100 Years of Breeding Guayule - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 352

guayule since many of the important characters in guayule appear to be multigenic
with low heritability, but as stated above success will be enhanced when a screening
procedure is developed that will allow for large numbers of plants to be sampled in
a timely manner. Mass selection should be successful since polyploid guayule
populations are fairly variable because (1) new genetic combinations are continu-
ally being produced, (2) the plants are highly heterozygous due to both self-
incompatibility and apomixes, and (3) germplasm lines in which mass selection
can be performed have not been selected for uniformity. Unfortunately, many of
these germplasm lines are already planted and have been grown for many years,
compounding the environmental effects and making selecting for genetic differ-
ences more difficult.
Hybridization of apomictic polyploids is a method that has been suggested, but
has been used sparingly because of the problems of separating the offspring that
arise from sexual reproduction from the apomicts. Plants expressing high levels of
sexuality could be identified using the method of Keys et al. [ 24 ] and crossed to
produce new genetic combinations from which further selections could be made.
Seed would be collected from the hybrid plants, planted, and tested for apomictic
potential. If the resulting progeny is predominately apomictic, seed from them
would be placed in progeny trials and tested for possible release as new lines. If
the plants are predominately sexual, they could be backcrossed to enhance certain
characteristics, self-pollinated to produce a segregating population from which
more selections could be made, or apply standard breeding strategies generally
not used in guayule.
Another tool has become available with the successful use of flow cytometry to
determine ploidy levels in guayule [ 17 , 18 ]. Segregating breeding populations can
be screened to identify diploid plants for use in crossing programs while at the same
time identifying polyploid plants for evaluation in yield trials. This would be
especially helpful if large numbers of plants could also be simultaneously screened
for rubber and resin content to identify the most promising plants.
Interspecific hybridization has been applied on only a limited scale [ 5 ]. None of
the other Parthenium species produce an appreciable amount of rubber, although
they should be considered as potential sources of vigor, increased resin content,
increased biomass, disease and insect resistance, regrowth ability after clipping,
and cold tolerance. The major disadvantage of interspecific hybrids is that it will
take a large number of backcross generations to guayule to increase the rubber
content as well as to keep the new desirable trait(s). The University of California-
Riverside has released three germplasm lines (Cal-1, Cal-2, and Cal-5) that were
developed from interspecific crosses of guayule with three different Parthenium
species [ 25 , 26 ]. These three have increased vigor, biomass production, and resis-
tance to Verticillium wilt. AZ-101, a vigorous natural interspecific hybrid, is an
open-pollinated cross between guayule and Parthenium tomentosum var. stramo-
nium [ 1 , 19 ]. This line was also known as Gila 1 and used extensively during the
third commercial effort. The low rubber content made successful rubber extraction
more difficult even though the high biomass indicated yields per hectare were
higher.