INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

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

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

MULTILINE VARIETIES

Breeding methods for self pollinated crops were developed around the "pure line" concept in which uniformity within selected lines was stressed.

In some cases this led to a threat of widespread disease, particularly where only one gene for resistance had been incorporated into the leading varieties.

The need for diversification in resistance genes was recognized. One solution proposed was the multiline variety.

Multiline variety (first proposed by Jensen in 1952 for oats and later by Borlaug as a means for combating stem rust in wheat).

= Composite of genetically identical lines, except that each line possesses a different gene for resistance to the disease (introduced via backcrossing) = composite of isolines.

An assumption is made that each introduced gene will contribute resistance to a separate physiologic race, or group of races, of the pathogen.

By mixing different combinations of the isolines, the multilines can be reconstituted each year, making it possible to change the component isolines as changes occur in the prevalent races of the pathogen.

In the event of occurrence of a new race some plants may be susceptible, but not all. This mix of susceptible and resistant plants should provide a buffering effect against rapid disease development and thus extend the life of resistance genes.

DISADVANTAGES OF THE MULTILINE CONCEPT:

1. Limited utility except in high risk areas;



2. No genetic improvement is made in yield or other characteristics except that provided by disease resistance and as long as the same variety is used as the recurrent parent;



3. Labor required to produce and maintain the isolines reduces the resources available for improvement in varietal characteristics other than disease resistance;



4. The multiline may need to be reconstituted each year to insure the correct proportion of lines.