100 Years of Breeding Guayule - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 351

winter cycle) before the first harvest at year 2; reproduction is essentially asexual
(asexual reproduction by facultative apomixis); and because breeding plots must be
maintained for many years to evaluate multiple harvests from regrowth. As
described above, at first glance the available germplasm from which selections
were made appeared rather narrow, but the first breeding successes were through
either mass selection or the selection of individual high-yielding plants [ 1 , 4 ,
15 ]. Another limitation to breeding progress is the lack of a nondestructive test to
screen for variation in rubber/latex content. Whole plants must be harvested and
then ground, subsampled, and analyzed for rubber, latex, and resin content. This
severely limits the plant breeder in the number of plant samples to screen from large
populations.
The most extensively employed breeding approach in guayule has been single-
plant selections from within apomictic polyploid populations. Selection of individ-
ual plants is usually the simplest and most rapid method when heritabilities for
desired characters are high. If heritabilities are high, increases can be made in a
short period of time, but the long-term potential is for only modest gains since new
genetic combinations are not being produced. Thus, the degree of success using this
method depends first upon the amount of heterogeneity in the population; second,
whether or not the differences are genetic; and third, on the number of plants that
can be screened [ 1 ]. This method increased annual rubber yields from approxi-
mately 300-1,000 kg/ha, by selecting for the components of yield described
previously, but predominately by selecting simultaneously for increased rubber
concentration (%) and dry matter or biomass production [ 19 , 22 ].
When heritabilities are low, single-plant selection is not as effective as family
selection [ 5 ]. In family selection, families of progeny, either full sibs or half sibs,
are used to evaluate the quality of the parent plants. Thus, parent plants are not
selected on their own merits but on those of their progeny. The disadvantage of
family selection is that there is a lengthened generation interval. However, because
guayule is a perennial plant with almost continuous flowering, many generations of
progeny can be obtained from a single plant once it has been selected as a suitable
parent.
Mass selection is one of the oldest plant breeding methods, and significant gains
can be achieved in a relatively short period of time because only the top yielding
plants in a population are selected to become the parents of the next generation.
Today, mass selection is used to enhance germplasm and develop cultivars, espe-
cially in crops where there are few individuals involved and cross pollination is the
major mode of reproduction. Mass selection has been used in sexual diploid
populations by Ray et al. [ 23 ], in which, after three cycles of selection, a diploid
line tolerant to Verticillium dahlia was developed.
Mass selection has never been used extensively in polyploid guayule because, to
enhance populations using this method, one must be able to screen effectively many
plants (hundreds at minimum, to thousands optimally), and cross pollination must
be the major mode of reproduction (facultative apomixes will slow the selection
process). However, mass selection has been used effectively to develop uniform
lines once selections have been made. Mass selection does have potential in