100 Years of Breeding Guayule - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 348

California [ 1 ]. A selection made by McCallum in Salinas, “593,” was the principal
germplasm utilized for guayule production in the 1920s, 1930s, and the Emergency
Rubber Project [ 1 ]. During the Emergency Rubber Project, a major activity was
germplasm and cultivar development. The breeding material developed during this
time became the basis for the research efforts starting in the 1970s and continuing to
the present [ 1 , 5 ].
There were two main germplasm collection expeditions during the Emergency
Rubber Project, the second major commercialization effort. LeRoy Powers,
W.B. McCallum, and D.S. Olson collected 66 accessions from 24 locations in
Mexico; and Powers and W. Federer collected 368 accessions from 21 locations
in Texas. These accessions were then planted and evaluated at Salinas in 1943. In
1948, B.L. Hammond and J. Hinton collected an additional 174 accessions from
93 locations in Mexico [ 1 ]. The USDA guayule breeding program at Salinas,
California, was terminated in 1959, and 24 germplasm lines, developed by
H.M. Tysdal from the Powers, Hammond, and Hinton collections, plus line “593”
developed by McCallum, were selected for storage at the USDA National Seed
Storage Laboratory (now the National Center for Genetic Resources Preservation)
at Fort Collins, Colorado, in 1965. These 25 lines, selected on the basis of their
rubber production and plant growth characteristics, were the only ones saved from
the hundreds of selections, breeding lines, and accessions stored at Shafter and
Salinas, California. These 25 lines plus the line “Bulk Richardson” (from
D.D. Rubis; a bulk seed collection from Mexico made by Richardson) became
what was commonly called the 26 USDA germplasm lines, from which the breed-
ing programs in the 1970s began [ 1 ].
Interestingly, 21 of the 26 USDA lines came from the state of Durango, Mexico.
The apparent narrow germplasm base is accentuated by the fact that 15 of the lines
descended from the Powers, McCallum, and Olson collection #4265, which was a
bulked seed collection from five plants at one location. The original diploid material
came from collection #4254, which was also bulked seed from five plants at one
location [ 1 ].
In 1976, R.C. Rollins made collections from 45 locations in Mexico. In 1977,
C.T. Mason collected related Parthenium species throughout Mexico, Naqvi and
Hanson collected guayule from 50 locations in Mexico (also in 1977), and in 1982
Tipton and Gregg collected seeds from 10 native populations in Texas. An exten-
sive effort was mounted in 1982 by Mexican scientists who collected 3,000
accessions from 310 locations from six states [ 1 ]. Unfortunately, it is unclear
where most of these accessions are today. This germplasm appears to have been
lost during one of the many periods of inactivity when funding for guayule breeding
programs was discontinued.
All breeding approaches depend upon the existing genetic variability found in
the available germplasm [ 5 ]. Even though this genetic base appears to be rather
narrow, it has not been a hindrance to guayule breeding programs. This is probably
because the highly heterozygous genetic makeup of the plants in the guayule
germplasm collection and the facultative nature of apomixis in polyploid guayule
continually release new variability with each seed harvest. In fact, with the limited