Novel Phaseolin Types in Wild and Cultivated
Common Bean (Phaseolus vulgaris, Fabaceae) 1
R. L. KOENIG,2 S. P. SINGH,3 AND P. GEZrS 2
Forty-one wild types and 41 cultivars o f common bean (Phaseolus vulgaris) f r o m
Meso- and South America were screened for variability o f phaseolin seed protein
using one-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis
(SDS/PAGE) and two-dimensional isoelectric focusing SDS/PAGE. Wild accessions f r o m the A ndean region showed phaseolin types which had not been previously
identified in wild material f r o m that region. Other wild accessions f r o m Argentina
exhibited novel phaseolin patterns collectively desig, nated as 'J" ('Jujuy') phaseolin
types, and one accession f r o m northern Peru exhibited a novel phaseolin type, the
7" ('lnca ') type. The 'H" and "C'phaseolins, previously identified only in cultivars,
were observed in several wild accessions ,from Argentina. Among cultivars, two
minor variants o f the "S" phaseolin type were identified. The 'Sb" ('S Brazil') was
characteristic o f a limited number o f cultivars f r o m Brazil whereas the 'Sd" ('S
Durango 222') predominated in cultivars o f the Mexican central highlands. The
distribution o f the previously described "B" phaseolin appeared to be larger than
jormerly known as it extended not only in Colombia but also in Central America.
It is possible to correlate the "Sb', 'Sd', and 'B" phaseolin types with certain agronomic traits.
Nuevos tipos de faseolina en frijoles (Phaseolus vulgaris, Fabaceae) silvestres
y cultivados. La variabilidad defaseolina, la proteina principal de la semilla, f u e
analizada en una muestra de 41 formas silvestres y 41 formas cultivadas del frfjol
comfm (Phaseolus vulgaris) de Meso- y Suramdrica mediante electroforesis a una
y dos dimensiones en gelos de poliacrilamida con dodecil sulfato de sodio ( S D S /
PAGE). Las formas silvestres de la regi6n andina enseharon tipos de faseolina que
no hab[an sido ident[ficados anteriormente en material silvestre de esta regi6n.
Formas silvestres de Argentina mostraron varios tipos nuevos de faseolina que
fueron llamados tipos 'J" ('Jujuy') y una accesi6n silvestre del norte de Peru mostr6
un tipo nuevo que fud llamado el tipo T ('Inca'). Dentro de las formas cultivadas,
dos formas variantes de la faseolina "S"fueron identificadas. El tipo "Sb" ('S Brazil')
j h { caracteristico de un nf~mero limitado de variedades de Brasil, mientras el tipo
"Sd" ('S Durango 222 ")predominaba en variedades del altiplano central de MOxico.
La distribuci6n de la faseolina 'B' descrita previamente parece ser m6s amplia que
lo que se habfa determinado anteriormente ya que se encuentra no solamente en
Colombia pero tambi(n en America Central. Es posible correlacionar los tipos de
Jaseolina 'Sb', 'Sd', y 'B" con ciertos rasgos agron6micos.
Seed protein and isozyme variability have been used extensively in m a n y crops,
including c o m m o n bean, to detect patterns and levels o f genetic diversity within
a species (Ladizinsky 1983; Loveless and Hamrick 1984). H a m r i c k and Allard
(1972) used protein p o l y m o r p h i s m s in A v e n a f a t u a L. and A. barbara Port. ex
Received 20 July 1988; accepted 26 December 1988.
-~Department of Agronomy and Range Science, University of California, Davis, CA 95616.
3 Department of Agronomy and Range Science, University of California, Davis, CA 95616. Present
address: Centro Internacional de Agricultura Tropical, Apartado a6reo 6713, Cali, Colombia.
