Table of Contents
Ray, D.T., D.A. Dierig, and A.E. Thompson. 1990. Facultative apomixis in
guayule as a source of genetic diversity. p. 245-247. In: J. Janick and J.E.
Simon (eds.), Advances in new crops. Timber Press, Portland, OR.
Facultative Apomixis in Guayule as a Source of Genetic Diversity
Dennis T. Ray, David A. Dierig, and Anson E. Thompson
- APOMIXIS IN GUAYULE
- VARIATION ANALYSIS
- Fig. 1
- Fig. 2
The United States is totally dependent on imports of natural rubber for
industrial purposes. The annual cost of importing rubber amounts to nearly $1
billion (Thompson and Ray 1988). Guayule (Parthenium argentatum
Gray, Compositae), a small rubber-producing perennial shrub native to the
Chihuahuan desert of north-central Mexico and southwestern Texas, has been
long-recognized as a promising alternative source of natural rubber with the
potential for cultivation in the arid and semi-arid environments of the
Successful commercialization of guayule depends largely upon the development of
higher yielding cultivars from the available germplasm. Initially, it appeared
that the generic base from which improvement depended was narrow. Fifteen of
the 26 available families of germplasm are selections from bulk-seed from a
five-plant population collected at Durango, Mexico (Thompson and Ray 1988).
Materials derived from this collection have exhibited extreme variability both
within and between lines for rubber quality and quantity, dry weight, resin
content, yield, and ploidy level (Ray and Thompson 1986, Thompson and Ray 1988,
Thompson et al. 1988).
Guayule reproduces predominantly by apomixis (asexual reproduction by seed).
Although it has been assumed in guayule that apomixis assures genetic
uniformity from generation to generation, its facultative nature (apomixis and
sexuality coexisting) and the high amount of heterozygosity in individual
plants and the heterogeneous make-up of populations, results in the release of
considerable variation whenever sexual reproduction (amphimixis) occurs (Powers
and Rollins 1945).
Polyploidy and aneuploidy are common in guayule (Powers 1945, Bergner 1946, Ray
and Thompson 1986). Generally, diploids reproduce sexually and polyploids
reproduce by apomixis. In apomictic guayule the embryo sac develops directly
without meiosis from the megaspore mother cell (MMC). Pollination is not
necessary for embryo development, but is needed for normal endosperm
development (Powers 1945). Meiosis in the pollen mother cells is normal,
resulting in reduction of the chromosome number in the male gametophytes.
The facultative nature of apomixis in guayule results in four classes of
progeny (Esau 1946). The origin and relative chromosome numbers of these four
classes from tetraploid (2n = 4x = 72) parents illustrates the
complexity of reproduction and the potential for release of genetic variability
in this species (Fig. 1). The predominant class of progeny arise from
non-reduction of the MMC, without fertilization. These are apomictic
tetraploid progeny and are identical generically to the maternal parent.
Progeny from fertilized, unreduced MMCs will include plants with increased
ploidy levels. In our example, these progeny would be hexaploid (2n =
6x = 108). Polyhaploid (2n = 2x = 36) plants are the
result of meiotic reduction of the tetraploid MMC, and embryo development
without fertilization. The final class would be amphimictic tetraploid
(2n = 4x = 72) progeny that arise from normal reduction and
fertilization. Thus, two reproductive modes produce tetraploid progeny, one by
apomixis and the other by sexual reproduction. The remaining two progeny
classes vary in chromosome number from the parental population.
Our objectives were to quantify variation existing within and between guayule
breeding lines derived from single plants, and to understand how apomixis may
act to affect this variation. In 1986, 42 breeding lines were selected for
further study on the basis of superior yield, rubber concentration, and top
regrowth. Plants consisted of four-year old progeny rows from single plant
selections of facultatively apomictic plants. In February 1987, ten randomly
selected plants were individually harvested from each of these 42 lines and 17
characters were measured and described by Dierig et al. (1989a, b). Rubber
content (%), rubber yield (g/plant), dry weight (kg/plant), and height (cm)
were analyzed (Fig. 2).
Variation for the reported characters suggest that there are genotypic
differences between lines that could be exploited through further selection
(Dierig et al. 1989a, b). For example, the means of the 42 lines for rubber
content ranged between 4.9 and 9.8% (Fig. 2). Significant variation was also
observed within lines. Variation within some lines was less than 1% rubber
(e.g. 8.5 to 9.2%), while others were over 5% rubber (e.g. 6.5 to 11.7%).
Variation between and within lines for the other characters also varied
significantly (Fig. 2). The completion of tests designed to elucidate the
contribution of the genetic and environmental components and the analysis of
chromosome numbers of progeny will enable us to estimate the amount of
variability due to genetic hybridity, recombination, euploidy, and/or aneuploidy
There are two means by which variability can be produced in guayule
populations. The first is by genetic segregation from meiotic reduction of a
MMC, either with or without fertilization. Since guayule plants are highly
heterozygous, this would account for many new gene combinations in the progeny
The second is by the genetic imbalance resulting from polyploidy and/or
aneuploidy. Thus, the high amount of variability we have found among and
within guayule lines indicates that continued progress by selection is feasible.
- Bergner, A.D. 1946. Polyploidy and aneuploidy in guayule. U.S. Dept. Agr. Tech.
- Dierig D.A., D.T. Ray and A.E. Thompson. 1989a. Variability among and between
guayule lines. Euphytica 44:265-271.
- Dierig D.A., A.E. Thompson and D.T. Ray. 1989b. Relationship to morphological
variables to rubber production in guayule. Euphytica 44:259-264.
- Esau, K. 1946. Morphology of reproduction in guayule and certain other species
of Parthenium. Hilgardia 17:61-120.
- Powers, L. 1945. Fertilization without reduction in guayule (Parthenium
argentatum Gray) and a hypothesis as to the evolution of apomixis and
polyploidy. Genetics 30:323-346.
- Powers, L. and R.C. Rollins. 1945. Reproduction and pollination studies on
guayule, Parthenium argentatum Gray and P. incanum H.B.K. J.
Amer. Soc. Agron. 37:96-112.
- Ray, D.T. and A.E. Thompson. 1986. Chemical and cytological characterization of
the original 26 USDA lines. Proc. Sixth Annu. Guayule Rubber Soc. Conf. College
- Thompson, A.E. and D.T. Ray 1988. Breeding guayule. In: Janick, J. (ed.). Plant
Breeding Reviews 6:93-165.
- Thompson, A.E., D.T. Ray, M. Livingston and D.A. Dierig. 1988. Variability of
rubber and plant growth characteristics among single-plant selections from a
diverse guayule breeding population. J. Amer. Soc. Hort. Sci. 113:608-611.
Fig. 1. Reproduction in guayule resulting in four classes of progeny.
Fig. 2. Variability for (A) rubber content(%),
(B) rubber yield
(C) dry weight (kg/plant), and
(D) height (cm) among and within 42
progeny rows from single plants. The lines are in the same order for each
character and within each line the square represents the mean, the vertical
line the range, and the dots the standard error for the population.
Last update August 27, 1997