|Ray, Dennis - THE UNIV OF ARIZONA|
Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 7, 2004
Publication Date: July 1, 2005
Citation: Ray, D.T., Coffelt, T.A., Dierig, D.A. 2005. Breeding guayule for commercial production. Industrial Crops and Products, 22(1):15-25. Interpretive Summary: Guayule is one of the most promising sources of hypoallergenic latex to replace latex products made from the Hevea plant that cause Type I allergies. For guayule to become commercially successful, significant advances need to be made in breeding and developing new higher yielding germplasm. Past efforts have been sporadic and as a result improvements in germplasm, while significant, have been slow. This current review of previous guayule breeding efforts indicates where progress has been made and where future studies need to be done. These results will be helpful to guayule breeders and administrators in identifying the most significant problems for future limited research efforts and areas that can use expanded funding and support.
Technical Abstract: Breeding a new domestic crop, such as guayule, is not appreciably different from enhancement and breeding of conventional crops. In both instances, plant breeders take the extant germplasm and search for genetic variability in the desired traits. The major differences are that in new crops plant breeders are often working with an unfamiliar species that is not yet fully domesticated and the available germplasm is often limited. The main objective of the guayule breeding program is to facilitate successful commercialization by developing higher yielding cultivars. Improvement has been accomplished, with newer lines yielding up to 250% more rubber than lines developed in the 1940s and 1950s. This is surprising because the genetic base from which improvements have been made appears to be very narrow, and because guayule reproduces predominately by apomixes (asexual reproduction by seed). Improvements through plant breeding are dependent upon genetic diversity within the available germplasm, and being able to identify different genotypes. It has been shown that the available germplasm exhibits extreme variability both within and between lines for morphological traits such as height, width, and biomass; chemical constituents such as rubber, resin, and latex contents; and genetic and chromosomal markers. The measured variation is due partly to the facultative (asexual reproduction and sexuality coexisting) nature of apomixes in guayule, which periodically releases genetic variation among progeny. It has also been shown that a great amount of this measured variation is due to environment, and selections, to take advantage of genetic differences, must be made within the first two-years of growth. There have been relatively few individuals involved in guayule breeding. Thus, with limited resources and time, most of the improvement has been made through single-plant selections from within populations. Although this method has the potential for only modest long-term gains, it requires a relatively short period to realize improvements. Indirect measures have been developed to increase breeding efficiency. For instance, most selections are made for plant height, width and biomass because they have been found to be highly correlated with rubber yield. As guayule approaches commercialization, breeding will become a priority and other breeding schemes will be tested and utilized such as: mass selection; recurrent selection among sexually reproducing diploids, followed by chromosome doubling; family selection; crossing high yielding apomictic plants; and crossing high yielding apomictic plants to sexual diploid plants, which should release significant genetic diversity.