INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

P. STEPHEN BAENZIGER, 338 Keim Hall, 472-1538

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

SYNTHETIC VARIETIES

Read Briggs and Knowles, Chapt. 18.


  1. Darwin noticed that cross pollination increased vigor.


  2. W. J. Beal in the 1880's recommended crossing varieties to increase vigor. Did not have inbred lines.


  3. Shull, 1909, recommended single cross hybrids, recognized:


    1. Must choose the parent carefully;

    2. Inbred produce faithfully;

    3. Hybrids from inbreds will be uniform over time;

    4. Economic problems with using low yielding inbreds.


  1. Jones, 1918, suggested double cross hybrids to lessen the economic problems of single cross hybrids.


    2. Problem is finding good combining ability.

  2. Number of single cross hybrids from N inbreds is:


N(N-1)/2Hence thirty inbreds can make 435 SC hybrids
50 inbreds can make 1225 SC hybrids.

N(N-1)(N-2)/3=Number of three way crosses.

N(N-1)(N-2)(N-3)/8=Number of double crosses,
hence 30 inbreds can make 82215 hybrids which is far too many to test.

Hybrid yield can generally be predicted by inbred yield and by top cross yield of inbreds for culling purposes.

Once have narrowed the set of inbreds, can predict yield of double crosses from the yield of single crosses. For example, if you want to estimate the yield of (A x B) X (C x D), it can be done by averaging the yield of the four single crosses not used in making the double cross, namely

[(A x C) + (A x D) + (B x C) + (B x D)]/4 = (A x B) x (C x D) for yield.

Similarly for three-way hybrids:

[(A x C) + (B x C)]/2 = (A x B) x C for yield.

These prediction equations are very important and have many uses. For example, if one wanted to try to determine which line (B) would be the best to cross with A to make an improved A for the single cross hybrid A x C, current theory suggests that the B should be the line that gives the best (A x B) x C hybrid yield [(A x C) + (B x C)]/2. Instead of making the three-way hybrid, it is possible to make all possible single cross hybrids and predict the best three-way hybrid. From the prediction of (A x B) x C, can then identify B.