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lion) was equal to that harvested for beans. The acreage harvested for

hay decreased steadily after 1940 and only about one-half million acres

have been harvested for hay annually since 1956. In contrast, the acreage

harvested for beans was nearly 14 million in 1950 and nearly 24 million

in 1960. An estimated 27 million acres were harvested for beans in 1961.

Soybeans are produced primarily for oil and protein. Seeds of

varieties produced in the United States average about 21 per cent oil

and 40 per cent protein on a dry weight basis. Nearly 90 per cent of the

soybean oil used in the United States is in human foods, primarily

shortening and margarine, and over 95 per cent of the protein used

domestically is in animal feeds.

The soybean has been used little as a research tool in genetics and

breeding research. However, research workers interested in improving

the species for economic use have made substantial contributions to

the literature on the genetics and breeding of soybeans. Notable breeding progress has been made in the United States and Canada in the past

25 years and this progress played a major role in the expansion of soybean acreage in the two countries. Breeding progress is becoming increasingly difiicult, however, because of an increased number of breeding

objectives, notably resistance to diseases, and because the gross gains

in the breeding of the introduced crop for a new production area have

been made. Future gains will be more difiicult than those of the past

and will require more refined techniques or procedures.

The purpose of this review is to summarize and present information

on the genetics and breeding of soybeans, particularly information relative to improving breeding procedures. In the attempt to make the

review as complete as possible, permission to use unpublished information was obtained from several individuals. This was done when the

unpublished information was considered to be especially pertinent in

rounding out the available information on a given subject or in a few

instances when the information was pertinent to a given subject and

would not be published elsewhere. The authors express their sincere

thanks to all the individuals contributing unpublished information.


The soybean belongs to the family Leguminosae, subfamily Papilionoideae, and the genus Glycinc L. The botanical classification of the

cultivated form has been controversial and the multiplicity of names

applied to it has created confusion as to its correct designation. However, Ricker and Morse (1948) contend that according to international

botanical rules the correct name of the soybean is Glycine max (L.)

Merrill, a viewpoint shared by most taxonomists.



The literature on the species situation within the genus Glycine has

been a grossly confused issue, and this confusion has complicated the

investigation of species in the genus. A recent concentrated attempt to

obtain seed of species of Glycine has met with some sucoess and a

taxonomic investigation by F. J. Hermann has greatly simplified the

species situation. He concluded that in addition to G. inux the genus

was made up of the following species: G. clandestina Wendl.; G. falcata

Benth.; G. ktrobeana (Meissn.) Benth.; G. tabacina (Labill.) Benth.; G.

tomentella Hayata; G. petitiana (A. Rich.) Schweinf.; G. javanica L.;

G. ussuriensis Regel & Maack; and a tenth species, G. sericea Benth. not

Willd., for which a new name is proposed in Dr. Hermann’s publication.

Seven subspecies or varieties are listed for G. javanica and one for G.


G. max and G. ussuriensis are known to have 40 chromosomes and

both behave as diploids. They are cross fertile and their hybrids usually

have normal fertility. Ramanathan (1950) listed G. javanica as having

20 pairs of chromosomes, the basic number of the genus being 10. H. L.

Weaver (personal communication) observed 20 pairs of chromosomes

in some types of G. javanica and 10 in others. He also observed 20 pairs

in G. falcata. Limited attempts in the United States to cross some of the

species (other than G. ussuriensis) listed above to G. mux have failed.

A type referred to frequently in the literature as G. gracilis was considered to belong to G. max in Hermann’s investigation of the genus.


The origin of the cultivated form of the soybean is unknown. “The

soybean is native to eastern Asia” is a statement frequently transferred

from one publication to another. Nagata (1960a) recently reviewed the

literature on the subject and considered the distribution of soybeans and

other ethnobotanical principles in an interesting study of the origin of

the cultivated type. Although he concluded that the origin of soybean

culture still remains obscure, his results indicated that the origin was

in China proper, especially in north and central China. He based his

conclusions in part on the distribution of G. ussuriensis, which he considers to be the progenitor of the cultivated form. According to Morse

(1950) there is little doubt that G. max was derived from G. ussuriensis

since apparently no other wild plant found can possibly be its ancestor.

Nagata postulated that the cultivated form was introduced into Japan


1 Information from a manuscript entitled “A Revision of the Genus Glycine and

Its Immediate Allies” prepared by F. J. Hermann for publications as U . 5’. Dept. Agr.

Tech. Bull. 1268. Sincere appreciation is extended to Dr. Hermann for permission to

use the information prior to publication.



via Korea and presented information to suggest that it was introduced

into Korea directly from north China sometime during the period 200 B.C.

to the third century. Hamada (1955) described preserved types stored

in the Shosoin Treasury since about the seventh century (along with

medicinal herbs introduced from China) that resembled the shortseason types currently grown in Kyushu and Loochoo Provinces of

Japan. Nagata (1960a) interpreted this to indicate that the short-season

types of Japan may have been introduced directly from central China

to south Japan. Additional detail on the ancient history of the soybean

may be obtained from Morse (1950).