Economic Botany, 44(1), 1990, pp. 50-60
9 1990, by the New York Botanical Garden, Bronx, NY 10458
1990]
K O E N I G ET AL.: P H A S E O L I N TYPES
51
Link. to estimate the average gene diversity per locus for five enzyme loci in 11
different locations in California. They were able to measure the average gene
diversity and relate this to the degree of aridity of the environment. Kesseli and
Michelmore (1986) used allozyme variation among 31 collections of cultivated
Lactuca sativa L. and wild Lactuca species to assess genetic diversities and establish phylogenies. Sullivan and Freytag (1986) used seed proteins of 120 accessions belonging to 17 wild species of Phaseolus to assess inter- and intraspecies
variation and determine interspecific relationships. Nevo et al. (1986) examined
genetic diversity and population structure in 52 populations of Hordeum spontaneum Koch., the wild progenitor of barley, using allozyme variation of 27 loci.
They identified spatial variation patterns and correlations between environmental
variables and isozyme variation; they concluded that genetic variation in wild
barley is high, partly adaptive, and predictable by ecology and allozyme markers.
Rick and Fobes (1975) and Rick et al. (1977) monitored allozyme variation in
Lycopersicon accessions from a wide geographic distribution to determine patterns
of genetic variation and the extent of genetic differentiation between populations
and races. Electrophoretic techniques have been widely employed to assess the
genomic and evolutionary relationships between different Aegilops and Triticum
species (Johnson 1967; Johnson and Hall 1966; Johnson et al. 1967).
Studies on crop domestication have been conducted for several crops, including
barley, tomato, radish, and rice, using molecular markers (Glaszmann 1987; Nevo
et al. 1986; Rick et al. 1977; Second 1982). Previous studies on domestication of
common bean (Phaseolus vulgaris L., Fabaceae) have used phaseolin, the major
seed storage protein of the common bean. Phaseolin displays a number of characteristic banding patterns depending on the accessions when subjected to onedimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS/
PAGE) and two-dimensional isoelectric focusing SDS/PAGE (IEF-SDS/PAGE).
Genetic studies have shown that the genes coding for different polypeptides of
each phaseolin pattern are tightly linked and inherited as a single Mendelian unit
with the alleles being co-dominant (Brown et al. 198 l a,b, 1982).
Earlier studies of wild and cultivated common bean genotypes have revealed
that: (1) wild genotypes of different geographic origins could be distinguished by
their phaseolin type ('S' and 'M' in Mesoamerica; 'B' and 'CH' in Colombia; and
'T' in the Southern Andes, i.e., Peru and Argentina); (2) cultivars displayed a
parallel geographic distribution in phaseolin types (the 'S' phaseolin type was
predominant in Mesoamerica whereas the 'T' type predominated in the Andean
region); (3) cultivars probably arose from multiple domestications along the geographical distribution area of wild beans (two major areas of domestication being
Mesoamerica and the southern Andean region and a minor area in Colombia);
and (4) domestication induced a reduction in phaseolin diversity, especially in
Mesoamerica (Gepts and Bliss 1986; Gepts et al. 1986).
Previous studies of Gepts et al. (1986) and Gepts and Bliss (1986) examined a
limited number of accessions from the Andean region, Central America, and
peripheral locations in Mexico. The availability of new materials resulting from
the explorations of Drs. Daniel Debouck (International Board for Plant Genetic
Resources, Rome, Italy) and G. Nabhan (Desert Botanical Gardens, Phoenix, AZ)
provided for additional electrophoretic analysis to contribute a more complete
picture of phaseolin diversity in wild common bean. In addition, cultivars were
52
ECONOMIC BOTANY
[VOL. 44
examined to identify minor variants of the 'S' phaseolin and clarify that the
geographical distribution o f ' B ' phaseolin in cultivars is not limited to Colombia
but extends into Central America.
