), Fusarium sp., anthracnose (Colleto-trichum lindemuthianum), angular leafspot(Isariopsis griseola), charcoal rot (Macrophomi-

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phaseolin Diversity m the Tepary Bean 293

a

1

bVI

IV

dII

* e * f g h

XV IV V!ll XIIn o p

VII XIII XIV XI XI

Fig, 1, One-d,imensional SD,S polyacr)']amide ge! electrophoresis of phaseolin of wi3d (a—o) and cultix'ated(p) Phaseolus acutifolius. a: phaseolin 1: accession G40044; b: VI: G40100; c: V: G40052; d: II: G40050; e:XV: G40121; f: IV: G40074; g: VII!: G40078;h: XII: G40102; i: X: G40072; j : IX: G40091; k: III: G40053;1: VII: G4005'5,: m: XIII: G40087; n: XIV: G40106; o: XI: G40103: and p: XI: PI310606

na phaseolma) and tmpoasca leafho'P'per (re-viewed m Ti-iOM.AS et al. 1983, PR.ATT andNABHAX 1988).

In order to iacilitate a wider use of thegenetic resources of tepar\' bean, it is necessaryto gam a better understanding of its geneticattributes and the organization of genetic var-labilit)' m this species. P. acutifolius is highlyself-poliinated (unpubl, observ.) and itschromosome number is In ~ 2.\ = 22 (KAR-PEC:HENKO 1922). Little is known, however,about Its genetics or its genetic diversit}'. Wereport here on a surve\' of phaseoiin in wildand culti\'ated P. acutifolius.. Phaseolin is themajor seed storage protein m Phaseolus beans.Elec'trophoretic studies of seed protein havebeen earned out mainly in the familiesGramineae and Legummosae (LADIZINSKY andFIYMOWITZ 1979). In bread wheat (Triticumaestivum), JOHNSON (1972) co^niirmed the ori-gin of the A- and B-genomes from T, dicoccumand of the D-genome from Aegilops squarrosa.HYMOWITZ and KAIZUMA (1979 and 1981)studied the frequenc)- distribution of two seedproteins to make inferences on the origin anddissemination of soybean (Glycine max) germ-plasm. In P. vulgaris, phaseolin consists of aseries of similar polypeptides whose molecularweight ranges between 45,000 and 52,000 dal-tons and isoelectric point ranges between 5,6and 5.8 (BROWN et al. 1981a). These polypep-tides are coded by a series of genes which areclosely linked and are inherited as a singleMendelian unit (BROWN et al, 1981 b, TAEBOT etai. 1984). The genes oi the 'T' phaseolin in1. viilgarii show a high level of sequence simi-

larity (98 %) and most of the hetero'geneityobserved atnong phaseolin polypeptides ap-pears to be due to post-translational changes,e.g., glycosyiations (Lioi and BOLLINI 1984,SLICHTOM et al. !985). Analyses of phaseolinelectrophoretic banding patterns m P. vulgariscultivars and wild lines have cO'nfirmed thehypothesis of multiple domestication centersfor the common bean, GLPTS et al. (1986) sug-gested 2 major centers in Middle America andin the Andes in South America and GEPTS andBLISS (1986) found a third minor one in Go-lombia. The p'haseolins of P. vulgaris andP. acutifolius are related as suggested by theirsimilar molecular weight (see below) and theexistence of serological cross-reactivity (KLOZ1971).

Results prese'nted here suggest a singledomestication event for the tepary bean result-ing in a strong reduction in pbaseohn variabili-ty upon domestication in this species.

Materials and MethodsPlant material: A sample of 63 P. aattifolius A.Gray accessions was studied, which included 55' wildlines obtained from the Centre Internacional deAgricukura Tropical (ClAT, Cali, Colombia) andeight culti^'ars obtained from J. G. 'W''ames and W,C. 'ft''ells of the University of California at Riverside.Information about the location of origin was avail-able for most of the accessions. Forty-four of the 55wild lines came from Mexico: 22 were collected inthe state of Durango, ten in Sonora, four in Jalisco,three in Sinaloa, and one each m Zacatecas and inNavarit, Five wild lines originated m the USA. Five

Plant Breeding, VoL IS! (4) 21

294 SCHINKEL and GEPTS

of the iandraces originated in Mexico and one each inNicaragua, the USA, and Puerto Rico (Table 1). \i'tdefine landraces as cultivars developed from wildmaterial by mass or single plant selection 'withoutartificial hybridization. In addition, landraces areadapted to low input agriculture and to a specificlocal environment; thus, their geographical distribu-tion tends to be limited.