According to Bening (1951) the first news of the soybean was

brought to the Western Hemisphere in the writings of Engelbert Kaempfer in 1712. Morse (1950) presented a detailed account of the modem

history of the soybean and recorded that the first published account of

the plant in the United States appeared in 1804. According to him not

more than eight varieties of soybeans were grown in the United States

prior to the numerous introductions by the U. S. Department of Agriculture beginning in 1898.

Introductions from Manchuria, China proper, Korea, and Japan have

played a predominant role in the soybean industry in the United States.

The early varieties and the germ plasm used in soybean breeding in

this country came from these introductions (see Section V, F).

II. Reproduction


Soybean flowers are normally about 6 to 7 mm. in length, and their

smallness imposes a limitation on the ease with which controlled pollinations can be made. Guard (1931) described the soybean flower as

having a tubular calyx terminating in five unequal lobes. The largest of

these is anterior, the next two lateral, and the smallest two, obliquely

posterior. The calyx is persistent, being intact on the ripe fruit, but

rapidly deteriorates and only fragments may be found on pods that

have been exposed to weather for a considerable length of time. The

corolla consists of five separate petals. The largest (standard) is posterior, the two next in size (wings) lateral, and the two keel petals

anterior. There is no fusion of the keel petals as in some other legumes.

The ten stamens are separate at first, but shortly before anthesis the

filaments of nine of them are elevated as a single structure by the

development of a basal region, leaving the posterior stamen separate.

Miksche ( 1961 ) recently reviewed the literature on morphological

studies with soybeans and presented the results of an interesting study

on the developmental anatomy of the plant. He studied organ and tissue



organization from dormant seed to floral initiation. Although this area

of work is beyond the scope of this review, Miksche’s results and the

literature cited by him are valuable for research workers interested in




The time of flowering of soybean plants depends largely on the

number of hours of darkness they receive each day. Other factors such

as temperature, nutrition, and light intensity and quality may influence

the response of soybeans to dark periods suitable for flowering; but in

the field the length of the dark period is usually the primary influence

in the induction of flowering. Plants of many varieties are completely

incapable of flowering unless they receive 10 or more hours of darkness

daily and plants of all varieties flower more quickly with daily dark

periods of 14 to 16 hours than with shorter ones (Borthwick and Parker,

1939; Parker and Borthwick, 1951). Since the length of the daily dark

period is a function of latitude, soybean varieties are adapted as a fullseason crop to narrow belts of latitude.

The effect of changes in natural photoperiods on the maturity of

soybeans occurs primarily prior to flowering. Rates of development in

subsequent stages also are influenced by photoperiod but the periods

from about seed set or end of flowering to maturity are similar for all

soybean varieties regardless of maturity. Natural photoperiods over a

wide range of latitude also are similar during the latter stages of

development of the soybean. When the natural photoperiods are manipulated to create substantial differences, differences among varieties in

their response to photoperiod in stages of development after flowering

are readily observed (Nagata, 1960b; Johnson et al., 1960). This is discussed in greater detail in the review by Cartter and Hartwig in this


Soybean plants normally produce many more flowers than pods that

finally mature. Shedding of 75 per cent or more of the flowers is not

uncommon, and even under the most favorable conditions the loss of a

substantial portion of the flowers can be expected. Flower and pod

shedding apparently are not due to a lack of viable pollen (Van Schaik

and Probst, 1958b) or to lack of fertilization (Kato .et d.,1955).


1. Natural

Soybeans are completely self-fertile and the amount of outcrossing

under natural conditions is about 0.5 per cent for plants in adjacent

rows and 1 per cent for plants grown in close contact (Weber and

Hanson, 1961).



Soybean breeders and geneticists have become increasingly concerned in recent years with techniques for increasing the amount of

“natural” outcrossing in soybeans. A search for male-sterile, female-fertile

types has been unsuccessful, and various other approaches have been


chemical referred to in the literature as FW-450 and two apparently

related chemicals ivere evaluated by Casas (1961) as selective gametocicles. Although pollen viability was reduced by the chemicals, the flowers failed to open properly and an actual decrease in outcrossing resulted.

Similar results were obtained with F\17-450by Hanson (personal communication ) . Casas obtained 5.2 per cent outcrossing of normal plants

in cages containing honey bees compared to only 0.6 per cent for

plants outside the cages.

\\‘eber and Hanson (1961) obtained a four- to sixfold increase in

outcrossing of plants from seed irradiated with different dosages of

X-rays and thermal neutrons. Outcrossing of the untreated checks was

approximCitely 1 per cent. From a theoretical consideration of the

amount of outcrossing required to have practical utility in intermating

populations, they concluded that the figure should be higher than the

approximately 4 to 6 per cent which they obtained.