MATERIALS AND METHODS
Plant materials'
The 41 wild accessions ofP. vulgaris were obtained from: the Phaseolus World
Collection at the Centro Internacional de Agricultura Tropical (CIAT), Cali, Colombia; Dr. H. Briicher (CRICYT, Mendoza, Argentina), Dr. R. Hannan (Pullman, WA) and Dr. J. G. Waines (Riverside, CA) (Table 1). Six accessions were
from Mexico, 4 from Guatemala, 1 from Costa Rica, 1 from Peru, and 29 from
Argentina. The 41 cultivars of P. vulgaris described in Tables 2, 3, and 4 were
obtained from the Phaseolus World Collection at CIAT, Cali, Colombia.
Preparation of flour samples for electrophoresis
Four seeds of each wild bean accession were analyzed by one-dimensional
sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS/PAGE). A flour
sample was taken from the raphe end of each seed and suspended for at least 30
min in a solution consisting of equal volumes of 0.5 M NaCI solution and cracking
buffer (0.625 M Tris-HCl; pH 6.8; 2 mM EDTA; 2% (w/v) SDS; 40% (w/v) sucrose;
1% (v/v) 2-mercaptoethanol; and 0.1% (w/v) bromophenol marker dye) (Brown
et al. 1981a; Gepts et al. 1986). The mixture was heat-treated at 100~ for 5 min
and then centrifuged at 15,000 g for 5 min at room temperature. Samples of the
supernatant were then subjected to one-dimensional or two-dimensional SDS/
PAGE electrophoresis.
Electrophoresis
One-dimensional SDS/PAGE was performed following the method described
by Laemmli (1970) modified by Ma and Bliss (1978). Electrophoresis was carried
out in 0.75 mm thick, 15% (w/v) polyacrylamide slab gels. Two-dimensional IEFSDS/PAGE was carried out as described by Brown et al. (1981 a) except that 15%
polyacrylamide slab gels were used for the SDS dimension.
Assigning names to the new phaseolin types
Assigning names to the new phaseolin types in wild common beans follows the
present system in which the locale (region or pre-Columbian civilization) of the
accessions is used to collectively designate all phaseolin types of the area: e.g., 'I'
(Inca). If a specific phaseolin type is found also in a cultivar, the initial of the
cultivar's name is used to specifically identify that phaseolin type: e.g., 'Sd' ('S'
phaseolin variant found initially in cultivar Durango 222).
RESULTS
Phaseolin diversity in wild P. vulgaris
Results of one-dimensional SDS/PAGE of phaseolin from Mexican wild bean
accessions were similar to those obtained previously (Gepts et al. 1986) in that
KOENIG ET AL.: PHASEOLIN TYPES
19901
TABLE 1.
CIAT number
G19887
G19888
G19889
G19890
G19891
G19892
G19893
G19894
G19895
G19896
G19897
G19898
G19899
G19901
G19902
G19903
G7469
G19906
G19907
G19908
G19909
G20559
G09997A
G09997B
G12862
53
PHASEOLIN TYPES OF WILD PHASEOLUS VULGARIS ACCESSIONS.
Other number
Country
DGD 622
DGD 623
DGD 624
DGD 626
DGD 628
DGD 629
DGD 630
DGD 636
DGD 637
DGD 639
DGD 643
DGD 644
DGD 647
DGD 649
DGD 650
DGD 651
DGD 0621
DGD 0632
DGD 0634
DGD 1711
DGD 1712
DGD 1713
DGD 1715
DGD 1716
Jujuy 621
Salta 1716
Tucuman 634
Tucuman 639
N129, L326
DGD 1962
DGD 1610
DGD 1611
DGD 1616
DGD 1619
P1246410
M7446A
M7446B
P318693
P319441
L625
LI3
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Argentina
Peru
Guatemala
Guatemala
Guatemala
Guatemala
Costa Rica
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Phaseolin type
T,
T
T
T
T
T
H
J
J
C
J
C
T
T
H
T
T,
T
T
J
T
T
T
T
T
T
T
T
T
I
S
M
M
S
M
S
M
M
M
S
M
the 'M' and 'S' phaseolin types predominated. Materials from the Andean region
showed, in addition to the 'T' types, unique phaseolin patterns not previously
observed. Figure 1 shows a one-dimensional SDS/PAGE of phaseolin patterns in
wild (lanes 2, 4, 5, 6, 7, 8, 12, and 16) and cultivated (lanes 1, 3, 9, 10, 11, 13,
and 15) common beans. Phaseolin types 'T' (lane 2), 'S' (lane 12), and 'M' (lane
16) had been described before among wild common beans (Gepts et al. 1986;
Gepts and Bliss 1986). New phaseolin types were observed among wild bean
accessions from the Andean Region. Accession D G D 1962 from northern Peru
54
ECONOMIC BOTANY
TABLE 2.