Each accession was classified as var. acutifolius orvar. tenuifolius was determined according to a mod-ification of the criterion of PRATT and NABHAN

(1988). At flo'tt'ering, the length and maximal widthof the terminal leaflet of the trifoliolate leaf 'ft'eremeasured on 1 to 4 greenhouse-grown plants peraccession and a length/width ratio was calculated foreach accession. Based on these calculations,, we could

Table 1. Identification, geographic origin, phaseolin type, and leaflet length/width ratio of Phaseolusaculifoliids accessions

CIAT Othernumber identi-

fication

Botanicalvarietv^

Status'' Countrv State Phaseolin Leaflettype' length,/

width ratio

G40044G40045G40046G40047G40048

C40049G40050C40051G40052G40053

G40054G40055G40056G40071G40072

G40073G40074G40075G40076G40077

G4OO78G40079G40080G40081G40082

G40083G40085G40086G40087

G40088

G40089G40090G40091G40092G40093

PI 146800PI263590PI312196PI312197PI312198

P1319438PI319439PI319440PI319441PI319442

P1319444PT319445NI502LI 24L137

L176L177

DGD/HN286

DGD/HN287DGD/HN295DGD/HN300DGD,/HN388DGD7g/057

DGD78/060DGD78/063DGD78/072D'GD78/075D'GD78/078

acuacuacuacuacu

acuacuacuacuacu

acuacuacutenten

4CU

acu

aeu»c«,acu

actiacuacuaCii

acu

acuacuacuacuacu

acutenacuacuacu

MexicoMexicoMexicoMexico

MexicoMexicoMexicoMexicoMexico

MexicoMexicoMexicoUSAUSA

MexicoMexico

MexicoMexicoMexicoMexico


MexicO'Me'xicoMexicoMexicoMexico

SonoraSonoraSonoraSonora

SonoraSonoraSonora

Sonora

SonoraSinaloa

ArizonaArizona

SonoraSonora

SinaloaDurangoDurango

DurangoDurangoDurangoDurangoDurango


1IVVP'II

IIIIIVV

111

XIVI1iV

XIX

IVIVIVIVIV

VIIIXVII

11VI

VIVIViXIIIIV

VlXIIIX*VHP''VI*

2.32.12.32.22.2

2.72.72.24.22.5

2.12,02.16.57.6

1.1

2,12,32.45.2

5.12.02.23.92.7

4.4

3.7

2.73.82.4

1.5

9.02.52.34.1

Phaseolin Diversity in the Tepary Bean 295

Table 1,

CIATnumber

G40094G40095G40096G40097G40098

G40099G40100G40101G40102G40103

G40104G40105G40I06G40107G40108

G40109G40113G40114G40117

G40121

G40035

G4001'6

(continued)

Otheridenti-fication

DGD78/081DGD78./084DGD7g/091DGD78,'O94DGD78/097

DGD78/099DGD78/102DGD78/104DGD78/109DGD78/111

DGD78,/126DGD78,/128DGD78,/136DGD78/137DGD78/'HN293

DGD/HN422ASU 36163

A 335 wildA-1399

Oaxaca 43PI319443PI310606L578PI310801

PI311897L242—26P1321638

Botanicalvariety^

acu

acu

acu

acu

acu

acu

acu

acu

ten

acu

acu

acu

acu

acu

acu

ten

ten

ten

ten

acu

acu

acu

acu

acu

acu

acu

acu

acu

Status*"

w

V.'

w

w

w

w

w

w

w

w

w

w

w

w

w

w

'W

w

ccc

c

c

c

c

c

Country


AlexicoMexico,MexicoMexicoMexico


MexicoUSAUSAUSAMexico

MexicoMexicoMexicoUSANicaragua

MexicoMexicoUSA

State


DurangoDurangoDurangoDurangoSinaloa

JaliscoJaliscoJaliscoJaliscoDurang'O

Na'S'antArizona

Zacatecas

OaxacaDurangoChiapasPuerto RICO

SonoraSonoraArizona

Pliaseolintype'