Athow (personal communication) has observed as much as 16 per

cent outcrossing of plants infected with tobacco ringspot virus. A small

percentage of the seed from infected plants normally give rise to virusfree normal plants (-4thow and Bancroft, 1959) and the possibility of

utilizing the normal plants as pollen parents in a virus-infected population caged with bees is intriguing. The virus-free plants could be used

to establish noinial lines after the desired number of generations of

in termating.

2. Artificial

Crossing soybeans is tedious. The procedure followed in emasculating is standard. but the time of emasculation, collection of pollen, and

pollination varies greatly. This variability depcnds to a large extent on

environment and to a lesser extent on the personal preference or needs

of indib idual workers.

The small size and fragileness of soybean flowers make it necessary

to use extreme care in emasculating. The only instrument used is a small

pair of forceps. The inner surfaces should be flat without corrugations

and the spring end should have light tension. Flowers which would

normally open the morning following the day the cross is made are

used as females. These are in the bud stage with the color of the petals

readily visible. The lobes of the calyx are removed by grasping them



individually with the forceps and pulling downward. The corolla is

then grasped with the forceps at a right angle to the axis and removed

with a slow pull, working the forceps gently from side to side during

the process. The corolla and all the anthers may be removed in one

motion; however, the keel and/or anthers often are not removed with

the first pull. The keel can be removed easily and the points of the

forceps can be used to remove the anthers. However, removal of the

anthers in this manner frequently results in injury to the remaining

parts of the flower and poor success in crossing. When a genetic marker

can be used to distinguish F1 plants from plants of the female parent,

most experienced operators make no attempt to remove the anthers with

the points of the forceps.

Local environmental conditions, including weather and insects,

determine the time of day when pollen is collected and crossing is

most successful. Generally, in the central and northern parts of the

United States, flowers that have opened the day the cross is to be made

can be used to furnish pollen and crossing can be done successfully

throughout the day. However, in the southern States it is extremely difficult to get viable pollen from open flowers. In this area pollen flowers

are collected early in the morning an hour or two before they would

have opened and stored in a cool, dry place for use later in the day.

A desiccator is usually used to ensure dry storage conditions. Pollinations

in mid to late afternoon are usually the most successful. In the southern

States the percentage of success from pollinations made in the forenoon

is extremely low.

When the flowers are ready for pollinating, the tips of the forceps

are inserted in the back of the keel of the pollen flowers and the pistil

and column of anthers removed. This is used as a brush to deposit pollen

on the exposed stigma of the emasculated flowers. Generally three or

four pollinations can be made with a single flower.

From one to three flowers at a node may be in the right stage for

crossing and all other flowers and buds should be removed. The flowers

or node should be tagged for identscation of the cross. The flowers

should be checked about a week after the pollinations are made and all

newly developed flower buds removed. Soybean flowers may continue

to develop after a cross has been made and when harvesting the crossed

seed it is sometimes difficult to determine whether the pod developed

from the emasculated flower or from one that developed later. Pods

resulting from a cross can readily be distinguished when small by the

absence of the calyx lobes on the base, and this distinction can usually

be made when the pods are mature.

Some published information indicates that flowers should be emascu-



lated one day and pollinated the next and that a leaf should be pinned

around each crossed flower for protection. Most agronomists in the

United States use no type of covering or protection for crossed flowers.

They emasculate the flowers desired for one cross and pollinate them


Even when the best available techniques are employed by experienced operators, the percentage of successful crosses varies greatly from

time to time. Too much or too little moisture, low night temperatures,

insects, manipulation of the photoperiod, and various other factors have

been observed to influence the success of crosses. The ideal environment

for crossing soybeans is unknown, but the success of the crossing program often can be increased greatly by a well-timed application of an

insecticide or supplemental irrigation.

Although the standard procedure of crossing soybeans has been used

for some large undertakings in recent years, much time and effort are

required in the actual crossing operation and in obtaining the desired

flowering plants over a sufficient length of time. Techniques for storing

pollen and means for speeding large-scale crossing operations would

therefore facilitate current research programs.

Hanson (personal communication ) materially increased the number

of pollinations that could be done per day by doing some of the operations in the laboratory in the morning when pollinations are least successful. Flowers were collected at the appropriate time in the morning

and the anthers and pistil were separated from the floral parts in the

laboratory. Up to 30 anther rings were stored in a 00 gelatin capsule

by sticking them around the edges of the capsule. The capsules were

stored over a mixture of 25 ml. of concentrated sulfuric acid and 75 ml.

of water in a refrigerator to maintain the desired humidity.