('IAT number
[ V O L 44
CULTIVARS DISPLAYING THE ' S B ' ( ' S BRAZIL') PHASEOLIN TYPE,
Other number
(304830
G05059
BZL-0349
G05173
BZL-987
ldentificalion
Country
'Rio Tibagi'
'H6 Mulatinho'
'Cachinho'
"Mulatinho Vagem Roxa'
' Pitouco'
"Seleccao L-39-PB'
'Black Turtle Soup'
Brazil
Brazil
Brazil
Brazil
Brazil
Brazil
U.S.A."
' Hedrick (1931) indicaled that 'Black Turtle Soup' may have originated in Brazil,
(Fig. 1, lane 8) displays a phaseolin type that lacks the highest (52 kD) molecular
weight band characteristic o f phaseolin in accessions o f southern Peru and Argentina (Gepts et al. 1986; present results); and resembles that o f Colombian wild
lines which also lack this band (Gepts and Bliss 1986). A m o n g the Argentinean
accessions, G19893 (Fig. 1, lane 3) and G19902 exhibited an ' H ' phaseolin type
which had previously only been observed in cultivars. Accessions G 19896, G 19898,
and D G D 0621 (not shown) exhibited a 'C' phaseolin previously unreported
a m o n g wild beans. Accessions G19887, G19894, and D G D 1711 (Fig. 1, lanes
5, 6, and 7) displayed phaseolin types not previously identified a m o n g wild beans
or cultivars.
The distinctness o f the presumed novel phaseolin types in wild beans was
confirmed by two-dimensional I E F - S D S / P A G E (Fig. 2). Based on these results,
it was decided to name the new phaseolin types formally as follows: T (Inca) for
phaseolin o f accession D G D 1962; 'J' (Jujuy) for phaseolin of accession D G D
T A B L E 3.
CULTIVARS DISPLAYING THE ' S D ' ( ' S D U R A N G O
('IAT number
Other number
G00278
G10957
G 10971
G10982
G11010
PI 165435
DGD 78/027
DGD 78/035(}
DGD 78/046B
DGD 78/006A
G00845
G00876
G02402
PI201349
PI203934
P1312092
G13676
G04399
F-056
222')
ldentillcation
'Ejotero'
'Bayo'
'Pinto'
'Bayo Regional'
'Apetito'
'Bayo Criollo'
'Bayo Durango"
'Cuarenteno'
Durango 222
'Garbancillo Zarco IV'
'Garrapato'
Mexico 222
'Wexano'
'Urubonobono'
'Manlequilla'
Tamaulipas 9
'Bayo Criollo'
PHASEOLIN TYPE.
Counlw
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
Mexico
1990]
K O E N I G ET AL.: PHASEOLIN TYPES
TABLE4.
CIAT number
G02511
G04691
G 12449
GO1344
GO1345
G 14027
G02923
G16267
G03017
G 17648
G02997
G05054
CULTIVARS WITH ' B ' PHASEOLIN TYPE.
Other number
PI 313590
P207262
CAT-0998
P208776
P208777
Identification
Country
Boyach 22, 'San Gileno'
'Matahambre'
Colombia
Colombia
Colombia
Ecuador
Nicaragua
Nicaragua
Nicaragua
E1 Salvador
Costa Rica
Guatemala
Guatemala
Guatemala
Brazil
U.S.A.