VIIIIXVVIVI

II-VIIIXIIXI

VIVIXIVIIIIX

IIIIXIIIXV

XIXIXIXIXI

XIXIXI

Leafletlength/

width ratio

2.12,72.62.42.6

3,93,64.2

10.42.3

4.5

4.8

2.6

2,13,4

12,712,76.69.02.9

2.7

2.6

2.12.42.7

2.42.32,1

^ acu: P. acutifolius var, acutifolius; ten: P. actUijohus \'ar. tenuifolius'' w: wild; c: cultivated' '•': accession heterogeneous for phaseolin t}'pe; onb' the most frequent type is indicated

determine two groups (Table 1): lines with subovateto broad obovate leaflets (ratio < 5.2) were classifiedas var. .acutifoltus and accessions with narrow linearleaflets (ratio > 6.5) belonged to the var. tenuifolius.This IS in broad agreement with the classification ofPRATT and NABHAN (1988), who classified lines witha ratio < 3.4 in var. acutifolius and lines with a ratio> 5 m var. tenuifolius. Differences in absolute valuesmight be due to different growing conditions ordevelopmental stages at the time of measurement,

LTsing this criterion, al! landraces belong toP. acutifoltus var. acutifolius. Among wild lines,eight accessions had a leaf ratio significantly higherthan 6.5 and were therefore classified as P. acutifolius

var. tenuifohus. Fi\'e var, tenuifolius accessions camefrom the USA, two from Durango and one fromNavant, The remaining wild lines had a leaf ratiosmaller than five and 'WTre classified as P. acutifoliusvar. acutifolius.

Preparation of flour samples for electrophoresis;Fi^'e seeds of each accession w,'ere analyzed by one-dimensional sodium dodec)-l sulfate polyacrylamidegel electrophoresis (SDS/PAGE), Flour from theraphe half of each seed was prepared and suspendedat room temperature in 0.5 M NaCl pH 2.4 for atleast 1 h. Subsequently, the same amount of crackingbuffer (0.625 U Tris-H'Gl, pH 6.8; 2 % (w/v) SDS;

21'*

296 ScHiN,KEL and GEPTS

i

m

Fig. 2. Two-dimensional IFF/SDS poh'acrylamide gel eiectrophoresis of phaseolin of wild (a—o) andcultivated (p) Phaseolus acutifoliui. Phaseolin patterns are shown in same order as in Fig, 1: a: phaseolin I:G40044; h: VL G40100; c; V: G40052; d: II: G40050; e: XV: G40121; f,: IV: G40074; g: VIII: G40078; h:XIL G40102; l; X: G40072; ]: IX: G40091; k: III: G40053; 1: VII: G40055; m: XIll; G40087; n: XIV:G40106; o: XI: G40054; and p: XI: PI310606

40 '% {vfix) sucrose; 1 % (v,'"'v) 2-mereaptoethanol; 2mM EDTA; 0.01 % (w/v) bromophenol blue markerdye (B,ROWN et al. 1981 a) was added. The mixturewas heat-treated at 100 °C for 5 min, eentrifuged for5 min and the supernatant was subjected to elec-trophoresis. One seed per phaseolin type was analy-zed by two-dimensional isoelectrie focusing SDS/PAGE (IEF-SDS/PAGE). The sample preparationwas similar to one-dimensional electrophoresis.

Electrophoresis; One-dimensional SDS'.'PAGE elec-trophoresis was performed according to the methodof LAEMML! (1970) modified by M A and BLISS(1978). Electrophoresis was carried out in 0.7-mmthick 13 % (w/v) polyacrj'lamide slab gels. Two-dimensional IFF-SDS/PAGE was carried out accord-ing to BROWN et al. (1981 a). Proteins were stainedwith Coomassie Brilliant Blue 250 dissolved m 5 wa-ter : 5 methanol : 1 glacial acetic acid and gels were

destained in 5 w-ateracid.

