Black Soybeans

[Wayne Parrott]

In article <0423hms-020395140523 at msp81-14.nas.mr.net> 0423hms at InforMNs.k12.MN.US (Jerry Hoikka) writes:
>From: 0423hms at InforMNs.k12.MN.US (Jerry Hoikka)
>Subject: Black Soybeans
>Date: 1 Mar 1995 19:48:34 GMT

>We are looking for information on Black Soybeans for a science fair
>project. Can anyone tell us where to obtain information as to why black
>soybeans are black and why they appear in a normal population of soybeans. 
>Any help would be appreciated.

>Dale Redetzke   Hutchinson Middle School, Hutchinson, MN

>-- 
>Hutchinson Middle School
>1365 South Grade Road
>Hutchinson, MN 55350

There is actually a book called "Soybeans:  Improvement, Production, and Uses, 
published by the American Society of Agronomy, Inc., Publisher,  Madison, WI, 
in 1973, and edited by B.E. Caldwell, R.W. Howell, R.W. Judd, and H.W. 
Johnson. (there is a second edition from 1987, but this latest one no longer 
has the color photographs of the black soybean seeds.]  The genetics of seed 
coat color are explained in chapter 4, "Qualitative genetics", written by R.L. 
Bernard and M.G. Weiss.  Quoting from the chapter (p. 141-143):

"Black and brown pigments may occur in the outer layer of soybean seed coats 
in varying amounts and patterns.  Virtually all wild soybeans and about 9% of 
the USDA germplasm collection, usually the weedy or hay varieties, have a self 
black seed coat (i.e., the entire seed coat surface is black].  A few wild 
soybeans and 6% of the germplasm collection, again mostly weedy or hay-type, 
have a brown seed coat.  Nagai (1921) showed that this difference was due to a 
single gene pair with black (R) completely dominant to brown (r).  He reported 
that the black pigment was an anthocyanin... "

There is another gene pair, (T-t) which affect the seed coat color (and other 
plant parts as well).  Thus, a soybeans of genotype:

RRTT   would have a black seed coat,
rrTT      have a brown seed coat
RR tt     have a buff seed (buff = what is now the standard soybean color),
rr tt        have a buff seed coat.

It is not quite this simple, as there is another gene (ie W1) that will 
convert the RRtt plant from buff seed coat to "imperfect black" seed coats 
(these have buff and black colors blendedd on the seed coat).  Also, I 
still have not answered why you would have found a black soybean in a field.

There is yet another gene, the "I" gene, which is responsible for that.  A 
black soybean is black only if it has the recessive i gene, that is, is of 
genotype RRTTii.  Soybeans of genotype RRTTII are again buff in seedcoat 
color.  There are other alleles (versions) of the I gene as well.  Those that 
are RRTT i^k i^k have what is known as black saddle seed coats-- the seed coat 
is buff, except for a black area around the hilum (the scar where the seed was 
attached to the pod).  Those that are RRTTi^i i^i have a black hilum (the 
hilum for RRTTII types is grey).