(Venezuela)
Seda 42 (Loja)
'Orgulloso'
P326346
P309842
55
'Chimbolo'
'Mezcla'
'Pata de Zope"
'Rabia del Gato'
Cornell 49-242
1711, G19887, and G19897. It should be kept in mind that the 'J' category
represents--as for the 'M' and ' C H ' t y p e s - - a heterogeneous class including those
phaseolin types present in wild beans but absent from cultivars.
Phaseolin diversity in cultivated P. vulgaris
Previous analyses o f phaseolin diversity among cultivars (Gepts et al. 1986),
indicated some heterogeneity a m o n g the 'S' phaseolins, which was investigated
further in the present study. The a m o u n t o f protein loaded on acrylamide gels
was reduced in the present study to obtain a better separation o f the individual
bands o f the 'S' phaseolin pattern. In doing so, two m i n o r variants--'Sb' and
' S d ' - - o f t h e 'S' phaseolin type were revealed (Fig. 1, lanes 13 and 14). The 'Sb'
pattern is best recognized in one-dimensional S D S / P A G E by the presence o f a
stronger band at the low molecular weight end o f the pattern (see arrow in Fig.
1, lane 14); occasionally, this band m a y appear as two separate bands (not shown).
The 'Sd' pattern can be identified in one-dimensional S D S / P A G E by the presence
o f a pair o f equally strained bands in the central part o f the pattern (see two arrows
in Fig. 1, lane 12). Analysis o f the 'Sb' (not shown) and 'Sd' (Fig. 3) phaseolins
by two-dimensional I E F - S D S / P A G E reveals that their pattern is very similar to
that o f the 'S' phaseolin; c o m p a r e d to the 'S' phaseolin type, the 'Sd' type lacks
a polypeptide (compare a and b in Fig. 3). This similarity suggests that the 'Sb'
and 'Sd' types are mere variants o f the 'S' phaseolin type.
The identification o f the 'Sb' and 'Sd' phaseolin types p r o m p t e d us to screen a
collection o f cultivars to investigate their geographic distribution. Seven cultivars
showed the 'Sb' phaseolin type, six o f which came from Brazil, hence 'Sb' (Table
2). The seventh cultivar is 'Black Turtle Soup', an old U.S. heirloom variety
thought to have been introduced from Brazil (Hedrick 1931). The 'Sd' pattern
was observed in 20 cultivars most o f which came from the north-central highlands
o f Mexico, hence 'Sd' (i.e., 'S Durango 222') (Table 3).
The i m p r o v e d electrophoretic separation afforded by the lower sample protein
56
ECONOMIC BOTANY
[VOL. 44
Fig. 1. One-dimensional SDS polyacrylamide gel electrophoresis of phaseolin (P) in wild (lanes 2,
4-8, 13, and 16) and cultivated (lanes 1, 3, 9-12, 14-15) accessions of Phaseolus vulgaris. Lanes 12: 'T' phaseolin; lanes 3-4: 'H'; lanes 5-7: 'J'; lane 8: 'I'; lane 9: 'C'; lane 10: 'A'; lane 11: 'S'; lane
12: "Sd'; lane 13: %'; lane 14: 'Sb'; lanes 15-16: 'M'.
concentration also allowed us to identify a number of additional cultivars with
the 'B' (Boyacfi 22) phaseolin (Table 4). This phaseolin type was first observed
among wild and cultivated common beans from Colombia (Gepts and Bliss 1986).
The additional cultivars include a number of accessions that had been previously
classified as having an'S' phaseolin type (Gepts et al. 1986). They include primarily
well-known common bean landraces of Central America such as 'Orgulloso', 'Pata
de Zope', and 'Rabia de Gato', and cultivars from northern South America (Colombia and Ecuador).
Finally, among Mexican cultivars, one accession was identified with an 'M'
phaseolin type (Fig. 1, lane 15). This is the first time that a Mexican cultivar has
been observed to have a phaseolin type other than a n ' S ' type (or any of its
variants).