nethanol : 1 glacial acetlu

Results

Electrophoretic analysis of phaseolin types

A preliminary electrophoretic analysis indi-cated that the P. acutifolius phaseolin has amolecular weight similar to the P. vulgarisphaseolin (not shown). Fifty of the wild lines(91 %) were homogeneous, all five seedsshowed the same banding pattern. Five lineswere heterogenous. Among wild lines, 15 dif-ferent phaseolin patterns could be distin-guished (Fig. 1, Table 1). These 15 patternswere mentio'ned with Roman numerals I toX'V to distinguish them from the patterns in

lin Diversity in th't! Tepary Bean 297

P. vulgaris, which are identified with letters.The patterns II, IV, and VI were exhibited by9, 9, and 11 lines respectively. All other typeswere found at a much lower frequency. Thevar. tenmfolius accessions exhibited fo'Ur dif-ferent phaseolin patterns, three of which werefound also among wild var. acutifolius acces-sions (II, X, XI) and one which w,as observedexclusively in var. tenuifolius accessions (XII),All landraces tested showed the same phaseolin

Table 2. Geographic distribution of phaseolin pat-terns in Phaseolus acutifolius

Phaseolint -pe

IIIIIIIVVVIVilVIIIIXXXIXIIXIIIXIVXV

West of the SierraMadre

Numb

434

J2

2

3

1

Occidental'

e r ')'o

100'

44

tooSO

2'5'icra

1001,00

too

East of theMadre Occi

Number

5

1

1

9

3

1

1

Sierradenta! '

0/

56

70

10075

100

100

100

100

100

'• States of Jalisco. Nayarit, Sinaloa, S,onO'ra, andArizona

' States of Dutango and Zacatecas

pattern which most closely resembledphaseolin type XI, observed in the wild lmesG4005'4, 'G40071, and G401C3,

Two-dimensional IEF-SDS/PAGE con-firmed the high level of phaseolin variabilityamong wild lines. Slight differences were ob-served within phaseolin group XI. The highestsimilarity to the cultivar pattern is shown byline G40103.

Geographic distribution of phaseolin types

Three phaseolin types — II, IV, and V! —which occurred at the highest frequencyamong wild lines (see previous section) origi-

nated on both sides of the Sierra Madre Occi-dental mountain range. The other phaseolintypes occurred either west or east of thismountain range (Table 2; Fig. 3). Phaseolintypes I, III, VII, X, XI, and XIV appearedexclusively in the accessions tracing to thewestern Mexican states of Sonora, Sinaloa, andJalisco and in Arizona. The phaseolin types V,VIII, IX, XII, XIII, and XV were found onlyin accessions from Durango and Zacatecas,east of the Sierra Madre Occidental.

Differentiation of leaflet morphology in var.tenuifoUus

The analysis of leaflet shape reveals a discon-tinuity 10 the leaf/width ratio between 5.2 and6.5 (Table 1). Accessions with ratios below 5.2have subovate to obovate leaflets (e.g.. Fig. 4aand b), whereas those with ratios above 6.2have elongated narrow linear-lanceolate leaf-lets (e.g.. Fig. 4c ,and d). The former groupcontains both cultivars and wild accessionsbelonging to P. acutifolius var. acu-tifohus; thelatter include wild accessions belonging toP. acutifolius vzr. ienuifolius. Among the var.tenuifolius forms, two variant leaflet typeswere observed with or without basal lobes(Fig, 4c and 4d, respectively). In our sample,variants with basal lobes ('G4OO71, 'G'4OC72,G40113, G40114, ,and G4C117) originated inArizona (USA) whereas variants without basallobes (G4009C, G40102, and G40109) origi-nated in Durango or Nayarit (Mexico). Theaccessions with lobed leaflets generally had aleaflet length/width ratio between 6.5 and 9.0(with the excep'tion ot G40113 with a ratio of12.7); the accessions with linear leaflets, on theother hand, were characterized by larger ratiosranging between 9.0 and 12.7. Two of the'three accessions with linear le,aflets exhibited aunique phaseolin type (XII) not observedamong var. acutifolius forms, whether wild orcultivated. All the accessions with lobed leaf-lets displayed phaseolin patterns observed alsoamong var. acutifolius accessions.