Kuehl (1961) recently obtained useful data on a number of questions

of importance in crossing soybeans: (1) Germination of pollen in a

30 per cent sucrose solution containing 120 p.p.m. of boric acid was

found to be a good indicator of the germination or effectiveness of

pollen in crosses; ( 2 ) pollen was stored successfully in a calcium chloride

desiccator at 3.3"C. for approximately 1 month. Storage at -20" was

less successful. The stored flowers were dry and brittle but storage for

about 30 minutes over water in a closed container restored moisture to

the tissues and induced the anthers to dehisce. ( 3 ) Pollen first became

viable about 10 hours prior to natural anthesis; and (4) the stigma of

emasculated flowers was most receptive to pollen on the day preceding

the morning of normal anthesis and remained receptive for 2 days

after anthesis.

In a report of a detailed study of the time required for various



stages of development from flower bud differentiation in the plant to

differentiation of the embryo in the seed, Kato et ul. (1954) presented

photographs interpreted to indicate fertilization on the day of flowering.

Because flowers normally open in the early daylight hours and the exact

time of collection of flowers was not given, the results can be interpreted

to indicate that fertilization takes place in about 10 hours or less after

natural pollination. If this same time sequence prevails in flowers used

in crosses 15 to 20 hours before anthesis would have occurred, there

would seem to be little likelihood of self-pollination even in nonemasculated flowers if viable pollen is used since the pollen of the

crossed flower would not be viable until 5 to 10 hours after the cross

was made (Kuehl, 1961).

111. Genetics of Qualitative Characters

Although the soybean has never been the subject for extensive

research by geneticists, the mode of inheritance of simply inherited

characters has been studied from time to time by agricultural workers

interested in the soybean as a crop. Reviews which include the results

of much of this work have been published by Owen (1928a), Woodworth (1932, 1933), Matsuura (1933), Morse and Cartter (1937), Weiss

(1949), Williams (1950), and Johnson (1961).

Genes reported for the soybean and the traits that they influence are

given in Table I. The list includes all genes reported except those

presented as only tentative suggestions and those apparently based on

expressions of previously reported gene pairs. Accordingly, in this

review, del (Stewart and Wentz, 1930) is considered to be the same

as t; de2 (Woodworth and Williams, 1938) to be p2; f ( Takahashi, 1934)

to be nu; h (Ting, 1946) to be t; lo (Domingo, 1945) to be o (oval leaflet); 90 (Stewart, 1930) to be o (reddish-brown seed coat); and y2

(Morse and Cartter, 1937) to be g. Table I includes the gene symbols

(where duplicate symbols have been assigned the ones used in the more

recent review papers are presented here), a brief descriptive phrase for

the contrasting traits, and the major reference( s ) identifying the gene

pair and assigning the symbols. A soybean strain carrying the specified

gene(s) is listed for each of the more uncommon traits. A few genes

such as df, st, and yl have been lost whereas others such as A, BZ, E, e,

L, 1, S, s, sh, sp, and w2 are probably present in available germ plasm

but have not yet been reidentified.


The variation among soybean varieties in pigmentation of various

parts of the plant, especially the seed, has provided the material for


of the I’h(wotylw.: and tlw 1iefrrcwc.c.s 13st:il)lishing the h l o d ~of

1nlicrit;inc.c nncl Assigninq thv S ~ m l m l s

‘4 1,ist of Cerics 13t*imrt(dfor thc Soy1)cwi Ilic.111diiig;i &scription















Bl 4 B ,

h , or h , o r 12,



c , c,

c , or c2




























Appww’cI p ~ i l w s ~ x * n ~ ~ ~ ~

Erwt p r i l ~ w c . ~ w x

1,c:nf abscission :it matririty

Dc4aycd nlwission ( T206)a

Bloom on srcd coat ( T 4 )

No bloom

Slrarp prilwsccncc: tip

Blurit pii1)c~sc.cnc.ctip

Cr;ick(d s ( d coat ( T217)

Entirc sccd coat

1hist;incc. to frogoy lcafspot


Yollow cotyicdons in srecl

Green cotylcdons (T38)

Normal plant

Dwarf plant

Indeterminate stem (T10)

Determinate stem ( T 6 )

Early maturity

Late maturity

Normal stem

Fasciated stem (T173)

Normal iron utilization

Inefficient iron utilization (T203)

. .-




~- --.




Kurasn\vn, 1036; Morsr and C:irtt(,r, 1937

l’rolnt, 1950

Woodworth, 10.32, 1933

Ting, 1946

Napii, 19%; hf;itsurira, 1933


Athow and Probst, 19S2

Woodworth, 1921; Owm, 1 9 2 7 ~ ; Veatch und

Woodworth, 1930

Stewart, 1927; Wooclworth, 1932, 1933

Woodworth, 1932, 1933

Owen, 1927b

Nagai, 1926; Takngi, 1929; Woodworth, 1932,


Weiss, 1943



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