DISCUSSION
Variation in phaseolin type among wild accessions
The results of our study provide additional information on the domestication
of common bean. The study confirms that plant material from Middle America
has the greatest amount of genetic variability of phaseolin (Gepts et al. 1986);
however, the new phaseolin types found in wild material from the Andean region
reveal that there exists a greater amount of genetic variability in this region than
previously identified. The fact that these new phaseolin types have been found
only in wild material confirms the previously described trend of reduction in
genetic diversity in cultivated forms of P. vulgaris (Gepts et al. 1986).
1990]
KOENIG ET AL.: PHASEOLIN TYPES
57
Fig. 2. Two-dimensional isoelectric focusing SDS polyacrylamide gel electrophoresis o f phaseolin
in wild Phaseolus vulgaris, a: 'J' phaseolin: G19897; b: 'J': G19887; e: 'J': D G D 1711; d: 'I': D G D
1962; e: 'C': G19896; f: 'H': G19893.
The genetic diversity in the Andes is not as high compared to that found in
Mesoamerica. The occurrence of lower levels of genetic diversity in the Andes
may arise from one or several of the following causes. Most of the wild species
of Phaseolus are distributed in North America (principally Mesoamerica) (Delgado
Salinas 1985; Marrchal et al. 1978). This suggests that the genus as a whole, and
also wild P. vulgaris, originated in North America. The presence of wild P. vulgaris
in the Andes would trace back to several dispersal events at unknown times in
the past. Genetic drift and selection to new environments associated with the
dispersal from Mesoamerica to the Andes would have resulted in a reduction of
genetic diversity. Second, phaseolin might be linked to loci that play an adaptive
role to the habitat of wild beans in the Andes, for example seed size. Species living
in a closed environment such as forests tend to have larger seeds than species
living in an open environment, such as grasslands or shrubby vegetation (Silvertown 1982). The habitat of wild P. vulgaris in Mesoamerica consists of secondary
vegetation whose principal components are perennial pioneering shrubs; wild P.
vulgaris was not observed in forests or other climax vegetation (Gentry 1969). In
the Andes, on the other hand, wild P. vulgaris grows in a mesothermic forest from
Venezuela to Argentina (Briicher 1988). Phaseolin, as one of the major constituents of the seed, can also influence seed size (Hartana 1983). Differences have
been observed between various phaseolin types for the amount of phaseolin and
total protein per seed (Hartana 1983, 1986). Because of the particular environment
wild P. vulgaris is faced with in the Andes, selection may have favored those
genotypes with a larger amount of seed storage reserves such as proteins (i.e.,
phaseolin) or carbohydrates. This hypothesis could be tested by introducing the
58
ECONOMIC BOTANY
[VOL. 44
Fig. 3. Two-dimensional isoelectric focusing SDS polyacrylamide gel electrophoresis of phaseolin
in cultivated Phaseolus vulgaris, a: 'Sd' phaseolin: Durango 222; b: "S': 'Sanilac'; c: mixture of'S' and
'Sd' phaseolins. The arrow in a points to the missing polypeptide (compare with b).
different phaseolin types in the same genetic background and comparing the
germination behavior of the isogenic lines with different phaseolin types. Traits
other than seed size may also provide better adaptation to Andean environments;
their linkage relationships with phaseolin need to be determined. Third, the Andean region would have less environmental diversity and offers therefore fewer
possibilities for diversifying selection. This is unlikely as Lycopersicon spp., which
originated in the Andes, appear to be very diverse (e.g., Rick and Fobes 1975).
Interestingly, the phaseolin pattern of the northern Peru accession DGD 1962
(province of Cajamarca) lacks the 52 kD high molecular weight subunits ofphaseolin and is therefore more similar to Colombian phaseolin types than to those of
southern Peru and Argentina. This accession may be within a geographical transition area (Colombia, northern Peru) between Mesoamerican and Andean populations. The boundaries of this transition area can be broadly estimated with a
genetic marker such as phaseolin; however, with additional markers such as isozymes the area can be more narrowly defined.