DiscussionThe analysis of phaseolin types and their geo-graphical distribution leads to the followingfindings. First, wild P. acutifolius exhibited awide range of genetic variability for phaseohn

298 ScHiNKEL and GEPTS

PACIFIC OCEAN

Fig, 3,, Geographic distribution of phaseoJin types in wild Phaseolus acutifohus of northwestern Mexico andArizona. The phaseolin patterns found in each state are represented diagrammaticaily: AZ: Arizona; DGO:Durango; JAL: Jalisco; NAY; Nayarit; SIN: Sinaloa; SON: Sonora; ZAC: Zacatecas, Phaseolin patternsfound in var, tenuifolius accessions are indicated by t underneath the phaseolin pattern

as no less than 15 different patterns were ob-seni-ed among 55 genotypes. This high level ofdiversity is similar to that observed amongwild P. vulgaris from Mesoamerica (GEPTS etal. 1986) and confirms earlier observationsmade by SULLIVAN and FREYTAG (1986). Sec-

ond, with the exception of three patterns (II,IV, and VI) which occurred at a high frequen-cy and were geographically widespread, allother patterns exhibited a more circumscribed

geographical distribution occurring either westor east of the Sierra Madre Occidental. Thisobservation suggests that this mountain rangeis an effective geographical barrier to geneflow. Alternatively, this differential distribu-tion may result from adaptation of the geno-types carrying the various phaseolin types todifferent environmental conditions on eitherside of the Sierra Madre Occidental. WildP. acutifolius accessions of two Mexican states


5 cmFig. 4. Variation for leaflet shape in Phaseolusacutifolius. a; cultivated P. acutifolius var.acutifoiius; b: ti'ild P. acatifolms; c: P. acutifoliusvar. tenuifolius with lobulated leaflets; d:P. acutifolius var. tenuifolms with linear leaflets

— Sonora on the 'western side and Durango onthe eastern side of the Sierra Madre Occidental— showed high levels of phaseolin variability.Gaution needs to be exercised, however, be-cause these two states were also the best rep-resented in our sample. This may indicate theneed for more exhaustive explorations in otherparts of the wild P. acutifolius distributionrange.

Third, the var. tenuifolius types exhibitedfour different phaseolin patterns, three ofwhich were also found in the wild var,acutifolius types. The existence of phaseohntypes common to the two botanical varietiesmay be explained in two not mutually exclu-sive ways. Leaf shape may evolve more quicklythan phaseolin type. Gertain var. tenuifohusand var. acutifolius accessions may haveevolved from a common ancestor and •wouldtherefore share a phaseolin type; subsequentselection for adaptation to contrasting environ-ments such as valley bottoms (var, acutifolius)and valley slopes {var. tenuifolius) (PRATT andNABHAN 1988) may have lead to accessionswith different leaflet shapes. Alternatively, thecommonahty in phaseolin types between var.tenmfolius and var. acutifolius can be attrib-uted to gene flow between the two forms.

Accessions from the western states of MexicO'(Nayarit and Sonora) and Arizona (USA) exhi-bited phaseohn types in common with wildP. acutifolius var. acutifolius (II, X,, and XI)and cultivars (XI). Morphologically intermedi-ate, putative hybrid populations have been re-ported near the Sonora-Arizona border (PR,fiTTand NABHAN 1988); in addition, the var.acutifolius and var. tenuifohus forms are sym-patric in the same area, i.e. 'western Mexicoand southwestern U.S. (NABHAN 1985; PRATTand NABH.IN 1988). It may be significant thatthe phaseolin type unique to var. tenuifohus(XII) is found only among accessions withlinear leaflets, i.e. with leaflets most dissimilarto those of wild and cultivated var. acutifoliustypes. Because var. tenuifolius types withlobed leaflets share phaseolin types with var.acutifoltus types, it may be worthwhile inves-tigating whether they actually represent hy-brids between var. acutifolius genotypes andthose var. tenuifolius genotypes that exhibitlinear ie,aflets.

Fourth, the eight cultivars exhibit only asingle phaseolin pattern (XI). Although oursample of cultivars 'was smaller than our wildgenotype sample, it is geographically represen-tative as it includes cultivars from Arizona tothe north to Nicaragtia to the south. The exist-ence of a single phaseolin type among teparycuhivars resembles the situation encounteredamong Mesoamerican domesticates of thecommon bean where a single phaseoUn type('S') is observed in contrast to the extensivephaseolin variability of the wild ancestor(GEPTS et al, 1986). As for the common bean,this strong reduction in variabihtv- could beexplained by postulating a single domesticationevent. Is it possible to identify a specific regionwhere this domestication may have takenplace? Three wild P. acutifolius lines exhibitphaseolin pattern XI observed also among cul-tivars; one of these originated in Arizona and isa var. tenuifolius type; the second and thirdones originated in Sonora and Sinaloa, respec-ti^'el)' (Fig. 3), and are wild var. acutifoliustypes. Because of the widely divergent leafletmorphology, it is thought that var. tenuifoliustypes may not have given rise to tepary cul-tivars; the presence of phaseohn pattern XI inthis var. tenuifoUus type may be due to anoccasional outcross between cultivars and avar. tenuifolius type both of which grow to-

