INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

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

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

DEFENSIVE CROSS POLLINATION BREEDING STRATEGIES

Cross F₁ back to Line A and to Line B. Continue to backcross to Line A and B. It will narrow the differences in the lines because the method will result in having some of the genes in Line A transferred to Line B and vice versa. If heterosis is due to overdominance, this method should not improve the resulting hybrid. If the hybrid is improved, it must be due to epistasis or the accumulation of additive effects.

Second Cycle Recoveries:

This method is similar to and often uses backcrossing in that an elite inbred line is improved by adding a limited number of useful traits. Generally the method involves crossing Elite A x Parent B. The following generations may include backcrossing to Elite A or selfing with selection to recover the phenotype of Elite A with the traits of interest. Whether backcrossing or selfing and selection are used, the end result is similar -- small improvements of an already existing elite inbred line. In corn, care is usually taken to chose a Parent B that is genetically similar to Elite A to reduce the number of backcrosses or to make the selection easier.

Example: B73 was released by Iowa State University and was derived from the Iowa Super Stiff Stalk Synthetic. It has tremendous yield potential for hybrids grown in Indiana, but lacks resistance to Helminthosporium turcicum. H100 is a second cycle recovery for H. turcicum resistance of a previous inbred that was also derived form the Iowa Super Stiff Stalk Synthetic. The easiest way to improve B73 was to cross it with H100 because the lines were already similar in background.

While this method is very defensive -- leading to small improvement, it, like backcrossing is a more predictable breeding method. Commercial breeders use this approach extensively which may explain why the main inbred lines used in hybrid corn are public inbreds or their derivatives. Note that only Pioneer insists that one inbred parent in their hybrids is proprietary (this may be changing -- Federal seed law does not permit naming the same genetic material the same name). Hence there are many look-alike hybrids.

A method to tell if a hybrid is the same as one of your hybrids is the following: Self the unknown hybrid and cross it your hybrid. If they are look-alike hybrids, then the self and the cross are identical. If they are different, the self will be different (usually yield less) than the cross. Can also use modern biochemical techniques (RFLPs).