Finally, the hypothesis of two independent centers of domestication--Middle
America and the Andean region--holds true in this study. The phaseolin types
of the two regions are markedly different with no overlaps of phaseolin types
within the wild materials.
Variation in phaseolin type among cultivars
The existence of cultivars with 'S' phaseolin variants or 'B' phaseolin is significant because these cultivars exhibit a defined geographic distribution and
agronomic traits that are important in the development of new cultivars. The 'Sb'
phaseolin cultivars originate in Brazil, are small seeded (<25 g/100 seeds) and
very early, and have a tolerance to low soil acidity. The 'Sd' phaseolin cultivars
originate in the highlands of north-central Mexico. They are characterized by a
prostrate growth habit (growth habit III: Singh 1982) and medium seed size (2540 g/100 seeds). They display good combining ability and a positive association
between seed size and yield. They are generally drought tolerant and at least one
cultivar (G02402; 'Garrapato') is resistant to bean golden mosaic virus.
The 'B' phaseolin cultivars, as identified in this article, are distributed in northern South America and Central America. They are also characterized by growth
1990]
KOENIG ET AL.: PHASEOLIN TYPES
59
habit III, but are small-seeded. They show early maturity, and some cultivars are
resistant against anthracnose (Collelotrichum lindemuthianum), common bacterial blight (Xanthomonas phaseoli), and Apion godmanni pod borer.
The existence of these correlations between phaseolin type and agronomic traits
does not imply that phaseolin is directly involved in these traits. The observed
correlations are probably a consequence of the self-pollinated nature ofP. vulgaris
which leads to strong multilocus associations (Allard 1975). These associations
in turn can lead to the formation of gene pools, i.e., groups of genotypes with
similar attributes (Gepts 1988; Gepts and Bliss 1985; Singh 1988).
The identification of an 'M' phaseolin type in a Mexican cultivar raises the
issue of gene flow between wild and cultivated forms of com m on bean. 'M'
phaseolins are normally found only among wild beans (Gepts et al. 1986; present
results). The presence of an 'M' type in accession G11025A suggests that this
accession is a hybrid between a wild and cultivated type. Aside from its normal
cultivated phenotype, this accession exhibits a speckled color pattern characteristic
of wild bean seeds, further suggesting its hybridity. O f the approximately 115
Mexican cultivars that have been analyzed, only one has displayed an 'M' phaseolin type. Two interpretations can be offered to explain this observation: either
gene flow from wild to cultivated types is fairly rare, in agreement with the
predominantly self-pollinating nature of common bean; or the phaseolin locus is
closely linked to a locus determining an essential aspect of the cultivated phenotype. Selection of the cultivated phenotype in the progeny of a cultivated •
wild hybrid would then cause an indirect selection against the 'M' phaseolin type.
A more definitive answer to the issue of gene flow between wild and cultivated
genotypes must await identification of additional highly variable molecular markers characterizing genetic variability and an analysis of the linkage relationships
between phaseolin and morphological traits determining the cultivated phenotypes.
ACKNOWLEDGMENTS
This work was supported in part by the Charles A. Lindbergh Fund. RLK is a recipient of a fellowship
of the International Agricultural Development Graduate Group, University of California-Davis. We
thank H. Brficher, D. Debouck, R. Hannan, R. Hidalgo, and J. G. Waines for providing seed samples.
We are grateful to M. Schmidt and L. Duval for typing the manuscript.
LITERATURE CITED
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globulin-1 storage protein of French bean. Theor. Appl, Genet. 59:83-88.
- - ,
F. A. Bliss, and T. C. Hall. 1981b. Linkage relationships between genes controlling seed
proteins in French beans. Theor. Appl. Genet. 60:251-259.
- - ,
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