300 SCHINKEL and GEPTS

gether on fJoodplams m that region (NABHAN1985). The wild accessions of Sonora andSinaloa, on the other hand, exhibit a leafletshape similar to that of cultivars. Nevertheless,these similarities do not allow us to concludethat tepary may have been domesticated inSonora and Sinaloa, because they can equallywell be explained by gene flow between cul-tivars and wild forms. An analysis of addition-al markers such as isozymes or restriction frag-ment length polymorphisms may providemore conclusive evidence.

The existence of a single phaseolin typeamong cultivars can also be explained by theexistence of a selection pressure acting onphaseohn or traits linked to phaseohn. Theparticular phaseolin type now found amongcultivars may have come to predominate in thecultivated gene pool because it has superiornutritional qualities or is more easil)'metabolized during germination in cultivatedfield conditions. Alternatively, traits confer-ring superior fitness in a cultivated environ-ment (such as reduced pod dehiscence or in-crease seed coat permeability) may be linked tothe particular phaseohn type found among cul-tivars. No data are available that allow us todistinguish between these possibilities.

We thank Giles 'W'AINES and B. 'W''ELLS (Universityof California, Riverside) and R. HiD.ALGO (GeneticResources Unit, CiAT, Cali, Colombia) for provid-ing seed samples of tepary cultivars and wild forms.respectively. Thanks are also due to Giles 'W'AIN'HSand R. PR.'\TT (Ohio State University) for helpfulsuggestions on the manuscript.

ZusammenfassungVariabilitat des Phaseolins bei der Texas-Bohne (Phaseolus acutifolius A. Gray)

Texas-Bohnen (Phaseolus acutifohus A. Gray)werden hauptsachhch m den trockenen Gebie-ten Mexikos und des Sijdwestens der U.S.A.von Kleinbatiern fiir den Figenbedarf ange-baut Sie stellen fiir Eigenschaften wie Resi-stenz gegen Schadlinge und Krankheiten undToleranz gegen Trockenheit ein Reservoir ge-netischer Variabihtat dar, das fiir die Verbesse-rung der gewohnlichen Gemiisebohne (Pha-seolus vulgaris L.) genutzt werden konnte. Umdie geographische Verteilung der Variabilitat

und den EinfluiJ der Domestikation auf diegenetische Variabilitat zu ermitteln, wurde dasPhaseohn — Hauptbestandteil des Reserve-Eiwesfies lm Samen — von 55 Wildformen und8 angebauten Sorten mit Hilfe der Polyacr'^'l-amidgel-Elektrophorese untersucht. FolgendeErgebisse zeichneten sich ab; 1. Wildformender Texas-Bohne wiesen eine erhebliche Varia-tion des Phaseolins auf. 2. Neben drei haufigenund weitverbreiteten Phaseolin-Typen gab esverschiedene Typen mit begrenzter Ausbrei-tung, die entweder ostlich oder westlich derGebirgskette der Sierra Madre Occidental imNordwesten Mexikos gefunden wurden. 3. BeiP. acutifohus var. tenuifohus, einer Wildformmit verlangerten Blattchen, wurden vier ver-schiedene Phaseolin-Typen festgesteilt, vondenen drei auch bei P. acutifohus var.acutifolius beobachtet worden smd, was auf dieMoglichkeit emes Genaustausches zwisehendiesen beiden Unterarten oder eine davon un-abhangige rasche Veranderung der Blattformhinweist. 4. Bei den 8 angebauten Sortenwurde nur em einziges Elektrophoresemusterdes Phaseolins gefunden, woraus zu schliefSenist, daE hier eine einmalige Domestikationstattgefunden hat, die zu erheblichenEmschrankungen der Variabilitat fuhrte.

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Authors' address: Christiane ScHINKEL, Dt. P.GEPTS, Department of Agronom}' and Range Sci-ence, Universit}' of California, Davis, CA 95616(USA).

phaseolin diversity in the tepary bean, phaseolus

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